GIS-Based Spatial Mapping of Flash Flood Hazard in Makkah City, Saudi Arabia
Gomaa M. Dawod1,2, Meraj N. Mirza3 , Khalid A. Al-Ghamdi 4
1 Survey Research Institute, Giza, Egypt
2 Umm Al-Qura University, Makkah, Saudi Arabia
3 Center of Research Excellence in Hajj and Omrah, Umm Al-Qura University, Makkah, Saudi Arabia
4 Geography Department, Umm Al-Qura University, Makkah, Saudi Arabia E-mail: dawod_gomaa@yahoo.com, meraj@mmirza.com, kaghamdi@uqu.edu.sa Received March 29, 2011; revised May 12, 2011; accepted May 20, 2011
Journal of Geographic Information System, 2011, 3, 225-231:
Abstract
Flash floods occur periodically in Makkah city, Saudi Arabia, due to several factors including its rugged topography and geological structures. Hence, precise assessment of floods becomes a more vital demand in development planning. A GIS-based methodology has been developed for quantifying and spatially mapping the flood characteristics. The core of this new approach is integrating several topographic, metrological, geological, and land use datasets in a GIS environment that utilizes the Curve Number (CN) method of flood modelling for ungauged arid catchments. Additionally, the computations of flood quantities, such as depth and volume of runoff, are performed in the attribute tables of GIS layers, in order to assemble all results in the same environment. The accomplished results show that the runoff depth in Makkah, using a 50-years return period, range from 128.1 mm to 193.9 mm while the peak discharge vary from 1063 m3 /s to 4489 m3 /s. The total flood volume is expected to reach 172.97 million m3 over Makkah metropolitan area. The advantages of the developed methodology include precision, cost-effective, digital outputs, and its ability to be re-run in other conditions.
Keywords: Flood Assessment, Rainfall-Runoff Model, NRCS, GIS, Saudi Arabia
1. Introduction Hazards of flash floods are vital in terms of human lives loss and economical damages. Makkah city, west of Kingdom of Saudi Arabia (KSA), exhibits two unique features that increase the hazardous flood consequences: (1) its topography is very complex; (2) about three million Muslims are gathered annually in Makkah to perform Hajj over a two-week period in the winter, which is the main rainfall season in Saudi Arabia. Due to the increasing interest in flood impacts over the last couple of decades, extensive flood estimation studies have been carried out in different countries, such as USA [1], Egypt [2,3], Nigeria [4], South Korea [5], China [6], and Saudi Arabia [7,8]. This paper aims to develop a GIS approach for assessing the flash flood hazards for Makkah metropolitan area, utilizing the most up-to-date and precise available data sets.
Flood estimation methods aim to model the rainfall-runoff relationships, and can be categorized into three groups according to their complexity. Simple approaches, such as the rational method and empirical formulas, estimate the peak discharge quickly and with little number of inputs. The Curve Number (CN) is an example of moderate flood estimation methods. Detailed, or complex, models are able to identify the causes of problems rather than producing a simple description of overall conditions [9]. The CN method is quite used in engineering design and flood management projects, particularly in the USA [10-13].
Geographic Information Systems (GIS) and Remote Sensing (RS) techniques have been utilized as efficient tools in flood risk assessment [14,15]. For example, Change et al. [16] applied GIS to study the time-based
relationship between flood hazards and land use changes. Also, Jasrotia and Singh [17] uutilized the CN method to study the runoff and soil erosion within a GIS environment. Moreover, Chen et al. [18] tested a GIS model, which consists of a storm-runoff model and an inundation model, to model flood hazards. In addition, Dongquan et al. [19] developed a GIS batch process to delineate catchments and compute their geomorphologic parameters. Furthermore, Guptaa, and Panigrahya [20] has utilized several data sources and two runoff models in a GIS platform to investigate the flood characteristics and variations of large basins in India. Additionally, Gogoase et al. [21] utilized GIS to develop inundation maps foe extreme flood events. Moreover, Karmakar et al. [22] proposed a methodology for six major damage centers in the Upper Thames River watershed, Canada to assess the flood risks, i.e. flood probability of occurrence, vulnerability to flood, and exposures of land use and soil type to flood
3. Flash Floods in Makkah City Makkah
city is located in the south-west part of KSA, about 80 Km east of the Red Sea (Figure 1). It extends from 39˚35’ E to 40˚02’E, and from 21˚09’ N to 21˚37’ N. The area of the metropolitan region (the study area) equals 1593 square kilometres. The topography of Makkah is complex in nature, and several mountainous areas exist inside its metropolitan area. The winter is considered as the main rainy season in Saudi Arabia. The annual rain over Makkah city, for a period extending from 1966 to 2009, varies from 3.8 mm to 318.5 mm, with an average of rainfall equals 101.2 mm (Figure 2). Due to the complexity of Makkah’s topography, flash floods occur periodically with significant variations in magnitude. Mirza and Ahmed [23] have reported that the extreme flood type is repeated with a return period of 46 years, while a second-order flood takes place occasionally with a return period of 33 years, and a low-dangerous flood comes about every 13 years. Using the magnitude of the annual rain average (which equals 101.2 mm) as a rain intensity factor might not be optimum in flood estimation process. The rain intensity of a single extreme storm may exceed the annual rainfall average for a year. For example, the 1969 storm records (Figure 3) showed that the rain intensity reaches 107.5 mm/hour during the first 10 minutes of that storm. Based on records of a single rainfall station, this extreme storm resulted in a runoff volume of more than 41 million cubic meters in the central area of Makkah city, with results of severe damages and human loses [23]
Analyzing the flood series frequency, the return period or recurrence interval can be computed. That period defines the average number of years during which a flood of a given magnitude will be equalled or exceeded once. The Welbull method, among several other formulas, computes the return period T as [24]:
T= (n + 1) /m
where n is the number of events, or number of records, m is the order or rank of the event (flood item) when flood magnitudes ranked in descending order
Figure 1. Study area.
Figure 2. Annual rains in Makkah city from 1966 to 2009
Figure 3. Rain intensity of the 1969 storm in Makkah city.
The computed return period of the 1969’s flood has been estimated to 44 years. That piece of information is quite helpful in flood assessment studies, as it means that: 1) that flood magnitude is expected to occur by about 2013; and 2) selected return period value for flood management projects should be equal or greater than 44 years. The rainfall intensity for a 50-years return period has been estimated as 200 mm/h [25] and is used in the current research study.
4. Data and Methodology Several
datasets have been collected for flood assessment in Makkah city. The main data set is a Digital Elevation Model (DEM) over the study area. This DEM was produced by the by King Abdulaziz City of Sciences and Technology (KACST) with a spatial resolution equals to 5 meters. A window covering Makkah metropolitan area (Figure 4) has been provided through the Center of Excellence in Hajj and Omrah, Umm Al-Qura university. Mirza et al. [26] confirm that this national DEM is 3 times more accurate than previously published global DEMs (e.g. ASTER and SRTM) over Makkah area. From Figure 4, it can be noticed that the heights of Makkah metropolitan area range from 80 to 982 meters. The other datasets include digital geological, soil, and
land use maps of the study area.The developed GIS-based flood assessment methodology consists of several stages. Figure 5 depicts an overview flow chart of that scheme. In the first stage, the Arc GIS software along with the Arc Hydro extension are used to obtain several shapefiles describing the geomorphology of the study area. These shapefiles include: the main basins and the sub-basins of each main catchment, along with drainage network using Strahler method (a simple widely-utilized network order method), and the longest stream path in each catchment. The second stage of the developed methodology is based on the flood assessment method developed by the US Natural Resources Conservation Service (NRCS), formerly known as the Soil Conservation Service (SCS). It worth mentioning that there are several hydrologic methods used for flood estimation, but the SCS method has been applied in the current research study. This method, also known as the Curve Number (CN), makes use of geological information to assign a unique CN value for each area, which will be further used to estimate the surface runoff depth and the peak discharge magnitude. VBA is used to compute the required flood defining parameters that consists of [27
Figure 4. The national 5-m DEM for Makkah city.
The developed GIS-based flood assessment methodology consists of several stages. Figure 5 depicts an overview flow chart of that scheme. In the first stage, the Arc GIS software along with the Arc Hydro extension are used to obtain several shapefiles describing the geomorphology of the study area. These shapefiles include: the main basins and the sub-basins of each main catchment, along with drainage network using Strahler method (a simple widely-utilized network order method), and the longest stream path in each catchment. The second stage of the developed methodology is based on the flood assessment method developed by the US Natural Resources Conservation Service (NRCS), formerly known as the Soil Conservation Service (SCS). It worth mentioning that there are several hydrologic methods used for flood estimation, but the SCS method has been applied in the current research study. This method, also known as the Curve Number (CN), makes use of geological information to assign a unique CN value for each area, which will be further used to estimate the surface runoff depth and the peak discharge magnitude. VBA is used to compute the required flood defining parameters that consists of [27]:
2) (Q = ( P –0.2 S )2 / ( P + 0.8 S)
3 ) (S = 25.4 ( (1000 / CN ) – 10
4 ) QT = Q A
5) qp = qu A Q
v = 0.2279 L / tc (6
tc = 1.67 [ L 0.8 (S+1)0.7 ] / [ 1900*SL0.5 ] (7
Sd = 0.133 Tc (8
Figure 5. The developed GIS-based flood assessment meth odology.
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