Preliminary Investigation of Wadi al Asla (Wadi Bani Malik) Dam Site, Jeddah Area, Kingdom of Saudi Arabia

SGS-OF-2001-1 William H. Shehata, Khalid A. Bankher, Samer A. Shouman, and Abdulaziz M. Al Solami, with the collaboration of Bandar Al Tunsi, Sultan Al Bahloul, Ali Al Balkhi, Yasser Zabrmawi, Khaled Al Ahmadi, and Khalid Al Wagdadi
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ABSTRACT

Most of the wastewater generated in the city of Jeddah is discharged into the ground in the hills east of Jeddah.  A non-engineered earthen dike exists to prevent the wastewater from flowing back to the city through Wadi al Asla (Wadi Bani Malek).
The volume of the wastewater stored in the reservoir behind the dike is expanding significantly and water levels are continuously rising.  The purpose of this study is to locate a site for construction of a dam in the downstream section of Wadi al Asla, where the wastewater can be treated and stored.
The scheme will require construction of a treatment plant between the dike and the treated wastewater lake, and stabilization of the existing earthen dike.
In future, the treated wastewater can be used for local agricultural and recreational purposes.
The Wadi al Asla area is rugged, with elevations ranging between 50 m and 181 m.
The wadi slopes at a rate ranging between 0.53 and 0.87 m/km.  Different igneous and metamorphic rocks cover the area and unconsolidated sediments cover the wadi course.
The selected dam site is covered by quartz diorite and granodiorite intersected by mylonite and intermediate rock dikes and a small body of basalt.
The north abutment is more coherent than the southern one.
They are intersected by three sets of joints in addition to random joints.
Two minor faults were mapped in the north abutment and one in the south.
Abutment rock masses could be classified as good to very good and are expected to improve with depth.
Geophysical investigations and subsequent core drilling confirmed surface geologic investigations. The bedrock in the wadi course, at the proposed dam site, are quartz diorite and granodiorite covered by 5 to 8 m of well-graded (SW) and occasionally silty (SP) sand.
Subsurface investigations also revealed the presence of an old shear zone along the main course of the wadi that was intersected by a younger shear zone downstream of the dam site.
If the height of the dam is no greater than 23 m, the spillway would be a natural one located on a tributary of Wadi al Asla.
The rock masses in the spillway site could also be classified as good rocks.
At a 23 m dam height, reservoir capacity would be around 40 million m3 covering an area of 4,900,000 m2.
As no clays are available within a 50-km radius of the dam site, the dam type is suggested to be either concrete or rock-fill with an upstream concrete slab.
Sufficient construction materials with suitable engineering properties are available close to the proposed dam site.
The environmental issues that must be addressed are the possible instability of the earthen dike and the potential subsurface flow beneath it.
If it is built, the dam will offer a number of benefits to local inhabitants, and its construction will solve the immediate wastewater problem in Jeddah until a more permanent solution can be found.

 

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Shehata, W.H., Bankher, K.A., Shouman, S.A., and Al Solami, A.M., 2001, Preliminary investigation of Wadi al Asla (Wadi Bani Malik) dam site, Jeddah area, Kingdom of Saudi Arabia, With the collaboration of Al Tunsi, B., Al Bahloul, S., Al Balkhi, A., Zabrmawi, Y., Al Ahmadi, K., and Al Wagdadi, K.: Saudi Geological Survey Open-File SGS-OF-2001-1, 28 p., 27 figs., 9 tables, 6 apps.