Floodplain Mapping for Indus River: Chashma –Taunsa Reach

Usman Khalil, Noor Muhammad Khan, Habib -ur- Rehman

Abstract


Floods are the most critical among all the natural calamities. Flood water inundate the floodplain areas and cause vast damages to life and property. It is thus essential to address this natural calamity to decrease the effect it causes to property and people. In this study, Indus River reach Chashma-Taunsa (252 km) was selected, that experienced exceptionally high flood in 2010, due to which huge damages occurred to life and property. In this study, flood hydraulic modelling and floodplain mapping has been performed to have initial assessment of flood vulnerability and to provide vision for emergency preparedness plan. Flood mapping can further precede flood hazard and risk maps. To perform hydraulic modelling and floodplain mapping HEC-RAS, ArcGIS and its extension Hec-GeoRas were used as tools. HEC-RAS model input cross-sections data were collected from physical survey and extracted from DEM SRTM 90 m by using Hec-GeoRas. Comparison was made between surveyed and SRTM DEM extracted cross-sections; to perform necessary verification and merging the channel data in DEM extracted cross-section. Frequency analysis has been conducted by using Gumbel’s and Log Pearson Type III distributions to determine the flood peaks for extreme events. Frequency analysis result shows that the flood of 2010 in study area has return period of 180 years. Cross-sections data and results of frequency analysis were used in HEC-RAS to perform unsteady flow analysis for low to extreme events. HEC-RAS model was calibrated and validated for year 2010 and 2006, respectively. Coefficient of determination (R2) 0.95 & 0.90 and Nash and Sutcliffe coefficient that 0.93 & 0.86 were calculated, that show good calibration and validation respectively. Results of HEC-RAS model were exported in ArcGIS to perform inundation mapping. Depth and area of flooded extent for 2010 flood were calculated as 8.1 m and 1900 km2, respectively. Flood inundation maps of the whole study area were prepared for 25, 500 year and 2010 year return period flood. Floodplain maps show flooded area depth from 7.1 to 9.1 m for 25 year to 500 return period floods, respectively. Flood inundation maps will be an important tool for Engineers, Planners for emergency actions plans and for flood management.

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