The worst of three parts: days with 50 mm of rain or more, the height of the place above the nearest large river (GloFAS), and low-lying coast.
Hazards
Heavy rain: ERA5 days per year with >= 50 mm. ERA5 is a ~30 km model and smooths peaks: real local downpours are stronger.
River: the largest river cell (mean flow >= 1 m3/s) within about 15 km, from GloFAS. What matters is how far the place sits above it: a hilltop village above an estuary is not exposed to river floods.
Coast: when a sea cell is within 10 km and the place is low (elevation from the Copernicus DEM).
See River discharge (GloFAS) for the river forecast.
Why it matters for Koh Samui
Floods damage homes, cut roads and ferries, and contaminate water. Check the exact plot before buying or renting.
Thresholds
Part
1
2
3
4
Heavy-rain days/yr
0.5-2
2-5
5-10
>= 10
Height above river cell
< 30 m
< 10 m
< 3 m
--
Coastal and elevation
< 20 m
< 10 m
< 5 m
--
Score = the highest part. Elevations are at the place point: individual streets can be lower. Exposure scale: 0 very low, 1 low, 2 moderate, 3 high, 4 very high. Higher = more exposed. The colour of the matrix cell follows the same scale.