El Bastawesy, M. El Harby, K., and Habeebullah, T. (2012). 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, 9(1), pp. 580-589. (ISI Impact Factor 2012: 0.5)
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 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.
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 runoffdischarge 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).
Generally, the hydrological data collected in drylands remain insufficient and limited due to technical, political and economical factors, and therefore, the hydrological processes are not fully understood and the hydrological models are uncalibrated (El Bastawesy et al., 2009). Nonetheless, most of these data are also limited and gathered by individuals or entities during pilot projects and various independent case studies, which cannot represent the diversity within dryland setting and processes. It is also difficult to monitor flash floods that occur suddenly, and the potential to capture during classical-field experiments is very rare. Therefore, the investigation of most flash flood-events was mainly based on post-event survey, which plays a critical role in gathering essential observations and data (Borga et al., 2008). The spatial and temporal variability of flash flood events even within a single catchment clearly demonstrate the need for the development of distributed model; to assess vulnerability and to mitigate against future damage.
Remote sensing data and Geographic Information System (GIS) techniques are widely used to estimate various distributed hydrological parameters for the investigation of catchment hydrology following given techniques and interpretations (Schultz, 1987; Scipal et al., 2005; Milzow et al., 2008 ). The most straightforward use of remote sensing images is to identify geologic, geomorphologic and landuse-land cover features, which in turn have a strong influence on overland flow generation. Several remote sensing- products such as the Tropical Rainfall Monitoring Mission (TRMM) and the Global Precipitation Climatology Project (GPCP) are increasingly available to measure the precipitation at semi-global coverage with a grid spatial resolution of 0.250 X 0.250 (Huffman et al., 2007; Hossain et al., 2011). The remote sensingrainfall estimates are widely utilized in the hydrological models, particularly over the areas with poor or no rain-gauges data (e.g. Milewiski et al., 2009; Abu El Magd et al., 2010). However, the uncertainty of remote sensing- rainfall estimates on flood prediction has to be considered; the calibration with in situ data in different gauged areas showed non-systematic overestimation and underestimation (Almazroui, 2011). Although, the remote sensingbased rainfall estimates may represents the sole source of precipitation input to any hydrological model for the dryland catchments.
The digital elevation models (DEM) are mainly obtained from different sources and at different spatial resolution, and therefore are widely being used in the various hydrological models. The automatic delineation of catchment-hydrographic parameters from the DEM has gradually replaced the traditional manual delineation of these parameters from the conventional topographic maps (Band, 1986; Chorowicz et al., 1992). The manual method is a tedious and error-prone technique in dryland alluvial areas, where the thalweg or active longitudinal channel courses are not marked on topographic maps and significant changes in these courses can take place over relatively short periods of time. The major issues associated with the derivation of surface drainage networks from DEM are related to the quality, source and resolution of the DEM and to the processing techniques and algorithms employed (Zhang and Montgomery, 1994; Wolock and Price, 1994). However, the delineation of various morphometric parameters (for a typical dryland catchment) was not very sensitive to the change of DEM resolution (from 20 m to 90 m) (El bastawesy, 2007).
The aim of this paper is to develop a hydrological model for a typical dryland catchment, which has undergone dramatic urban expansion, in order to assess the impact of flash flood hazard on urban areas, and also to determine the effect of constructed mitigation measures on the fragile surface and groundwater resources of the area. Herein the Wadi Ibrahim catchment of Makkah city in the Kingdom of Saudi Arabia is selected for this study, due to its importance as it hosts the Holy Mosque of Makkah and considerable in situ data are available.
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