Global and Planetary Change 70 (2010) 64–75 Contents lists available at ScienceDirect Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha Reconstructing an extreme flood from boulder transport and rainfall–runoff modelling: Wadi Isla, South Sinai, Egypt Alan E. Kehew a,⁎, Adam Milewski a, Farouk Soliman b a Geosciences Dept., Western Michigan Univ., Kalamazoo, MI 49008, USA b Geology Department, Faculty of Science, Suez Canal University, Ismailia, Egypt article info abstract Article history: The Wadi Isla drainage basin, a narrow steep bedrock canyon and its tributaries, rises near the highest elevations Accepted 6 November 2009 of the Precambrian Sinai massif on the eastern margin of the tectonically active Gulf of Suez rift. The basin area Available online 17 November 2009 upstream from the mountain front is 191 km2 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 Keywords: indicate extremely high competence. In one reach, a 60-m-long boulder berm, ranging in height from 3 to 4 m, lies palaeoflood along the southern wall of the canyon and contains boulders 2–3 m in diameter. Boulder deposits beyond the Sinai rainfall–runoff model mouth of the canyon generally appear to be less than several metres thick and are composed of imbricated, well- boulder transport sorted boulders. The last flood that deposited these boulders is believed to have been a debris torrent with a low flash floods 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 s1 and a corresponding discharge range of ∼1550 m3 s−1 to 1660 m3 s−1. A lower limiting discharge of 1320 m3 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 m3 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 m3 s−1 to 1660 m3 s−1. Results show that rainfall must range between ∼102 mm and 125 mm to produce this discharge. © 2009 Published by Elsevier B.V. 1. Introduction arid fluvial systems include those of Ely and Baker (1985), Webb et al. (1988), Wohl (1992), House and Pearthree (1995), and Greenbaum Fluvial activity in arid regions is typically dominated by large flash et al. (1998, 2000, 2006). The most common method of estimating floods that occur very infrequently, and whose geological record may peak discharges from palaeofloods is the step-backwater method, in persist for long periods of time (Tooth, 2000). Deposits of these events which palaeostage indicators of various types along with measured include slackwater sediment in high-level protected areas of bedrock channel slope and geometry are used to reconstruct the water-surface canyons, coarse channel sediment deposited in response to changes profile during the flood and its associated discharge. in hydraulic conditions during the flood, and sediment transported In this study, a more rudimentary approach was used to estimate to depositional sites such as alluvial fans or plains. Because gauging the magnitude of a flood emanating from a wadi located in the rugged, stations or direct measurement of these events are extremely rare, high-relief terrain of the south Sinai Peninsula of Egypt. Preservation palaeoflood hydrology is utilized to reconstruct the discharge, velocity, of a large accumulation of boulders assumed to have been transported and other variables of these rare flows. Palaeoflood studies focusing on and deposited by a single flood event immediately downstream from the mouth of the canyon enabled velocity calculations of the transport- ing flood to be made using empirical formulas. In addition, observa- ⁎ Corresponding author. tions of boulder size, palaeostage indicators, and channel dimensions E-mail address: [email protected] (A.E. Kehew). at several reaches in the lower canyon were used to calculate velocity 0921-8181/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.gloplacha.2009.11.008 A.E. Kehew et al. / Global and Planetary Change 70 (2010) 64–75 65 and discharge from the Manning equation. The values obtained by region, including Gebel Umm Shomar (2586 m), which is just south of both methods were comparable, within the uncertainties of parameter Gebel Katherina (2637 m), the highest peak in the Sinai Peninsula. estimation. The rock walls of the canyon and its tributaries are extremely steep and rise 700 m or more above the floor of the wadi. 2. Tectonic and geological setting of the Wadi Isla drainage basin 3.1. Morphology of the lower canyon Wadi Isla, a steep canyon and its tributaries, occupies a 348–km2 drainage basin that drains a portion of the western side of the Sinai The magnitude of large floods that have moved through Wadi Isla massif, a faulted and uplifted block of Precambrian terrain located in can be estimated by the sizes of transported boulders both within the the south Sinai Peninsula (Fig. 1). Of this area, 191km2 comprises canyon and within a boulder fan deposit at the mouth of the canyon a steep bedrock canyon and its tributaries in the massif and the re- where the wadi is incised into the alluvial plain. Along the lower- mainder encompasses the alluvial reach of the wadi as it crosses the most reach of the canyon, older deposits of coarse, partially cemented broad El-Qaa alluvial plain on the west side of the massif. Uplift of this alluvium (Fig. 3) are truncated by erosion during more recent floods. region is related to rifting of the Gulf of Suez, which lies just to the Similar cemented deposits in Nahal Zin in the Negev Desert were west, and is the northern extension of the Red Sea Rift. The Sinai sub- considered to be Pleistocene in age (Greenbaum et al., 2000). Boulder plate, bounded by the Suez rift to the west and the Dead Sea transform deposits form a variety of bedforms in the canyon including pendant on the east, formed by the separation of the Arabian and African plates bars associated with sharp bends in the canyon and boulder berms. beginning in the Tertiary. Flanking the Gulf of Suez on the eastern side Transport of boulders as large as 5 m in diameter is inferred by imbri- of the Suez rift is a series of normal faults and folds associated with cation with other boulders in these bedforms (Fig. 4). With the excep- rifting. Although Tertiary rocks in some of these blocks are exposed tion of one exposure, the boulder deposits lack finer, flood-deposited east of the Gulf of Suez in parts of the South Sinai, in the Wadi Isla area, sediment, such as the sandy and gravely alluvium that cover the wadi the El-Qaa Plain rises gradually from sea level to the steeply sloping bottom. The lack of interstitial fines indicates a change in flood regime mountain front composed of uplifted Precambrian igneous and meta- from the flood that transported the largest boulders to the smaller, morphic rocks. The Precambrian rocks of the Sinai Massif constitute more recent floods in the wadi which, based on regional investi- the uplifted eastern shoulder of the Suez Rift. The structural relief gations, should be hyperconcentrated (Lekach and Schick, 1982; of the Suez Rift shoulders reaches 4–5 km, which is a combination of Alexandrov et al., 2003). direct tectonic uplift of the rift flanks and isostatic uplift caused by erosion of tectonically uplifted crustal block (Garfunkel, 1988). The 3.2. Boulder berms topographic relief of the rift shoulder is significant, rising from sea level to peaks of 2500 m or more within a distance of about 50 km Boulder berms are linear ridges of coarse clasts deposited in a from the Gulf of Suez. streamwise orientation along channel margins by debris torrents— The El-Qaa Plain is approximately 20 km wide and slopes uni- high-velocity flash floods with sediment concentrations below the formly from elevations around 400 m at the mountain front to the transition to non-Newtonian debris flows. Debris flows deposit analo- Gulf of Suez. Structurally, the El-Qaa Plain is a secondary rift basin gous features known as debris flow levees (Carling, 1987). Boulder (Ghodeif, 2002) or half graben bounded by normal faults along the berms are formed in regions of flow separation associated with valley mountain front. Tertiary rocks are exposed on the western side of the widening or on the convex sides of channel bends (Carling, 1987, El-Qaa Plain north of the Wadi Isla area.