ASSESSING FLASH FLOOD RISK IN JEDDAH, SAUDI ARABIA UTILIZING SPACE DATA AND HYDROLOGICAL MODELING

Flash floods are a deadly and costly event that can be exacerbated in arid environments. On November 25, 2009, the city of Jeddah, Saudi Arabia, received more than 80mm of rainfall in a short period of time causing massive flooding, extreme destruction of property, and an indiscriminate loss of life. In December 2010 and again in January 2011, more severe flood events wreaked havoc on Jeddah. Such extreme events have been a catalyst into furthering the understanding of flash flood events and how to properly manage their sudden and destructive nature.

Wadi Qous, one of many dry valleys that exit the mountain chains east of Jeddah, was reported to be the main cause for destruction to the city. Here, a hydrologic model driven by information on the topographic and drainage characteristics (generated from SRTM data), precipitation (constructed from TRMM data in 3-hour increments), and land cover/use plus soil properties (generated from satellite images and geological maps) was generated for this valley. Utilizing HEC-HMS, hydrographs were produced to illustrate the response of the valley to rainfalls and aid in locating high risk areas.

Analysis shows that Qous basin was affected by a high magnitude rainstorm with short duration. This resulted in a torrent of water with a short time to peak, 10 hours, and sharp peak flow of 98 m³⋅s⁻¹, respectively. This resulted in the appearance of surface runoff within a narrow time slot at the valley outlet. This might be explained by the short distant and travel time of surface runoff toward the city, the funnel-shape of the basin and its narrow drainage course (less than 100 m in a few locations), and the expansion of bedrock outcrops and urban land cover as reflected by high Curve Number value. Due to the relative lack of hard structures, such as dams, there were no obstacles or impediments to slow the flood waters prior to entering the city. The area at the valley outlet was particularly hard hit as was the area around King Abdulaziz University.

A comparison of the hydrographs shows that a reduction of floodwaters on the city of Jeddah is possible. Constructing a dam and reservoir system at the confluence of the three upper reaches and the establishment of a spillway underneath the city located near the outlet of the basin and emptying into the Red Sea would greatly diminish the impact of flash flood rain events.