Abstract
A technique that utilizes the SCS curve number and dimensionless unit hydrograph and GIS technology has been adopted in this study in order to reconstruct urbanized arid region flash flood magnitude with minimum available data. Estimated values of hydrograph ordinates were routed by utilizing HEC RAS software to reconstruct the flood hydrograph and flood characteristics of the devastating flood event of 25 November 2009 on East Jeddah districts to investigate the effect of levee breach and narrow town streets on water level in the town. The calculated maximum stage of this flood was estimated at 1.34 m in the main wadi channel in the natural condition, but it exceeded 2 m in the narrow streets, reaching about 2.1 m at street entrances in the worst-case scenario of the wet condition; this was found to be due to levee breach and the effect of narrow streets. The latter situation agrees well with actual stage observations in Gueeza streets contradicting the assumption that the flood event was entirely due to natural causes.