Assessment of flood inundation mapping of Surat city by coupled 1D/2D hydrodynamic modeling: a case application of the new HEC-RAS 5

Surat city of India, situated 100 km downstream of Ukai dam and 19.4 km upstream from the mouth of river Tapi has experienced the largest flood in 2006. The peak discharge of about 25,770m³/s released from the Ukai dam was responsible for a disaster. To assess the flood and find inundation in low lying areas, simulation work is carried out under the 1D/2D couple hydrodynamic modeling. 299 cross sections, 2 hydraulic structures and 5 major bridges across the river are considered for 1D modeling, whereas a topographic map at 0.5 m contour interval was used to produce a 5 m grid and SRTM (30 & 90 m) grid has been considered for Surat and the Lower Tapi Basin. The tidal level at the river mouth and the release from the Ukai dam during 2006 flood is considered as the downstream and upstream boundaries respectively. The model is simulated under the unsteady flow condition and validated for the year 2006.The simulated result shows that 9th August was the worst day in terms of flooding for Surat city and a maximum 75-77 % area was under inundation. Out of seven zones, the West zone had the deepest flood and inundated under 4-5m. Furthermore, inundation is simulated under the bank protection work (i.e. levees, Retaining Wall) constructed after the 2006 flood. The simulated results show that the major zones are safe against the inundation under 14,430 m³/s water releases from Ukai dam except for the West zone. The study shows the 2D capability of new HEC-RAS 5 for flood inundation mapping and management studies.