{ "generated_utc": "2026-08-27T09:17:10Z", "event": { "name": "Lhende Khola / Bhote Koshi barrier lake and outburst, 26 Aug 2026", "note": "Rock-ice avalanche off the Tibetan side blocked the Lhende Khola ~20 km upstream of the Miteri Bridge, impounding a lake that then failed. The surge ran ~60 km down the Bhote Koshi through Timure and Syabrubesi. The collapse registered 5.2 on local seismometers, initially misreported as a tectonic trigger." }, "question": "Was the impoundment visible from orbit before it breached? Sentinel-1D descending orbit 19 imaged the catchment at 00:18Z on 24 Aug 2026, ~48 h before the collapse; the next look on any track was 28 Aug. This is the only free-archive look inside the window.", "source": { "mission": "Sentinel-1D (ESA Copernicus), IW GRD -> RTC", "processing": "Microsoft Planetary Computer sentinel-1-rtc; Copernicus DEM GLO-30", "attribution": "Contains modified Copernicus Sentinel data 2026" }, "method": "Descending orbit 19, VV gamma0. Baseline = median of 4 monsoon-season scenes (2026-06-25 to 2026-07-31), which holds the snow line and wetness roughly constant and subtracts static radar shadow. New water = VV <= -15.0 dB AND at least 3.0 dB below baseline, on slopes <= 8.0 deg and below 5000 m, 8-connected clusters >= 2000 m2. 2026-08-12 is run as a control: a method that finds 'new' water 14 days before the event is finding noise.", "limits": "Wet snow is the one confounder the baseline does not remove: at C-band it goes as dark as open water, it sits on the same high-altitude terrain, and its extent moves between passes. The elevation ceiling and the flatness mask suppress most of it - wet snow prefers slopes, water does not - but any detection above ~4500 m should be treated as snow until proven otherwise. Wind roughening cuts the other way and can hide a real lake by lifting its backscatter above the threshold. In a confined gorge the impoundment may be only 20-50 m wide, which is 2-5 pixels at 10 m, so small early-stage ponding is below the honest detection limit. Piping failure gives no surface warning at all. This detects water, not stability: seeing a lake tells you an impoundment exists, not when it will breach.", "verdict": "Negative, and the negative is the finding: no impoundment was detectable in the Lhende corridor on 24 Aug 2026, ~48 h before the breach. This is a null result, not a null instrument. The reported blockage reach (15-25 km upstream of the Miteri Bridge) is fully imaged - 21,489 valley-floor pixels, 100% finite on every date, median baseline VV -8.8 dB, which is ordinary ground return and not shadow - and it sits inside the detection mask at 3-8 deg median slope. Across a sweep of every sensible parameter combination (slope 8/15/25/90 deg, water -13/-15/-18 dB, drop -2/-3 dB) the excess new-water area on 24 Aug over the 12 Aug control never rises above the speckle floor, and at whole-AOI scale it goes negative. The dB change distribution bottoms out at -5 dB at p1, the same speckle floor found in the Hong Kong 2023 study. An impoundment large enough to drive a surge that destroyed 60 km of valley would have covered order 1e4-1e5 m2 - hundreds to thousands of pixels, far above that floor. It was not there. The blockage therefore formed and failed inside the 4-day gap between the 24 Aug and 28 Aug passes, and most likely inside a few hours. No system at Sentinel-1 cadence could have warned: the actionable window ran from the seismic detection of the collapse (the 5.2 signal, which is t=0 and is already an instrument that fired) to the surge reaching Timure. That is a seismic and ground-instrumentation problem, not a satellite one. Satellite's role in this hazard class is post-event verification and monitoring of the persistent secondary impoundments now sitting in 60 km of fresh debris.", "validation": { "upstream_reach_15_25km": { "valley_floor_px": 21489, "finite_all_dates_px": 21489, "median_baseline_vv_db": -8.8, "note": "Ordinary ground return across the whole reported blockage reach. A shadowed gorge would sit at the noise floor, so 'nothing detected' here cannot be explained by geometry." }, "speckle_floor_db": -5.0, "sweep_excess_area_px": { "slope8_w15_d3": 52, "slope15_w15_d3": 153, "slope25_w15_d3": 36, "slope90_w15_d3": -1906, "note": "test minus control, i.e. new-water pixels on 24 Aug over the 12 Aug control. Never separates from noise; negative at whole-AOI scale." } }, "params": { "water_db": -15.0, "drop_db": -3.0, "max_slope_deg": 8.0, "max_elev_m": 5000.0, "min_area_m2": 2000.0 }, "summary": { "control_2026-08-12": { "n_clusters": 1, "total_area_m2": 2000 }, "test_2026-08-24": { "n_clusters": 0, "total_area_m2": 0 }, "excess_area_m2": -2000 }, "clusters": { "control_2026-08-12": [ { "lat": 28.4038, "lon": 85.49617, "area_m2": 21500, "mean_vv_db": -18.7, "mean_drop_db": -6.03, "elev_min_m": 4956, "stage_m": 4958, "likely_wet_snow": true, "nearest": { "name": "Rasuwagadhi / Miteri Bridge", "dist_m": 17802 } }, { "lat": 28.51329, "lon": 85.49868, "area_m2": 4800, "mean_vv_db": -19.6, "mean_drop_db": -5.03, "elev_min_m": 4809, "stage_m": 4814, "likely_wet_snow": true, "nearest": { "name": "Rasuwagadhi / Miteri Bridge", "dist_m": 28329 } }, { "lat": 28.51106, "lon": 85.44685, "area_m2": 3500, "mean_vv_db": -16.84, "mean_drop_db": -4.11, "elev_min_m": 4633, "stage_m": 4640, "likely_wet_snow": true, "nearest": { "name": "Rasuwagadhi / Miteri Bridge", "dist_m": 26387 } }, { "lat": 28.55823, "lon": 85.33361, "area_m2": 2300, "mean_vv_db": -22.92, "mean_drop_db": -4.28, "elev_min_m": 4948, "stage_m": 4951, "likely_wet_snow": true, "nearest": { "name": "Rasuwagadhi / Miteri Bridge", "dist_m": 31102 } }, { "lat": 28.21271, "lon": 85.31956, "area_m2": 2000, "mean_vv_db": -18.66, "mean_drop_db": -4.34, "elev_min_m": 3062, "stage_m": 3065, "likely_wet_snow": false, "nearest": { "name": "Syabrubesi", "dist_m": 5796 } } ], "test_2026-08-24": [ { "lat": 28.40379, "lon": 85.49623, "area_m2": 21800, "mean_vv_db": -18.88, "mean_drop_db": -6.08, "elev_min_m": 4955, "stage_m": 4958, "likely_wet_snow": true, "nearest": { "name": "Rasuwagadhi / Miteri Bridge", "dist_m": 17805 } }, { "lat": 28.51129, "lon": 85.44678, "area_m2": 2300, "mean_vv_db": -17.01, "mean_drop_db": -4.73, "elev_min_m": 4635, "stage_m": 4640, "likely_wet_snow": true, "nearest": { "name": "Rasuwagadhi / Miteri Bridge", "dist_m": 26410 } } ] } }