Flood inundation map of Sirajgonj district using mathematical model

Abstract

Bangladesh is a flood-prone country as being located at the confluence of three large rivers the Ganges, Brahmaputra and Meghna. About 92.5 percent of the combined basin area of the three rivers lies outside of the country. Furthermore, about 80 percent of the annual rainfall occurs in the monsoon (June to September) across the river basins. Therefore, Bangladesh is forced to drain out huge cross-border monsoon runoff together with its own runoff through a network of rivers. Most of the time, the volume of generated runoff exceeds the capacity of the drainage channels and this makes it one of the most flood vulnerable countries in the world. The objective of the study is to develop flood inundation map in a low-lying riverine flood prone area of Bangladesh at Sirajgonj District as this district suffered flooding due to excess runoff of trans-boundary Jamuna River. An early warning system with sufficient lead-time developed with hydrologic and hydrodynamic model will help to mitigate the flood damages. In this study, a weather forecast model was coupled with a hydrologic model and a hydrodynamic model for predicting floods in Jamuna River at Sirajgonj district. Weather Research and Forecasting model (WRF 3.0) was used to predict rainfall over the basin with lead-time of 6 days in this study. At first the hydrological model HECHMS 4.0 (Hydrologic Engineering Center - Hydrologic Modeling System) was calibrated and validated with discharge at Bahdurabad Station which is derived from Global Weather rainfall Data with Clark’s Unit Hydrograph transformation method. All basin component models runs Soil Moisture Accounting loss model that was executed with Linear Reservoir base flow model to simulate long-term response of a watershed to wetting and drying. Then output from the WRF model was coupled with hydrologic model HEC-HMS. Before using the model for prediction, the HEC-HMS model was calibrated with WRF output by observed discharge at Bahdurabad Station. WRF predicted rainfall for 1st June 2014 to 9th October 2014 was introduced to HEC-HMS and the generated river discharges of subbasin were ingested to the HECRAS 4.1.0 (Hydrologic Engineering Center-River Analysis System) hydrodynamic model for water profile computations along the Jamuna River. This hydrodynamic model was again calibrated and validated with observed water level at Bahdurabad station. Output of HEC-RAS was exported to ArcMap 10.1 where it was visualized as a flood inundation map with the use of the extension of HEC-GeoRAS. These maps have been developed for each day integrating the Digital Elevation Model (DEM) data of Shuttle Radar Topo-graphic Mission (SRTM) and interpolation of water level height obtained from HEC-RAS output at different cross-sections. From these maps, it has been found that Sirajgonj experienced less rainfall in the month of June than others. Only Southwest part of this district experienced flooding from the end of the month June. As Brahmaputra Right Embankment (BRE) was damaged near the Shahjadpur Thana, water entered through this thana and inundated the low-lying area of Shahjadpur Thana, Ullah Para Thana and Tarash Thana. Again, the Chauhali Thana, which is situated in the east of BRE, suffered severe flooding during almost whole monsoon period. Whereas, Roygonj Thana, Sirajgonj Sadar Thana, Kazipur Thana and Belkuchi Thana are comparatively dry which might be caused by their relative higher elevation. In month of July and August, intensity of rainfall was increased gradually and as a result, low-lying area of Shahjadpur Thana, Ullah Para Thana, Tarash Thana and Chauhali Thana were inundated with more flood water. Observing these maps, it has been found that this district suffered highest inundation in the month of August. On 24 August 2014 these four thana were completely inundated with flood water and other thana named Kamarkhand Thana and Royganj Thana were inundated partially. Although there is some damaged portion in the embankment in northeast direction, the adjacent area was comparatively dry because of their relative higher elevation. Naturally, floodwater moves towards downward slope and hence inundates the area having lower elevation. From 3 September 2014 flood inundated area had started decreasing and this decreasing pattern continued upto 18 September 2014 because of less rainfall. Then on 23 September 2014 flood inundated area had increased again and then flood inundation depth had started decreasing afterwards. Here, the inundation map prepared by HEC-GeoRAS, is mainly done by the water occupied channel area as areal extent due to rainfall. Only flood due to overtopping is considered here, flooding due to breaching of embankment or other reasons is not considered in this study. The main purpose of developing these flood inundation maps is to predict the flood with sufficient lead time due to rainfall with existing condition.