The flood event
On 26 August 2026, a destructive outburst flood affected the Bhote Koshi–Trishuli river corridor across the Nepal–China border. The Planet Crisis Response catalogue describes the event as a flood following a preliminary assessment of an ice-and-rock avalanche in the upper catchment; it also explicitly states that the trigger remained under investigation at publication time.
This project focuses on the spatial context: a narrow river valley, surrounding steep terrain, and changes visible between the available baseline and post-event satellite acquisitions. It does not claim to identify the exact trigger, calculate damage, or verify casualty figures.
Why use 3D?
A 2D image can show a changed river channel, but it cannot easily communicate how tightly the river is confined by surrounding terrain. A 3D viewer helps reveal the relationship between slope, tributary confluences, valley confinement, and the likely route of flood material.
The opacity slider blends baseline and post-event imagery; orbit and grab-pan navigation let the user inspect the valley from different viewpoints. The top-down control fits the mapped area, while the measurement tool distinguishes slant distance across terrain from horizontal map distance.
How the viewer was made
- Image selection: Sentinel-2 Level-2A scenes were queried through a Google Earth Engine workflow. A 12 August 2026 scene was selected as the nearest usable baseline; a post-event composite spans 27 August–15 September because monsoon cloud cover limited optical imagery.
- Terrain preparation: Copernicus GLO-30 elevation data was exported for the area of interest and converted into a lightweight height grid.
- 3D web rendering: A Three.js mesh is built from the height grid and textured with aligned before/after imagery. The post-event layer uses controllable transparency.
- Interaction design: The viewer adds orbit, grab-pan, auto-fit bird's-eye view, labels, and terrain-aware measurement for exploratory use.
Benefits and intended use
The result is a compact way to communicate an event's landscape context in a portfolio, briefing, or teaching setting. It can help a non-specialist understand why valley geometry matters, while giving a GIS-oriented audience direct controls to inspect the imagery and terrain.
Interpretation boundary
This is an exploratory visualization, not an official damage assessment or incident report. Cloud cover, acquisition timing, 10 m imagery resolution, and unverified event information constrain interpretation. The viewer supports discussion and further investigation; it must not be used to establish causality, quantify losses, or make operational decisions.
Sources
- Planet Crisis Response — Bhote Koshi–Trishuli Outburst Flood catalogue (event metadata, imagery catalogue, and stated uncertainty).
- Copernicus Emergency Management Service — EMSR927 (official mapping activation reference).
- Nepal Floods 2026 Disaster Imagery Viewer (open imagery discovery reference).
- Copernicus Sentinel-2 and Copernicus DEM (source datasets).