**3.2.4 Conclusions and recommendations**

150 Urban Development

This Rational formula is characterized by consideration of the entire drainage area as a single unit, estimation of flow at the most downstream point only, and the assumption that rainfall is uniformly distributed over the drainage area. The Rational Formula is as follows:

Qp=0.278 C\*I\*A

the predicted peak discharge has the same probability of occurrence as the used rainfall intensity (I), the runoff coefficient (C) is constant during the rain storm and the recession

Peak runoff rates have been calculated by the Rational Formula using the maximum total daily rainfall records for the period 1985-2005 (PME, 2006). The runoff coefficient, (C) was taken to be equal to 0.75 corresponding to residential area/business area/asphalt streets. The total daily rainfall was converted into rainfall intensity (I) in mm/h. The results of

Jan 27.9 1.16 2.73 Feb 28.3 1.18 2.77 Mar 32.4 1.35 3.17 Apr 25.4 1.06 2.48 Nov 30 1.25 2.93 Dec 47.8 1.99 4.67

Table 4. Peak runoff rates for maximum daily total rainfall Period (1985-2005)

runoff under these assumed conditions will be as shown in Table 5:

Table 5. Peak runoff assuming high values of rainfall

According to maximum total daily rainfall record reached on December 20th 1995 at 47.8 mm/day, the peak runoff has been calculated using the rational formula to be 4.67 m3/sec. Assuming higher values of rainfall storms at 2.5 mm/h and 5 mm/h, recalculation of peak

> Rainfall Intensity (mm/h) Q, peak flow (m3/sec) 2.5 5.86 5 11.73

(mm/h) Peak runoff (m3/sec)

where:

Qp = Peak runoff rate (m³/sec)

I = Rainfall intensity (mm/hr) A = Drainage area (km²)

time is equal to the time of rise.

calculation are shown on Table 4.

C = Runoff coefficient (dimension less)

The Rational Formula follows the assumption that:

Month Rainfall (mm/day) Rainfall intensity

The assessment of topography, drainage and meteorological data of the in northwest of Riyadh City in Saudi Arabia showed that the site represents the mouth of a catchment area that drains towards the main course of Wadi Hanifa. The surface area of the catchment was found to be about 11.2 km2 with an average slope of 0.02. The analyses of the flow path indicated that the flow is towards the southeast. The climate is characterized with a very hot summer, mild winter and little rain. The prevailing wind direction is towards the north and northeast. Total rainfall data showed that the maximum daily rainfall for the period 1995- 2005 was 47.8 mm. Peak runoff was calculated using the Rational formula to range between 4.7 and 11.73 m3/sec.

Based on the results of this study, hydraulic design of storm water drainage system should take in consideration that almost all of storm water will flow through the middle of the site. This study forms a small model showing the importance of characterizing hydrological elements as part of a comprehensive environmental analysis for future urban planning. Historic meteorological data and particularly rainfall, estimation of peak runoff integrated together with satellite image interpretation and digital elevation model are important for any detailed urban design.

#### **3.3 Identification of areas prone to hydrological hazards in Riyadh city, Saudi Arabia (Faisal K. Zaidi, et al, 2011)**

#### **3.3.1 Introduction**

Saudi Arabia is one of the most arid regions of the world however this has not prevented it from the growth of big cities along the coasts like Jeddah and Damam and along the ancient Wadi system such as Riyadh, Madinah and Makkah. Though the average annual rainfall in Saudi Arabia is only about 100 mm/year,(PME, 2005), it is not free from hydrological hazards especially in the big cities like Jeddah and Riyadh mainly due to rapid urbanization which has led to the development of housing colonies in topographically low lying regions and obstruction of the natural drainage systems. The flood hazards in the city of Jeddah in November, 2009 as result of heavy rainfall and blockage of natural drainage system is a good example in the recent times.

The present study focuses on the city of Riyadh, (Figure 25) which has grown rapidly over the past few years and can be subjected to flooding hazards in events of heavy rainfall. The objective of the study is to identify the areas in Riyadh city which may be prone to such hydrological hazards.

Riyadh city has grown from an area of 1 km2 in 1901 to about 2435 km2 in 2010, (www.arriyadh.com). The population of the city is about 4.8 million. The temperature varies from 43° C in July to about 8° C in January. The overall climate of the city is arid with average annual rainfall not exceeding 105 mm/year, (PME, 2006). The city is typically bordered by a complex system of valleys (known as wadis in local language) along its western limits. The average elevation of the city is about 690 meters above mean sea level with the main drainage following a Northwest-Southeast pattern and is typically controlled by the Najd Fault system, (Powers et al, 1966).

Assessing Hydrological Elements as Key Issue for Urban Development in Arid Regions 153

and lies in the western part of the study area and the Sulay Basin which trends in a more or less North-South direction. Hanifa basin occupies an area of 4199.51 km2 whereas the Sulay basin has an area of 1514.43 km2 .The various morphometric parameters for the two basins were calculated and have been discussed in the following sections, (Table 6. Figure 28 shows

the two basins with the stream patterns and the city of Riyadh.

Fig. 26. DEM and the streams in the study area

Fig. 27. Hanifa and Sulay Basins

### **3.3.2 Methodology**

The methodology involved carrying out a detailed morphometric analysis of the basins within the Riyadh city. Morphometric parameters such as basin shape and basin relief influence the nature of hydrographs and hydrological variables. Drainage basin morphometric analysis of the study area was carried out using the SRTM DEM data, (http://srtm.usgs.gov/data/obtaining.html). The DEM from SRTM is available on 90 m by 90 m spatial resolution. The DEM was treated before it was subjected for hydrological processing in GIS.

Fig. 25. Location of Riyadh city (study area) within Saudi Arabia

Based on the result of the hydrological processing which involved the identification of flow direction, flow accumulation and stream generation, (Figure 26), the Riyadh city was divided into two basins which are identified as the Hanifa Basin and Sulay Basin, (Figure 27). The drainage channels were classified into different orders using Strahler's 1964 classification. Other basin parameters such as basin area, basin perimeter, basin length and stream length were obtained which was further used to obtain the different ratios such as Drainage Density, Bifurcation Ratio, Stream Frequency, Form Factor, Elongation Ratio, and Circulatory Ratio.

#### **3.3.3 Results**

Based on the results of the hydrological processing, the DEM of the study area was divided into two basins namely the Hanifa basin which trends in a Northwest-Southeast direction

The methodology involved carrying out a detailed morphometric analysis of the basins within the Riyadh city. Morphometric parameters such as basin shape and basin relief influence the nature of hydrographs and hydrological variables. Drainage basin morphometric analysis of the study area was carried out using the SRTM DEM data, (http://srtm.usgs.gov/data/obtaining.html). The DEM from SRTM is available on 90 m by 90 m spatial resolution. The DEM was treated before it was subjected for hydrological

Fig. 25. Location of Riyadh city (study area) within Saudi Arabia

Based on the result of the hydrological processing which involved the identification of flow direction, flow accumulation and stream generation, (Figure 26), the Riyadh city was divided into two basins which are identified as the Hanifa Basin and Sulay Basin, (Figure 27). The drainage channels were classified into different orders using Strahler's 1964 classification. Other basin parameters such as basin area, basin perimeter, basin length and stream length were obtained which was further used to obtain the different ratios such as Drainage Density, Bifurcation Ratio, Stream Frequency, Form Factor, Elongation Ratio, and

Based on the results of the hydrological processing, the DEM of the study area was divided into two basins namely the Hanifa basin which trends in a Northwest-Southeast direction

**3.3.2 Methodology** 

processing in GIS.

Circulatory Ratio.

**3.3.3 Results** 

and lies in the western part of the study area and the Sulay Basin which trends in a more or less North-South direction. Hanifa basin occupies an area of 4199.51 km2 whereas the Sulay basin has an area of 1514.43 km2 .The various morphometric parameters for the two basins were calculated and have been discussed in the following sections, (Table 6. Figure 28 shows the two basins with the stream patterns and the city of Riyadh.

Fig. 26. DEM and the streams in the study area

Fig. 27. Hanifa and Sulay Basins

Assessing Hydrological Elements as Key Issue for Urban Development in Arid Regions 155

Stream frequency is the ratio of the total number of stream segments of all the orders in the basin to the total area of the basin, (Horton, 1945). The stream frequency for the Hanifa Basin is 0.39/km2 and 0.42/km2 for Sulay basin. The stream frequency is dependent on the

Basin length is the longest length of the basin from the head waters to the point of confluence, (Gregory and Walling, 1973). Hanifa basin has a length of 125.50 km and Sulay

Form factor is the ratio of the basin area to the square of the basin length. The form factor varies inversely to the basin length. Circular basins have a form factor close to 1. The form factor for Hanifa basin is 0.26 and 0.46 for Sulay basin indicating the presence of elongated

It is the ratio of the diameter of a circle having the same area as the basin to the basin length. The elongation ratio for the Hanifa Basin is 0.58 whereas it is 0.57 for the Sulay Basin. The elongated shapes of the basins are a result of the guiding effect of thrusting and faulting in

Basin Hanifa Sulay Area (km2) 4199.51 1514.43 Total Stream Length (km) 3707.55 1389.71 Drainage Density(km/km2) 0.88 0.92 Total number of Streams 1651.00 630.00 Stream frequency 0.39 0.42 Drainage Texture 0.35 0.38 Bifurcation Ratio 6.45 3.58 Length (km) 125.50 76.50 Basin Perimeter (km) 410.68 289.52 Ht Max (mts) 1033.00 714.00 Ht Min (mts) 438.00 484.00 Basin Relief (mts) 595.00 230.00 Form Factor 0.26 0.46 Elongation ratio 0.58 0.57 Circulatory Ratio 0.31 0.23 Slope (%) 6.60 2.30 Relief Ratio (m/Km) 3.30 2.29

Table 6. Morphometric parameters of Hanifa and Sulay Basins

rainfall and the temperature of the region.

basins which is quite evident from the Figure 3.

basin has a length of 76.50 km.

the basin, (Vaslet et al, 1991).

Stream frequency

Basin length

Form factor

Elongation ratio

Drainage density

Drainage Density is the ratio of the total stream length in a given basin to the total area of the basin, (Strahler. 1932, 1945). The drainage density of the Hanifa Basin is 0.88 km/km2 as compared to 0.92 km/km2 for the Sulay Basin.

#### Bifurcation ratio

Bifurcation Ratio for the Hanifa basin is 6.45 where as it is 3.38 for the Sulay basin. Bifurcation ratio is the number of streams of any given order to the number of streams in the next higher order, (Horton, 1932). A high bifurcation ratio indicates more chances of flooding as water from different channels tend to accumulate in a single channel rather than spreading out.

#### Stream order

Strahler's, 1964 system was taken for the stream ordering. The number of streams gradually decreases as the stream order increases. Hanifa basin is a 5th order basin whereas the Sulay basin is a 6th order basin.

#### Stream length

The total stream length for the Hanifa basin is about 3700 km whereas the total stream length for Sulay basin is about 1390 km. The length of stream is maximum in case of first order in both the basins.


Table 6. Morphometric parameters of Hanifa and Sulay Basins

#### Stream frequency

154 Urban Development

Drainage Density is the ratio of the total stream length in a given basin to the total area of the basin, (Strahler. 1932, 1945). The drainage density of the Hanifa Basin is 0.88 km/km2 as

Bifurcation Ratio for the Hanifa basin is 6.45 where as it is 3.38 for the Sulay basin. Bifurcation ratio is the number of streams of any given order to the number of streams in the next higher order, (Horton, 1932). A high bifurcation ratio indicates more chances of flooding as water from different channels tend to accumulate in a single channel rather than

Strahler's, 1964 system was taken for the stream ordering. The number of streams gradually decreases as the stream order increases. Hanifa basin is a 5th order basin whereas the Sulay

The total stream length for the Hanifa basin is about 3700 km whereas the total stream length for Sulay basin is about 1390 km. The length of stream is maximum in case of first

Fig. 28. Extent of Hanifa and Sulay Basin and the city of Riyadh

compared to 0.92 km/km2 for the Sulay Basin.

Drainage density

Bifurcation ratio

spreading out. Stream order

Stream length

basin is a 6th order basin.

order in both the basins.

Stream frequency is the ratio of the total number of stream segments of all the orders in the basin to the total area of the basin, (Horton, 1945). The stream frequency for the Hanifa Basin is 0.39/km2 and 0.42/km2 for Sulay basin. The stream frequency is dependent on the rainfall and the temperature of the region.

#### Basin length

Basin length is the longest length of the basin from the head waters to the point of confluence, (Gregory and Walling, 1973). Hanifa basin has a length of 125.50 km and Sulay basin has a length of 76.50 km.

#### Form factor

Form factor is the ratio of the basin area to the square of the basin length. The form factor varies inversely to the basin length. Circular basins have a form factor close to 1. The form factor for Hanifa basin is 0.26 and 0.46 for Sulay basin indicating the presence of elongated basins which is quite evident from the Figure 3.

#### Elongation ratio

It is the ratio of the diameter of a circle having the same area as the basin to the basin length. The elongation ratio for the Hanifa Basin is 0.58 whereas it is 0.57 for the Sulay Basin. The elongated shapes of the basins are a result of the guiding effect of thrusting and faulting in the basin, (Vaslet et al, 1991).

Assessing Hydrological Elements as Key Issue for Urban Development in Arid Regions 157

This implies that during the same intensity of rainfall in the region, water in the Hanifa Basin will be collected from a larger catchment as compared to Sulay Basin, thus bringing a greater volume of water to the main drainage. Secondly the Hanifa Basin has a more rugged topography as compared to Sulay basin, (see Figure 27 and Table 8) which means the velocity of the water will be much higher. Third and most importantly the bifurcation ratio of Hanifa Basin is much higher as compared to Sulay Basin, (Table 8). This is true specially for the 5th order streams which clearly means that more number of 4th order streams meet at different points to form a 5th order stream in Hanifa Basin, (Figure 28). From the perspective of hydrological hazards a high bifurcation ratio increases the chances of flooding as water from a given stream order, rather than spreading out when they meet the

Based on this observation, all the 4th order sub-basins of the Hanifa basin have been identified and demarcated (Figure 29). The mouth of these basins are the potential sites for hydrological hazards in case of heavy rainfall and should be carefully monitored during the

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**4. References** 

#### Circulatory ratio

It is the ratio of the basin area to the area of the circle having the same perimeter as the basin. The circulatory ratio for Hanifa basin is 0.31 and the circulatory ratio for Sulay Basin is 0.23. This factor is influenced by the lithological characteristics of the basin

Slope

The Hanifa basin shows a high relief ratio (3.30 m/km) and has an average slope percentage of 6.6 as compared to Sulay Basin which has a relief ratio of 2.29 m/km and an average slope percentage of 2.3. The results clearly suggest that Hanifa basin has a more rugged terrain as compared to Sulay basin.
