Flood hazard mapping of dharla river floodplain using HEC-RAS 1d/2d coupled model

Abstract

Development of flood hazard map in Dharla River floodplain, located in the north-west zone of Bangladesh using 1D/2D couple hydrodynamic model simulation has been reported. Maps have been developed with data of administrative upazila and landuse pattern of the study area using flood depth as a hydraulic characteristic factor of flood. The hydrodynamic model for mapping were developed using the Hydrologic Engineering Center River Analysis System (HEC-RAS) in concert with HEC-GeoRAS. HECGeoRAS set procedures, tools, and utilities for processing Geographic Information Systems (GIS) data by using a graphical user interface on a GIS platform. Automated GIS processing procedures in HEC-GeoRAS provided a useful and expeditious method for repetitive hydraulic model development during analysis of the Dharla River floodplain. Reach length, stream centerline, main channel bank, flow path lines and cross sections have been determined using HEC-GeoRAS. The geometric data has been imported into HEC-RAS using a data exchange format developed by HEC. The resultant water depth exported from HEC-RAS simulations has been processed by HEC-GeoRAS for flood inundation delineation and hazard map generation. Calibration and verification of the hydrodynamic model were performed in 2013 and 2014 respectively with observed water level data using Manning’s roughness coefficient (n). Model simulation result has showed that 23.8% and 34 % of total study area were inundated under water in 2017 and 1998 respectively. According to the analysis of flood water depth in year 2017 and 1998, it was found that area of F1 (0 m- 0.9 m) was significant from May to September. From the hazard mapping, out of ten upazilas, Lalmonirhat Sadar, Phulbari and Kurigram Sadar along the Dharla River were found to be the most vulnerable to flood hazard. It was also found that Chilmari, Bhurungamari and Kaliganj upazilas which are the outermost upazilas of Dharla River floodplain were very less susceptible to flooding. Considering the agriculture landuse pattern, Boro - Fallow - T.aman was found to be the most vulnerable crop and Rabi Crop - B.aus - Fallow was the less vulnerable crop to the flood events of 2017 and 1998 in the study area. Generally, the study showed that the methodology for river flood analysis using the 1D–2D coupled hydrodynamic model is generic and can be applied to similar geographical conditions.