Flash Flood Risk Assessment in Egypt

Abstract

The subject of the flash flood risk assessment is an inclusive task that relies on the characteristics of the study area and the nature of previously recorded incidents. The Egyptian Nile Wadies (East Nile, and West Nile) are draining toward the highest population density and associated assets, while the Red Sea and Sinai wadies are draining toward high-density touristic compounds and scattered big cities and connecting roads. The existence of high urban densities and associated assets in the highest discharge locations (at wadies outfalls) without adequate consideration of wadi paths led to a considerable wadies encroachments and catastrophic recorded incidents. All recorded incidents are either due to unplanned urban and agricultural expansion, or insufficient flood mitigation measures, or lack of maintenance. Due to the freshwater stress in Egypt, the rainfall harvesting in the form of dams or artificial lakes should be considered as a top priority flood mitigation measure wherever applicable. The total capacity of all flood protection dams and artificial lakes all over Egypt is about 70 million m³ (MWRI 2016) that raises the potentiality for more similar measures to increase the rate of investment return from both flood mitigation and reduction in freshwater stress. The available data for this study were sufficient enough to calculate the catchments peak discharge and runoff volume. The 100 year return period was selected for the peak discharge calculations. Many thresholds have been tested for catchment delineation in order to obtain a reasonable number of catchments suitable for such a regional-scale study. The SRTM 90 × 90 DEM file was utilized as an input in the delineation procedure, with selected threshold was set to 50 km². Due to the large variance of the catchments peak discharge and runoff volume, the box plot technique was employed to eliminate the ranking outlier values. The catchments were classified into five categories very high risk, high risk, moderate risk, low to moderate risk, and low risk. This categorization was done for the Peak Flow Standardized Risk Factor (PFSRF) and Runoff Volume Standardized Risk Factor (RVSRF) in order to prioritize the flood mitigation measures required for projects. The classification based on the runoff volume can guide the designer accounting for rain harvesting projects that would increase the rate of investment return from both flood mitigation and the reduction of freshwater stress. A two-dimensional HEC-RAS rainfall-runoff modeling is conducted for Ras Gharib city by using updated 30 × 30 DEM files to contain the manmade topographical modifications. The model was verified versus aerial photos for the 2016 incident. In order to assess the effectiveness of the newly constructed culvert (16 vents, 3 m × 3 m box culvert) with attached two dikes, another updated two-dimensional HEC-RAS rainfall-runoff model has been conducted and the results showed significant improvement in flood intensity values in Ras-Gharib city.