Flood hazard assessment using Geographic Information Systems of wadi El-dukhan, western desert, Sohag Governorate, Egypt

Abstract

The study aims to investigate Wadi Dukhan in the Western Desert in Sohag Governorate, which has an area of 50 square kilometers in terms of its nature and the rainfall that falls on it, in order to identify the places at risk of flooding. This study employed grid-based Curve Number (CN) method through the GIS drainage analysis operating on digital elevation models (DEM) to estimate surface runoff depth in Wadi El-Dukhan catchment, Sohag, Egypt. In the SCS (Soil Conservation Service) based hydrologic modelling system, the CN plays a significant role in determining runoff. It takes into account factors like slope, vegetation cover, and catchment area. The curve number values from NRCS standard tables were assigned to the intersected hydrologic soil groups and land use maps to generate CN values map. Effect of slope on CN values and runoff depth was determined. Topographic, soil and land use maps data were used in preprocessing in the Arc GIS 9.3 and Arc Hydro 9 for computing Hydrologic parameters. To estimate the curve number from which the daily runoff was approximated, GIS software was used for data generation, storage, manipulation, and integration. Calculations were made for surface storage volume and initial abstraction. It was found that during the last six years (2010-2016), Wadi Dukhan rained six times, three of these events were during 2012, while the rest of the times were once a year. The rain at 2012 was the highest (1 mm / hour). According to the geological assessment of the area it has been found that there are two main areas. The first is made of hardened limestone, which reduces water intrusion and increases runoff. The hydrological soil group 'C' was chosen for this area, so the corresponding CN was equal to 86. This value increases with increasing slope; it was in some places up to 86. The second category consisted of Quaternary flood sediments, which consist of grains sand and gravel, causing a decrease in runoff rates and an increase in the possibility of water infiltration into groundwater; this class is classified as 'A' hydrologic soil group type. The corresponding CN value was 63. This value also differed according to the slope difference in this region. Also, in the present study Wadi Dukhan was divided into 29 subcatchments. It was found that a very large area of the region has a high CN value of 86, which means that the study area is in a poor hydrological condition that makes the water run-off high. This is due to the type of soil and poor vegetation.