abeylicious/Maiduguri-flood-sar-2024-changes

Cloud-proof Synthetic Aperture Radar (SAR) flood mapping pipeline in Google Earth Engine using Sentinel-1 C-band (VV polarization) to quantify urban inundation from the September 2024 Alau Dam collapse in Maiduguri, Nigeria # Satellite Radar (SAR) Flood Extent Mapping: Maiduguri 2024 > 📖 **Read the full step-by-step case study on Medium:** Cloud-Proof Flood Extent Mapping with Sentinel-1 SAR in Google Earth Engine ## Overview This repository contains a cloud-proof Synthetic Aperture Radar (SAR) change-detection pipeline implemented in **Google Earth Engine (GEE)** to quantify the flood inundation caused by the **Alau Dam collapse in Maiduguri, Borno State, Nigeria (September 2024)**. Because standard optical sensors (e.g., Sentinel-2, Landsat) were obstructed by heavy monsoonal cloud cover during the event, **Sentinel-1 C-band Synthetic Aperture Radar (GRD, VV Polarization)** was utilized to penetrate clouds and map open surface water extent. --- ## Key Results & Metrics * **Pre-Flood Baseline:** July 1 – August 25, 2024 * **Post-Flood Assessment Window:** September 5 – September 25, 2024 * **Optimal Backscatter Threshold:** `-16 dB` (VV Polarization) * **Calculated Surface Inundation Area:** **~20 km²** within the metropolitan ROI --- ## Methodology & Workflow 1. **ROI Definition:** Maiduguri Metropolis, Alau Dam reservoir, Ngadda River channel, and the downstream Jere Bowl. 2. **SAR Filtering:** Sentinel-1 IW (Interferometric Wide Swath) mode in `VV` polarization. 3. **Temporal Compositing:** Median reductions for baseline and immediate post-breach periods to minimize radar speckle. 4. **Thresholding & Masking:** Backscatter threshold applied at `-16 dB` to segment calm surface water. 5. **Change Detection:** Spatial logic isolating newly inundated pixels while excluding permanent water bodies: $$\text{Flood Extent} = \text{Water}_{\text{Sept}} \land \neg \text{Water}_{\text{August}}$$ 6. **Area Reduction:** Spatial aggregation using `ee.Reducer.sum()` at 10 m native pixel resolution. --- ## Threshold Calibration & Sensitivity Analysis A sensitivity comparison was conducted to eliminate dry soil/sand false positives in semi-arid terrain: | Threshold | Calculated Inund …