Disaster Response

How to Detect Floods with Satellite Radar

Learn how to visually interpret flood extent in Sentinel-1 SAR imagery. This guide explains why water appears dark in radar and how to compare before/after scenes.

TL;DR: SAR detects floods through clouds because water appears dark (specular reflection). Use Sentinel-1 before/after comparison. VH polarization best for flood extent. 6-day revisit. Free in Off-Nadir Delta.

Works Through Storms

Radar penetrates clouds, rain, and darkness - exactly when floods occur.

Rapid Assessment

Get flood extent information within hours of satellite overpass.

Wide Area Coverage

Single Sentinel-1 scene covers 250km swath for regional assessment.

How SAR Detects Water

SAR sends microwave pulses and measures the reflected signal. Calm water surfaces act like mirrors, reflecting the radar away from the satellite, making water appear very dark in SAR imagery.

By comparing pre-flood and during-flood imagery, new dark areas indicate flood extent. This works even when the area is covered by clouds or at night.

SAR Flood Interpretation Guide

What Appears Dark

• Open Water: Calm surfaces reflect radar away (specular reflection)
• Flooded Fields: Agricultural areas covered by water
• Smooth Surfaces: Roads, runways, calm lakes

What Appears Bright

• Urban Areas: Buildings cause corner reflections
• Flooded Vegetation: Double-bounce from water under trees
• Rough Surfaces: Waves, rough terrain, forests

Polarization Tips

VV Polarization

Best for open water detection. Strong contrast between water and dry land.

VH Polarization

Better for flooded vegetation. Cross-polarization sensitive to volume scattering.

Applications

Emergency Response

Prioritize rescue and relief efforts based on flood extent

Damage Assessment

Document affected areas for insurance and recovery planning

Floodplain Mapping

Historical flood extent for risk assessment and planning

Flood Monitoring

Track flood progression and recession over time

How It Works

Identify Affected Area

Navigate to the flooded region on the map.

Get Pre-Flood Baseline

Load Sentinel-1 SAR imagery from before the flood event.

Load Current SAR Data

Search for the most recent Sentinel-1 imagery during/after the flood.

Compare & Map Extent

Use difference analysis to identify flooded areas (water appears dark in SAR).

Frequently Asked Questions

Why use SAR for flood mapping?▼

SAR (Synthetic Aperture Radar) can see through clouds and works at night, making it essential during storm events when optical satellites cannot capture clear imagery.

How do I identify water in SAR?▼

Water appears very dark (low backscatter) in SAR imagery because smooth water surfaces reflect radar away from the sensor. Compare with pre-flood imagery to identify new water extent.

How quickly is new data available?▼

Sentinel-1 has a 6-day revisit time. During major disasters, the International Charter may task additional satellites for faster coverage.

Can I map floods in urban areas?▼

Urban flood mapping is more challenging due to building reflections, but still possible. Look for areas where normal urban texture is replaced by dark (water) returns.

What about flooded vegetation?▼

Flooded vegetation can show increased backscatter (brighter) in some cases due to double-bounce effects. Time series analysis helps identify these areas.

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