Flood Modelling and Inundation Mapping of Meenachil River Using HEC-RAS and HEC-HMS Software

Abstract

AbstractThe flood modelling and inundation mapping study was done for the Meenachil river basin in Kerala. The study shows the usage of HEC-RAS and HEC-HMS tools in determining the flood hazard and vulnerability. Flood inundation maps for different return periods were developed to analyze the future flooding scenarios. Abnormal heavy rainfall in July-August 2018 which leads to severe flooding caused loss of many lives and damages to properties in Kerala. This study also analyzed the 2018 flood event to develop the flood inundation maps and to identify the flood affected area of different land use classes as well as the flood affected road infrastructure. The Meenachil river basin was delineated using HEC-HMS and the topographic characteristics were extracted from SRTM DEM. Hydrologic model was developed using SCS Curve number method, SCS unit hydrograph method and Muskingum method for loss, transform and routing method respectively. The simulated hydrologic model was manually calibrated using discharge data. The precipitation data from the year of 1990 to 2017 was used to run the hydrologic simulation to obtain the discharge values which were used as input for hydraulic analysis using HEC-RAS. The discharge values obtained were then used to find the peak discharge for different return periods of 5, 10, 50, 100, 500 years using Gumbel’s method. The simulated discharge is fed to HEC-RAS as the upper boundary condition for identifying the flood affected area. One dimensional steady state analysis was carried out to obtain the flood depth for different return periods. Similarly, hydraulic analysis was carried out for the year of 2018. The peak annual discharge in cumecs was calculated using Gumbel’s method and found to be 7484.468 m3/s. The flood plain map covered an area of 24.2465 km2. It was observed that depth of flow increased towards the downstream end. The developed flood inundation map was superimposed on road map to find the length of different classes of roads that got submerged. Land use map was generated for the year of 2018 through Isocluster method of unsupervised classification of LISS III image. This was used to identify the area of different land use classes that were affected during 2018 flood as per the simulation.KeywordsSRTM DEMHEC-RASHEC-HMSGumbel’s methodFlood inundation mapping