Abstract
Makkah city is considered as a geologically transitional region that combines topographic and terrain features of the Sarawat Mountains to the east and the coastal plain of the Red Sea to the west and it is consisted of a semi-circular plateau cut by several structural valleys that were defined by faults and cracks. The surface runoff resulting from the rainstorms over the mountainous areas lead to the occurrence of flashfloods that debouch into the tributaries of valleys towards estuaries that encompass residential areas and municipal infrastructures. The present study assesses the negative impacts of flash floods on under-construction high-voltage towers in the valleys surrounding Makkah city using topographic analysis, Ground Penetrating Radar (GPR) surveys and field checks. Topographic modeling was implemented to delineate risky zones that are vulnerable to flash floods at the intersections between major valleys in Wadi Numan and Wadi Al Sharaya Basins and high voltage power lines. The GPR results showed the presence of three geological layers of sedimentary rocks: the first surface layer (A) is composed of alluvial sediments of fine to coarse sand with a thickness of 2 m. Followed by the second layer (B) of sand and gravel, which attains 3 m thickness. The third layer (C) consists of alluvial sediments mixed with rock fragments (diorite and quartzite diorite) and extend to a depth of more than 10 m. The GPR sections indicated that the concrete tower bases are different in thickness and depth from site to other. Different protection measures for the tower bases were suggested to avoid the adverse impacts of the flash flood. The measures varied from site to site and included the establishment of soil barriers, implementation of rocky hills, the defensive collision nets system and the concrete walls system.