HEC-RAS Model for Mannnig’s Roughness: A Case Study

Abstract

Channel roughness is considered as the most sensitive parameter in development of hydraulic models for flood forecasting and flood inundation mapping. Hence, it is essential to calibrate the channel roughness coefficient (Mannnig's "n" value) for various river reaches through simulation of floods. In the present study it is attempted to calibrate and validate Mannnig's "n" value using HEC-RAS for Mahanadi River in Odisha (India). For calibration of Mannnig's "n" value, the floods for the years 2001 and 2003 have been considered. The calibrated model, in terms of channel roughness , has been used to simulate the flood for year 2006 in the same river reach. The performance of the calibrated and validated HEC-RAS based model has been tested using Nash and Sutcliffe efficiency. It is concluded from the simulation study that optimum Mannnig's "n" value that can be used effectively for Khairmal to Barmul reach of Mahanadi River is 0.029. It is also verified that the peak flood discharge and time to reach peak value computed using Mannnig's "n" of 0.029 showed only an error of 5.42% as compared with the observed flood data of year 2006.