Rapid Upstream Rainfall Induced Flood Mapping and Monitoring Using Open Source Data: A Case of The July 2020 Flood In the Jamuna Basin

Abstract:

Monitoring rainfall induced monsoon flooding in the Ganges-Brahmaputra-Meghna (GBM) is challenging given factors such as excessive cloud cover in monsoon season and absence of geostationary RADAR satellite in developing countries such as Bangladesh. This makes the use of open-source data inevitable. The July 2020 monsoon flooding is set to become one of the most prolonged floods in Bangladesh and expected to last till the end of August. The Jamuna Basin in Bangladesh is particularly prone to flooding due to intense rainfall in the upstream catchments of Assam and Meghalaya. Past studies have suggested hydrological modelling approaches that were data-intensive and required additional hydrological data that are often less frequent and unreliable in Bangladesh. This study presents an approach for comprehensive rapid flood monitoring using readily available open-source data for the downstream middle Jamuna basin. Flood extent at 6-day interval is delineated using Sentinel 1 C-Band SAR data in the Google Earth Engine (GEE) Platform. The Python-based Floodwater Depth Estimation Tool (FwDET v2.0) is used to estimate the Flood Depth in the Middle Jamuna basin using the flood extent polygons and freely available ALOS PALSAR 12.5 meter DEM data. Satellite retrieved GPM IMERG daily rainfall data is obtained for upstream (Assam and Meghalaya) and downstream basin for identifying the Spatio-temporal patterns of rainfall throughout the month of July. Freely available and regularly updated AWS rainfall data for stations in Assam and Meghalaya is used for validation of satellite-derived rainfall. The flood extent for 3, 15, and 21 July was 18.95%, 30.38%, and 40.11% respectively. A maximum flood depth of 114 meters is recorded for the flood on July 21. The mean rainfall in upstream Assam from 8th to 14th July was 521 mm which corresponds to the initial flooding on the July 15 in the downstream basin whereas 600 mm rainfall in Meghalaya corresponds to the flood extent in 21st of July. Flood extent, depth, and rainfall distribution statistics and maps were developed for the 300 Unions (lowest administrative area) in the middle Jamuna basin to facilitate emergency response. Therefore, flood extent, depth, and rainfall distribution at the Union level could be monitored using this open-data based approach as the flood progresses.