FLOOD INUNDATION MODELING & MAPPING IN A REACH OF RIVER JAMUNA IN BANGLADESH

Abstract

Bangladesh is a flood-susceptible country. Jamuna is one of the most flood vulnerable River in Bangladesh. Hydrodynamic modelling of about 172 Km River reach of Jamuna from Bahadurabad to Aricha has been undertaken as this river reach is vulnerable to recurring flooding every year. The extent of flooding and velocity with and without embankment condition have been studies by developing hydrodynamic model using HEC-RAS 1D & 2D. As the observed discharge data for gauging station was available for limited number of times but stage observations were available at 12 hourly interval, a stage discharge relationship at Bahadurabad gauging station has been developed to convert stage readings into corresponding discharge values required as upstream boundary for hydrodynamic model. Using the available 28 cross sections, 1D hydrodynamic model was developed using HEC-RAS. Twelve hourly flow Hydrograph and normal depth has been used as upstream and downstream boundary condition respectively. Unsteady flow state has considered in this simulation and the model was calibrated and validated using observed stage data at two locations in the study reach for the year 2009 and 2010 hydrological event correspondingly. The Manning’s n value 0.026 and 0.027 for channel and both banks respectively was adjudged best to simulate observed water level data at Sirajganj and Aricha gauging sites. The NSE values obtained for both station is above 0.77 for calibration and validation period. The 2D hydrodynamic model was also setup both for with and without embankment scenario using the SRTM 30 m DEM which was resampled into 5 m grid cell size using ArcGIS. A simplified representation of Brahmaputra Right bank Embankment (BRE) was developed and interpolated at 5m grid resolution and fused with resampled DEM at 5 m resolution to generate terrain for setting up hydrodynamic model with BRE. Hourly flow hydrograph and normal depth were used as the upstream and downstream boundary conditions in the unsteady flow simulation. The results obtained for 2D hydrodynamic model indicate that 6, 11, 20, 37, and 26% area of the total flooded area was inundated with embankment condition by F0 (0 – 0.3 m), F1 (0.31 – 0.9 m), F2 (0.91 – 1.8 m), F3 (1.81 – 3.6 m), and F4 (> 3.6 m) flood depths, respectively. However without embankment 7, 13, 19, 39, and 23 % of the total flood area would be flooded with water depth class F0 (0 – 0.3 m), F1 (0.31 – 0.9 m), F2 (0.91 – 1.8 m), F3 (1.81 – 3.6 m), and F4 (> 3.6 m), respectively. In both scenarios (with and without BRE), velocity of water on floodplain remains below 0.5 m/s but the flow velocity is above 1m/s in the River reach and higher velocity of flow were simulated in middle path of the river where depth ABSTRACT ii of water is generally high. The spatial distribution of the simulated maximum flood area of inundation is about 14378 Km2 and 12248 Km2 out of 22167 Km2 without and with embankment condition respectively. The results also shows that 64.5 and 55 % of the total study domain area are inundated by without and with embankment condition respectively. It also shows that about 2130 Km2 area is protected due to embankment.