Flood inundation mapping and flood mitigation of Ghaghat river floodplain using HEC-RAS 1D/2D coupled model

Abstract

Ghaghat River flows through North West region of Bangladesh in Nilphamari, Rangpur and Gaibandha district. It is a distributary of Teesta River which originates at Jaldhaka in Nilphamari district. It carries flow from Teesta to Brahmaputra River thus it has a major influence during flood period. During monsoon, when water level of Ghaghat River at Gaibandha crossed the danger level, the floodplain of this river is inundated.At almost every year people of Gaibandha municipality experienced high magnitude flood which is the most devastating natural hazard of this region. The aim of this study is to develop a Hydrodynamic model for Ghaghat River basin area using HEC-RAS 1D/2D coupled model to prepare flood inundation mapping.Flood inundation mapping of Ghaghat River has been carried out to identify the flood affected area for this river basin. A HEC-RAS 1D model of Ghaghat River has been set up using bathymetry of 2013 and the discharge and water level hydrographsinupstream and downstream boundaries respectively. A HEC-RAS 1D/2D coupled model of Ghaghat river basinhas been set up for the same condition for flood mapping. HEC-RAS 1D model has been calibrated for 2013 (June to November) for the water level data at Gaibandha and validated for the period of 2015 (June to November) for same condition.Finally, a 1D-2D coupled model of Ghaghat River floodplain has been set up in HEC-RAS. This coupled model is calibrated and validated for Manning's roughness coefficient ‘n’ = 0.020for the year of 2013 and 2015 respectively.The values of R2 and NSE have been found 0.9596 and 0.956 for calibration and 0.9027 and 0.730 for validation respectively.The qualitative comparison hasbeen done among FFWC observed map, satellite image and simulated model.After calibration and validation of the 1D-2D coupled model, it has been simulated for the historical flood events of 2004, 2007, 2011, 2012 and 2017.The study area (total 1532 km2) consist of three districts where Nilphamari, Rangpur and Gaibandha covered13.81 km2, 951.75km2and 566.42 km2 respectively. This study proposed three scenarios for mitigating floods in the study area. The first scenario involved extending and increasing the height of the left and right embankments of the Ghaghat River from downstream. The second scenario involved dredging the cross-sections at the downstream and nearest one-third length of the river. The third scenario involved extending and raising the height of the embankment along with dredging the cross-section at the nearest one-third length of Ghaghat River.Approximately 19.27%area of the floodplain of the Ghaghat River has been affected by floodingfor 2004. From all the floodmitigation scenarioson the basis of cost estimation and reduced percentage of flood mitigation,scenarios3.2has been suggested(raising the existing embankment height by 1 meter up to 20 kilometers from downstream at left bank and right bank and raising the existing embankment height by 1.2 meter from 20 kilometers to 37 kilometers from downstream at left bank and from 20 kilometers to 27.5 kilometers at right bank and construction of new embankment of 1.2m height from existing right bank from 27.5 kilometers to 37 kilometers at right bank and dredging 1.0 meter for cross-sections 15 to 22 (49.5-73 kilometers)).