HYDROLOGICAL MODELLING AND FLOOD HAZARD MAPPING OF NULLAH LAI

Floods are among the most devastating natural hazards in the world causing huge losses of lives and infrastructure. Flooding can be partially avoided but flood hazards can never be ruled out. However, future flood prevention measures require a stronger stress on integrated approaches incorporating flood forecasting and risk uncertainties. Therefore, an appropriate flood modeling and mapping approach is needed to assess the potential damages. This study is carried out by integrating hydrological models with GIS to estimate the flood zone of Nullah Lai in Rawalpindi. HEC-RAS and HEC-GeoRAS hydrological models have been used to delineate the areas vulnerable to flood at different discharge values. A topographic survey of fine resolution of the target area (Kattarian to Gawalmandi Bridges) was used to generate the DEM of the area. Krigging was used to interpolate the elevation data. GIS technology has been used to delineate the variation of topography and to find the inundation depths at various locations in the study area. Inundation area estimated at the discharge value of 3000 m3/sec is 3.4 km2 out of which 2.96 km2 is occupied under the inundation depth from 1 to 5 meters. Maximum inundation depth can go up to 20 meters for this discharge value. Output of the study using HEC-RAS shows that inundated areas and inundation depths are in close approximation with survey based inundation results obtained by JICA. This shows that the integrated modeling approach used in the present study works well in order to delineate areas vulnerable to flood with a good estimation of inundation depths at a specific discharge value.