goatai.io/glof · Case Study 02
Nepal 2026 — Langtang Lirung Ice–Rock Avalanche and Bhote Koshi–Trishuli Cascade
From a ~103 Mm³ rock–ice slope failure to debris flow and downstream flash flood
A ~103 Mm³ bedrock-and-glacier slope failure on Langtang Lirung became, within minutes, an ice-rock avalanche and then a debris flow, which travelled on into the Bhote Koshi–Trishuli corridor. The initiating process is now much better constrained. The main remaining uncertainty is how rock, ice and liquid water changed on the way to the downstream flood, and how they were progressively entrained, deposited and separated.
The water observed downstream cannot be read as a reservoir that existed at the source. The event has to be treated as a multiphase cascade that kept changing as it moved.
Preliminary — source constrained, transformation unresolved
Evidence updated: 21 Sep 2026
Classification: Langtang Lirung ice-rock avalanche → debris flow / debris flood → water-dominated flash flood farther downstream. Not a GLOF: no lake was found at the origin point. Every external value is dated, tiered and tied to an evidence-register entry; every model value is tied to a named simulation output. The four-member screening ensemble on this page predates the 20 September source reconstruction and is kept as provenance, uncalibrated and not re-run. What changed since 11 September · Changelog
- 1 · ~103 Mm³ initial mass · rock + ice (bulk volume, not water)
- 2 · ~12% ice · ~12.6 Mm³ (~88% non-ice, mostly rock)
- 3 · 0.92 km² glacier tongue struck (~30 s into the descent · area, not volume)
- 4 · ~20 Mm³ excess water downstream (estimated at Devghat, ~200 km on)
The Central Question
Where did the water come from?
The source failure was overwhelmingly rock, not water. The latest published reconstruction puts the initial volume at ~103 Mm³, of which only ~12.6 Mm³ — about 12% — was glacier ice. Yet about 20 Mm³ of excess water was later estimated passing Devghat. These two figures cannot be reconciled by treating the event as the instantaneous melting of the ice that first detached.
First-order check · GOAT AI derivation from published values
12.6 Mm³ ice × 917 kg/m³ ÷ 1,000 kg/m³ ≈ 11.6 Mm³ water [E-132 · E-160]
— an upper limit: it assumes 100% of the initial ice melted, which did not happen.
The initially detached ice alone cannot explain ~20 Mm³ of downstream excess water.
The gap is not an estimate of a missing lake. Nothing observed so far requires one: ICIMOD reports no lake at the origin point[E-144], and every lake with a reported volume formed after the flood[E-015 · E-056]. The gap is spread across several exchange terms, none of which has yet been measured.
Why 20 Mm³ does not imply a 20 Mm³ lake
A flood does not need to start as a lake
The ~20 Mm³ excess-water estimate describes water passing a gauge about 200 km downstream, after the cascade had crossed glacierised terrain and a river network. It is not an estimate of the liquid water present at the source.
Water could have built up progressively from the initial glacier ice, from ice in the glacier tongue the mass struck, from partial melting, from subglacial and englacial storage, from river and channel water, from saturated sediment and from tributaries — while solids were being deposited along the way.
Downstream flood water is an emergent product of the cascade, not necessarily a pre-existing reservoir released at T0.
Water-source ledger · five candidate contributors · none is quantified as a contribution
Source A
Initially detached glacier ice
12.6 Mm³ ice → ≤ 11.6 Mm³ water
Published volume · contribution unknown
The complete-melting limit. The actual contribution is lower than that, because complete melting did not occur — unless ice beyond the initial 12.6 Mm³ was also melted. [E-132 · E-160]
Source B
Glacier tongue struck during descent
0.92 km² · volume not constrained
Area published · no volume registered
An additional ice reservoir available for entrainment, fragmentation and partial melting. The published figure is an area. No volume is assigned on this page, because none has been published that we can register. [E-133]
Source C
Englacial, subglacial and permafrost water
magnitude unresolved
Physically plausible contributor
Water stored beneath the glacier, in its drainage system, in saturated basal material, in fractures and permafrost, and carried within the moving mixture. WWA names these among the likely sources. [E-148]
Source D
Existing river and channel water
quantity unresolved
Established process
The mixture entered a live stream. ICIMOD describes it picking up gravel, boulders and water from the riverbed as it became a debris flow; WWA adds river water pushed ahead of the flow. [E-141 · E-148]
Source E
Saturated sediment and channel storage
not separately quantified
Physically plausible
Water held in alluvium, saturated channel deposits and floodplain sediment along the route, plus tributary inflow between the source and Devghat.
Every published quantity, by what it physically is
| Quantity | Current value | Type | Where · when | Confidence | Can it be added directly? | Evidence |
|---|---|---|---|---|---|---|
| Initial failed volume | ~103 Mm³ | rock + ice, bulk | Source slope, ~5,190 m · at failure | Latest peer-reviewed reconstruction | No | [E-131] |
| Initial ice | ~12.6 Mm³ | solid ice | Source slope · at failure | Latest peer-reviewed reconstruction | No | [E-132] |
| Initial non-ice remainder | ≈90 Mm³ | bulk, mostly rock | Source slope · at failure | GOAT AI derivation | No | [E-161] |
| Maximum water equivalent of the initial ice | ≈11.6 Mm³ | theoretical liquid upper limit | — (assumes 100% melting) | GOAT AI derivation | No | [E-160] |
| Lower glacier tongue struck | 0.92 km² | AREA, not volume | ~4,010 m · ~30 s after failure | Published reconstruction | No | [E-133] |
| Early deposition | ~67 Mm³ | debris, bulk | Chhochen Khola · within ~100 s | Published reconstruction | No | [E-134] |
| Continuing / remobilised mass | ~36 Mm³ | debris-flow bulk | Into Lende Khola · minutes | Published reconstruction | No | [E-134] |
| Excess water at Devghat | ~20 Mm³ | water (rating-curve estimate) | Devghat, ~200 km downstream · 14:10–18:00 NPT | Provisional observational estimate | No | [E-032 · E-146] |
| Corridor sediment deposition | ~30.5 Mm³ | deposited solids / debris | Along the corridor · after the event | Reported synthesis; method not stated | No | [E-147] |
These values belong to different state variables, locations and times. Adding or subtracting them does not make a mass balance.
The only arithmetic in this section is within a single study’s own partition (67 + 36 = 103 Mm³, as the abstract gives it) and the two GOAT AI derivations above. The Devghat figure is a clear-water rating applied to a sediment-laden flood [E-032]. The 30.5 Mm³ deposition figure reaches us through WWA; the underlying NDRRMA document has not been retrieved [E-147].
Source To Flood
How a rock–ice failure became a flood
A mass-transformation diagram, read top to bottom. Solid-bordered values are published; dashed amber exchanges are processes known or likely to have occurred whose size nobody has measured. The ~103 Mm³ that detached is not the ~103 Mm³ that moved downstream: most of it stopped within two minutes.
Stage 1
Source failure
North flank of Langtang Lirung · ~5,190 m · 0.76 km²
Stage 2
Descent
~1,200 m fall · ~30 s
ice → water: impact and frictional melt — amount unknownStage 3
Glacier interaction
Detached glacier tongue · ~4,010 m
The slide struck and entrained the tongue; Mw 5.1-equivalent signal [E-133]
0.92 km² tongue — area, not volume[E-133]+ additional glacier ice — volume unknown+ stored subglacial / englacial water — unknownStage 4
Chhochen Khola
Within the following ~100 s
− ~67 Mm³ debris deposited[E-134]Stage 5
Lende Khola
14.2 km in ~350 s · past Gyirong Port
Debris-flow bulk peak 2.46 × 10⁵ m³/s at the gorge near Gyirong Port — not a water discharge [E-135]
+ river water, gravel and boulders from the bed — unknown[E-141]+ partial ice melt — unknown− solids deposited — unknown
This is a transformation diagram, not a closed quantitative mass balance. Several exchange terms remain unknown.
Stages 1–5 follow Li et al. (Journal of Mountain Science, online 20 Sep 2026), from its abstract; the full text has not been reviewed here. Zhao et al. (16 Sep) describe the same chain as ice–rock avalanche (>7 km) → debris flow (>13 km) → flood (>80 km) [E-139], and name the source as the glacier toe of Tsangbu Ri, a neighbouring peak — a disagreement recorded, not resolved[E-138].
Three state variables
Follow the state, not the hazard label
Solid rock
- failure
- fragmentation
- entrainment
- deposition
Solid ice
- detachment
- entrainment
- fragmentation
- melting
Liquid water
- pre-existing storage
- melt generation
- river entrainment
- downstream routing
rock ⇄ moving flow · entrainment and deposition
ice → water · melting, one way
water ⇄ moving flow · storage, entrainment, separation
What travels downstream is not the original source mass, unchanged. Its physical state changes the whole way.
This is the representational problem behind physics-grounded AI: a model whose state variables conserve rock, ice and water separately and exchange them by physical laws can follow this cascade. A model that carries one hazard label and one volume cannot.
The principal unresolved mass-balance problem
Can we close the water budget?
Published information now constrains both ends of the cascade. The poorly observed part is everything in between.
Transformation terms — poorly observed
- additional ice entrained
- fraction of ice melted
- pre-existing liquid-water storage
- river-water entrainment
- sediment–water exchange
The downstream water volume emerged from a sequence of phase transformations and water entrainment whose individual contributions are still only partly quantified.
Epistemic State
What is established, and what is not
Updated 21 September. The initiating process is now substantially constrained. The uncertainty has moved down the cascade — from what failed to how rock, ice and water were exchanged on the way to the river. Confidence refers to the evidence for each statement, not to any model.
High confidence
- source failure on the north-facing flank of Langtang Lirung, in Nepal [E-140 · E-130 · E-150]
- a large failure of bedrock and glacier ice [E-143 · E-140]
- ~103 Mm³ as the latest published initial-volume reconstruction, with ~12.6 Mm³ of ice — the estimate exists and is peer-reviewed; its uncertainty is not stated in the abstract [E-131 · E-132]
- interaction with a lower glacier tongue [E-133]
- rapid transformation into an ice-rock avalanche and debris flow [E-133 · E-144 · E-139]
- substantial early deposition, and debris-rich propagation into Lende Khola [E-134 · E-135]
- about 20 Mm³ of excess water estimated at Devghat [E-032 · E-146]
- no extreme rainfall capable of explaining the flood [E-145 · E-137 · E-120]
- barrier lakes formed after the flood, not before it [E-015 · E-054 · E-055]
Moderate confidence · working interpretation
- partial melting contributed water [E-148 · E-141]
- subglacial and permafrost storage contributed water [E-148]
- river and channel water was incorporated into, or displaced by, the debris flow [E-141 · E-148]
- changing solid concentration drove a progression from debris flow to debris flood to a more water-dominated flood [E-149 · E-139]
No percentages are assigned to any of these.
Still unresolved
- volume of the lower glacier tongue entrained
- how much ice actually melted
- pre-existing liquid water in glacier and permafrost storage
- water entrained from the Lende, the Bhote Koshi and tributaries
- sediment–water exchange along the route
- short-lived obstruction, if any [E-078 · E-040 · E-041]
- whether transient impoundment materially altered the principal wave
- time-varying solid concentration
- the source hydrograph entering the river domain
- which peak the source lies on: Langtang Lirung or neighbouring Tsangbu Ri [E-130 · E-138]
- why seismic-inversion volumes are 2–5× the published reconstruction [E-070 · E-071 · E-131]
Current working cascade · principal physical narrative
- 01Bedrock–ice slope failure
- 02Ice-rock avalanche
- 03Lower-glacier impact and entrainment
- 04Rapid debris-flow transformation
- 05Water and sediment entrainment
- 06Downstream debris flood
- 07Progressive transition toward a water-dominated flood
Continuous propagation, branch A in the earlier ensemble, is now the principal working process. Sustained blockage (branch C) is no longer presented as a co-equal primary interpretation; see the earlier hypothesis-testing ensemble.
Secondary hydraulic modifier — unresolved
Transient obstruction
Did an obstruction lasting minutes form and release, and change the principal debris-flood wave? It remains physically possible: HiRISK reports the flow dammed the Lhende Khola “potentially only for a matter of minutes”, and a DHM official gave two blockage locations on the day[E-078 · E-040 · E-041]. It is carried as a question to investigate, not as the source of the flood.
Published source estimates, in the order they appeared
| Study / source | Date | Source volume | Ice fraction | Method | Status | Evidence |
|---|---|---|---|---|---|---|
| ICIMOD specialist, via Kathmandu Post | 28 Aug 2026 | 100–200 Mm³ (quantity type not stated) | — | not given | Early expert estimate · secondary | [E-072] |
| CLaSH StoryMap (Calvelage), regional stations | 2 Sep 2026 | 284–492 Mm³ bulk | — | long-period seismic force inversion, bulk density 2,200 kg/m³ | Not peer-reviewed · unreconciled | [E-070] |
| CLaSH StoryMap (Calvelage), teleseismic | 2 Sep 2026 | 197–349 Mm³ bulk | — | as above, global stations | Not peer-reviewed · unreconciled | [E-071] |
| Dahal (Tribhuvan University), via OnlineKhabar | 2 Sep 2026 | ≥ 25 Mm³ snow and ice (“very conservative”) | — | satellite-based, details not given | Early estimate · secondary | [E-073] |
| CU Boulder Today | 3 Sep 2026 | “50 million metric tons” of ice, rock and water | — | unattributed | Tertiary · not used | [E-074] |
| Zhao et al., Journal of Mountain Science | 16 Sep 2026 | no volume in abstract · source zone ~0.75 km² | — | multisource observation | Peer-reviewed journal · names the source as Tsangbu Ri | [E-138] |
| World Weather Attribution | 17 Sep 2026 | no volume · “roughly 2 km²” collapsed | — | multidisciplinary synthesis | Rapid assessment · secondary | [E-150] |
| Li et al., Journal of Mountain Science | 20 Sep 2026 | ~103 Mm³ rock + ice | ~12.6 Mm³ (≈12%) | remote sensing, seismology, hydrology, field evidence; Voellmy and Saint-Venant models | Latest peer-reviewed reconstruction · current best estimate | [E-131 · E-132] |
Adopting ~103 Mm³ as the current best estimate reflects its method and its peer review, not agreement: the seismic force inversions published three weeks earlier are two to five times larger, and that gap is not explained here. Until 20 September this page carried the range 25 to ~490 Mm³ with no preferred value. The earlier figures stay visible because they are part of how the reconstruction evolved.
Terms used on this page
Primary event: the Langtang Lirung ice-rock avalanche [E-140]
Transformation: ice-rock avalanche → debris flow / debris flood
Downstream: flash flood, water-dominated farther down [E-149]
Not used as established: GLOF, glacier collapse alone, landslide-dam outburst, dam-break flood. Where older wording appears in sources or in this page’s own history, it is labelled with its date.
Before The Failure
The slope may have been observable before it failed
Retrospective precursor observation — not demonstrated operational prediction
Up to ~30 cm of movement in the three to four weeks before failure
ICIMOD reports that satellite radar shows the slope had moved by up to 30 cm in the three to four weeks before it failed [E-142]. The statement comes from analyses shared in a scientific coordination group. The sensor, the processing and the error are not stated, and it has not been peer-reviewed.
It suggests the source zone was not necessarily physically static before failure. Whether that deformation could have been detected, interpreted and turned into an actionable warning in real time is a separate problem.
Many slopes creep and very few fail, so a retrospective signal on the one slope that did fail says little about how often such a signal would have produced a false alarm elsewhere.
Observing slow state change at a source zone
- InSAR — wide-area displacement; C-band decorrelates over snow and ice, and revisit is days
- GNSS — continuous point displacement where a station can be installed and powered
- Fibre sensing, where practical — strain and vibration along a cable
- Ground-based radar — near-continuous displacement of a visible face
- Optical displacement tracking — feature tracking from repeat imagery or fixed cameras
- Seismic and acoustic monitoring — cracking and rockfall activity
- Thermal and meteorological context — permafrost temperature, melt and water input
Event Reconstruction
What happened on 26 August 2026
Event origin, published reconstruction timings, reported onset, gauge rise onset and rise duration are different quantities and are labelled as such. Conflicting accounts are recorded side by side, not reconciled. A working timeline, not a validated model output — no row below is a timing from this page’s own model.
Shortly after 08:37 NPT on 26 August 2026, a very large mass of rock and ice failed from the north-facing flank of Langtang Lirung. The latest published reconstruction puts its initial volume at about 103 Mm³, dominated by rock, with about 12.6 Mm³ of glacier ice [E-140 · E-131 · E-132]. On the way down, the mass struck a lower glacier tongue, broke up and became a highly mobile ice-rock avalanche[E-133]. Much of it was deposited early, and part continued into the Lende Khola as a debris flow [E-134]. From there the moving mixture progressively took in and displaced water and sediment from the channel system as it travelled into the Bhote Koshi–Trishuli corridor[E-141 · E-148].
So what reached the downstream gauges was not simply the original landslide, nor simply meltwater, nor necessarily the breach of a lake that already existed. It was what a multiphase cascade had become by the time it got there.
25 Aug
Contested account
Dam formed the previous afternoon? (IRDR via ADRC)
A rapid analysis describes an ice-rock failure at 13:00–14:00 NPT on 25 August damming the river for 18–19 hours. The underlying report could not be retrieved, and the account conflicts with the USGS and Chinese timings below. Recorded, not adopted. [E-019]
08:37:10
Event origin
Landslide-type seismic signal, M5.2
USGS ComCat origin time, event type “landslide”, reviewed — first reported as an M4.4 earthquake and reclassified. NDRRMA describes the signal as generated by the mass movement itself. DHM separately relays a local M2.6 signal at 08:20 (single source, no attribution); ICIMOD reports anomalous signals at the Jilong and Zhangmu seismometers, with no causal link established. The ADRC chronology places a dam breach at 08:30–08:37. Li et al. attribute an Mw 5.1-equivalent signal to the glacier-tongue impact ~30 s into the descent, so whether T0 marks the start of failure or that impact is not resolved here — a difference of under a minute. [E-002 · E-003 · E-015 · E-012 · E-013 · E-018 · E-133]
failure + seconds
Published reconstruction
Slope failure begins on the north flank of Langtang Lirung
A 0.76 km² mass of bedrock and glacier ice, ~103 Mm³ with ~12.6 Mm³ of ice, fails at ~5,190 m and falls towards the Chhochen Khola. Timings in this and the next two rows are from Li et al.’s reconstruction, counted from failure onset — not clock times, and not model output from this page. [E-130 · E-131 · E-132]
~+30 s
Published reconstruction
Impact on the lower glacier tongue
The slide strikes and entrains a detached glacier tongue of 0.92 km² at ~4,010 m and develops into a large ice-rock avalanche. [E-133]
next ~100 s
Published reconstruction
Major deposition and transformation in the Chhochen Khola
~67 Mm³ of debris is deposited; the remaining ~36 Mm³ is remobilised as a highly mobile debris flow and enters the Lende Khola. [E-134]
~09:00
Reported flood onset
11:10–11:20
Gauge rise onset
Malekhu begins to rise; station destroyed mid-rise
+7.13 m by the last sample at 11:40, after which the station was lost — so the recorded rise is a lower bound. DHM’s warning and danger crossings are given as 11:20/11:40 in its press release and 11:26/11:43 in its technical report. [E-021 · E-024 · E-029 · E-030]
11:45:35
Second signal
Second landslide-type signal, M4.2
3 h 08 min after the first; location fixed to the first event; what failed is not stated. Distinct from the barrier-lake events of 27–30 August. [E-004]
14:20–16:00
Downstream gauge
Devghat peaks at 6.57 m
168 km below the border and beyond the modelled corridor. DHM rates the peak at about 5,850 m³/s and the excess volume above base flow at 19.96 Mm³ — a clear-water rating applied to a sediment-laden flood. WWA reports the same ~20 Mm³ as passing in under four hours. It is water passing Devghat, not water present at the source. [E-031 · E-032 · E-146]
27 Aug–4 Sep
Post-event secondary hazard
Newly formed barrier lakes
A lower lake near the Chhochen–Purepu confluence was put at ~2 Mm³ of water and overflowing on 27 August, with later estimates of more than 2.5 and up to 4 Mm³; China’s Ministry of Natural Resources put a funnel-shaped water body below the source at more than 120,000 m², depth unknown, on 28 August; UNOSAT mapped two new lakes on 28 August imagery; an overflow on 28 August was followed by a reported 0.6 m river rise (army spokesperson, no gauge location). The lower lake was reported drained by 30 August–1 September. A crater pond below the source was put at 1.4 Mm³ on 30 August and 0.65 Mm³ on 31 August; Nepal’s FFD reported an upper lake refilling on 4 September. None of these is the modelled T+3 h secondary pulse. [E-056 · E-068 · E-069 · E-053 · E-054 · E-055 · E-060 · E-058 · E-059 · E-050 · E-051 · E-062]
NPT is UTC+05:45; Beijing time is NPT+02:15. Human impact figures are still changing and are deliberately not published here. They are also excluded from model calibration — a reconstruction validated against casualty counts is not a physical model.
Corridor History
Same corridor. Two years. Two different source mechanisms.
On 8 July 2025 and on 26 August 2026 floods reached the Rasuwagadhi–Trishuli corridor from different glaciers in the same catchment, by different mechanisms.
A warning architecture built around a catalogue of known glacial lakes could recognise the first hazard class and still be blind to the second.
This does not imply that either event could have been predicted operationally. ICIMOD makes the same distinction: lakes can in principle be inventoried and watched, unstable rock faces are far harder to track[E-144].
The July 2025 event in detail
When
8 July 2025; first reported impact at the Rasuwagadhi border post about 03:15 NPT [E-093]
Source
Supraglacial lake system on the Purepu Glacier, upper Lende catchment, Tibet (China), ~5,160 m [E-094 · E-090]
Trigger
Supraglacial lake outburst — High confidence: NDRRMA situation report and a peer-reviewed field and remote-sensing study. A 2026 preprint notes englacial or subglacial water cannot be excluded; that concerns flood volume, not the source. [E-090 · E-091 · E-092]
Damage corridor
Nepal–China Friendship Bridge destroyed; Rasuwagadhi dry port and three hydropower plants damaged; 16 km of road [E-095]
Relation to 2026
Same catchment and corridor, different glacier and a different mechanism. The 2026 failure is on the north flank of Langtang Lirung; HiRISK reports the 2026 mass flow met the 2025 flood deposits. USGS notes that the same mountain produced the 2015 earthquake-triggered avalanche — the same mountain, not the same slope. [E-130 · E-078 · E-098 · E-100 · E-101]
Two major cryosphere-related cascade events reaching the same corridor within roughly 14 months illustrate why monitoring the corridor and its evolving source zones may be more useful than monitoring named lakes alone. For this reconstruction the 2025 flood matters twice over: its channel changes post-date the SRTM terrain the routing uses, and — if DHM records exist — it is the independent event on which the routing could be calibrated before 2026 is re-run.
Monitoring Lesson
The monitored object is physical state
Monitor the corridor, not the named lake — that lesson stands. This event extends it: a source-to-asset warning architecture has to observe three timescales.
Weeks to months
State evolution
- slope displacement
- glacier geometry
- permafrost and temperature
- fracture development
- hydrological loading
Seconds to minutes
Failure detection
- seismic
- fibre / DAS
- radar
- camera and computer vision
- acoustic signatures
Minutes to hours
Cascade propagation
- river stage
- corridor sensors
- radar and cameras
- surviving gauges
- physics-grounded routing
Hazard classification can come later. Warning has to follow the evolving physical state.
Monitoring Gap
What the corridor could not see
Four facts about this corridor on 26 August, each tied to an evidence-register entry. They are the reason the monitoring argument is about corridors rather than lakes.
A lake inventory would not have flagged the source
China’s Ministry of Water Resources reports no moraine lake larger than 0.1 km² on the Nepal side above Gyirong Port, and no pre-existing glacial lake failure has been confirmed. [E-066]
The source is in Nepal; the corridor crosses a border
The failure was on Langtang Lirung, in Nepal. The mass then entered the Lende Khola, which marks the Nepal–China border, and passed Gyirong Port on the Chinese bank — so part of the upstream corridor is observed through another jurisdiction’s reporting. Early reports placing the source or blockage on the Tibetan side are kept in the register as superseded. [E-140 · E-130 · E-141 · E-041 · E-066]
The unit of risk is the corridor and its evolving source zones, not a named lake.
Measurability
Why this event was hard to measure
Every quantity on this page descends from the two gauges that survived out of five. That is worth stating plainly, because it bounds what any reconstruction of this event can claim.
The forward map is many-to-one
Different mixes of melt, stored water and entrained river water — like the earlier A/B/C branches before them — produce downstream signatures the surviving observations cannot separate. That is non-identifiability, not noise: more gauges at Galchhi would not fix it. [E-148 · E-078 · E-009]
Before — susceptibility, not prediction
Retrospectively, this slope is reported to have moved by up to 30 cm in the three to four weeks before it failed [E-142] — a signal found afterwards, not one that was acted on (see above). InSAR and offset tracking can detect progressive deformation, but C-band decorrelates over snow and ice, revisit is 6–12 days against a final acceleration that can run in hours, and the base rate is unfavourable — many slopes creep, very few fail. Until 21 September this panel said no usable short-term precursor exists; that is revised to: none has been shown to be usable in real time.
The honest product of this phase is siting and susceptibility: which reaches deserve instruments, which source zones a lake inventory misses. This corridor was identifiable as high-priority in advance — the 2015 avalanche came off the same mountain [E-100] and the 2025 flood came down the same corridor from a different glacier [E-078 · E-090]. Nothing would have given the date.
During — not Earth observation
Revisit in days, latency in hours, and near-permanent monsoon cloud: across the whole post-event window to 13 September, no Sentinel-2 scene over this corridor came in below 40% cloud[E-046].
This phase belongs to seismic and infrasound at the source, to in-corridor sensing that either survives or fails informatively, and to the telemetry-loss logic above. Seismic also does not respect the border: the corridor crosses it and the signal was recorded and timestamped regardless [E-002 · E-010].
After — where measurement is possible
The only phase that can be instrumented retrospectively, because the archive already holds both 8 July 2025 and 26 August 2026. Corridor state, geomorphic change, exposure — and the calibration that would make any timing claim trustworthy.
The 2025 analogue is already measurable: its post-event window cleared to 1% cloud by late September, while the 2026 window has not yet reached that point[E-046]. That asymmetry is the organising fact of this section — Earth observation carries the before and after phases, and carries nothing during.
Warning Lead Time
How much warning existed
Reconstructed from the gauge record and DHM’s own account. T0 is the USGS origin of the slope failure at 08:37:10 NPT, and every offset is arithmetic on it. Lead time is measured from the first warnings issued at 09:15 NPT to the local gauge rise onset.
| Corridor point | Clock (NPT) | Offset from T0 | Lead time after first warning | Evidence |
|---|---|---|---|---|
| Ice-rock avalanche seismic origin (T0) | 08:37:10 | — | — | [E-002] |
| Rasuwagadhi gauge · last telemetry, no rise recorded | 08:40 | +3 min | no warning yet | [E-026] |
| Gyirong Port / border reached, ~22 km | ~08:44 | ~+7 min | no warning yet | [E-016 · E-017] |
| Syabrubesi gauge · last telemetry, no rise recorded | 08:50 | +13 min | no warning yet | [E-027] |
| Flood Forecasting Division notified | 09:00–09:05 | +23 to +28 min | — | [E-033 · E-037] |
| First DHM warnings issued | 09:15 | +38 min | 0 — the reference point | [E-033 · E-037] |
| Betrawati gauge · last telemetry, no rise recorded | 09:20 | +43 min | no rise recorded | [E-028] |
| Galchhi rise onset | 10:20–10:30 | +1 h 43 m to +1 h 53 m | 65–75 min | [E-020 · E-038] |
| Malekhu rise onset | 11:10–11:20 | +2 h 33 m to +2 h 43 m | 115–125 min | [E-021 · E-038] |
| Devghat rise onset · 168 km, outside the modelled corridor | 14:10–14:20 | +5 h 33 m to +5 h 43 m | about 295–305 min | [E-036 · E-038] |
Above Betrawati: no warning at all
Downstream: lead time existed
Below Galchhi the gap between the first warning and the local rise was over an hour and grew with distance — 65–75 min at Galchhi, 115–125 min at Malekhu, about 5 h at Devghat[E-038]. It is in the record, not produced by any model.
What the detection chain cost
About 23–28 min from failure to institutional awareness and about 38 min to a first warning[E-037], on a corridor that crosses an international border below its source and where no gauge survived above Betrawati.
Lead time is a property of location, not of technology
Timure and Syabrubesi had effectively none. Galchhi had over an hour and a half from the failure, Devghat most of a working day [E-038]. No improvement in sensing, classification or dissemination changes those numbers, because they are set by the distance between the source and the asset. The corollary is uncomfortable: some assets cannot be protected by a warning system at all, and need structural or operational measures instead.
Do not generalise these figures. They are observed at four real stations on one corridor. Extending them to any other location needs a travel-time model, and this page’s own ensemble misses the observed arrivals by hours — see the routing result. Until that routing is calibrated, lead time cannot be stated for a location without an observed arrival.
These are the lead times this event’s detection chain happened to produce, reconstructed after the fact. They are not a system performance claim and not a forecast of what any warning system would achieve.
The corridor did detect the wave — in the loss of the signal, not in the signal
Three stations stopped transmitting in strict downstream order — Rasuwagadhi 08:40, Syabrubesi 08:50, Betrawati 09:20 — and none of them recorded a rise first[E-026 · E-027 · E-028 · E-039]. The sequence is consistent with the wave passing each site. The second loss, at 08:50, came 25 minutes before the first warning was issued at 09:15; the third came 5 minutes after it. No supervisory logic existed to treat coincident, correctly ordered loss of telemetry as an affirmative input.
The limits matter as much as the finding. A last transmission is an upper bound on the time of loss, resolved only to the 10-minute polling interval. Loss of transmission has causes other than destruction — power, communications, the station itself — so the ordering is consistent with propagation rather than proof of it, though DHM separately states the Rasuwagadhi station was swept away [E-026]. An apparent mean celerity of about 19 m/s over the 45.8 km between the first and last loss follows from these times, but it rests on model chainage rather than surveyed distances and is order-of-magnitude only [E-039]. This is a design finding recovered from the record afterwards — not a method that was available on the day.
The gap was not a sensing gap
The physical signal existed at T+0 and was recorded by existing regional and teleseismic networks, with an origin time resolved to the second [E-002]. The first warning went out at 09:15, about 38 minutes later [E-045], and it went out because people telephoned each other: DHM was notified at 09:00–09:05 by the district administration and by station staff[E-033].
What that does not mean: that a warning could have been issued minutes after the failure. An automatic seismic solution takes minutes rather than seconds; this event was initially catalogued as an M4.4 earthquake[E-012 · E-003] and only later reclassified as landslide-type[E-002]; and the published classification wording was still being revised on 28 August [E-006 · E-005]. The defensible statement is narrower and more useful: the signal was available long before the warning, real-time discrimination of landslide-type signals from earthquakes is the missing capability, and no route existed from a seismic detection into this corridor’s warning chain.
River Corridor
Source area to Malekhu — the reach under reconstruction
Roughly 100 km of channel from a glacierized source slope to the Trishuli gauges, crossing a transboundary headwater, a contested blockage reach and a densely used highway corridor. Three of the corridor gauges were destroyed before recording the wave.
04 · First major impact zone
Rasuwagadhi / Miteri Bridge
Gauge lost after 08:40, no rise recorded
[E-026]Exposure
What was in the path
Asset classes along the corridor, from what the evidence register supports. Where a quantity is not supported, the asset class is listed as not assessed rather than left out. Human-impact figures are excluded from this page.
Corridor gauges
Four lost: Rasuwagadhi, Syabrubesi and Betrawati stopped transmitting before recording any rise; Malekhu was destroyed mid-rise, so its +7.13 m is a lower bound [E-026 · E-027 · E-028 · E-021]
Bridges
Damage reported along the corridor; count and identity not assessed here. In July 2025 the same corridor lost the Nepal–China Friendship Bridge at Rasuwagadhi [E-121 · E-095]
Highway corridor
Damage reported; length not assessed here. The July 2025 event left 16 km of the Syafrubesi–Rasuwagadhi road impassable [E-121 · E-095]
Dry port (Rasuwagadhi)
Not assessed for 2026. Large sections were damaged in July 2025 [E-095]
Hydropower assets
Damage reported; capacity, outage duration and per-scheme identity not assessed here. Three plants were damaged in the July 2025 event [E-121 · E-095]
Settlements along Rasuwagadhi–Timure–Syabrubesi
Severe damage reported, qualitative only. Human-impact figures are excluded from this page by policy [E-121]
Worked illustration · not a result for any real asset
For a hypothetical intake or barrage sited on the Galchhi reach, the observed forcing is a stage rise of +8.83 m in 30 minutes[E-022], with a screening water-equivalent peak band of ≈4.6–8.3 × 10³ m³/s from the order-of-magnitude hydraulic check (model output, sim-claude edff56c · outputs 4 Sep 2026 · diagnostics 11 Sep 2026). Dam-break amplification of the kind reconstructed at Chungthang for Sikkim is not assessed here, and neither is debris impact loading, sediment burial or intake blockage. This is an illustration of what a consequence assessment takes as input, not an assessment of any asset.
Method
What is transferable here is the procedure, not this event
The reconstruction below runs a fixed procedure: an evidence register in which every datum carries tier, confidence and permitted use; a tiered epistemic state separating what is established from what is unresolved; an end-member ensemble spanning the mechanism branches rather than a single preferred scenario — kept visible, not removed, once the evidence moves past it; per-member consistency checks against observations the model was not built to match; a published missing-data register; and a dated changelog that keeps superseded interpretations visible. The same procedure applies to any asset corridor, and it produces those six artefacts each time. Everything on this page is screening-level and preliminary.
Superseded as primary event interpretation — retained for auditability
Earlier hypothesis-testing ensemble
These simulations were built when the source process was much less constrained. They stay published as provenance, showing how the reconstruction changed as evidence arrived. The three branches below are shown as they stood on 11 September, with where each stands after the 21 September evidence. Both events are cryosphere-driven cascades, but only South Lhonak is established as a classical GLOF pathway.
Why the unresolved mechanism did not block operational use · 11 September, still holds
The required downstream response is the same under all three branches. A hydraulic wave of this magnitude arrives at Galchhi and Malekhu whether it came from continuous propagation, a minutes-scale plug or a sustained blockage [E-020 · E-021 · E-022]. So a warning chain has to trigger on source signal and observed corridor propagation, not on a mechanism classification: that classification was not available on 26 August, is still not available now, and is not needed in order to act. The branches below matter for reconstructing the event and for deciding what to monitor at the source — not for deciding whether to warn.
Sikkim 2023 · reconstructed
Moraine-lake outburst archetype
- 01Moraine / ice-rock collapse
- 02Impulse wave into existing proglacial lake
- 03Moraine overtopping and breach
- 04Outburst hydrograph
- 05Downstream river routing
- 06Hydropower cascade failure
Nepal 2026 · Branch A · earlier hypothesis
Continuous propagation
- 01Ice-rock slope failure
- 02Direct channel entry
- 03Entrainment and melt
- 04Debris-rich hydraulic wave
Members: direct — mobilised from T+0
21 Sep · Now the principal working process — see the working cascade above.
As of 11 Sep · Currently more consistent with the available timing and event-classification evidence, while the hydraulic pathway remains unresolved. USGS describes the debris entraining water and material “in the existing stream and river channels”; one Chinese expert states there was no barrier lake in the original process; the reported runout to the border post is 6–7 minutes. [E-009 · E-065 · E-016]
Nepal 2026 · Branch B · earlier hypothesis · not modelled
Transient plug (minutes)
- 01Ice-rock slope failure
- 02Channel obstruction lasting minutes
- 03Rapid release
- 04Debris-rich hydraulic wave
Members: not represented in the current ensemble
21 Sep · Retained as an unresolved secondary hydraulic modifier, not as the source of the flood.
As of 11 Sep · Physically distinct from both A and C, and the gap in the present ensemble. HiRISK reports the mass flow dammed the Lhende Khola “potentially only for a matter of minutes”. Until members with obstruction durations of minutes are run, the ensemble cannot claim to span the mechanism space. [E-078]
Nepal 2026 · Branch C · earlier hypothesis
Sustained blockage
- 01Ice-rock slope failure
- 02Channel blockage
- 03Impoundment
- 04Breach or overflow
Members: low · medium · high — release 0.5–2 h after the failure
21 Sep · Superseded as a primary event interpretation — retained for auditability.
As of 11 Sep · Less consistent with the current timing and classification evidence after the refresh. The 6–7 minute runout, the landslide-type classification of the M5.2 signal and NDRRMA’s placement of the lakes after the flood leave little room for a blockage that stored water for hours after 08:37. A variant with the dam formed on 25 August rests on one rapid analysis whose report could not be retrieved. [E-016 · E-002 · E-015 · E-019]
A named lake is not the unit of risk
China’s Ministry of Water Resources reports no moraine lake larger than 0.1 km² on the Nepal side above Gyirong Port [E-066], and ICIMOD reports no lake at the origin point[E-144], so the event is not labelled a GLOF. The distinction is operational rather than semantic: monitoring has to include unstable glacierised slopes, transient channel obstruction and downstream cascade propagation alongside persistent glacial lakes. The corridor evidence behind that conclusion is set out near the top of this page.
Historical Screening Simulation
The earlier ensemble, running
The four-member ensemble represents two modelled end-member branches within a three-branch physical hypothesis space: branch A continuous propagation, branch B a minutes-scale transient obstruction not yet modelled, and branch C sustained blockage. Routed down the real corridor — OpenStreetMap river centerlines, an SRTM long profile, landslide-dam breach hydrographs and debris-bulked Muskingum-Cunge routing to the Trishuli gauges. Preliminary and uncalibrated: the point is where the members disagree, not any one line.
Historical screening ensemble — retained for provenance
These four simulations were built before the 20 September quantitative source reconstruction was available. They test earlier hydraulic end members and should not be read as the current best reconstruction of the source process. Interactive access is kept on purpose. Why they are being superseded ↓
Corridor Modelled
127.9 km
Source reach → Malekhu
Ensemble Members
4 members
End-member experiments · 2 of 3 branches
Peak at Source
0.6k–10.7k m³/s
Water-equivalent; bulked ×1.3–3.0 in routing
Observational Checks
5 evaluable
Timing now evaluable · rapid onset moved to internal
Branch C · sustained blockage
Branch A · continuous propagation
The embedded view plays the ensemble as run on 4 September. The evidence-consistency assessment reflects the 11 September evidence; the physical interpretation on this page reflects the 21 September evidence and supersedes the embedded panels where they differ — in particular, sustained blockage is no longer a primary interpretation. No parameter was fitted to a gauge observation and no source volume enters the model.
Open Simulation →Results
What the ensemble produces
Historical ensemble, as run on 4 September. Two modelled end-member branches within a three-branch physical hypothesis space: the members bracket continuous propagation (branch A) and sustained blockage (branch C), while a minutes-scale transient obstruction (branch B) is not yet modelled. Every value below is model output from sim-claude edff56c · outputs 4 Sep 2026 · diagnostics 11 Sep 2026. All four were specified after the ICIMOD stage reports were public, so none is an independent prediction of stage; none was specified with knowledge of the DHM timing.
| Member | Hypothesis | Source volume (type) | Blockage | Release mode | Bulking | Peak at source · water / bulked | Galchhi arrival | Galchhi stage rise · as run |
|---|---|---|---|---|---|---|---|---|
| low | Sustained blockage | 0.6 Mm³ impounded water (scenario) | 15 m | overtopping erosion | ×1.3 | 634 / 824 m³/s | T+8.2 h | 1.3 m |
| medium | Sustained blockage | 1.4 Mm³ impounded water (scenario) | 25 m | progressive debris release | ×1.8 | 2,364 / 4,255 m³/s | T+6.6 h | 3.5 m |
| high | Sustained blockage | 6 Mm³ impounded water (scenario) | 40 m | rapid dam collapse | ×2.5 | 10,736 / 26,839 m³/s | T+5.1 h | 9.9 m |
| direct | No sustained blockage | 5 Mm³ assumed hydraulically mobilised volume (scenario) | none | no impoundment | ×3 | 9,564 / 28,691 m³/s | T+4.0 h | 13.4 m |
Water peak: water-equivalent discharge (breach release + avalanche surge + baseflow). Bulked peak: water × bulking, the debris-bulked volume flux that is routed. The direct member’s 5 Mm³ is an assumed hydraulically mobilised volume — the scenario label from which its routed 4 Mm³ water-equivalent surge was set. It does not enter the solver, and it is not an estimate of the total failed rock-and-ice mass. Geological source volume, hydraulically mobilised volume and water-equivalent flood volume are three different quantities. The latest published reconstruction, ~103 Mm³ of rock and ice, is the first of these [E-131]; at the 11 September freeze the estimates available spanned 25 to ~490 Mm³ of bulk material [E-070 · E-073]. No member is parameterised from an observed failure volume or an observed store.
The finding worth arguing about
Independent arrival observations reject something all four members share — before they resolve the source mechanism
The 4 September version argued that the reported gauge response needed an order of magnitude more water than the “reported ~1.4 Mm³ impoundment”. That figure describes a pond measured four days after the flood [E-050], so the argument compared the flood with a store that did not exist when it happened. It is withdrawn.
What the miss costs operationally. Every member, including the one released at T0, reaches Galchhi hours after the observed onset[E-020]. What the observation rejects is the routing configuration common to all four. For anyone building a warning system on the same ingredients — OpenStreetMap centrelines, an SRTM-era long profile, conventional Manning values and a bulking proxy — that is a systematic overstatement of the travel time available. A system parameterised that way would promise lead time it cannot deliver, and the error stays invisible until an event tests it. Corridor-specific calibration against a recorded event on the same reach is not a refinement; it is the precondition, in the order set out below.
Independent timing test
The ensemble was configured on 4 September, before the DHM gauge rise-onset timestamps used here were recovered on 11 September. No member was fitted to arrival timing, and the tolerances (±30 min on an arrival, ±20 min on the travel time) were fixed before any timestamp was known. These observations are therefore an independent diagnostic of the routing configuration rather than a test the model was built to pass — which is why the result below is read as previously unused observations exposing a shared structural deficiency, not as a simulation that simply failed. [E-020 · E-021 · E-002]
What the observations show
Galchhi began rising between 10:20 and 10:30 NPT and Malekhu between 11:10 and 11:20 — T+1.7–1.9 h and T+2.5–2.7 h after the USGS origin. Each rise lasted about 30 minutes; the onsets are about 50 minutes apart. [E-020 · E-021 · E-025 · E-002]
Chinese accounts put the impact at Gyirong Port 6–7 minutes after the collapse [E-016 · E-017]. Every lake with a reported volume (0.65–4 Mm³ of water) was observed from 27 August onward, and NDRRMA places the lakes after the flood [E-015 · E-051 · E-056 · E-069]. The only account of an impoundment before the flood could not be traced to its source report [E-019].
At the 11 September freeze, source-volume estimates from identifiable scientists ranged from 25 to ~490 Mm³ of bulk material, none peer-reviewed [E-073 · E-072 · E-071 · E-070]. The first peer-reviewed reconstruction (~103 Mm³, 20 September) came after the ensemble [E-131]. All are bulk source volumes, not water volumes, and none is compared numerically with the ensemble.
What the model shows
Timing. Every member misses both arrival checks. The direct member releases at T0 and still reaches Galchhi 2.1 h after the observed onset window. Because the bias is common to both mechanism branches, the observed timing rejects the shared routing configuration as presently parameterised. Which part of that configuration is responsible is not yet isolated: candidates include the channel geometry and assumed hydraulic widths, the SRTM-derived long profile and slopes, Manning resistance, the debris-bulking representation, reach discretisation, source hydrograph shape, flow rheology, the arrival-detection criterion, and transient storage or acceleration the model does not represent.
Stage. As run, only the high member reaches both stage bands, on a 6 Mm³ impoundment, a 40 m dam, rapid collapse and ×2.5 bulking — its water peak is 5.75× the Costa & Schuster estimate. That is assumption-intensive, not invalid. After the baseline defect is removed, the direct member reaches both bands too; low and medium reach neither under any metric.
Order-of-magnitude hydraulic consistency. Inverting Manning normal depth at Galchhi’s +9 m on assumed geometry puts the flow at roughly 2 × 10⁴ m³/s of clear water, or ≈1.2–1.5 × 10⁴ m³/s as a debris-bulked volume flux at ×1.8–2.5, which is ≈4.6–8.3 × 10³ m³/s water-equivalent. Held for a triangular 1–3 h wave, the water-equivalent volume spans roughly 7 to 115 Mm³ across the three roughness cases. The Galchhi stage stayed more than 1 m above its pre-flood level for about 3 hours [E-035], which favours the longer durations. This is a screening check on whether the simulated flood magnitude is physically compatible with the observed stage — not a discharge reconstruction or a gauge rating, and there is no rating curve to check it against.
What the model cannot establish
Which mechanism occurred. After correction, the stage checks accept one member from each family, and the timing checks reject all four for a reason unrelated to mechanism. Since 21 September there is a further limit: no member carries rock, ice and water separately, so the ensemble cannot test the transformation now favoured.
Whether a transient plug formed and failed within minutes, as HiRISK suggests [E-078]. The blockage members hold water for 0.5–2 h and the direct member holds none; neither resolves a minutes-long blockage.
Any flood volume. The inversion volumes depend on assumed width, slope, roughness, baseflow and hydrograph shape, and they share the baseline convention of the stage defect. They are not comparable to the post-event lake storages or to the source bulk-volume estimates without a conversion model.
What would discriminate
A flow record at or above Rasuwagadhi for 25–26 August. A dam that held water for 18–19 hours should have cut flow downstream for most of a day. The public DHM endpoint no longer holds those days; Chinese-side records and 25 August imagery would serve too.[E-019 · E-034]
Timing, once the routing is constrained independently. The members release at different times, so arrival would separate them — but only in this order: constrain the routing on the July 2025 corridor event, freeze it before any exposure to the August 2026 timing observations, then add the missing transient-obstruction mechanism and re-evaluate the A/B/C branches against 2026 with no further parameter adjustment. Introducing a transient-plug member first would let a mechanism change absorb a routing error. That work is awaiting approval and is not part of this update.
Evidence Consistency
Evidence consistency, per member
Each check is assessed per ensemble member against that member’s own value — never against the envelope. A check with no observation is not-evaluable, not a pass. The point is which observations a member explains or contradicts, not a score. Casualty and missing-person counts are not used as checks.
| Member | Mechanism branch | Timing (3 checks) | Stage (2 checks) | Interpretation |
|---|---|---|---|---|
| low | C · sustained blockage | 0 of 3 consistent | 0 of 2 as run | Too small on stage everywhere, and its wave does not survive to Malekhu. Inconsistent with the observations. |
| medium | C · sustained blockage | 0 of 3 consistent | 0 of 2 as run | Too small on stage at both gauges. Inconsistent with the observations. |
| high | C · sustained blockage | 0 of 3 consistent | 2 of 2 as run | Reaches both stage bands, on upper-end assumptions (6 Mm³ impounded, 40 m dam, rapid collapse, ×2.5 bulking). Arrives hours late. |
| direct | A · continuous propagation | 0 of 3 consistent | 0 of 2 as run · 2 of 2 baseline-corrected | Reaches both stage bands once the baseline defect is removed; overshoots as run. Arrives earliest of the four, still ~2 h late. |
Counts of consistent checks, not a quality rating: a member is not “better” for passing more of five unequal tests. What matters is which observations each member explains and which it contradicts, and the per-check detail below carries that. For reference, the counts of consistent checks out of five evaluable are low 0/5 as run, 0/5 baseline-corrected · medium 0/5 as run, 0/5 baseline-corrected · high 2/5 as run, 2/5 baseline-corrected · direct 0/5 as run, 2/5 baseline-corrected.
Denominator: 2 stage checks + 2 arrival checks + 1 travel-time check; rapid onset moved to internal consistency. It was 3 on 4 September, when the timing checks had no observation; it grows when a check becomes evaluable and is never frozen. The five checks are not independent: Galchhi and Malekhu sample the same propagating wave, and the travel time is the difference of the two arrivals. [E-020 · E-021 · E-025]
Observational checks — per member (as run)
| Check | Observed | low | med | high | direct | Result |
|---|---|---|---|---|---|---|
| Galchhi arrival | Rise onset 10:20–10:30 NPT = T+1.71–1.88 h · tolerance ±30 min [E-020 · E-002] | ✗ | ✗ | ✗ | ✗ | Modelled T+8.19 h · T+6.63 h · T+5.06 h · T+3.96 h. Every member is late; the direct member, released at T0, reaches Galchhi 2.1 h after the observed onset window. |
| Malekhu arrival | Rise onset 11:10–11:20 NPT = T+2.55–2.71 h [E-021 · E-002] | ✗ | ✗ | ✗ | ✗ | Modelled no arrival · T+8.69 h · T+6.59 h · T+5.39 h. The low member has no arrival: its routed wave peaks 2.1 m³/s above baseflow at Malekhu, below the 5 m³/s arrival floor — scored a fail, not excluded. |
| Galchhi → Malekhu travel time | 40–60 min between onsets · tolerance ±20 min [E-025] | ✗ | ✗ | ✗ | ✗ | Modelled — · 123 min · 92 min · 86 min. Insensitive to the onset anchor. The direct member misses the tolerance by 6 min — close, and within the ±10 min sampling uncertainty of each onset. |
| Galchhi stage jump | +8.83 m in 30 min · band 6–12 m [E-022 · E-023] | ✗ | ✗ | ✓ | ✗ | As run: 1.3 m · 3.5 m · 9.9 m · 13.4 m. Only the high member lands in the band as run — on upper-end assumptions (see the Costa & Schuster check). Corrected for the baseline defect, the direct member lands in the band as well. |
| Malekhu stage jump | ≥+7.13 m in 30 min (censored) · band 4–10 m [E-021 · E-024] | ✗ | ✗ | ✓ | ✗ | As run: 0.7 m · 3.4 m · 9.0 m · 12.4 m. Same pattern as Galchhi. The recorded rise is a lower bound — the station was destroyed mid-rise — so the band’s upper limit is not observationally supported. |
Arrival rule (model): the first time the routed flood wave exceeds the larger of 5% of that member’s own peak excess at the station and 5 m³/s, sustained for 5 minutes. Observed arrival: the 10-minute bracket between the last flat and first risen gauge sample. T0 is the slope failure, anchored to the USGS origin. The 4 September page quoted clock times of 12:38–17:11 NPT; that span was built as 08:40 + the fastest member (3.96 h) to 09:00 + the slowest (8.19 h), mixing two onset anchors, and is retired.
Known defect · stage-rise baseline
The routing measures each stage rise from a baseline computed with clear-water roughness on unbulked baseflow, but computes the flood stage with debris roughness on bulked baseflow. Part of every published rise is that switch — a rise that would appear with no flood wave at all. It was found in review on 11 September, after the as-run scores were known. The table quantifies it without re-running anything; adopting a corrected metric and re-running is awaiting approval.
| Station | Member | As run | Baseline artefact | Wave-only rise | Fastest 30-min rise | Band |
|---|---|---|---|---|---|---|
| galchhi | low | 1.31 m | 0.65 m | 0.66 m | 0.52 m | 6–12 m |
| galchhi | medium | 3.53 m | 1.77 m | 1.76 m | 1.41 m | 6–12 m |
| galchhi | high | 9.89 m | 2.89 m | 7.00 m | 6.30 m | 6–12 m |
| galchhi | direct | 13.45 m | 3.58 m | 9.87 m | 9.36 m | 6–12 m |
| malekhu | low | 0.68 m | 0.67 m | 0.01 m | 0.00 m | 4–10 m |
| malekhu | medium | 3.39 m | 1.83 m | 1.56 m | 0.95 m | 4–10 m |
| malekhu | high | 8.99 m | 2.93 m | 6.06 m | 4.32 m | 4–10 m |
| malekhu | direct | 12.36 m | 3.64 m | 8.72 m | 6.80 m | 4–10 m |
The low member’s 0.68 m at Malekhu is 0.67 m of artefact: its wave is effectively gone by Malekhu, which is why it has no arrival there. The 4 September statement that the direct member “overshoots” the band does not survive the correction.
Archived blockage hypothesis only — Costa & Schuster (1988) landslide-dam regression [E-110 · E-111 · E-112]
The relation evaluates a landslide-dam-breach hypothesis. It does not describe the currently favoured ice-rock avalanche → debris flow transformation, and is kept only as an earlier consistency experiment.
| Member | PE = h · V · 9,800 N/m³ | Costa Qp | Model water peak | Ratio | Breach-only water peak | Model bulked peak (not comparable) |
|---|---|---|---|---|---|---|
| low | 8.82e+10 J | 486 m³/s | 634 m³/s | 1.3× | 594 m³/s (1.22×) | 824 m³/s |
| medium | 3.43e+11 J | 847 m³/s | 2,364 m³/s | 2.79× | 2,324 m³/s (2.74×) | 4,255 m³/s |
| high | 2.35e+12 J | 1,866 m³/s | 10,736 m³/s | 5.75× | 10,696 m³/s (5.73×) | 26,839 m³/s |
Qp = 0.0158 PE^0.41, fitted to 12 landslide dams (r² = 0.81, standard error 185%). Compared only with water discharge: the regression predicts water peaks, and the bulked flux is a different quantity. The direct member has no impoundment, so the regression has no argument there. The high member reaches the downstream stage band only while combining a large impoundment, rapid release and high bulking.
Internal consistency — not scored, not evidence
| Check | Observed | low | med | high | direct | Result |
|---|---|---|---|---|---|---|
| Rapid onset | Sudden release, not a rainfall hydrograph | ✓ | ✓ | ✓ | ✓ | 3.2–20.9 min at the source reach. Every member is a sudden release by construction, so the check confirms the boundary condition and cannot separate members. Moved here from the observational score on 11 September. |
| Travel distance | About 100 km (USGS, provisional) | — | — | — | — | 127.9 km in every member — set by where the corridor stops, not where the wave dies. Not evidence. |
| Mechanism labelled, not asserted | Completeness check on our own reporting | ✓ | ✓ | ✓ | ✓ | Carried as provisional scenario labels: two modelled branches (A continuous propagation, C sustained blockage) within a three-branch hypothesis space. |
| Secondary signal representable | Second landslide-type signal at 11:45:35 NPT | ✓ | ✓ | ✓ | ✓ | A secondary pulse at T+3 h; its volume is assumed. Not related to the barrier-lake overflow of 28 August. |
| Impacted corridor represented | Damage along Rasuwagadhi–Timure–Syabrubesi | ✓ | ✓ | ✓ | ✓ | All five corridor settlements carried as routing stations. |
What the ensemble says
No member passes a timing check, and the member released earliest is still more than two hours late at Galchhi. The bias is shared by both mechanism branches, so it is not evidence for either: it points to the corridor-routing configuration common to all four members, without yet isolating which part of it is wrong. On stage magnitude, the low and medium blockage members fail under every metric. The high blockage member passes on upper-end assumptions; the direct member fails as run and passes once a known baseline defect is removed. The stage checks therefore cannot choose between the high blockage member and the direct member, and the timing checks cannot be used for that until the routing is calibrated on an independent event. Neither statement is a conclusion about what happened on 26 August.
Reading The Model
What this model does and does not resolve
A screening solver used for hypothesis testing and diagnostics. Stated plainly so that no capability is attributed to it that it does not claim.
The model resolves
- network-scale flood propagation down a 127.9 km corridor
- alternative release scenarios across mechanism families
- first-order attenuation between stations
- arrival and peak timing per member
- approximate stage / discharge consistency at the gauges
The model does not resolve
- 2D inundation extent or channel avulsion
- sediment erosion and deposition
- granular debris mechanics and non-Newtonian rheology
- an evolving channel blockage or dynamic breach formation
- true cross-section hydraulics and high-resolution valley geometry
- phase state: rock, ice and liquid water are not tracked separately — see the next experiment
How debris is represented
The routed quantity is a bulked volume flux: Qbulked = Qwater × B, with B constant per member (×1.3 to ×3.0) and applied to both discharge and wave volume. Resistance is raised with it, neff = n (1 + 0.6 (B − 1)). Both are empirical screening conventions, not derived relations: the flow is still solved hydraulically as water with modified resistance. Solids are not conserved separately, density is not tracked, and B does not vary by reach.
The solver does not resolve non-Newtonian debris-flow rheology. Sediment and debris enter only through bulk-volume and effective-resistance proxies, so the results are flood-propagation screening rather than a debris-flow simulation.
What “uncalibrated” means here
No model parameter was adjusted to match the Galchhi or Malekhu stage or arrival observations. Scenario parameters — impounded volume, blockage height, breach mode and duration, bulking, the mobilised volume of the direct member — were chosen as screening assumptions before those observations were used, and the arrival tolerances were fixed before any timestamp was recovered.
Two things do come from outside this event, and are not observations of it: channel widths and monsoon baseflow are assumed by reach class and seasonal magnitude, and Manning resistance follows conventional values for the reach types. Scenario selection itself is disclosed rather than treated as neutral: all four members were specified after the reported stage rises were public.
Order-of-magnitude hydraulic consistency check
Not a discharge reconstruction or a gauge rating: a screening calculation that tests whether the simulated flood magnitude is physically compatible with the observed stage rise. It inverts Manning normal depth at the reported rise on a compound section, and it is sensitive to every assumption listed beside it, so only the order of magnitude is meaningful.
| Station | Observed rise | Implied peak · clear-water discharge | Implied peak · debris-bulked volume flux (×1.8–2.5) | Implied peak · water-equivalent discharge (×1.8–2.5) | Water-equivalent flood volume, 1–3 h wave | Assumptions |
|---|---|---|---|---|---|---|
| galchhi | +9 m | ≈2 × 10⁴ m³/s | ≈1.2–1.5 × 10⁴ m³/s | ≈4.6–8.3 × 10³ m³/s | ≈7–115 Mm³ | compound section: rectangular channel 110 m wide to 4.0 m bankfull, then valley berm 4.0x width with n x 1.3; slope 0.004142 (SRTM 90 m long profile); n 0.038; baseflow 700 m³/s; triangular excess wave above bulked baseflow |
| malekhu | +7 m (censored) | ≈1 × 10⁴ m³/s | ≈7.7–9.9 × 10³ m³/s | ≈3.1–5.5 × 10³ m³/s | ≈4–75 Mm³ | compound section: rectangular channel 130 m wide to 4.0 m bankfull, then valley berm 4.0x width with n x 1.3; slope 0.002523 (SRTM 90 m long profile); n 0.035; baseflow 750 m³/s; triangular excess wave above bulked baseflow |
The baseline stage in this inversion uses the same clear-water convention as the stage-rise defect below, so it carries the same bias. Volumes are water-equivalent; the bulked-flux volumes are larger by the bulking factor.
Reconstruction Method
How the reconstruction was built
South Lhonak gave VARUNA a lake-breach pathway. Rasuwa needed a landslide-dam and debris-rich routing pathway alongside it, so the case was built as its own study rather than by bending the Sikkim defaults. Every stage is screening-level.
Evidence register
Refreshed 21 SepEvery external datum carries source URL, publisher, published date, event time, accessed date, source tier, confidence and permitted use. Tier 2 and 3 sources never support an observational check. Casualty, missing-person and displacement figures are excluded entirely. In operator terms: every published value is dated, tiered, sourced and version-linked, and every superseded interpretation stays visible with the reason it was withdrawn. After an event a warning system is reviewed by a board, an insurer or a regulator, and the question is what was known when — this structure is what answers it.
Corridor and long profile
Done127.9 km from the source reach to Malekhu: planform from OpenStreetMap river centerlines, long profile sampled from SRTM 90 m. SRTM-era topography predates the 2015 earthquake and the channel-changing floods since, including July 2025. The suspected blockage sits at the reported ~20 km above the Miteri Bridge; a second report of ~10 km is not used.
Trigger module — scenario bands
DoneThe trigger is carried as labelled scenario bands — impounded water, blockage height, breach mode, bulking — not as an identified mechanism, and the bands populate branches A and C only. No failure polygon is digitised and no collapse volume is claimed. The medium member was centred on a figure now known to describe a post-event pond; the band is retained as a scenario, not as a reported value. Since 21 September the published ~103 Mm³ source reconstruction exists, but it has not been used as a model input: these bands predate it.
Landslide-dam breach hydrographs
DoneReservoir routing through a growing trapezoidal breach with a broad-crested weir law, mass-conserving, across three blockage modes. A fourth member assumes no impoundment and is driven by an assumed hydraulically mobilised volume. Breach-only water peaks are cross-checked against Costa & Schuster (1988), verified from the original paper.
Debris-rich routing
Done · defect foundMuskingum-Cunge routing from the South Lhonak case with a bulking ratio and debris roughness; stage from a compound channel-and-valley Manning proxy. A screening model — a final reconstruction needs a two-phase mass-flow solver. Known defect: the stage baseline uses clear-water roughness on unbulked baseflow while the flood series uses debris roughness on bulked baseflow.
Evidence-consistency and uncertainty report
Re-assessed 11 SepEvery check assessed per member, never against the envelope. Timing checks are now evaluable against DHM telemetry, with the tolerances fixed on 4 September before any timestamp was known. Rapid onset joins travel distance as internal consistency because it cannot separate members. “Validation” is reserved for a later stage, when the model architecture is frozen and evaluated against independent events or withheld data.
Review diagnostics
New · read-onlyA post-processing script over the committed 4 September outputs: volume and discharge types, the arrival rule, the stage-baseline defect, the Costa & Schuster cross-check and the timing scores. It re-runs nothing and changes no parameter.
Multiphase source-to-river experiment
Specified · not runRock, ice and liquid water carried as separate state variables through source mass movement, debris-flow transformation and river routing, with conservation and exchange diagnostics. Specified on 21 September; no result exists and none will be published until it is run.
Provenance
Why the ensemble is being superseded
The four members were end-member experiments on a source process that was then poorly constrained. The 16–20 September evidence changes the question they were built to answer. Nothing below re-runs, re-scores or recalibrates them.
What no longer holds as a primary frame
The varied quantity is the wrong one. Three members vary an impounded-water volume behind a sustained blockage. The source is now reconstructed as a ~103 Mm³ rock–ice failure with no lake at the origin point [E-131 · E-144].
No member tracks the phases. Every member routes a water hydrograph multiplied by a constant bulking factor. None carries rock, ice and water separately, so none can represent deposition of ~67 Mm³ in two minutes, entrainment, or melt [E-134].
The timing leaves little room for hours of storage. 14.2 km of the Lende in ~350 s, and Gyirong Port reached in about 6–7 minutes[E-135 · E-016].
What carries forward unchanged
The routing-timing finding. Every member, including the one released at T0, reaches Galchhi hours after the observed onset. That finding never depended on the mechanism, so the new evidence leaves it standing: the corridor routing configuration still has to be constrained on an independent event before any timing claim is made.
The stage-baseline defect disclosure, the order-of-magnitude hydraulic check and the observed lead-time ledger are unaffected.
The Costa & Schuster (1988) check stays as an earlier consistency experiment on the archived blockage hypothesis only. It evaluates a landslide-dam breach, not the currently favoured transformation.
No previous modelling result has been silently recalibrated or deleted. The members, their scores and the embedded simulation are all still published in the section above.
Modelling Roadmap
Next numerical experiment
The next stage does not run another breach hydrograph that varies a hypothetical impounded-water volume. It is specified here and has not been run; no numerical result will be published until it is.
Specified · not run · no results
Objective: test whether the observed downstream wave can arise from a ~103 Mm³ rock–ice failure through progressive mass transformation and water entrainment, without needing a large pre-existing reservoir of impounded water.
Domain A
Source mass movement
rock · ice · liquid water
- acceleration
- fragmentation
- entrainment
- deposition
- ice melt
Domain B
Debris-flow transformation
solid concentration · water
- sediment concentration
- bed entrainment
- glacier interaction
- deposition
- liquid-water incorporation
Domain C
River routing
input: a time-dependent mixed-phase flux, not a clear-water release
- downstream routing
- further sediment entrainment and deposition
- tributary inflow
- phase dilution and separation
Minimum state variables
rock / sediment · ice · water
Minimum tracked diagnostics
solid mass conservation · ice mass · melt generation · liquid-water mass · entrainment · deposition · evolving solid fraction · source-to-river transfer hydrograph · downstream water-equivalent discharge · arrival timing
The order set on 12 September still applies to Domain C. First constrain the corridor routing on the July 2025 event, and freeze it before it sees any of the August 2026 timing observations. Only then evaluate 2026. The new source domains are no reason to tune routing against the 2026 arrivals. This work is awaiting approval and is not part of this update.
Comparison
Sikkim 2023 and Nepal 2026, side by side
Two cascades, one cascade class. The comparison is what turns a single reconstruction into a monitoring requirement.
| Feature | Sikkim 2023 · South Lhonak | Nepal 2026 · Rasuwa / Bhote Koshi |
|---|---|---|
| Main trigger | Lateral moraine / ice-rock collapse into lake | Bedrock and glacier-ice slope failure on Langtang Lirung, ~103 Mm³ with ~12% ice (published reconstruction, 20 Sep) |
| Source process | Collapse into an existing lake | Ice-rock avalanche → glacier-tongue impact → debris flow → debris flood → water-dominated flood. A minutes-scale obstruction is an unresolved secondary modifier; sustained blockage is archived |
| Primary water store | Existing proglacial lake | No single store: initial and entrained ice, melt, subglacial and permafrost water, river and channel water — each unquantified. No lake at the origin; post-event lakes observed from 27 Aug are not the main-event store |
| Breach / release type | Moraine dam overtopping and erosion | No breach needed: progressive transformation, with ~67 Mm³ deposited early and ~36 Mm³ continuing as a debris flow |
| Routing corridor | Teesta River | Lende Khola, Bhote Koshi, Trishuli |
| Key consistency checks | Breach Q, lake volume drained, Chungthang timing, Rangpo stage | Galchhi and Malekhu rise onset and stage jumps (DHM telemetry); both sample one wave |
| Infrastructure cascade | Chungthang / Teesta III dam-break amplification | Bridges, highway, hydropower and gauges; dam-break amplification not assessed |
| Product lesson | Monitor unstable moraine-lake systems before overtopping | Monitor physical state — slope, failure, cascade — across the corridor, not a named lake |
The Sikkim reconstruction is live and interactive — open the South Lhonak cascade simulation.
Where This Leaves The Case
What the reconstruction yields so far
The Rasuwa event shows why a Himalayan cascade cannot always be represented by one source volume and a downstream flood-routing model. The source began mostly as rock and ice. The downstream hazard became increasingly hydraulic. Between those two states lay fragmentation, glacier interaction, melting, entrainment, deposition and phase separation.
The first ensemble did its job, and it stays on this page as the record of that. It showed where the evidence could discriminate and where the model could not. It also exposed a routing-timing bias that does not depend on the mechanism and still has to be removed on an independent event. The next iteration is a multiphase source-to-river experiment, not a tuned roughness value.
For warning systems this changes the monitored object. A lake is an object. A cascade is a changing physical state.
Observe the source. Detect the transition. Track the corridor. Forecast the consequence.
For A Corridor Of Your Own
What a fleet operator can establish now
Screening-level, on the same procedure as this page. Each line is an outcome, not a service.
- which of your corridors have source zones a lake inventory would miss
- where your corridor is instrumentally blind, and which gauges survive a wave
- what lead time your current detection chain would actually deliver, reconstructed against a recorded event on your own reach
- which reaches need corridor-calibrated routing before any warning threshold can be trusted
- what a consequence assessment at a specific intake or barrage would require as input
Research collaboration, data sharing or an operational assessment for the Bhote Koshi and Trishuli corridor — or for your own reach.
Get in TouchEvidence Snapshot
What is known, and how well
A subset of the evidence register, rendered from it. Tier 2 and 3 sources are used for narrative context or scenario bounds only; formal observational checks rely on primary or suitable near-primary evidence. The full register (122 entries) is at the foot of the page.
| ID | Item | Value | Event time | Tier | Confidence | Use | Source |
|---|---|---|---|---|---|---|---|
| E-001 | Event date | 2026-08-26 date | 2026-08-26 | Primary | High | narrative only | Government of Nepal, Ministry of Home Affairs ↗ |
| E-002 | ComCat us7000tbwb origin time | 2026-08-26T02:52:10Z (= 2026-08-26 08:37:10 NPT, UTC+05:45) UTC | 2026-08-26T02:52:10Z | Primary | High | timing reference | USGS NEIC (ComCat) ↗ |
| E-004 | ComCat us7000tc90 origin time | 2026-08-26T06:00:35Z (= 2026-08-26 11:45:35 NPT) UTC | 2026-08-26T06:00:35Z | Primary | High | narrative only | USGS NEIC (ComCat) ↗ |
| E-005 | USGS event classification (current page wording, findings as of 8/28/26) | catastrophic debris flow and flood likely triggered by rapid slope failure involving a glacier | 2026-08-26 | Primary | Medium | narrative only | USGS Landslide Hazards Program ↗ |
| E-008 | USGS travel distance of debris flow and flood | approximately 100 km | 2026-08-26 | Primary | Medium | narrative only | USGS Landslide Hazards Program ↗ |
| E-020 | Galchi (Trishuli at Galchi) gauge rise onset | last pre-rise 360.623 m @10:20 NPT; first risen 363.583 m @10:30 NPT m (station datum; values ~361 suggest elevation-referenced) | 2026-08-26T10:30:00+05:45 | Primary | High | timing diagnostic | DHM / CHWRR (telemetry) ↗ |
| E-022 | Galchi gauge peak and rise duration | peak 369.451 m @10:50 NPT; rise +8.83 m from 10:20 to 10:50 (30 min); 11:00 369.381; back to 363.0 by 12:50 m | 2026-08-26T10:50:00+05:45 | Primary | High | stage observational check | DHM / CHWRR (telemetry) ↗ |
| E-021 | Malekhu (Trishuli at Furke Khola) gauge rise onset | last pre-rise 3.351 m @11:10 NPT; 3.963 m @11:20; 7.781 m @11:30; 10.482 m @11:40 (last value) m | 2026-08-26T11:20:00+05:45 | Primary | High | timing diagnostic | DHM / CHWRR (telemetry) ↗ |
| E-023 | ICIMOD: Galchhi water-level rise (magnitude and duration) | ~9 ("as much as nine metres") within 30 minutes m | 2026-08-26T10:20:00+05:45/2026-08-26T10:50:00+05:45 | Near-primary | Medium | corroboration | ICIMOD ↗ |
| E-026 | Rasuwagadi (Bhotekoshi at Rasuwagadi) last telemetry | 1.620 m @08:40 NPT (02:55Z); 18 values 05:50-08:40, slowly falling from 1.776 m; nothing after m | 2026-08-26T08:40:00+05:45 | Primary | High | narrative only | DHM / CHWRR (telemetry) ↗ |
| E-040 | DHM: debris blocked river ~20 km upstream of Miteri Bridge (Chinese imagery) | 20 km upstream of Miteri Bridge | 2026-08-26 | Secondary | Low-Medium | scenario bound | The Kathmandu Post (DHM FFD chief Binod Parajuli) ↗ |
| E-041 | DHM: Lende Khola blockage ~10 km upstream of Miteri Bridge, Tibetan side | 10 km upstream of Miteri Bridge | 2026-08-26 | Secondary | Low-Medium | narrative only | HimalPress English (DHM Flood Forecasting Division, Binod Parajuli) ↗ |
| E-050 | Impact-crater pond storage volume (the '1.4 Mm3' figure) - English origin | 1.4 million m3 | 2026-08-30T11:23:00+08:00 | Near-primary | Medium | narrative only | Xinhua (relaying China Ministry of Water Resources) ↗ |
| E-054 | UNOSAT 4260 barrier lake 1 (near Nepal-China border) area | approximately 12 (map inset label 11.92 ha; 990 m x 253 m) ha | 2026-08-28 | Primary | High | narrative only | UNOSAT (UNITAR) ↗ |
| E-056 | Lower (Puripu Qiangzangbu / Chhochen-Purepu confluence) barrier lake volume, 27 Aug morning | 2 million m3 | 2026-08-27 | Near-primary | Medium | narrative only | Global Times (citing MWR via CCTV) ↗ |
| E-032 | DHM report: Devghat flood volumes (total and excess over base flow) | total ~57.40 Mm3; excess ~19.96 Mm3; base flow 2,600 m3/s; window 14:10-18:00 NPT million m3 | — | Primary | Medium | narrative only | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-070 | Seismic-inversion source volume (regional stations) | 284-492 Mm3 bulk source volume | 2026-08-26T08:37:10+05:45 | Near-primary | Low-Medium | narrative only | CLaSH StoryMap (analysis/figure by Christopher Calvelage, EarthScope) ↗ |
| E-074 | CU Boulder Today: 50 million metric tons of ice, rock and water (earliest instance found) | 50 million metric tons (equated to ~13 billion US gallons) | 2026-08-26 | Tertiary | Low | narrative only | CU Boulder Today (university news Q&A with Alton Byers, INSTAAR) ↗ |
| E-120 | Rainfall role | not a regular rainfall flood; no heavy rainfall reported immediately before the event | 2026-08-26 | Secondary | Medium | narrative only | Kathmandu Post (reporting DHM) ↗ |
| E-130 | Source failure: location, area and elevation (Li et al.) | 0.76 km² ice-rock mass failed on the north flank of the Langtang Lirung massif at ~5,190 m a.s.l., Chhochen Khola catchment, Nepal km²; m a.s.l. | 2026-08-26 | Primary | High | narrative only | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-131 | Initial failed volume, rock + ice bulk (Li et al.) | ~103 Mm³ bulk source volume (rock + ice) | 2026-08-26 | Primary | Medium | narrative only | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-132 | Initial ice volume and ice fraction (Li et al.) | ~12.6 Mm³ ice (≈12% of the initial volume) Mm³ ice | 2026-08-26 | Primary | Medium | narrative only | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-133 | Lower-glacier interaction (Li et al.) | after ~30 s of descent the slide struck and entrained a detached glacier tongue of 0.92 km² at ~4,010 m a.s.l.; seismic signal equivalent to Mw 5.1 s; km² (area, not volume); m a.s.l. | 2026-08-26 | Primary | Medium | narrative only | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-134 | Early deposition and remobilisation (Li et al.) | within the following ~100 s: ~67 Mm³ deposited in the Chhochen Khola; remaining ~36 Mm³ remobilised as a highly mobile debris flow into the Lende Khola Mm³ debris bulk | 2026-08-26 | Primary | Medium | narrative only | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-138 | Source zone (Zhao et al.) | source zone ~7.5 × 10⁵ m² at the glacier toe of Tsangbu Ri, Nepal; head scarp 5,120 m a.s.l. m²; m a.s.l. | 2026-08-26 | Primary | Medium | narrative only | Zhao B., Tian H., Chen H., Su L., Chen W., Wang H. — Journal of Mountain Science, doi:10.1007/s11629-026-1475-x (abstract) ↗ |
| E-140 | Source identification and proposed event name (ICIMOD) | north-facing slope on Langtang Lirung, North-Central Nepal; bedrock collapse triggered an ice-rock avalanche; proposed name “Lirung ice-rock avalanche” | 2026-08-26 | Near-primary | High | narrative only | ICIMOD (news, 16 Sep 2026: expert convening on the Rasuwa–Gyirong flood) ↗ |
| E-142 | Pre-failure slope deformation (ICIMOD Q&A, satellite radar) | slope moved by up to 30 cm in the three to four weeks before failure cm | 2026-07/2026-08-26 (3–4 weeks before failure) | Near-primary | Low-Medium | narrative only | ICIMOD, Gyirong-Rasuwa flood 2026 resource page — “Q&A: The Gyirong - Rasuwa flood” ↗ |
| E-146 | Excess water at Devghat (WWA, citing CHWRR) | ~20 Mm³ of excess water passed Devghat in under four hours; flow more than doubled to ~5,850 m³/s Mm³ water (hydrological estimate) | 2026-08-26 | Secondary | Medium | narrative only | World Weather Attribution — “Rapid Warming in the Himalaya Exacerbates Geohazard Cascades Beyond Adaptation Limits” ↗ |
| E-147 | Corridor sediment and debris deposition (WWA, citing NDRRMA) | 30.5 Mm³ of sediment and debris deposited along the corridor Mm³ deposited solids/debris | 2026-08-26/2026-09 | Secondary | Low-Medium | narrative only | World Weather Attribution — “Rapid Warming in the Himalaya Exacerbates Geohazard Cascades Beyond Adaptation Limits” ↗ |
| E-160 | Derived: maximum liquid-water equivalent of the initial ice | ≈11.6 (12.6 Mm³ ice × 917 kg/m³ ÷ 1,000 kg/m³ = 11.55); an upper limit that assumes 100% melting Mm³ water (theoretical upper limit) | 2026-08-26 | Primary | Medium | narrative only | GOAT AI derivation from E-132 ↗ |
Source Register
Where every number on this page comes from
Key sources below; every externally sourced value resolves to an entry in the full evidence register (122 entries, external evidence as of 21 Sep 2026, register updated 21 Sep 2026; the previous freeze was 11 Sep 2026). Model values resolve to sim-claude edff56c · outputs 4 Sep 2026 · diagnostics 11 Sep 2026.
Full evidence register · 122 entries · JSON
| ID | Item | Value | Event time | Published | Tier | Confidence | Class | Use | Dependence | Source |
|---|---|---|---|---|---|---|---|---|---|---|
| E-001 | Event date | 2026-08-26 date | 2026-08-26 | 2026-08-26 | Primary | High | — | narrative only | — | Government of Nepal, Ministry of Home Affairs ↗ |
| E-002 | ComCat us7000tbwb origin time | 2026-08-26T02:52:10Z (= 2026-08-26 08:37:10 NPT, UTC+05:45) UTC | 2026-08-26T02:52:10Z | 2026-08-26 | Primary | High | — | timing reference | independent | USGS NEIC (ComCat) ↗ |
| E-003 | ComCat us7000tbwb magnitude | 5.2 (magType ms_vx; magnitude-error 0.09; 26 stations) magnitude | 2026-08-26T02:52:10Z | 2026-08-26 | Primary | High | — | narrative only | — | USGS NEIC (ComCat) ↗ |
| E-004 | ComCat us7000tc90 origin time | 2026-08-26T06:00:35Z (= 2026-08-26 11:45:35 NPT) UTC | 2026-08-26T06:00:35Z | 2026-08-27 | Primary | High | — | narrative only | — | USGS NEIC (ComCat) ↗ |
| E-005 | USGS event classification (current page wording, findings as of 8/28/26) | catastrophic debris flow and flood likely triggered by rapid slope failure involving a glacier | 2026-08-26 | 2026-08-27 | Primary | Medium | — | narrative only | — | USGS Landslide Hazards Program ↗ |
| E-006 | USGS event classification (EARLIER wording, findings as of 8/27/26; later revised) | catastrophic debris flow and flood likely triggered by a glacial collapse | 2026-08-26 | 2026-08-27 | Primary | Low-Medium | — | narrative only | — | USGS Landslide Hazards Program ↗ |
| E-007 | USGS statement on uncertainty of initiation type (8/28 version) | unclear whether landslide incorporating part of a glacier or a glacial collapse | 2026-08-26 | 2026-08-27 | Primary | Qualitative | — | narrative only | — | USGS Landslide Hazards Program ↗ |
| E-008 | USGS travel distance of debris flow and flood | approximately 100 km | 2026-08-26 | 2026-08-27 | Primary | Medium | — | narrative only | — | USGS Landslide Hazards Program ↗ |
| E-009 | USGS description of flow transformation (mechanism-relevant) | glacial debris with water and ice melted and entrained material/water in existing channels -> fast-moving far-travelled landslide and flood | 2026-08-26 | 2026-08-27 | Primary | Qualitative | — | narrative only | — | USGS Landslide Hazards Program ↗ |
| E-010 | ComCat us7000tbwb location | lat 28.271, lon 85.515, depth 0 km; place "55 km NW of Kodari, Nepal"; location estimated from satellite images degrees | 2026-08-26T02:52:10Z | 2026-08-26 | Primary | Medium | — | narrative only | — | USGS NEIC (ComCat) ↗ |
| E-011 | DHM press release: avalanche source coordinates | snow avalanche near border at N 28°16'50", E 85°32'15.45" | — | 2026-08-27 | Primary | Low-Medium | — | narrative only | — | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-012 | DHM press release: M2.6 and M4.4 earthquakes near Rasuwagadhi (source DMG) | M2.6 at 08:20; M4.4 at 08:37 NPT (Richter), near Rasuwagadhi border area | 2026-08-26T08:37:00+05:45 | 2026-08-27 | Primary | Medium | — | narrative only | — | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-013 | ICIMOD: anomalous seismic signals at Jilong and Zhangmu | anomalous signals at Jilong seismometer (~12 km from affected area) and unusual activity at Zhangmu | 2026-08-26 | 2026-08-26 | Near-primary | Qualitative | — | narrative only | — | ICIMOD ↗ |
| E-014 | ICIMOD: flood onset and stage rise (reported) | Bhote Koshi rise began ~09:00 NPT; Trishuli at Galchhi rose up to 9 m within 30 min; Malekhu 7 m over similar period | 2026-08-26 | 2026-08-26 | Near-primary | Low-Medium | — | narrative only | — | ICIMOD ↗ |
| E-015 | NDRRMA SitRep #01: avalanche location, detached area, lake formation after the event | 20 km upstream of Rasuwagadhi; ~1 km2 detached section | 2026-08-26T08:40:00+05:45 | 2026-09-01 | Primary | High | — | narrative only | — | NDRRMA Situation Report #01 ↗ |
| E-016 | Collapse time and runout to Gyirong Port (CAS IMHE via Xizang press conference) | collapse ~10:52 BJT (08:37 NPT) at ~5,200 m; ~22 km to Gyirong Port in about 6-7 min | 2026-08-26T10:52:00+08:00 | 2026-08-30 | Near-primary | Medium | — | narrative only | — | CGTN (Xizang news conference; CAS Institute of Mountain Hazards and Environment) ↗ |
| E-017 | Guo Zhaocheng: signal start 10:52, Gyirong video impact 10:59; source ~5,140 m | 7 minutes | 2026-08-26T10:52:00+08:00 | 2026-08-27 | Secondary | Medium | — | narrative only | — | CCTV/Xinhua via Dazhong Daily live blog (expert Guo Zhaocheng; affiliation not captured in retrieved text) ↗ |
| E-018 | ADRC overview: temporary dam breach time | temporary dam breach 08:30-08:37 NPT, 26 Aug | 2026-08-26T08:30:00+05:45 | 2026-09-08 | Secondary | Low-Medium | — | narrative only | — | Asian Disaster Reduction Center (sourcing IRDR rapid analysis, 28 Aug) ↗ |
| E-019 | ADRC/IRDR: mass failure and dam formation 25 Aug | mass failure and dam formation 25 Aug 13:00-14:00 NPT; impoundment ~18-19 h (IRDR via ADRC) | 2026-08-25T13:00:00+05:45 | 2026-09-08 | Secondary | Low | — | narrative only | — | ADRC (citing IRDR 28 Aug) ↗ |
| E-020 | Galchi (Trishuli at Galchi) gauge rise onset | last pre-rise 360.623 m @10:20 NPT; first risen 363.583 m @10:30 NPT m (station datum; values ~361 suggest elevation-referenced) | 2026-08-26T10:30:00+05:45 | — | Primary | High | — | timing diagnostic | same event correlated | DHM / CHWRR (telemetry) ↗ |
| E-021 | Malekhu (Trishuli at Furke Khola) gauge rise onset | last pre-rise 3.351 m @11:10 NPT; 3.963 m @11:20; 7.781 m @11:30; 10.482 m @11:40 (last value) m | 2026-08-26T11:20:00+05:45 | — | Primary | High | — | timing diagnostic | same event correlated | DHM / CHWRR (telemetry) ↗ |
| E-022 | Galchi gauge peak and rise duration | peak 369.451 m @10:50 NPT; rise +8.83 m from 10:20 to 10:50 (30 min); 11:00 369.381; back to 363.0 by 12:50 m | 2026-08-26T10:50:00+05:45 | — | Primary | High | — | stage observational check | same event correlated | DHM / CHWRR (telemetry) ↗ |
| E-023 | ICIMOD: Galchhi water-level rise (magnitude and duration) | ~9 ("as much as nine metres") within 30 minutes m | 2026-08-26T10:20:00+05:45/2026-08-26T10:50:00+05:45 | 2026-08-26 | Near-primary | Medium | — | corroboration | same event correlated | ICIMOD ↗ |
| E-024 | ICIMOD: Malekhu water-level rise (magnitude and duration) | ~7 over 'a similar period' (~30 min) m | 2026-08-26T11:10:00+05:45/2026-08-26T11:40:00+05:45 | 2026-08-26 | Near-primary | Medium | — | corroboration | same event correlated | ICIMOD ↗ |
| E-025 | Derived: gauge rise-onset differences along corridor (not travel speeds) | Galchi->Malekhu ~50 min (+/-10); Malekhu->Kali Khola ~2 h (+/-10 min); Kali Khola->Devghat ~1 h (+/-20 min) min | 2026-08-26T10:20:00+05:45/2026-08-26T11:20:00+05:45 | — | Primary | Medium | — | timing diagnostic | partially dependent | GOATAI derivation from DHM 10-min telemetry ↗ |
| E-026 | Rasuwagadi (Bhotekoshi at Rasuwagadi) last telemetry | 1.620 m @08:40 NPT (02:55Z); 18 values 05:50-08:40, slowly falling from 1.776 m; nothing after m | 2026-08-26T08:40:00+05:45 | — | Primary | High | — | narrative only | — | DHM / CHWRR (telemetry) ↗ |
| E-027 | Syabrubesi (Bhote Koshi at Shyaprubesi) last telemetry | 3.804 m @08:50 NPT (03:05Z); no rise; nothing after m | 2026-08-26T08:50:00+05:45 | — | Primary | High | — | narrative only | — | DHM / CHWRR (telemetry) ↗ |
| E-028 | Betrawati (Trishuli at Betrawati) last telemetry | 3.549 m @09:20 NPT (03:35Z); flat 3.52-3.56 m since 08:30; nothing after m | 2026-08-26T09:20:00+05:45 | — | Primary | High | — | narrative only | — | DHM / CHWRR (telemetry) ↗ |
| E-029 | DHM press release: Furke (Malekhu) threshold crossings and max | warning (7 m) at 11:20; danger (8 m) at 11:40; max 10.48 m, then station swept away m | 2026-08-26T11:40:00+05:45 | 2026-08-27 | Primary | High | — | narrative only | — | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-030 | DHM report: Phurke (Malekhu) warning-level crossing | 11:26 NPT | 2026-08-26T11:26:00+05:45 | 2026-08-26 | Primary | Medium | — | narrative only | — | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-031 | DHM report: Devghat peak stage | 6.57 m at 16:00 NPT, then falling m | 2026-08-26T16:00:00+05:45 | 2026-08-26 | Primary | High | — | narrative only | — | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-032 | DHM report: Devghat flood volumes (total and excess over base flow) | total ~57.40 Mm3; excess ~19.96 Mm3; base flow 2,600 m3/s; window 14:10-18:00 NPT million m3 | — | 2026-08-26 | Primary | Medium | — | narrative only | — | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-033 | DHM: information of flood entry received by Flood Forecasting Division | 09:05 NPT (sec.1); timeline row 09:00 (DAO Rasuwa + Betrawati station staff); warnings from 09:15 | 2026-08-26T09:05:00+05:45 | 2026-08-26 | Primary | High | — | narrative only | — | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-034 | Pre-event stage trend, 05:50 NPT to flood onset | falling at Rasuwagadhi (1.776 -> 1.620 m by 08:40), Galchhi (361.97 -> 360.62 m by 10:20) and Malekhu (4.32 -> 3.35 m by 11:10) m | 2026-08-26T05:50:00+05:45/2026-08-26T11:10:00+05:45 | — | Primary | Medium | — | narrative only | — | DHM Nepal (10-min telemetry, provisional) ↗ |
| E-035 | Galchhi stage duration above pre-flood level | more than 1 m above the 10:20 pre-rise level (360.623 m) from the 10:30 sample through the 13:30 sample; below it by 13:40 h (about 3) | 2026-08-26T10:30:00+05:45/2026-08-26T13:40:00+05:45 | — | Primary | Medium | — | narrative only | — | DHM Nepal (10-min telemetry, provisional) ↗ |
| E-036 | Devghat (Narayani) gauge rise onset | last flat 4.639 m @14:10 NPT; rising from 14:20 (4.656 m), clear by 14:40 (4.800 m) m | 2026-08-26T14:20:00+05:45 | — | Primary | Medium | — | narrative only | — | DHM Nepal (10-min telemetry, provisional) ↗ |
| E-037 | Derived: detection-chain intervals from T0 | T0 to institutional awareness 23-28 min (09:00-09:05 NPT); T0 to first warnings issued 38 min (09:15 NPT) min | 2026-08-26T08:37:10+05:45/2026-08-26T09:15:00+05:45 | — | Primary | Medium | — | narrative only | — | GOATAI derivation from E-002 and E-033 ↗ |
| E-038 | Derived: lead time between first warning issued and local gauge rise onset | Galchhi 65-75 min; Malekhu 115-125 min; Devghat about 295-305 min (E-025 reach offsets give a consistent 265-335 min window) min | 2026-08-26T09:15:00+05:45/2026-08-26T14:20:00+05:45 | — | Primary | Medium | — | narrative only | — | GOATAI derivation from E-033, E-020, E-021, E-036, E-025 ↗ |
| E-039 | Derived: telemetry-loss sequence along the corridor | Rasuwagadhi last transmission 08:40 (T+3 min), Syabrubesi 08:50 (T+13 min), Betrawati 09:20 (T+43 min); strictly downstream order; none recorded a rise. Apparent mean celerity over the 45.8 km between the first and last loss, on model chainage, 19 m/s (24.6 m/s Rasuwagadhi to Syabrubesi, 17.2 m/s Syabrubesi to Betrawati) min; m/s | 2026-08-26T08:40:00+05:45/2026-08-26T09:20:00+05:45 | — | Primary | Low-Medium | — | narrative only | — | GOATAI derivation from E-026, E-027, E-028 and the corridor chainage of sim-claude edff56c ↗ |
| E-040 | DHM: debris blocked river ~20 km upstream of Miteri Bridge (Chinese imagery) | 20 km upstream of Miteri Bridge | 2026-08-26 | 2026-08-26 | Secondary | Low-Medium | — | scenario bound | — | The Kathmandu Post (DHM FFD chief Binod Parajuli) ↗ |
| E-041 | DHM: Lende Khola blockage ~10 km upstream of Miteri Bridge, Tibetan side | 10 km upstream of Miteri Bridge | 2026-08-26 | 2026-08-26 | Secondary | Low-Medium | — | narrative only | — | HimalPress English (DHM Flood Forecasting Division, Binod Parajuli) ↗ |
| E-042 | NDRRMA: snow-rock landslide ~20 km NE of Rasuwagadhi (26 Aug) | 20 km northeast of Rasuwagadhi border crossing | 2026-08-26 | 2026-08-26 | Secondary | Medium | — | narrative only | — | OnlineKhabar English (NDRRMA, citing Planet Labs imagery) ↗ |
| E-043 | ICIMOD statement on possible blockage (mechanism-relevant) | experts assessing whether debris temporarily blocked river at a narrow section forming a landslide dam | 2026-08-26 | 2026-08-26 | Near-primary | Qualitative | — | narrative only | — | ICIMOD ↗ |
| E-044 | DHM report: blockage/lake status from China | 13:35 NPT: per information from China side, lake at river-blockage site not yet fully drained | 2026-08-26T13:35:00+05:45 | 2026-08-26 | Primary | Qualitative | — | narrative only | — | Flood Forecasting Division, Centre for Hydrology and Water Resources Research (CHWRR, formerly DHM), Govt of Nepal ↗ |
| E-045 | Derived: interval between seismic origin and first warning issued | 37.8 min (08:37:10 NPT origin to 09:15 NPT first warnings) min | 2026-08-26T08:37:10+05:45/2026-08-26T09:15:00+05:45 | — | Primary | Medium | — | narrative only | — | GOATAI derivation from E-002 and E-033 ↗ |
| E-046 | Sentinel-2 L2A scene availability over the corridor, by window | 27 Aug-13 Sep 2026 (post-event): 0 scenes below 40% cloud. 1 Apr-20 Jun 2026 (pre-event): 55. 1 Jun-7 Jul 2025 (pre-2025-event): 8. 9 Jul-30 Sep 2025 (post-2025-event): 16, best 1.0% on 26 Sep 2025 scene count | 2026-08-27/2026-09-13 | — | Primary | High | — | narrative only | — | GOATAI census of the Element84 Earth Search STAC catalogue (Sentinel-2 L2A, AWS open data) ↗ |
| E-047 | Sentinel-2 spectral water indices fail over this turbid corridor (negative result) | At the Galchhi channel centre green 895 vs NIR 2334 (NDWI max -0.074 over a 300 m box, fraction >= 0 is 0.000); MNDWI with SWIR-1.6 gives max 0.017 at Galchhi and -0.082 at Malekhu, and an off-channel hillslope control is indistinguishable at -0.044 index value | 2026-05-24 | — | Primary | High | — | narrative only | — | GOATAI measurement on Sentinel-2 L2A scene S2C_45RTL_20260524_0_L2A (AWS open data) ↗ |
| E-050 | Impact-crater pond storage volume (the '1.4 Mm3' figure) - English origin | 1.4 million m3 | 2026-08-30T11:23:00+08:00 | 2026-08-30 | Near-primary | Medium | — | narrative only | — | Xinhua (relaying China Ministry of Water Resources) ↗ |
| E-051 | Impact-crater pond storage volume, 31 Aug (SAR) | 0.65 million m3 (65万立方米) | 2026-08-31T05:14:00+08:00 | 2026-08-31 | Near-primary | Medium | — | narrative only | — | Dazhong Daily live blog (CCTV News relaying MWR) ↗ |
| E-052 | Implied mean depth of impact-crater pond (derived check of '0.11 km2 x 13 m' hypothesis) | 13 m | 2026-08-30T11:23:00+08:00 | — | Near-primary | High | — | narrative only | — | Arithmetic by this analysis from MWR figures ↗ |
| E-053 | Funnel-shaped water body beneath ice-avalanche source area (upper blockage >120,000 m2) | 120000 m2 (lower bound); depth unknown | 2026-08-28 | 2026-08-29 | Near-primary | Medium | — | narrative only | — | CCTV News 'News 1+1' (MNR Geological Hazard Technical Guidance Center, Chen Hongqi) ↗ |
| E-054 | UNOSAT 4260 barrier lake 1 (near Nepal-China border) area | approximately 12 (map inset label 11.92 ha; 990 m x 253 m) ha | 2026-08-28 | 2026-08-31 | Primary | High | — | narrative only | — | UNOSAT (UNITAR) ↗ |
| E-055 | UNOSAT 4260 barrier lake 2 (near source) area | approximately 19 (map inset label 19.51 ha; 625 m x 413 m) ha | 2026-08-28 | 2026-08-31 | Primary | High | — | narrative only | — | UNOSAT (UNITAR) ↗ |
| E-056 | Lower (Puripu Qiangzangbu / Chhochen-Purepu confluence) barrier lake volume, 27 Aug morning | 2 million m3 | 2026-08-27 | 2026-08-27 | Near-primary | Medium | — | narrative only | — | Global Times (citing MWR via CCTV) ↗ |
| E-057 | Lower barrier dam radar geometry and storage | 2.2 million m3; mean dam height 60 m; dam length ~1,100 m; mean width ~300 m; lake area ~0.1 km2 | — | 2026-09-05 | Near-primary | Medium | — | narrative only | — | Science and Technology Daily (Xinhua; Xizang command HQ, Ren Wei) ↗ |
| E-058 | Lower barrier lake 'basically drained' (30 Aug) | lower barrier lake basically drained (MWR) | 2026-08-30 | 2026-08-30 | Near-primary | High | — | narrative only | — | Xinhua (MWR) ↗ |
| E-059 | Lower barrier lake 'completely released' (1 Sep) | lower barrier lake completely released (Xizang command HQ) | 2026-09-01 | 2026-09-01 | Near-primary | High | — | narrative only | — | Xinhua via Dazhong Daily live blog (Xizang '8.26' command HQ) ↗ |
| E-060 | River rise from 28 Aug barrier-lake overflow | 0.6 m | 2026-08-28 | 2026-08-28 | Secondary | Low-Medium | — | narrative only | — | Al Jazeera (quoting Nepali Army spokesperson) ↗ |
| E-061 | NDRRMA-reported lake area behind Lhende Khola blockage (Nepali original) | 0.11 km2 | 2026-08-27T11:44:00+05:45 | 2026-08-27 | Secondary | Medium | — | narrative only | — | OnlineKhabar (Nepali), reporting NDRRMA statement ↗ |
| E-062 | Nepal FFD (Parajuli): upper lake area and volume, 4 Sep | 0.6 million m3 (६ लाख घनमिटर); area ~0.19 km2 | 2026-09-04 | 2026-09-04 | Secondary | Low-Medium | — | narrative only | — | OnlineKhabar (Nepali), quoting DHM Flood Forecasting Division chief Binod Parajuli relaying Chinese team ↗ |
| E-063 | New landslide-dammed lake 2.8 km downstream of Puripu lake (29 Aug), breached | 2.8 km downstream of Puripu Qiangzangbu barrier lake | 2026-08-29 | 2026-08-30 | Near-primary | Medium | — | narrative only | — | Tide News (MWR citing Ministry of Emergency Management) ↗ |
| E-064 | CLaSH: two post-event lakes (major axes) | 770 m (lower lake major axis); 530 m (upper lake major axis) | 2026-08-28 | 2026-09-02 | Near-primary | Medium | — | narrative only | — | Center for Land Surface Hazards (CLaSH), NSF-funded; ArcGIS StoryMap ↗ |
| E-065 | Chinese scientist: no barrier lake in the original (main) process | no barrier lake in the original process; the lake formed later behind a small adjacent landslide | 2026-08-28 | 2026-08-29 | Secondary | Medium | — | narrative only | — | Wen Wei Po quoting Xu Qiang (President, Chengdu University of Technology) ↗ |
| E-066 | No moraine lakes >0.1 km2 on Nepal side above Gyirong Port | 0 count (Nepal side); 13 on Chinese side | — | 2026-08-30 | Near-primary | Medium | — | narrative only | — | Tide News (MWR) ↗ |
| E-067 | Nepal FFD (Parajuli): lake water ~10% of main flood water (27 Aug) | 10 % of Wednesday's flood water | 2026-08-27 | 2026-08-27 | Secondary | Low | — | narrative only | — | OnlineKhabar English, quoting Binod Parajuli (DHM FFD) ↗ |
| E-068 | Lower barrier lake storage >2.5 Mm3 (CCTV, 28 Aug) | 2.5 million m3 (lower bound) | 2026-08-28 | 2026-08-28 | Secondary | Low-Medium | — | narrative only | — | Global Times (citing CCTV News) ↗ |
| E-069 | Lower barrier lake maximum storage (China Anneng expert) | 4 million m3 (max, 27 Aug); >2.5 million m3 (28 Aug afternoon) | 2026-08-27 | 2026-08-29 | Secondary | Low-Medium | — | narrative only | — | CCTV 'News 1+1' (China Anneng 3rd Bureau chief expert Zhou Zhidong) ↗ |
| E-070 | Seismic-inversion source volume (regional stations) | 284-492 Mm3 bulk source volume | 2026-08-26T08:37:10+05:45 | 2026-09-02 | Near-primary | Low-Medium | — | narrative only | — | CLaSH StoryMap (analysis/figure by Christopher Calvelage, EarthScope) ↗ |
| E-071 | Seismic-inversion source volume (teleseismic stations) | 197-349 Mm3 bulk source volume | 2026-08-26T08:37:10+05:45 | 2026-09-02 | Near-primary | Low-Medium | — | narrative only | — | CLaSH StoryMap (Christopher Calvelage, EarthScope) ↗ |
| E-072 | ICIMOD cryosphere specialist estimate of collapse size | 100-200 Mm3 collapse size (quantity type not stated) | 2026-08-26 | 2026-08-28 | Secondary | Low | — | narrative only | — | The Kathmandu Post quoting Dr Mohd. Farooq Azam (ICIMOD) ↗ |
| E-073 | Tribhuvan University (Dahal) conservative satellite-based estimate | 25 million m3 (up to 40; others up to 100) | 2026-08-26 | 2026-09-02 | Secondary | Low | — | narrative only | — | OnlineKhabar English quoting Prof Ranjan Kumar Dahal (TU) ↗ |
| E-074 | CU Boulder Today: 50 million metric tons of ice, rock and water (earliest instance found) | 50 million metric tons (equated to ~13 billion US gallons) | 2026-08-26 | 2026-09-03 | Tertiary | Low | — | narrative only | — | CU Boulder Today (university news Q&A with Alton Byers, INSTAAR) ↗ |
| E-075 | Britannica failure mass | 55 million short tons (about 50 million metric tons) of ice and rock | 2026-08-26 | — | Tertiary | Low | — | narrative only | — | Encyclopaedia Britannica (John P. Rafferty); read via Wayback 2026-09-09 ↗ |
| E-076 | Geopera model-calibrated flow volume | 100 million m3 (±40%) | 2026-08-26 | 2026-08-28 | Tertiary | Low | — | narrative only | — | Geopera blog (Darcy Weedman) ↗ |
| E-077 | UNOSAT detachment zone (Landsat-9, 26 Aug 2026) | 1.957 (GDB Area_m2 1,956,822.875; geodesic 1,957,373 m2) km2 | 2026-08-26 | 2026-08-31 | Near-primary | Medium | — | narrative only | — | UNOSAT (UNITAR) ↗ |
| E-078 | Relation of 2026 source to 2025 source (HiRISK NP3) | 2026: rock-ice detachment of glacier RGI2000-v7.0-G-15-05732 north of Lirung massif, Lhende catchment, 28.288708°N 85.528159°E, ~5200 m; mass flow met 2025 Purepu GLOF deposits on valley floor <3000 m and dammed Lhende Khola qualitative; decimal degrees; m | 2026-08-26 | 2026-08-28 | Near-primary | Medium | — | narrative only | — | HiRISK RHA Report No. NP3 (Q. Liu, A. Khatri, A. Lord, M. Ghouli, N. Thapa, J. Steiner, W.W. Immerzeel, S. Allen) ↗ |
| E-080 | USGS inundation-boundary mapping status | underway (stated); no downloadable boundary dataset linked from page as of last update 8/28/2026 | — | 2026-08-27 | Primary | High | — | narrative only | — | USGS Landslide Hazards Program ↗ |
| E-081 | UNOSAT Product 4260 identity | ID 4260; "Satellite-Detected Newly Formed Barrier Lakes Following an Ice-Rock Avalanche in Rasuwa District, Nepal, as of 28 August 2026"; GLIDE FL20260826NPL | 2026-08-28 | 2026-08-31 | Primary | High | — | narrative only | — | UNOSAT (UNITAR) ↗ |
| E-082 | UNOSAT GDB FloodExtent layer area (cumulative floodwater/mudflow/rockflow, 26-28 Aug) | 64.9 (geodesic; attribute Area_m2 65,660,992) km2 | 2026-08-26/2026-08-28 | — | Near-primary | Medium | — | narrative only | — | UNOSAT (UNITAR) ↗ |
| E-083 | Copernicus EMS Rapid Mapping activation EMSR927 | activated 2026-08-26T09:53; eventTime listed 2026-08-25T22:00; activator EC Services / DG ECHO; category Flood / Flash flood; Charter 1052; GDACS FL1104124 | 2026-08-26 | 2026-08-26 | Primary | Medium | — | narrative only | — | Copernicus EMS (European Commission) ↗ |
| E-084 | Sentinel Asia: JAXA ALOS-2 Flood Proxy Map | SAR flood-extent proxy from ALOS-2 acquisition 2026-08-28 (file stamp 202608280620); GeoJSON + shapefile; posted 2026-08-31 | 2026-08-28 | 2026-08-31 | Near-primary | Medium | — | narrative only | — | JAXA via Sentinel Asia ↗ |
| E-085 | Charter narrative: surge onset time at Bhote Koshi (Tibet side) | approximately 09:15 local time (NPT) on 26 Aug 2026 NPT | 2026-08-26T03:30Z | 2026-08-26 | Secondary | Low-Medium | — | narrative only | — | International Charter Space and Major Disasters ↗ |
| E-090 | 2025 trigger: official Nepal attribution (NDRRMA) | Rapid discharge/outburst of supraglacial lake on Purepu Glacier; at least part supraglacial drainage qualitative | — | 2025-07-10 | Primary | High | — | narrative only | — | Government of Nepal, NDRRMA Situation Report #1 ↗ |
| E-091 | 2025 trigger: peer-reviewed confirmation (field + remote sensing) | Supraglacial GLOF from Purepu Glacier lake on 8 July 2025; outflow initially over glacier surface then gorge qualitative | 2025-07-08 | — | Primary | High | — | narrative only | — | Zhang T.-G., Wang W.-C., Khadka N., ... Yao T.-D., Advances in Climate Change Research (Elsevier/KeAi), doi:10.1016/j.accre.2026.06.022 ↗ |
| E-092 | 2025 trigger caveat: incomplete drainage and possible englacial/subglacial contribution | 2025 drainage incomplete; englacial/subglacial contributions 'cannot be excluded' qualitative | — | 2026-08-12 | Near-primary | Medium | — | narrative only | — | Xu Q., Kang S., Du W., et al., EGUsphere preprint egusphere-2026-4065 (Brief communication; not yet peer reviewed) ↗ |
| E-093 | 2025 Lende/Bhote Koshi flood: first reported impact time at Rasuwagadhi border post | 2025-07-08 03:15 NPT (05:30 China Standard Time) local time | 2025-07-08T03:15:00+05:45 | 2025-07-09 | Near-primary | High | — | narrative only | — | HiRISK (Qiao Liu, Amrit Thapa, Jakob Steiner), Rapid Hazard Assessment Report No. CN1 ↗ |
| E-094 | 2025 source lake coordinates and elevation (HiRISK) | 28.402561°N, 85.646705°E; 5160 m a.s.l. decimal degrees; m a.s.l. | — | 2025-07-09 | Near-primary | High | — | narrative only | — | HiRISK RHA CN1 ↗ |
| E-095 | 2025 damage summary (NDRRMA, as of 10 July 2025) | Friendship (Miteri) Bridge destroyed; Rasuwagadhi dry port damaged; 3 hydropower plants; 16 km road counts; km | 2025-07-08 | 2025-07-10 | Primary | High | — | narrative only | — | Government of Nepal, NDRRMA Situation Report #1 ↗ |
| E-096 | 2025 published gauge hydrographs at Galchhi, Malekhu, Betrawati, Syabrubesi | NOT FOUND availability | 2025-07-08 | — | Primary | Qualitative | — | narrative only | — | Department of Hydrology and Meteorology (DHM), river-watch portal ↗ |
| E-097 | 2025 availability: DHM Flood Report 2082 annex - 8 Jul 2025 preliminary report (issued 2082/03/24 10:00) | Timure hydropower gauge +~3.5 m at 03:10; Syaphrubesi 3.65 m @03:30 -> 5.37 m @03:40 then transmission stopped; Betrawati 1.60 m (~220 m3/s) @05:00 -> 3.63 m @05:10, max 4.18 m (~1600 m3/s) @05:50 NPT | 2025-07-08T03:10:00+05:45 | 2026-05-08 | Primary | Medium | — | narrative only | — | DHM Flood Forecasting Division ↗ |
| E-098 | ICIMOD statement linking 2026 event site to 2025 flood | 'The same site devastated by last year's Rasuwa flood' qualitative | 2026-08-26 | 2026-08-26 | Primary | Qualitative | — | narrative only | — | ICIMOD media advisory, 26 Aug 2026 (quote: Qianggong Zhang) ↗ |
| E-099 | KP 28 Aug: DHM & ICIMOD supraglacial-lake GLOF attribution refers to JULY 2025 event | the supraglacial-lake GLOF attribution in this article refers to July 2025 | — | 2026-08-28 | Secondary | High | — | narrative only | — | The Kathmandu Post (Subeksha Poudel) ↗ |
| E-100 | USGS statement on 2015 Gorkha-earthquake debris avalanche from same mountain | several million cubic metres of ice and debris; start ~5,000 m elevation; fell ~1,900 m | 2015-04-25 | 2026-08-27 | Primary | Medium | — | narrative only | — | USGS Landslide Hazards Program ↗ |
| E-101 | 2015 Langtang avalanche: DEM-derived deposit volume (Fujita et al. 2017) | 6.81 ± 1.54 × 10^6 m³ (primary event); 0.84 ± 0.92 × 10^6 m³ (succeeding rockfalls) m³ | 2015-04-25 | 2017-05-22 | Primary | High | — | narrative only | — | Fujita K. et al., Natural Hazards and Earth System Sciences 17:749-764 ↗ |
| E-110 | Costa & Schuster (1988) landslide-dam peak discharge regression | Q = 0.0158 (PE)^0.41 Q in m³/s; PE in joules | — | 1988-07 | Primary | High | — | narrative only | — | Costa J.E. & Schuster R.L., GSA Bulletin 100(7):1054-1068 (Table 6) ↗ |
| E-111 | Costa & Schuster (1988) definition of potential energy PE | PE = dam height (m) × lake volume (m³) × specific weight of water (9,800 N/m³) J | — | 1988-07 | Primary | High | — | narrative only | — | Costa & Schuster 1988, GSA Bulletin, p.1065 ↗ |
| E-112 | Costa & Schuster (1988) stated applicability / limits | Large scatter (SE 64-185%); regressions suggested for reconstructing past peaks; envelope for conservative rapid prediction; sediment bulking can increase peaks manyfold downstream qualitative | — | 1988-07 | Primary | High | — | narrative only | — | Costa & Schuster 1988, GSA Bulletin, pp.1065-1067 ↗ |
| E-120 | Rainfall role | not a regular rainfall flood; no heavy rainfall reported immediately before the event | 2026-08-26 | 2026-08-26 | Secondary | Medium | — | narrative only | — | Kathmandu Post (reporting DHM) ↗ |
| E-121 | Infrastructure exposure | buildings, bridges, highway and hydropower assets damaged | 2026-08-26 | 2026-08-26 | Near-primary | Qualitative | — | narrative only | — | ICIMOD; USGS ↗ |
| E-130 | Source failure: location, area and elevation (Li et al.) | 0.76 km² ice-rock mass failed on the north flank of the Langtang Lirung massif at ~5,190 m a.s.l., Chhochen Khola catchment, Nepal km²; m a.s.l. | 2026-08-26 | 2026-09-20 | Primary | High | remotely derived | narrative only | — | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-131 | Initial failed volume, rock + ice bulk (Li et al.) | ~103 Mm³ bulk source volume (rock + ice) | 2026-08-26 | 2026-09-20 | Primary | Medium | author derived | narrative only | — | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-132 | Initial ice volume and ice fraction (Li et al.) | ~12.6 Mm³ ice (≈12% of the initial volume) Mm³ ice | 2026-08-26 | 2026-09-20 | Primary | Medium | author derived | narrative only | — | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-133 | Lower-glacier interaction (Li et al.) | after ~30 s of descent the slide struck and entrained a detached glacier tongue of 0.92 km² at ~4,010 m a.s.l.; seismic signal equivalent to Mw 5.1 s; km² (area, not volume); m a.s.l. | 2026-08-26 | 2026-09-20 | Primary | Medium | author derived | narrative only | — | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-134 | Early deposition and remobilisation (Li et al.) | within the following ~100 s: ~67 Mm³ deposited in the Chhochen Khola; remaining ~36 Mm³ remobilised as a highly mobile debris flow into the Lende Khola Mm³ debris bulk | 2026-08-26 | 2026-09-20 | Primary | Medium | author derived | narrative only | — | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-135 | Lende Khola debris-flow travel and gorge discharge (Li et al.) | 14.2 km along the Lende Khola in ~350 s; peak discharge 2.46×10⁵ m³/s with cross-sectional mean velocity 36.2 m/s at a bedrock-gorge section near Gyirong Port km; s; m³/s (debris-flow bulk); m/s | 2026-08-26 | 2026-09-20 | Primary | Medium | author derived | narrative only | — | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-136 | Numerical simulation results (Li et al.) | maximum slide velocity 129.2 m/s (1-D depth-averaged Voellmy); Gyirong-section peak discharge attenuated to ~11% of that at the Chhochen–Lende confluence (Saint-Venant) m/s; ratio | 2026-08-26 | 2026-09-20 | Primary | Medium | model reconstructed | narrative only | — | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-137 | Preconditioning and absence of a single trigger (Li et al.) | no single extreme rainfall or temperature anomaly and no recent tectonic trigger; compound event from long-term warming, glacier retreat, geological preconditioning and freeze–thaw | 2026-08 (preceding records) | 2026-09-20 | Primary | Medium | author derived | narrative only | — | Li H., Li P., Hu K., Liu J., Liu S., Kang S., Su P., You Y. — Journal of Mountain Science, doi:10.1007/s11629-026-1482-y (abstract) ↗ |
| E-138 | Source zone (Zhao et al.) | source zone ~7.5 × 10⁵ m² at the glacier toe of Tsangbu Ri, Nepal; head scarp 5,120 m a.s.l. m²; m a.s.l. | 2026-08-26 | 2026-09-16 | Primary | Medium | remotely derived | narrative only | — | Zhao B., Tian H., Chen H., Su L., Chen W., Wang H. — Journal of Mountain Science, doi:10.1007/s11629-026-1475-x (abstract) ↗ |
| E-139 | Cascade stage lengths (Zhao et al.) | ice–rock avalanche >7 km → debris flow >13 km → flood >80 km; overall runout >100 km; debris flow evolved into a flood on reaching Gyirong Port at 21.5 km km | 2026-08-26 | 2026-09-16 | Primary | Medium | author derived | narrative only | — | Zhao B., Tian H., Chen H., Su L., Chen W., Wang H. — Journal of Mountain Science, doi:10.1007/s11629-026-1475-x (abstract) ↗ |
| E-140 | Source identification and proposed event name (ICIMOD) | north-facing slope on Langtang Lirung, North-Central Nepal; bedrock collapse triggered an ice-rock avalanche; proposed name “Lirung ice-rock avalanche” | 2026-08-26 | 2026-09-16 | Near-primary | High | author derived | narrative only | — | ICIMOD (news, 16 Sep 2026: expert convening on the Rasuwa–Gyirong flood) ↗ |
| E-141 | Descent and riverbed entrainment (ICIMOD Q&A) | rock and glacier ice fell ~1,200 m into the Lhende Khola valley, shattered into pulverised rock, ice and meltwater, and picked up gravel, boulders and water from the riverbed to become a debris flow m (vertical) | 2026-08-26 | 2026-08-28 | Near-primary | Medium | author derived | narrative only | — | ICIMOD, Gyirong-Rasuwa flood 2026 resource page — “Q&A: The Gyirong - Rasuwa flood” ↗ |
| E-142 | Pre-failure slope deformation (ICIMOD Q&A, satellite radar) | slope moved by up to 30 cm in the three to four weeks before failure cm | 2026-07/2026-08-26 (3–4 weeks before failure) | 2026-08-28 | Near-primary | Low-Medium | remotely derived | narrative only | — | ICIMOD, Gyirong-Rasuwa flood 2026 resource page — “Q&A: The Gyirong - Rasuwa flood” ↗ |
| E-143 | Failure interpretation: bedrock failure taking the glacier (ICIMOD Q&A) | a bedrock failure that took the glacier with it, rather than a glacier collapsing on its own | 2026-08-26 | 2026-08-28 | Near-primary | Medium | author derived | narrative only | — | ICIMOD, Gyirong-Rasuwa flood 2026 resource page — “Q&A: The Gyirong - Rasuwa flood” ↗ |
| E-144 | Not a GLOF: no lake at the origin point (ICIMOD Q&A) | no lake found at the origin point; not a GLOF in the technical sense; ice-rock avalanche that turned into a debris flow | 2026-08-26 | 2026-08-28 | Near-primary | High | author derived | narrative only | — | ICIMOD, Gyirong-Rasuwa flood 2026 resource page — “Q&A: The Gyirong - Rasuwa flood” ↗ |
| E-145 | Rainfall role (ICIMOD Q&A) | rainfall and river gauges show nothing that could explain a flood of this size; only light rain in a side catchment | 2026-08-26 | 2026-08-28 | Near-primary | High | observed | narrative only | — | ICIMOD, Gyirong-Rasuwa flood 2026 resource page — “Q&A: The Gyirong - Rasuwa flood” ↗ |
| E-146 | Excess water at Devghat (WWA, citing CHWRR) | ~20 Mm³ of excess water passed Devghat in under four hours; flow more than doubled to ~5,850 m³/s Mm³ water (hydrological estimate) | 2026-08-26 | 2026-09-17 | Secondary | Medium | observed | narrative only | partially dependent | World Weather Attribution — “Rapid Warming in the Himalaya Exacerbates Geohazard Cascades Beyond Adaptation Limits” ↗ |
| E-147 | Corridor sediment and debris deposition (WWA, citing NDRRMA) | 30.5 Mm³ of sediment and debris deposited along the corridor Mm³ deposited solids/debris | 2026-08-26/2026-09 | 2026-09-17 | Secondary | Low-Medium | author derived | narrative only | — | World Weather Attribution — “Rapid Warming in the Himalaya Exacerbates Geohazard Cascades Beyond Adaptation Limits” ↗ |
| E-148 | Water-source interpretation (WWA) | melting glacier ice; water stored beneath the glacier and in permafrost; ice and water carried within the debris; river water pushed ahead of the flow | 2026-08-26 | 2026-09-17 | Secondary | Medium | author derived | narrative only | — | World Weather Attribution — “Rapid Warming in the Himalaya Exacerbates Geohazard Cascades Beyond Adaptation Limits” ↗ |
| E-149 | Cascade classification (WWA) | rock–ice avalanche → debris flood → water-dominated flash flood; average downstream speed 188 km/h | 2026-08-26 | 2026-09-17 | Secondary | Medium | author derived | narrative only | — | World Weather Attribution — “Rapid Warming in the Himalaya Exacerbates Geohazard Cascades Beyond Adaptation Limits” ↗ |
| E-150 | Collapse area and fall height (WWA) | roughly 2 km² of rock wall and glacier ice; fell ~1,400 m, from ~5,150 m to the valley floor at ~3,750 m km²; m | 2026-08-26 | 2026-09-17 | Secondary | Low-Medium | author derived | narrative only | — | World Weather Attribution — “Rapid Warming in the Himalaya Exacerbates Geohazard Cascades Beyond Adaptation Limits” ↗ |
| E-160 | Derived: maximum liquid-water equivalent of the initial ice | ≈11.6 (12.6 Mm³ ice × 917 kg/m³ ÷ 1,000 kg/m³ = 11.55); an upper limit that assumes 100% melting Mm³ water (theoretical upper limit) | 2026-08-26 | — | Primary | Medium | goatai derived | narrative only | — | GOAT AI derivation from E-132 ↗ |
| E-161 | Derived: non-ice remainder of the initial failed volume | ≈90 (103 − 12.6 = 90.4), ≈88% of the initial volume Mm³ bulk (predominantly rock; includes any debris, snow and pore space) | 2026-08-26 | — | Primary | Medium | goatai derived | narrative only | — | GOAT AI derivation from E-131 and E-132 ↗ |
Missing-Data Register
What we do not have yet
Publishing the gaps is part of the product. An item is closed only when it is genuinely sourced; each carries why it matters and the interim workaround.
Flow record above Rasuwagadhi, 25–26 August
Why The observation that would test a dam formed on 25 August: a sustained impoundment should cut flow downstream for hours. The public DHM endpoint no longer holds series before 26 August.
Interim Pre-event stage was falling at three gauges on 26 August — non-diagnostic without the preceding days and hydropower operating records
[E-034 · E-019]Water-budget exchange terms
New 21 SepWhy The principal unresolved problem: how ~20 Mm³ of excess water reached Devghat from a source that was ~88% non-ice. Needs the volume of the glacier tongue entrained, the fraction of ice melted, stored subglacial and permafrost water, river water entrained from the Lende, the Bhote Koshi and tributaries, and sediment–water exchange
Interim Carried as named, unquantified exchange terms; nothing is fitted to close the budget
[E-133 · E-148 · E-032]Source hydrograph and solid concentration entering the river domain
New 21 SepWhy The input the multiphase river routing needs: a time-dependent mixed-phase flux rather than a clear-water release
Interim None yet. Li et al. publish one debris-flow bulk peak at the Gyirong gorge, not a hydrograph
[E-135]Full text of Li et al. and Zhao et al.
Abstracts onlyWhy The methods and uncertainties behind the ~103 Mm³, ~12.6 Mm³, 67/36 Mm³ and 0.92 km² figures, and how the entrained tongue enters the partition
Interim Registered from the abstracts, with confidence Medium and evidence_class author_derived where the method is not stated
[E-131 · E-138]Main-event water store (blockage hypothesis)
Superseded 21 SepWhy Release volume under the archived sustained-blockage hypothesis. No pre-flood lake or impoundment has been observed; every quantified lake was measured from 27 August onward, and ICIMOD reports no lake at the origin point.
Interim Blockage members kept as scenario bands (0.6–6.0 Mm³ impounded water) in the historical ensemble, not as reported values
[E-050 · E-056 · E-144]Failure volume / mass
Partly closed 21 SepWhy Debris momentum, entrainment and flood energy
Interim Latest peer-reviewed reconstruction ~103 Mm³ rock + ice (Li et al.). Seismic force inversions of 197–492 Mm³ are 2–5× larger and unreconciled. Not yet a model input: the multiphase experiment is specified, not run
[E-131 · E-070 · E-071]Exact failure polygon
ConflictWhy Collapse volume and runout geometry
Interim Three areas for the source: 0.76 km² (Li et al.), ~0.75 km² (Zhao et al.), and ≈1.96 km² (UNOSAT detachment zone, matching WWA’s ‘roughly 2 km²’) — probably different definitions. No polygon adopted
[E-130 · E-138 · E-077 · E-150]Pre-failure deformation series
New 21 SepWhy Whether the reported ~30 cm of movement was detectable in real time, and how often similar signals occur on slopes that do not fail
Interim Carried as a retrospective observation relayed by ICIMOD; sensor, processing and error are not public
[E-142]Full gauge hydrographs and rating curves
Why Converting stage to discharge at Galchhi and Malekhu; testing the Manning inversion
Interim Normal-depth proxy under stated assumptions; Galchhi is an elevation-type gauge with no published rating
[E-022]Observed inundation boundary
Products exist, none adoptedWhy Spatial consistency checking
Interim Not yet adopted — definitions differ: UNOSAT floodwater/mudflow/rockflow extent (64.9 km², cumulative, rolling), Copernicus EMS mass-movement extent, JAXA ALOS-2 flood-water proxy; USGS mapping underway
[E-082 · E-083 · E-084 · E-080]High-resolution pre/post DEM
AcquirableWhy Source volume, erosion and deposition; the routing terrain predates the 2015 earthquake and the 2025 flood
Interim SRTM for the first pass; stereo photogrammetry requested. Acquirable in principle from commercial stereo, not from free imagery
Earth-observation corridor change set
BlockedWhy The one quantity still measurable at every point, including the three reaches where telemetry died: what the corridor looks like now, and the same for July 2025 as an independent calibration target
Interim Not deliverable from free imagery. Sentinel-2 indices cannot delineate this turbid channel — a 110 m perennial reach and a hillslope return the same class, so a first width attempt was discarded rather than published. Needs Planet or WorldView optical, or calibrated SAR (no terrain-correction toolchain here; Planetary Computer RTC assets refuse anonymous access)
[E-047 · E-046]EO standing limit
Method noteWhy Earth observation constrains corridor geometry and change extent. It never constrains discharge or timing, and the before-phase supports susceptibility and siting rather than prediction
Interim Any EO product adopted here inherits that boundary; timing claims stay tied to observed gauge arrivals
[E-046]2025 gauge records, Galchhi and Malekhu
Why An independent same-corridor routing calibration, so 2026 timing can be tested without tuning to 2026
Interim Not public; DHM data request. The calibration exercise itself is approval-gated
[E-096]Warning message timestamps
Partly availableWhy Early-warning performance analysis
Interim DHM reports warnings issued from 09:15; official alert logs still requested
[E-033]Cross-border observations
Why The source is in Nepal, but the Lende Khola forms the border and Gyirong Port lies on the Chinese bank; reach observations there come through another jurisdiction
Interim Chinese statements recorded as relayed by Xinhua, CCTV and MWR; primary MWR pages not fetched
[E-056 · E-066 · E-140]Update · 21 September 2026 · UPD-02
What changed since 11 September
Newly published evidence, changes to interpretation, and what was deliberately left unchanged — kept apart, because they bear on credibility differently.
Newly published evidence
- Source localised to a north-facing slope on Langtang Lirung, in Nepal; ICIMOD proposes the name “Lirung ice-rock avalanche”. The initiating mechanism is narrowed to a bedrock failure that took the glacier with it. [E-140 · E-143 · E-130]
- Li et al. (Journal of Mountain Science, online 20 Sep): ~103 Mm³ initial volume, ~12.6 Mm³ (≈12%) of it ice, from a 0.76 km² failure at ~5,190 m; a 0.92 km² glacier tongue struck ~30 s in; ~67 Mm³ deposited in the Chhochen Khola and ~36 Mm³ remobilised into the Lende; 14.2 km in ~350 s. [E-131 · E-132 · E-133 · E-134 · E-135]
- Zhao et al. (same journal, 16 Sep) describe the same cascade but name the source as the glacier toe of neighbouring Tsangbu Ri — recorded as a disagreement. [E-138 · E-139]
- WWA (17 Sep): ~20 Mm³ of excess water past Devghat in under four hours, corroborating DHM’s 19.96 Mm³; 30.5 Mm³ of sediment and debris deposited along the corridor; several water sources named. [E-146 · E-147 · E-148]
- ICIMOD relays satellite-radar evidence of up to 30 cm of slope movement in the three to four weeks before failure — a retrospective observation. [E-142]
Changes to interpretation
- The central question moves from which source mechanism to how rock, ice and water were transformed between source and flood. New sections: the water-source problem, the transformation diagram, the mass-budget table, and the three-state panel.
- Geological volume, ice volume, debris volume and water volume are now labelled and never combined. A GOAT AI derivation shows the initial ice could supply at most ≈11.6 Mm³ of water. [E-160 · E-161]
- Continuous propagation becomes the principal working process. Sustained blockage moves to the historical ensemble; transient obstruction is kept as an unresolved secondary hydraulic modifier. Costa & Schuster is demoted to that archived hypothesis.
- Language placing the source “on or beyond” the Nepal–China border is corrected. Earlier Tibetan-side reports stay in the register. [E-041 · E-140]
- The next modelling stage is specified as a multiphase source-to-river experiment; nothing has been run.
Unchanged, deliberately
- No simulation was re-run, no parameter changed, no score recomputed; the four members and the embedded simulation stay published. No previous modelling result has been silently recalibrated or deleted.
- The routing-timing finding stands: it never depended on the mechanism. [E-020 · E-021]
- The observed lead-time ledger, the telemetry-loss finding and the stage-baseline defect disclosure are unchanged. [E-038 · E-039]
- Human-impact figures remain excluded, including those in the new sources.
Does the main conclusion change? Yes, in where the uncertainty sits. The source process is now substantially constrained; what is unresolved is how rock, ice and water were exchanged between the source and the flood.
Previous update · what changed between 4 and 11 September (UPD-01), as published on 11 September
Newly published evidence
- USGS reclassified both seismic signals as landslide-type and revised its trigger wording on 28 August from “glacial collapse” to “rapid slope failure involving a glacier”. [E-002 · E-004 · E-005 · E-006]
- UNOSAT mapped two newly formed barrier lakes (≈12 ha and ≈19.5 ha) on 28 August imagery; Chinese authorities reported the lower lake drained by 30 August–1 September. [E-054 · E-055 · E-058 · E-059]
- NDRRMA’s first situation report (1 September) places the temporary lakes after the avalanche and debris flood. [E-015]
- Source-volume estimates from identifiable scientists now exist — none peer-reviewed, spanning 25 to ~490 Mm³ of bulk material. [E-070 · E-071 · E-072 · E-073]
Available before, recovered on 11 September
- DHM 10-minute gauge telemetry: Galchhi began rising between 10:20 and 10:30 NPT, Malekhu between 11:10 and 11:20. Arrival timing — carried as not-evaluable on 4 September — is now observed. [E-020 · E-021 · E-025]
- The “~1.4 Mm³ reported impoundment” is China’s Ministry of Water Resources estimate for a pond in the avalanche impact crater, measured on 30 August. It is a post-event store, not the reservoir of the 26 August wave. [E-050 · E-051 · E-052]
- Two blockage locations were given by the same DHM official on 26 August — about 20 km and about 10 km above the Miteri Bridge. [E-040 · E-041]
- A rapid analysis (IRDR, via ADRC) describes a dam formed on 25 August and breached at 08:30–08:37. It conflicts with the USGS and Chinese timing accounts; its source report could not be retrieved. [E-018 · E-019 · E-016]
- The July 2025 flood on the same corridor — a supraglacial lake outburst on the Purepu Glacier — is now documented here. [E-090 · E-091]
Corrections to interpretation
- “+30 min — Trishuli gauges respond” was wrong: ICIMOD’s 30 minutes is how long the rise lasted at each gauge, not a travel time. [E-023 · E-022]
- The headline argument — the gauges needed far more water than the reported impoundment — compared the flood with a lake that formed afterwards. It has been withdrawn and replaced, not defended. [E-050]
- All four members miss the observed arrival timing by more than their tolerance. The miss is common to every member, including the one released at T0, so the observed timing rejects the shared routing configuration as presently parameterised — it does not pick a mechanism. [E-002 · E-020 · E-021]
- A stage-metric defect was found: part of every published stage rise is a baseline switch, not flood wave. The “direct member overshoots” statement does not survive the correction.
As written on 11 September: “Does the main conclusion change? The mechanism remains unresolved. What changed is why: the timing observation the 4 September page was waiting for now exists, and this model cannot use it to separate the mechanisms until its routing is constrained independently.”
Changelog
Revisions to this reconstruction
Dated, so a reader can see what the page said before and why it changed. Fix identifiers refer to the 11 September review.
- Evidence freeze moved from 11 to 21 September. New register entries E-130–E-150 from Li et al. and Zhao et al. (Journal of Mountain Science, abstracts), ICIMOD (16 Sep news; Q&A) and WWA (17 Sep); E-160–E-161 are GOAT AI derivations. New entries carry an evidence class (observed, remotely derived, model reconstructed, author-derived, GOAT AI derived). No earlier entry was changed or removed.
- Source localised to Langtang Lirung, in Nepal, and the initiating mechanism narrowed to a bedrock failure that took the glacier with it. Zhao et al.’s Tsangbu Ri naming is recorded as a disagreement.
- Added: the ~103 Mm³ initial-volume estimate and the ~12.6 Mm³ initial ice, the lower-glacier interaction, and the ~67/36 Mm³ deposition and remobilisation reconstruction. The ~20 Mm³ downstream excess-water estimate is promoted to a headline quantity and kept strictly apart from source volume.
- Mass-state terminology corrected throughout. New: the “Where did the water come from?” section, the water-source ledger, the source-to-flood transformation diagram, the mass-budget table, the three-state panel and a comparison of published source estimates.
- Sustained-blockage scenarios moved to the historical ensemble, labelled “superseded as primary event interpretation — retained for auditability”. Transient blockage kept only as an unresolved hydraulic modifier. Costa & Schuster (1988) demoted to the archived blockage hypothesis.
- Multiphase modelling roadmap and next experiment specified; not run, no results.
- Pre-failure deformation added as a retrospective observation, with the prediction caveat. The measurability section’s earlier statement that no usable short-term precursor exists is revised accordingly.
- Event renamed “Nepal 2026 — Langtang Lirung Ice–Rock Avalanche and Bhote Koshi–Trishuli Cascade”. The 2025/2026 comparison and the monitoring lesson are strengthened, and the page order changed so the physical interpretation comes before the old simulations. The illustrative hero image is replaced by a conceptual schematic.
- No previous modelling result has been silently recalibrated or deleted. No simulation was re-run; references remain sim-claude edff56c. Human-impact figures in the new sources were not recorded.
- No model change: no re-run, no parameter change, no new external facts. Simulation references remain sim-claude edff56c.
- New finding — the corridor detected the wave in the loss of its telemetry, not in the signal: three stations stopped in strict downstream order, the second 25 minutes before the first warning and the third 5 minutes after it. Published with its limits (upper-bound loss times at 10-minute resolution, causes other than destruction, model chainage) and explicitly as a design finding recovered afterwards, not a method available on the day. E-039.
- New finding — the 38-minute gap between the seismic origin and the first warning was not a sensing gap: the signal existed at T+0 and the warning travelled by telephone. Stated with the automatic-solution latency and the initial earthquake misclassification in the same block, so it cannot be read as a claim that a warning was possible in minutes. E-045.
- New section “Why this event was hard to measure”: six structural reasons, then a three-phase frame — before (susceptibility, not prediction), during (not Earth observation), after (where measurement is actually possible).
- Lead time reframed as a property of location rather than technology, with an explicit instruction not to generalise the observed figures to locations without an observed arrival.
- Registered a census of Sentinel-2 availability over the corridor (E-046): no scene below 40% cloud in the 2026 post-event window, against 1% cloud in the 2025 analogue — which is why the 2025 event is measurable now and the 2026 corridor state is not.
- No model change: no re-run, no parameter change, no new external evidence. Simulation references remain sim-claude edff56c.
- New section “How much warning existed”: a lead-time ledger from T0 (08:37:10 NPT, E-002) with the offset and evidence for every corridor point, and the lead time between the first warning and each local rise onset.
- Three register entries added and used for those numbers — E-036 (observed Devghat rise onset), E-037 (detection-chain intervals) and E-038 (warning-to-onset lead time) — each naming its parent entries. Register count 92 → 95; external evidence still as of 11 September.
- New sections: “What the corridor could not see”, “What was in the path” (asset classes, with not-assessed stated where the register cannot support a quantity, plus one labelled worked illustration), and a closing “What a fleet operator can establish now”.
- Cascade Class now opens with why the unresolved mechanism does not block operational use: the downstream response is the same under all three branches.
- “The finding worth arguing about” gains the operational consequence of the routing miss — a systematic overstatement of available travel time for any system built on the same ingredients.
- Section order changed so the stakes precede the epistemology. Nothing epistemic was removed or softened: the 1.4 Mm³ withdrawal, the stage-baseline defect, the “uncalibrated” definition and the missing-data register are unchanged.
- Cascade-class heading is now “Same cascade class as Sikkim, different initiating process”, and the three hydraulic pathways are named in the opening sentence.
- The GLOF comparison reads “Both are cryosphere-driven cascades, but only South Lhonak is established as a classical GLOF pathway.”
- Evidence-register roles split further: timing reference (the T0 anchor), timing diagnostic (observed arrivals), stage observational check (measured stage), corroboration (an independent report of the same quantity). The register records how each former value maps.
- Branch wording no longer ranks hypotheses: branch A is “currently more consistent with the available timing and event-classification evidence, while the hydraulic pathway remains unresolved”, and branch C is “less consistent … after the refresh”.
- Page title and heading changed from “Glacier-Collapse Flood” to “Ice-Rock Slope-Failure Cascade”, matching the classification and the USGS reclassification documented on this page. The social card and the evidence register’s case field follow.
- “Validation” retired from the evidence register: observations now carry use = observational check (stage) or timing diagnostic (arrival timing and the T0 anchor). Validation is reserved for a frozen model architecture evaluated against independent or withheld data, and the register records how the former values map.
- “Tier 2 and 3 sources … never for validation” reworded: observational checks rely on primary or appropriate near-primary evidence.
- Cascade-class opening sentence disambiguated — the initiating process is Rasuwa’s, not something shared with South Lhonak.
- Classification wording finalised as “rapid ice-rock slope failure, debris avalanche and flash flood — hydraulic mechanism provisional”.
- The A/B/C mechanism taxonomy is now used from the ensemble introduction onward: two modelled end-member branches within a three-branch hypothesis space.
- The header summary drops “assumed failure volume” in favour of the branch names and “assumed hydraulically mobilised volume”.
- “Cascade class” used consistently in place of “hazard class”.
- The hydraulic consistency table now separates clear-water discharge, debris-bulked volume flux and water-equivalent discharge, with ranges at two significant figures instead of collapsing to a single figure.
- “Validation” reserved for a later stage; this stage is evidence consistency, observational checks and independent diagnostics. The #validation anchor is kept so existing links keep working.
- The transfer-validation sequence is stated in order: constrain routing on July 2025, freeze it, then add the transient-obstruction mechanism and re-evaluate 2026 without further adjustment.
- Classification changed to “rapid glacierised slope failure, debris avalanche and flash flood — hydraulic mechanism provisional”. The former wording put a contested mechanism (landslide dam) in the event name.
- “The routing is falsified” replaced throughout: the observed timing rejects the shared routing configuration as presently parameterised, and the candidate contributors are listed rather than implying the cause is isolated.
- The direct member’s 5.0 Mm³ is renamed an assumed hydraulically mobilised volume — a scenario label that does not enter the solver and is not an estimate of the total failed mass.
- The four members are stated to be end-member screening experiments; a minutes-scale transient obstruction is named as the missing mechanism and carried as branch B, not modelled.
- The archetype section is restructured into one initiating process with three possible hydraulic pathways (A continuous propagation, B transient plug, C sustained blockage).
- The Manning inversion is bounded as an order-of-magnitude consistency check, with values rounded to one significant figure.
- The independence of the timing test is elevated: the ensemble predates the recovered timestamps, so the observations are a diagnostic rather than a test built to pass.
- Score framing replaced with an evidence-consistency matrix per member; individual pass/fail results retained.
- New sections: the epistemic state (high / moderate / unresolved), and what the model does and does not resolve — including the debris-bulking treatment, a definition of “uncalibrated”, and the three distinct volume quantities.
- Added an illustrative hero reconstruction of the source-area cascade. An illustration rather than data, so it carries no evidence-register entry. The social-card image remains the ensemble hydrograph graphic from the simulation output.
- Case study published with a three-member ensemble; fourth member (direct) and per-member validation added the same day.
- Member count reconciled to four in the hero, simulation panel and results table; volumes labelled by type (F-01, F-02, F-21).
- Header chips restated as hypotheses; archetype rewritten as two branches (F-03, F-04).
- “~1.4 Mm³ reported impoundment” re-identified as a 30 August post-event crater pond; the headline argument built on it withdrawn and replaced (F-05, F-19).
- Blockage-location conflict published (F-06). “+30 min” tile corrected to a rise duration (F-07).
- Timing checks made evaluable from DHM telemetry; rapid onset moved to internal consistency; scores re-computed per member (F-08, F-13).
- Costa & Schuster (1988) relation verified and compared against water-only breach peaks (F-09).
- USGS origin times registered; conflicting timing accounts recorded without reconciliation (F-10).
- Low-member Malekhu wording tied to the model’s actual arrival rule (F-11).
- Stage-baseline defect disclosed with a diagnostic re-score; correction and re-run left for approval.
- Post-event barrier lakes, July 2025 event, inundation products and source-volume estimates added (F-14–F-17). KP 28 Aug passage resolved as referring to 2025 (F-18).
- Page-specific social metadata; evidence register, changelog and this section published (F-12, F-20, F-22).
Collaboration
Data that would sharpen this reconstruction
If you hold any of these, get in touch — they would change what this page can say.