Modeling flood inudation of the Jamuna river

Abstract

Bangladesh lies at the con uence of worlds three major rivers, namely the Ganges, the Jamuna and the Meghna. In Bangladesh, normally four types of ood expressly river

ood, ash ood, tidal ood and storm surge ood appear. The Jamuna River is most vulnerable to river ood. As a low lying country, ood occurs repeatedly in Bangladesh and cause tremendous losses in terms of property and life, particularly in the low land areas. At least 20 % areas are ooded every year and in case of severe ood 68 % areas are inundated. Therefore, the study is carried out to develop oodplain extend maps and inundation maps of the Jamuna River. The present study also deals the

ood pattern change with time and impact of levee on ood inundation area. One dimensional hydraulic model HEC-RAS with HEC-GeoRAS interface in co-ordination with ArcView is applied for the analysis. The bathymetric data collected from Institute of Water Modelling (IWM) is used for river grid generation. The collecting DEM from NASA is resampled into 10 10 m res- olution. The bathymetric grid has been merged with the topographic DEM to produce the complete DEM of the river. Using this complete DEM, required data sets such as stream centerline, banks, ow paths and cross sections are prepared in HEC-GeoRAS pre-processing option, which are nally imported in HEC-RAS. In HEC-RAS, bound- ary conditions for upstream and downstream are de ned by discharge and water level data respectively. After boundary condition setup, the model calibration and valida- tion are performed using known hydrological data collected from BWDB. Performance of the HEC-RAS models are evaluated using standard and widely used indicators. The coe cient of determination (R2) has been found as 0.985, 0.977, 0.821 and 0.811 for steady calibration, steady validation, unsteady calibration and unsteady validation re- spectively. The NSE values have been found greater than 0.60 for both calibration and validation. After calibration and validation ood inundation and ood hazard map are generated using post-processing of HEC-GeoRAS . The results of ood frequency analysis by Log Normal method showed discharges of 65772, 76871, 83519, 91299, 96703 and 101790 m3/sec for 2, 5, 10, 25, 50 and 100-year return periods oods, where Log Normal method is best tted among the using other methods and used in modeling. The percentages of area inundated by 2, 5, 10, 25, 50 and 100-year return periods oods are 38.28, 46.10, 51.14, 54.63, 56.89 and 59.19% respectively. The ood inundation area increase with the increase of return period. It followed into the log curve y = 249:84ln(x) + 1789:3. The classi cation of ood depth area showed most of the ooding area had water depth between 1.2 m to 3.6 m. The assessment of the ood inundated area showed that 41.99% and 30.83% area are of high hazard and very high hazard respectively for the 100-year return period

ood. The ood inundation area's changing pattern with time are studied by ve years unsteady simulation ow data at Bahadurabad station. From unsteady simulation, an empirical equation is developed between water level and ood inundation area. The empirical equation is y = 4:77x2 ð€€€ 138:76x + 1008:30. The levee has impact on ood inundation area. The ood inundation areas decrease with rise of levee elevation . For the 100 year return period, if levee elevation is raised up to 2.13 m from existing levee elevation, then ood inundation land area decreased from 59.19% to 40%. No land will be inundated, if the levee elevation raised up to 2.56 m. Thus, nding of the study may help in planning and management of ood plain area of the Jamuna River to mitigate future probable disaster through technical approach. Finding of the study may also help to determine suitability of building ood control structure like embankment, detention ponds for prevention purposes. The automated

oodplain mapping and analysis using these tools provide more e cient, e ective and standardized results and saves time and resources. In future study, this model results can be compared with the studies with SOBEK or HEC-RAS (1D/2D) model results. Flood risk maps, others structures like ood control dam, reservoir impact on ood can be studied in future studies.