Inundation modelling of the surma and kushiyara river floodplain due to monsoon flooding

Abstract

Bangladesh has long been facing floods during the monsoon seasons, which is beneficial up to a normal level, but detrimental if exceeds. The Surma and Kushiyara river floodplain located in the Northeast region of the country within the Upper Meghna basin is one of the vast floodplains of the country with complex hydrometeorological system. In this study, an effort has been made to model the inundation of Surma-Kushiyara floodplain due to recurrent monsoon flooding with a coupled hydrological and hydrodynamic model. The hydrological model was developed with HEC-HMS software for the Barak basin, flow governing entity of the Surma-Kushiyara. The one-dimensional hydrodynamic model was developed for the Surma-Kushiyara rivers including all major tributaries from Barak bifurcation up to the upstream of Haor basins with HEC-RAS software. Flood frequency analysis based on annual peak water level and discharges was carried out for selection of a normal and above normal flood scenario to identify inundation patterns. After calibration and validation, the simulated water surface and extent profiles from hydrodynamic model were processed in Geographic Information System with Multi-Error-Removed Improved-Terrain (MERIT) as the Digital Elevation Model to obtain flood maps for selected scenarios. The maps were later verified with Sentinel-1 and Landsat-8 satellite images and MIKE 11 model generated maps. Frequency analysis revealed year 2017 as a moderate to severe flood year of modal return period between 2.33-10 years, and 2018 as a normal flood year of average return period 2 years. Use of coupled hydrological model was not found effective compared to rated boundary inflows specially in absence of high spatiotemporal resolution rainfall data. Analysis showed the peak flooded area in 2017 to be 57% of the floodplain (3598 sq-km), with F2 (0.9-1.8m) being the dominant flood depth type. 23% of the floodplain area was estimated as the normal monsoon inundated area. Sylhet district as a whole is in more risk of flooding due to the Surma-Kushiyara, with 55% of the district area (1787 sq-km) within floodplain being affected in 2017 compared to the normal monsoon inundation of 15%. Production of Aush and specially Transplanted (T.) Aman rice can be significantly hampered due to monsoon floods with affected areas for Fallow-Aush-T. Aman and Fallow-T. Aman land types being 115,335 Ha and 77,790 Ha respectively in 2017. Flooding trend of the Surma-Kushiyara rivers in terms of water level is rising.