Optimal Design of Dams Located in Steep Slope Areas (Khor Arbaat in Eastern Sudan)

Abstract

Khor Arbaat is the main source of water supply for Port Sudan City, which is the capital of the Red Sea State, and second City in Sudan after Khartoum. Khor Arbaat originates from the Red Sea Hills and empties its water into the Red Sea Coast. Khor Arbaat is about 50 km to the city, and the Wadi has a length of 160 km with steep slope of 6 – 10 m per one kilometer which to high velocity of up to 4 m/sec. Therefore the Khor carries large quantities of sediments (up to 1553113.44 tons annually), which deposit in dams' reservoirs during the flood periods. Khor Arbaat receives high flood in summer (June, July, and August) and winter (November, December) seasons, but these quantities of water are useless. There are three dams constructed on the Khor; Upper Gate Dam 1, Sea Port Corporation Dam 2, and Fourth Reservoir Dam 3 were designed for the water supply of Port Sudan town, they generally suffer from siltation and their storage capacity is decreased due to floodwater. The steep slopes of the watershed, the high rainfall, the soil type of the watersheds area, the less vegetation cover make the situation more conductive for greater erosion to take place in the Khor Arbaat catchment during rainy seasons. This study aims to reach the optimum design of dams located on steeper slope areas such as Khor Arbaat in Eastern Sudan, and contribute to any mitigation measures that can be taken in order to reduce the amount of sediment inflow. This study proposed sediment settling basin to remove sediments by settling them into the basin. The physical , topographical and hydrological characteristics of the watershed was related to the design of settling basin through developing the digital elevation model (DEM), watershed delineation, stream networks using GIS capabilities. In this research, the total sediment loads were computed and the annual quantities of sediment which will be deposited in settling basin in future were predicted. In this study analysis was carried for thirty two sediment bed materials samples from different locations within upstream and downstream of Khor Arbaat dams. Sediment grading curves of bed materials samples were determined using sieve analysis, hydrometer test, and Atterberg limits. The grading curves described the sizes, distribution and pattern of deposition of sediment particles. The field investigation also included the siltation measurements inside Upper Gate Dam1 and dry siltation inside SPC Dam 2. GPS and GIS applications were used for determine samples location and other measurements.