Impacts of urban growth on flood hazards in Makkah City, Saudi Arabia

Abstract

Summary of the employed processing procedures . MATERIALS AND METHODS The compiled available spatial datasets constitute of two categories for 1990 and 2010. The first element consists of 105 cadastral maps in AutoCAD format for 1990, obtained from the Makkah governorate. The second constituent consists of a land use map developed by the Saudi geological survey authority dated 2010. Additionally, a national 5-m resolution DEM for the study area (Figure 3) has been obtained from King Abdulaziz City of Sciences and Technology (KACST). The Arc GIS software (v. 10) has been used in the present study to combine all obtainable data in a unique environment. The processing phase includes (Figure 4): rectifying the printed maps, digitizing shapefiles for residential, converting AutoCAD files to GIS shapefiles, unifying the spatial reference frames for all datasets, estimating flood characteristics, particularly the flood volume, through the curve number methodology, and comparing the attained quantitative for 1990 and 2010. The ArcGIS and ArcHydro programs were utilized to extract the main hydrological basins in the study area, and their sub-basins. The spatial analysis tools have been utilized to assign the CN value of each sub-basin based on its geological, soil, and land use properties. Visual basic application (VBA) was utilized to compute sub-basins' flood characteristics, for 1990 and 2010, within the attribute table of GIS shapefiles. Countless approaches have been proposed globally for estimating flood characteristics based on topographic and morphometric properties of hydrological regions. The NRCS, formerly known as the soil conservation service (SCS), might be considered as the most widely-utilized flood modelling methodology. It utilizes geological information to assign a unique CN coefficient value for each area that will be further used to estimate the surface runoff depth and the peak discharge magnitude. The NRCS method is utilized in engineering design and flood management projects in several countries (Masoud, 2011; Al-Jabari et al., 2009; Adebayo et al., 2009; Elaji, 2010; Xianzhao and Jiazhu, 2008; Gul et al., 2009), and particularly in USA (US ACE, 2004; US DoT, 2002). The basic formulas of the NRCS approach are (US NRCS, 1986):