Developing models and envelope curves for extreme floods in the Saudi Arabia arid environment

Forecasting and monitoring extreme floods in arid regions like Saudi Arabia (SA) are a big challenge for engineers and hydrologists. It is difficult to derive reliable flood estimates at any site without adequate flood measurements. Therefore, envelope curves were developed for reliable estimates of flood peaks. Relaying on recorded flood events in SA, Francou-Rodier approach is used to develop the Regional Maximum Flood (RMF) for some wadis and for SA as a whole. A total of 3121 flood events in 32 arid basins of sizes varying from 99 to more than 4500km(2) are collected and analyzed. Results show that established regional coefficients (K) range between 2.76 and 5.5. The RMF formula for the Saudi regions is Q=251A(0.45). The flood-frequency analysis showed that the Log-Pearson Type III is best. The extreme observed floods for the envelope curve for K=5.5 accommodate floods of recurrence interval ranging between 1000 and 100,000years. The study results provide more realistic runoff peaks for a design of flood protection works for SA watersheds and for the similar environment. Consequently, it is recommended to use the developed envelope curves and models for efficient, safe and precise hydraulic structures design in SA.