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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 30th 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.