Reconstructing an extreme flood from boulder transport and rainfall–runoff modelling: Wadi Isla, South Sinai, Egypt

Abstract

The Wadi Isla drainage basin, a narrow steep bedrock canyon and its tributaries, rises near the highest elevations of the Precambrian Sinai massif on the eastern margin of the tectonically active Gulf of Suez rift. The basin area upstream from the mountain front is 191 km 2 and downstream the wadi crosses a broad alluvial plain to the Red Sea. Stream-transported boulders within the lower canyon (up to 5 m in diameter) and in a fan downstream indicate extremely high competence. In one reach, a 60-m-long boulder berm, ranging in height from 3 to 4 m, lies along the southern wall of the canyon and contains boulders 2–3 m in diameter. Boulder deposits beyond the mouth of the canyon generally appear to be less than several metres thick and are composed of imbricated, well-sorted boulders. The last flood that deposited these boulders is believed to have been a debris torrent with a low content of fines. Mean intermediate diameter decreases from about 1.5 m just beyond the mouth of the canyon, where the channel width expands to 300 m, to about 0.5 m downstream to the point at which the valley is no longer confined on its south side. Using the empirical formula of Costa (1983), these clasts represent velocities decreasing from about 6.5 m s − 1 to 3.7 m s − 1 . Flood velocity and discharge were also calculated within the canyon, using Costa's (1983) method and by the Manning equation. Parameters for the calculation include a channel width of 65 m, flow depth of 3.5 m (average height of the boulder berm), an average slope of 0.038 and a roughness coefficient of 0.06. The resulting values include a velocity range between 6.8 m s − 1 and 7.3 m s 1 and a corresponding discharge range of ∼ 1550 m 3 s − 1 to 1660 m 3 s − 1 . A lower limiting discharge of 1320 m 3 s − 1 was obtained with an assumption of critical flow conditions. The boulder fan is much coarser than the older alluvial plain sediment, suggesting an increase in flood magnitudes in more recent times — perhaps in response to renewed uplift of the mountain front and/or climate change. A calibrated rainfall–runoff model was developed using SWAT to estimate modern flood discharges in Wadi Isla. The magnitude of discharge from 1998–2006 did not exceed 500 m 3 s − 1 . Artificial precipitation amounts (90 and 150 mm) were used to extrapolate from recent rainfall amounts to the amount of rainfall needed to produce a discharge range of 1320 m 3 s − 1 to 1660 m 3 s − 1 . Results show that rainfall must range between ∼ 102 mm and 125 mm to produce this discharge.