On the morning of 26 August 2026 a mountainside above the Lhende Khola, in Rasuwa, fell into the gorge of a small river. Seven minutes and forty seconds later a front of ice, rock and mud, moving at about 170 km/h, destroyed the border crossing at Rasuwagadhi, 22 km downstream, met the main river there, and ran the length of the Bhote Koshi and the Trishuli as a flood. I ask one narrow question with physics: where did all that water come from? It is answered below, and the answer is an energy budget rather than a model. A second question — how big the collapse was — is now answered by measurement: 110–125 million cubic metres, preliminary, from David Shean and Shashank Bhushan's elevation data, and the upper half of my model agrees. What the mass was made of is not answered.
As of 8 September, 1,342 people are confirmed dead in Nepal and 4,886 missing, with 43 dead and 519 missing on the Chinese side, and more than 5,300 injured. The missing count has begun to fall while the death toll rises — that is bodies being recovered, not people being found alive. Of roughly 900 recovered by 1 September, about 4% had been identified. Many of the dead were hydropower workers, from the plant at the border to the tunnels at Upper Trishuli-1. Every figure on this site is provisional.
The wave carried away bridges, vehicles, homes and people. No term in this arithmetic stands for them, and none of the numbers on these pages should be read as if it did.
दिवंगतहरूको स्मृतिमा — in memory of those lost.
What this is, before you read any of it. Independent, AI-assisted analysis, published days after the event by one person working outside his own field. I am an engineer in New Zealand's electricity sector, not a geoscientist. It is not peer-reviewed. No Nepali scientist has read it. By its own licence it is not fit to sit under an operational warning system, an evacuation decision or a hazard zoning without independent expert review. Five results have been withdrawn so far and all five are still readable. Everything here is preliminary, and the numbers that depend on the routing model are more preliminary than the rest. The contribution is arithmetic on figures already public — the relative sizes of the terms — not a finding about how well anyone else has done their job.
Every finding carries a confidence chip, and the colour is the part that means something. green the conclusion survives every combination of inputs I can still defend. blue it has survived the tests run against it, including a named challenge, but it rests on at least one input that could still move. amber unresolved, under revision, or partly withdrawn. The chip says how hard a finding has been shot at, not how much I believe it.
The water was already in the riverholds on best evidence
Melting ice is expensive and falling is cheap. A kilogram
falling 1,200 m releases about a 28th of the heat needed to melt a
kilogram of ice. At the collapse size I measured on 5 October from
David Shean and Shashank Bhushan's elevation data — 110–125 Mm³,
preliminary — frictional melt comes to 3–8 Mm³ on
best evidence, whatever the mass was made of. Nepal's Flood Forecasting
Division measured ~20 Mm³ above base flow at Devghat between 14:10 and
18:00 on the day. On best evidence melt is less than half of that, and the
rest was the monsoon-swollen river itself, swept up and delivered at once —
from the border junction down. Above the junction the Lhende held about
0.5 Mm³ of water, so the first 22 km were the mountain, not a
flood.
Caveats. This is an energy budget and it does not depend on the
routing model. It does depend on the collapse size and on what the mass was
made of, and nobody has measured the second. Melt reaches 20 Mm³ at
this size, and up to 24, if three things hold at once: the full
4,000 m drop, a heat partition of 0.70 at the very top of the published
range, and a mass between a fifth and two-fifths ice. Melt here is limited
by energy, not by ice, and rock is three times as dense as ice, so a
rockier mass releases more heat. Whether a mass that rocky can put 70% of
its heat into its ice, I have not examined. Corrected
5 October: this finding said melt capped at 1.1 Mm³, or
2.5 with every dial turned, and reached 20 only at 200 Mm³ and 80% ice.
The first pair was computed at a collapse size since ruled out. The second
had the ice dial backwards: at a fifth ice, those inputs give 38.
It did not fall like dry rockdry rock excluded
The border camera's clock — 7 min 40 s for
22 km — turns out to measure what the falling mass was made of,
not how big it was. Dry-rock versions arrive at 17–30 minutes whatever
size I make them; ice-rich ones arrive within minutes of the observed time.
The arithmetic works without any model: the drop over the run is
H/L = 0.14, so it only arrives at all if its effective friction is
below 0.14, and Scheidegger's rule gives dry rock 0.28–0.30 at this size,
which stops it after about 10 km. This ran 22.
Caveats. The clock rests on a CCTV timestamp and a seismic origin
time, both of which I have had to reconcile, and an earlier version of this
page overstated how independently it was corroborated. One published speed
(19 m/s, peer-reviewed) contradicts the ones I scored against; I think
it measures the water after the turn rather than the front, but that
reconciliation is mine and unreviewed, so both scorings are published.
The method travels — one test stands, one is withdrawnone of two stands
Against Chamoli 2021 the model reproduces the arrival time
at Tapovan once frictional melting is given a thermal lag, with one fitted
number and the rest held. Against Seti 2012 it does not: that test was
withdrawn on 3 September after I found a bad channel profile underneath it —
straight lines flown across a mountain rim, then flattened to zero slope by
the routine meant to clean them. Rerun on a corrected channel the test
fails and its second conclusion reverses.
Caveats. Nothing about the Trishuli findings depends on Seti. The
Chamoli test shares a channel-profile pipeline with this analysis, so it is
not fully independent of it.
How big the collapse was: 110–125 million cubic metres, measuredmeasured, preliminary
Subtracting the mountain after from the mountain before,
in the elevation data David Shean and Shashank Bhushan released on
18 September, puts the collapse at 110–125 Mm³,
preliminary: a slab mostly 90–160 m deep over about a square
kilometre, with the glacier's own thinning since 2015 taken off. It holds on
two other pre-event surfaces, one of them radar, and on two independent
post-event ones. The data is theirs, and if they publish a volume it
replaces mine.
The upper half of my model agrees from the other side. Run at the measured
size it fits better than at its own number: 15 runs of 300 match everything
measured above Betrawati, against 4 of 300 at 135–200. It will not go below
about 115, because a smaller mass reaches the border too late. So the hole
caps the collapse at about 125 and the border clock puts a floor at about
115, by routes that share nothing. Four of the 15 also land the downstream
measurements I hold back and never score — the arrival at Malekhu and
Kalikhola, and the peak at Devghat. That is the model's first support out of
sample.
Caveats. The measurement counts everything that left the scar
between late 2015 and 6–8 September, so for the 08:37 collapse alone it
is an upper bound. A third of the scar fails the data's own quality test; I
checked those cells against two other surfaces and they move the total by −4
to +2 Mm³. Four runs of 15 is thin. Galchhi fails in every run, 14–17 m
against 3.5–9.9 on the mud lines, and the smaller collapse barely moves it,
so that problem is in the lower river. It says nothing about how much was
ice. This finding said 14–34 Mm³ until September, which seismic
weighing ruled out on 1 October; then 110–175 from the mud-line map;
then 140–160 from the model's upper half, which turned out to be the bottom
of the range I had let it try. All three are in the changelog.
I proved my own model wrong, then fixed itfalsified, then fixed
The model could drop sediment but never pick any up.
Adding erosion — with formulas published for other rivers on other
continents and nothing tuned to this event — gave 3.8 Mm³ of scour
against 3.2 measured by satellite stereo. It also broke the model: with
deposition working properly the flood stalled before reaching the towns,
and 150 runs across every plausible combination could not both arrive on
time and leave the right debris. The fix was to stop treating sediment as
one substance — boulders settle and grind, silt and ice ride with the water
and leave no measurable deposit.
Caveats. An independent analyst, Darcy Weedman at geopera, reached
the same conclusion the same week by a different route. I lean on geopera's
satellite work heavily — the erosion figure, the border superelevation
speed, the deposit location — so "a different method" is true of the solver
and not of the inputs.
What is not settledopen
What the mass was made of is not resolved: wetness and ice content span nearly the whole range I allowed, and an earlier version of this page appeared to settle them, which turned out to be an artefact of holding one dial fixed. The modelled deposit sits at km 0–36 where stereo measurement puts it at 40–43. The height the river rose at Galchhi fails out of sample, now in every run at the measured size. The Devghat peak passes on a factor-of-2 test; in September every run that passed landed below the observation, and at the measured size three of the four do. And no Nepali scientist has read any of this.
My contribution is quantification, not correction. The scientists working on this event — Kargel, Shugar, Petley, ICIMOD and others — have consistently described river water and entrained sediment in their accounts of the flood. What nobody had published was the arithmetic: the relative sizes of the terms. That is the gap this project set out to fill, and where my numbers disagree with an agency's, both are shown. Nothing here is a finding about how well anyone else has done their job, and the warning-system section of the plain-English page is about a gap in the design of national systems generally, not about the people who were operating one that morning while four of its gauges were being destroyed.
9 October. Paused for some months — see the
next section. Nothing new was computed.
6 October. The collapse size goes into finding 04:
110 to 125 million cubic metres, measured, preliminary. After a night,
as the rule requires. The measurement is arithmetic on David Shean and Shashank
Bhushan's elevation data; the model agrees with it and, in 4 runs of 15, with
the downstream measurements it never sees. Galchhi still fails, and composition
is still open.
5 October, afternoon. Finding 01 is weaker than it
was, and its chip has gone from green to blue. Carrying the measured
size into the melt budget showed that I had the ice dial backwards. More ice
means less mass and less heat, so the case most favourable to melt is a rockier
mass, not an icier one. At 110–125 Mm³ melt is 3–8 Mm³ on best
evidence, but with every dial at its limit and a mass a fifth to two-fifths ice
it reaches the ~20 Mm³ of extra water. I cannot rule that corner out.
5 October. I measured the collapse directly
by subtracting the mountain after from the mountain before, using elevation
data David Shean and Shashank Bhushan released on 18 September. Preliminary:
110 to 125 million cubic metres left the scar, with the
glacier's own thinning since 2015 taken off. It holds on two other pre-event
surfaces, one of them radar, and on two independent post-event ones. That is
below the 140 to 160 my model gave, and below the bottom of the range I let it
try. Later the same day I ran the model at the measured size: it fits,
and better than at its own number — 15 runs of 300 match everything
above Betrawati, and 4 of those also land the downstream measurements I hold
back. Galchhi still fails in every run, and the smaller collapse barely moves
it, so that problem lies in the lower river, not the size. None of this is in
the findings yet, and it says nothing about how much was ice.
4 October. The top half of the model fits.
Splitting the problem at the border left an upper half that can be tested on
its own, and six runs of 300 now match everything measured above Betrawati at
once — putting the collapse at 140 to 160 million cubic metres.
It is not going into the findings, because the three held-back measurements
that caught the last false answer all sit below the border, so splitting the
model switched off the only check that has ever protected me. Composition is
still unresolved, and the ice-rich reading rests entirely on one lower-bound
rock measurement.
1 October. Three weeks of other people's work, and one
retraction of mine: these pages said the front did not slow down over the
first 22 km, and seismic tracking shows it accelerated off the headwall
and then slowed. A group has also weighed the moving mass from the force it
exerted on the Earth; that does not contradict my figure, but it rules out the
14–34 million cubic metres I published in early September.
Earlier: 5 findings withdrawn, several corrected, 7 whole-river
model versions built and none promoted.
The full record, with dates and reasons →
The work is paused from 9 October 2026, for some
months. The two questions it set out to answer have answers as good as
public data allows for now: where the water came from (finding 01) and how big
the collapse was (finding 04). What is left either needs one more piece of
modelling or waits on measurements nobody has made yet.
The first job when it restarts is the test at the junction.
The upper half of the model fits at the measured size, so the flood it sends
past the border is pinned. The test asks whether any lower river the mud lines
allow can carry that flood to Galchhi at 3.5–9.9 m without the water
reaching Malekhu too early. Every run fails Galchhi today, and the size of the
collapse is not the reason. Either answer is worth having.
What would restart it sooner: a repeat survey of the deposit
across a full melt season, roughly May–September 2027, which is the only clean
way to tell how much of the mass was ice and the main thing finding 01's corner
waits on; a volume or composition published by David Shean and Shashank
Bhushan, or anyone working from their data; the DHM gauge records at Betrawati
and Galchhi becoming public; or a correction. The issue tracker stays open, and
corrections still go up with the date and the reason.
Not a request — a statement of what the work does not have, so that a reader can weigh it. The DHM gauge records at Betrawati (station 447) and Galchhi both survived the flood; a peak discharge at either would settle the one question this model cannot, which is how much of the flood that passed Betrawati reached Galchhi and how fast. I do not have them, and I have not asked for them. The same goes for the hydropower headworks and SCADA records across the thirteen affected projects, which plausibly hold the only high-frequency measurements of the upper reach where the public gauges were destroyed; and for India's CWC hourly archive on the Gandak, which would extend the routing past the border. Everything on this site is built from what is already public, and its limits are the limits of that.
No Nepali scientist has reviewed this. It is the most conspicuous gap in the work. The Nepali translation is withdrawn rather than published on machine translation.