Life Science Journal, 2012;9(1) http://www.lifesciencesite.com 580 The Hydrology of Wadi Ibrahim Catchment in Makkah City, the Kingdom of Saudi Arabia: The Interplay of Urban Development and Flash Flood Hazards Mohammed El Bastawesy 1 , Khalid Al Harbi 1,2 , Turki Habeebullah 2,3 1 Geography Department, Umm Al-Qura University, Makkah, Saudi Arabia 2 Centre of Research and Excellence in Hajj and Omrah (HajjCore), Umm Al-Qura University, Makkah, Saudi Arabia. 3 The Custodian of the Two Holy Mosques Institute for Hajj Research, Umm Al-Qura University, Makkah, KSA mabastawesy@uqu.edu.sa ; bastawesy_mmm@yahoo.com Abstract: This paper investigates the development of a hydrological model for the dryland catchment of Wadi Ibrahim, which hosts the Holy Mosque of Makkah, in order to assess the interaction of urbanizing the alluvial channels and the flow discharges of occasional flash floods. The original landscape and landcover of the catchment have witnessed significant changes during the past few decades, where most of the alluvial channels and the mountain footslopes have been covered by urban. The infrequent threat of flash floods has prompted the development of a mitigation measure including; installation of rainfall-sewage system and subsurface culverts, in addition to the conveyance of flows from the upper sub-catchment into another drainage basin. However, the latest flash flood event of 30 th of December 2010 has resulted in fatalities and demonstrated the insufficiency of the current mitigation system to control flash floods. The runoff coefficient was estimated from the opportunistic observations and measurements of the flow discharge parameters for the latest event, in addition to the recorded rainfall parameters. The digital elevation model (DEM) was analyzed using Geographic Information System (GIS) to determine the spatially distributed time-areas zones of the catchment, which were used to simulate the runoff hydrographs under certain runoff coefficients and designed storms of long return periods. The development of urban areas on expense of the alluvial channels resulted in a significant surge of runoff discharge, and therefore increasing the threat of flash floods on urban areas downstream. As a result the transmission loss is diminishing; thus raising the alarm on the potential recharge to the underlying alluvial aquifer of the sacred well of Zamzam. Therefore, it is suggested that several small dams to be constructed at the fingertip drainage channels; to retain considerable amount of water and sediment within the catchment and to act as point-source recharge to the alluvial aquifer. [Mohammed El Bastawesy, Khalid Al Harbi and Turki Habeebullah. The Hydrology of Wadi Ibrahim Catchment in Makkah City, the Kingdom of Saudi Arabia: The Interplay of Urban Development and Flash Flood Hazards. Life Science Journal 2012; 9(1):580-589]. (ISSN: 1097-8135). http://www.lifesciencesite.com.86 Paper information: Submitted 12 November 2011; Accepted 2 March 2012. Key Words: flash floods, Makkah, Kabaa, Wadi, Remote Sensing, GIS, Hydrology, Urban, Alluvial channels. 1. Introduction: The drylands are characterized by droughts and the scarcity of water resources, occasional heavy storms often develop severe flash floods that devastate the inhabited areas (Cooke et al., 1982; Tooth, 2000; Foody et al., 2004). The records of destructive flash floods and the notable regional frequencies have largely raised the public awareness and motivated more scientific research on the hydrological processes within the fluvial system (e.g. Walling and Gregory, 1970; Chin and Gregory, 2001). Originally, flash flood frequencies and magnitudes are controlled by the interplay of different natural variables, including precipitation, antecedent conditions of the catchment, distribution of alluvium and water storage areas, etc. However, the growth of urbanization within the catchments can be added as an additional significant controlling factor for the development of flash flood. Urbanization, through the construction of impervious surfaces; building, roads, storm sewers and paving usually decreases the infiltration capacities of the underlying soils, and it significantly increase runoff- discharge downstream. The estimated increase of runoff coefficients; higher flow peaks and the decrease of time to peak in urban catchment Depends on the extent of urbanization and the anthropogenic managements of runoff of these catchments ( Suriya and Mudgal, in press (2011). In completely impervious and fully sewered areas, peak discharge increases 6 times more than in non-urbanized areas (leveson, 1980), and 90 % of the total rainfall may be converted into urban runoff (Shang and Wilson, 2009). However, the assessment of urbanization impact on the hydrological response of the developed catchments is complex and hampered by the lack of hydrological parameters and measurements, and the non-systematic temporal changes of the landuse and landcover due to urbanization (Smith and Bedient, 1981).