Hydrological Analysis for Floodplain Hazard of Jeddah’s Drainage Basin, Saudi Arabia

All streams and rivers naturally overflow periodically. In the study area, flooding is a result of rainfall exceeding the absorptive capacity of soil and the flow capacity of waterways. In November, 2009, more than 90 mm of rain fell in Jeddah during just 4 h; this was nearly twice the average rainfall for an entire year and the heaviest rainfall in Saudi Arabia in a decade. Synthetic unit hydrograph theory is utilized in the hydrologic analysis in order to determine the peak flow discharge that hit Jeddah city. Flood hydrograph determination is essential for the assessment of water resource potential and for the design of various hydraulic structures. Unit hydrographs were derived for twelve main wadies in the Jeddah basin and the peak flow discharge was estimated by utilizing two models: routing unit hydrograph and Snyder unit hydrograph. Regression relationships as a simple power function between drainage area, number of the channels in the catchment, their total length, and the flow discharge were developed. In addition, the relationships between drainage density, drainage frequency, and infiltration capacity were founded. The results were discussed and compared with those derived and developed by other investigators. The relations between these variables were found to be so strong, that it was possible to estimate peak runoff for ungauged drainage basins.