Qualitative and Quantitative Study of the Hydraulic Response and Flood Probability of Wadi ash Shaqqah Al Yamaniyah and Wadi ash Shaqqah ash Shamiyah, Kingdom of Saudi Arabia

SGS-TR-2010-1 Faisal A. Al-Ammawi, Ahmed M. Youssef, Salah A. Sefrey, Saad G. Al-Harthi, Mohammed I. Al-Zahrani, Basem K. Aldhahri
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ABSTRACT


The wadis of ash Shaqqah (Wadi ash Shaqqah al Yamaniyah and Wadi ash Shaqqah ash Shamiyah) are located about 250 km south of Jeddah, Kingdom of Saudi Arabia between lat 19°44'N. and 20°43'N. and long 40°30'E. and 41°20'E. These wadis run towards the Red Sea coastal zone.  The aim of the present work is to study and estimate the hydraulic response and flood discharges of the these wadis and their effect in the downstream flat areas.  Detailed investigations concerning the geological, geomorphological and hydrological aspects prevailing therein were carried out. This work mostly provided data that can be used in flood protection measures and flood discharge measurements.  The flood hazard degree of these basins was determined according to the morphometric parameters by using the model of El Shamy (1992).  The probability distributions of the maximum daily rainfall records and the probability records for different stations within and surrounding the area were used to generate the average rainfall values at each return period. Rainfall frequency curves were determined using the Log Pearson method, Type-III.  The hydraulic responses of these wadis were determined using two methods; (1) Snyder synthetic unit hydrograph and (2) Soil Conservation Services (SCS) based mostly on morphometric parameters of the basins. The results are presented in the forms of runoff hydrographs for different return periods (5, 10, 25, 50, 100 years). The lag times, time to peak, and flood peaks as well as flood and infiltration volumes were also evaluated. The output of this study is given in tabular data and presented graphically. Volumes and rate of sediment loads carried with runoff were estimated.  The results of the current study show that these basins are located in the moderate flood hazards zone based on the El Shamy’s model. In addition, these basins are rich in surface water in the form of runoff that is generated after rainfall. The peak discharge and water volume for Wadi ash Shaqqah al Yamaniyah and Wadi ash Shaqqah ash Shamiyah are estimated to 1,752 m3/s, 141 x 106 m3 and 1,787 m3/s, 153 x 106 m3, for a rain intensity of 154 and 156 mm/day for 100 return period respectively. Two cross sections were made at the downstream areas for both wadis and the data is presented graphically and as in tabular form. By comparing the peak discharge calculated from the Snyder method and the cross sectional equations, it was found that the depths of the water for Wadi ash Shaqqah al Yamaniyah and Wadi ash Shaqqah ash Shamiyah are 1.99 m and 2.24 m respectively.

 

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Al-Ammawi, F.A, Youssef, A.M., Sefrey, A.S., Al-Harthi S.G., Al-Zahrani, M.I., Aldhahri, B.K., 2011, Qualitative and quantitative study of the hydraulic response and flood probability of Wadi ash Shaqqah al Yamaniyah and Wadi ash Shaqqah ash Shamiyah, Kingdom of Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2010-1, 42 p., 36 figs., 20 tables, 3 apps.