Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Flash flood modelling in Oman wadis

Auteur : Al-Rawas, Ghazi Ali Abdullah

Université de soutenance : University of Calgary

Grade : Doctor of Philosophy (Ph.D.) 2010

Résumé
Oman is one of several countries located in an arid zone that is subject to flash flooding. Records show that major flash floods occurred in Oman in 1989, 1997, 2002, 2003, 2005, and 2007. Few flash flood studies in the literature have focused on the issue of flash flooding in an arid environment. Consequently, flash flooding in arid regions affecting Wadis like Oman, which is the focus of this research, is poorly understood. A review of the research gaps demonstrates that rainstorm and watershed characteristics are the most influencing factors on urban flash flood studies for an arid environment like Oman. The main objective of this research is to improve flash flood prediction by providing new knowledge and better understanding of the hydrological processes governing flash floods in arid regions like Oman. This includes developing rainstorm time distribution curves that are unique for this type of study region ; and analyze, investigate, and develop a relationship between arid watershed characteristics (including urbanization) and wadi flow flood frequency in Oman. Data from 2042 rainstorm events in the Rustaq watershed were used to develop heavy rainfall temporal distribution curves characteristic of arid climates. Orographic effects on rainfall were also investigated by separating the data into two regions, mountainous and coastal, and hyetographs were developed for both regions. The curves for both regions are similar and display a very high intensity at the beginning of the storm, which is known to be a characteristic of storms in arid regions. The new distributions were compared to other standard established distributions derived for regions in Canada and the United States. In addition, curves were developed for an area of a similar climate in southern Alberta. The greatest similarity was found between the Oman and Calgary curves but there was significantly higher intensity earlier in the storm in the Oman curves. Relationships between 12 watershed characteristics and mean wadi flood-peaks in northern Oman are investigated. Drainage area (DA), wadi slope (WS), watershed mean elevation (BE), and agricultural/farm area (FR) were found to be the key variables affecting flood flows, with DA having the strongest relationship. Bigger watersheds with high wadi slope, low altitudes, and less farms tend to have higher mean peak flow discharge (QMPF). Unlike past literature, this research shows that DA is positively related to flood peak discharge rates. A new approach is introduced by including FR impacts on runoff in Oman. The approach showed that FR improved the variance explanation by 11% over models using only traditional variables such as DA and BE. Rapid urban expansion in the Wadi Aday watershed in Oman and its impact on wadi peak flow generation and flood frequency is investigated. New hydrologic curve number (CN) and runoff coefficient (C) tables were created specifically for this type of region and for its unique residential characteristics and arid soils. The effect of urbanization on wadi peak flow between 1960 and 2003 shows that the urbanized area increased by 92% ; the average simulated wadi peak flows increased by 68%, time to peak decreased by 22.2 min ; the weighted C increased from 0.302 to 0.417, and the weighted CN increased from 76 to 79.

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