Abstract:
Monsoon flooding inundates a substantial part of Bangladesh, where 80% of the areas are floodplains. Sirajganj, located beside the Jamuna River in northwestern Bangladesh, is home to many communities living in the low-lying unprotected floodplains. The Ranigram village of Sirajganj has a hydraulic connection with the Jamuna River and is flooded almost every year. Proper assessment of the flooding process in inundated areas is a prerequisite for appropriate flood forecasting. The Flood Forecasting and Warning Center (FFWC) of the Bangladesh Water Development Board (BWDB) provides information on the river stage at some locations along the major rivers in Bangladesh. FFWC provides the forecasts by rainfall-runoff modeling and one-dimensional (1D) hydrodynamic modeling using MIKE 11 software which does not provide any flood information for the floodplain localities. Consequently, people of the localities cannot take necessary precautions before the floodwater enters their localities as they do not get any proper location-specific forecasts. This study explores how flood propagates in the floodplain, determines the water level variation in the floodplain concerning the Jamuna River stage, and develops a 2D model to forecast the flood in the floodplain of a riverside area in Sirajganj where there is an interaction between the locality and the river.
Water level gauges were installed at strategically selected locations in Ranigram, and flood data were collected during the 2018 and 2020 monsoons. A statistical equation of floodplain water level is derived from the river water level for the 2018 data and is validated with the observed data of 2020. With the R2, NSE, MSE and RMSE analyses, the observed floodwater level in Ranigram shows an excellent dynamic relation with the water level at Sirajganj on the Jamuna River. A 2D hydrodynamic model is developed with HEC-RAS using a high-resolution Digital Terrain Model (DTM) generated with surveyed bathymetry and Unmanned Aerial Vehicle (UAV)-based data, and is calibrated and validated with the observed water level data and satellite-based flood images. The model performance is also assessed with globally available WorldDEMTM, ALOS PALSAR, SRTM, MERIT and ASTER Digital Elevation Models (DEMs). The 2D model incorporated with high-resolution UAV-based DTM is found to better represent the flood scenario in the locality. The results of the flood arrival time, duration, maximum flood extent and depth are extracted from the model. In the 2D model, the boundary condition is developed from the FFWC’s forecasted water level in the Jamuna River and the result of the model represents the forecasted flood scenario in the Ranigram area for the corresponding FFWC forecast. This developed methodology can be helpful in forecasting floods in any riverside locality using the FFWC forecasted data. It will thus help the local people to take the necessary precautions before floodwater enters their lands. Furthermore, the outcomes of this study will be helpful in forecasting and assessing flood risk and damage for the floodplain areas.
