ANALYSIS OF FLOODPLAIN ENCROACHMENT USING HEC-RAS 2D UNSTEADY FLOOD FLOW MODELING

摘要

The study attempts to provide a methodology for a rapid assessment of flood dynamics over an area using HEC-RAS 5.0.5. HEC-RAS has been improved and includes new abilities of performing 2D flood routing over a large area at variable time steps. Its computational engine also has been parallelized in a 64-bit environment resulting to reduced run times even for large watershed areas. It would be sufficient to provide an initial assessment of flow extent and depth using generalized workflow especially for areas where there are no available extensive data on rainfall, land use and soil types. The study was performed on a 210-km" watershed with LiDAR-derived DEM covering almost 70% of the area including its floodplain. The LiDAR DEM was resampled to 5-m resolution and fused with a 5-m resolution IFSAR DEM for the remaining area of the watershed. The whole watershed was considered as 2D flow area at 30m x 30 m flow grid with a breakline on the highway centerline. HEC-HMS is used to estimate excess precipitation over the area. Several simulations are performed at established rainfall probabilities of 4%. 2% and 1%. Results are compared from flood simulations performed on terrains before and after the presence of major built infrastructures on the floodplain. Results show that full 2D unsteady flood modeling over a whole watershed could be used to have a rapid assessment of flood dynamics over an area.