ABSTRACT
Wadi Mikeimin, a 13 km2 watershed in the hyper-arid steep crystalline mountains of SE Sinai, experienced a local high magnitude flood in January 1971. A detailed reconstruction of the event was undertaken on the basis of: (1) high water marks; (2) geomorphic evaluation; (3) assumptions based on data transferred from comparable instrumented watersheds; (4) considerations of competence; and (5) analysis of the total sediment load deposited as an alluvial fan at the junction of Wadi Mikeimin with the large Wadi Watir.
The flow in Wadi Mikeimin peaked to 68.5 m3s−1 at the outlet and to 80 m3s−1 for a 5 km2 tributary within it. Total flow was in excess of 100,000 m3. The flow deposited 6,200 m3 of coarse stratified sediment in the form of an alluvial fan on the channel bed of Wadi Watir. The January 1971 flood did not activate Wadi Watir itself, and its channel remained dammed by the Mikeimin fan until November 1972, at which time it was breached by a major flood which did not activate Wadi Mikeimin. For the 21 months that elapsed between these two events, a lake up to 400 m long existed, fed by the perched water table of the nearby oasis of Ein Fortagha.
Breached remnants of similar sediment dams were found in many localities in the main wadis of the mountainous Sinai. Their role as obstructions at junctions, caused by the localized flood pattern characteristic of deserts, is important in understanding some aspects of floods, especially their fronts. In evaluating the probability of their formation, factors to be considered include the junction angle, the difference in size and in channel slope between the junction tributaries, and the temporal and spatial distribution of rainfall. Such obstructions may also operate for short periods within a single flood event wherever the size difference between tributary and main stream insures the required lag between the recession in the tributary and the onset of the floodwave in the main stream.
