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Flooding impacts can be reduced through application of suitable hydrological and hydraulic tools to define flood zones in a specific area. This article proposes a risk matrix technique which is applied on a case study of Taibah and Islamic universities catchment in Medina, Kingdom of Saudi Arabia (KSA). The analysis is based on integration of the hydrologic model hydraulic models to delineate the flood inundation zones. A flood risk matrix is developed based on the flood occurrence probability and the associated inundation depth. The risk matrix criterion is classified according to the degree of risks as high, moderate and low. The case study has indicted low to moderate risk for flood frequencies of 5 years return periods and moderate to high risk may exist for flood with rerun period of 50 and 100 years. The results are projected on a two-dimensional satellite images that shows the geographical locations exposed to flooding. A quantitative summary of the results have been presented graphically to estimate the magnitude of the inundation areas that can assess the degree of damage and its economic aspects. The developed flood risk matrix tool is a quantitative tool to assess the damage which is crucial for decision makers. ARTICLE HISTORY

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a b s t r a c tDue to the scarcity, randomness, and extremity of rainfall events in arid regions, planning and management of water resources are essential. Rainfall in many arid regions such as Saudi Arabia is characterized by high intensity and short duration during which flash floods occur and cause not only major loss in life and structures but also a huge loss of clean water. Understanding the relationship between rainfall and runoff is the key issue in the management and control of water resources. In this study, two approaches have been applied using Hydrologic Engineering Center's Hydraulic Modeling System model to simulate flood hydrographs of a mountainous watershed located on the west side of Saudi Arabia. The first approach was based on incorporating losses through the soil conservation service (SCS) curve number and SCS unit hydrograph. The second approach was based on effective rainfall in which excess rainfall was computed by Horton's infiltration method and the Phi index method. Results revealed that the performance of losses incorporation approach was poor in simulating runoff hydrographs in all studied storms. Its main drawback was the ineffective representation of flow mass conservation and the early generation of runoff due to rainfall input. In contrast, the effective rainfall approach simulated runoff hydrographs efficiently; moreover, results were comparable with many of those reported in the literature. The two critical hydrograph parameters of peak flow and time to peak were simulated accurately by Phi index method and Horton's infiltration method. The sensitivity analysis showed that the peak flow is directly proportional to the curve number and inversely proportional to the initial abstraction. From water management point of view, the simulated hydrographs added a valuable piece of information about the quantification of lost and stored rainwater. About 55%-70% of rainwater infiltrates through the soil profile and recharges the underlined groundwater reservoir, hence becomes a major source of water in the region.

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