Reducing the flash flood hazard in the Sinai

Abstract

In 2010 a large flash flood hit El Arish City, killing at least 6 people and causing a lot of damage. This was not the first time the Sinai was affected by these flash floods, more than 40 flash floods have been recorded in the El Arish watershed in the last 100 years. The enormous amount of water and the flow velocity of these floods are a hazard to its surroundings and to the rehabilitation plan of the Weather Makers. On the other hand the water is scarce in the arid Sinai and the flash floods could contribute to the plan to create a robust water cycle. The underlying goal of this research is to investigate whether a robust water cycle can be created by reducing the flash flood hazard and increasing the available water for the creation of biomass. Measures will have to be taken to start the transition and tackle issues such as drought, unfertile soil, water regulation and to properly address the social benefits to involve the inhabitants of the area. It is therefore essential to assess the hazards of the current and future situation of the Sinai and to apply appropriate measures on the hazardous locations.

                The goal of this research is to propose a strategy to reduce the flash flood hazard, by first identifying hazard prone areas and then propose measures to counter these hazards. This strategy takes into account a scenario on change in catchment hydrology, gives guidance on land use for the region and will propose feasible measures to reduce flash flood hazards. It does not involve the design of the feasible measures nor a cost benefit analysis.

                To assess the hazards in the El Arish watershed, the USACE method is used. The USACE method uses HEC-HMS for the hydrologic modelling and HEC-RAS for the hydraulic modelling. With a DEM sub basins and wadi channels are delineated and measured, precipitation data is collected from the TRMM and analyzed, the retention of the soil is simulated with curve numbers and transmission losses are estimated. The sensitivity of the hydrological model is checked and is used to simulate the 2010 flash flood to compare with imagery of the event. Measures usable in arid areas like the Sinai to reduce flash flood hazards are analyzed on their feasibility and their effects are modelled with the hydrologic model.

The current flash flood hazard is assessed by simulating an extreme event and the region with the highest relative hazard is selected. Measures are proposed based on their applicability and the physical attributes of the surface with a decision tree. The following measures are suggested: spreading dams in flat wadis (<2%), impervious dams in mild sloping wadis (2-5%) and pervious dams on steep sloping wadis (>5%). The effect of these measures combined on the peak discharge is estimated to be 70-75% for an extreme precipitation event. The measures decrease the total- and peak discharge but they also spread the flash flood making the duration of the flow longer. To check whether the suggestions are future-proof a scenario of change in the hydrological cycle is considered by increasing the extreme precipitation with 15%. This causes the peak discharge to increase with 30-35% if no measures would be applied. By applying measures the peak discharge decreases with 60-65%, which is a smaller percentage than without an increase of extreme precipitation (70-75%).

Challenges of the research are that measurement data was lacking to calibrate the conceptual hydrological HEC-HMS model, the large amount of uncertain parameters therefore cause equifinality. The estimated values of the parameters and the results of the simulation are however consistent with literature and gives a good indication on the hazard.

This research shows that the flash flood hazard in the El Arish watershed can be reduced by using feasible measures. The proposed method to reduce the flash flood hazards can contribute to the plan of the Weather Makers to rehabilitate the Sinai by slowing down the flood and to enhance the infiltration. It also creates suitable locations for vegetation by capturing sediment and water this could help to create a robust water cycle.