**Average Rainfall (1985-2005)**

Fig. 18. Average total monthly rainfall

The maximum total daily rainfall records at King Khalid Airport station is shown in Figure 19: Maximum daily total rainfall The maximum recorded daily rainfall was 47.8 mm and it was on December, 20th 1995. The minimum was 25.4 mm and was recorded on April, 11th 1991.

Fig. 19. Maximum daily total rainfall

#### Air temperature

146 Urban Development

Climate is characterized by a very hot summer, mild winter and little irregular rain with much variation in quantity. In general Riyadh area is influenced by the Mediterranean winter, precipitation and by local factors, such as the relief and distance from the sea. During winter time (November-February) the middle latitude cyclones tend to travel from the Mediterranean Sea towards the equator and then travel inland reaching the Najd Plateau. Monsoonal rains are caused by the tropical type cyclones in the Indian Ocean and travel over the Red Sea. The coldest month is January. Summer extends from sometime in

In Riyadh area the amount of rainfall is irregular through the years and through the months. Winter and spring is the rainy season, there is almost no rain between May and September. Average monthly rainfall for the period 1985-2005 is shown in Figure 18. Rain occurs mainly in November-January, through February and relative higher quantities of some 25 mm occur in March and April period. Less than 3 mm may occur during the month of October. The amount of rainfall is extremely variable from year to year and from month to month.

**Average Rainfall (1985-2005)**

**1 2 3 4 5 6 7 8 9 10 11 12 Month**

Table 3. Sub basin characteristics

April to the beginning of September (PME, 2005).

Annual rainfall rarely exceeds 125 mm (PME, 2005).

Fig. 18. Average total monthly rainfall

**0**

**5**

**10**

**15**

**Rainfall (mm)**

**20**

**25**

**30**

Climate

Rainfall

Area (km2) 11.26 Slope 0.02 Shape Factor 3.87 Sinuosity 1.18 Perimeter 27369.96 Mean elevation (m) 649.47

> The maximum air temperatures are reached during summer (June, July, and August) and minimum temperatures are attained during winter (December and January). Air temperature ranges from 8◦ C in winter to some 43◦ C in summer (Figure 20). The average monthly temperature is in the range 14.1◦ C to 43◦ C. The annual average temperature is 24.6◦ C. The coldest month is January while the hottest months are June, July and August.

Fig. 20. Minimum, maximum, and average monthly temperature.

Assessing Hydrological Elements as Key Issue for Urban Development in Arid Regions 149

**Wind Mean Speed (1985-2005)**

Pan evaporation rates are very high in Riyadh throughout the year (Figure 24). Annual average evaporation has been measured at King Khalid Airport station as 2910 mm. During rainy months of December, January, February, March and April, rainfall exceeds evaporation.

**Evaporation**

**1 2 3 4 5 6 7 8 9 10 11 12 Month**

Surface runoff in the site was calculated by the Rational Method (U.S. Soil Conservation Service, 1964; Chow et al., 1988; McCuen, 1998; Willson, 1990; McCuen, 1998). The Rational method is used primarily for computing peak flows for small urban and rural watersheds.

1 2 3 4 5 6 7 8 9 10 11 12 **Month**

Fig. 23. Average monthly wind speed

**k**

**m/hr**

Fig. 24. Evaporation (1985-2005)

**(mm)**

Runoff

Evaporation

#### Relative humidity

Since Riyadh city is located on the Najd Plateau, away from any water body, the relative humidity is very low. The Average values for relative humidity ranges from 19.5% in June and 52.5% January (Figure 21). Annual average relative humidity is 34.4%. These values reflect very dry or hyper arid climate.

#### **Relative Humidity M ean (1985-2005)**

Fig. 21. Average monthly relative humidity

#### Solar radiation

Solar radiation is an important factor that influences evaporation in the area. In Riyadh area the monthly solar radiation ranges from 328 cal/cm2 per day (January) to 597 cal/cm2 per day (June). Cloudness ratio is 0.721 (January) to 0.765 (June). The average annual value for solar radiation is 477 cal/cm2 per day.

#### Wind

Mean monthly speed value ranges from 3.8 km/hr in October to some 6.8 km/hr in July. In March it reaches its maximum at some 6.9 km/hr. The average annual wind speed is 5.1 km/hr. The prevailing wind directions are primarily North and Northeast (Figure 22 and Figure 23).

Fig. 22. Prevailing wind direction (Years 1985-2005)

**Wind Mean Speed (1985-2005)**

#### Evaporation

148 Urban Development

Since Riyadh city is located on the Najd Plateau, away from any water body, the relative humidity is very low. The Average values for relative humidity ranges from 19.5% in June and 52.5% January (Figure 21). Annual average relative humidity is 34.4%. These values

**Relative Humidity M ean (1985-2005)**

Solar radiation is an important factor that influences evaporation in the area. In Riyadh area the monthly solar radiation ranges from 328 cal/cm2 per day (January) to 597 cal/cm2 per day (June). Cloudness ratio is 0.721 (January) to 0.765 (June). The average annual value for

**1 2 3 4 5 6 7 8 9 10 11 12 Month**

Mean monthly speed value ranges from 3.8 km/hr in October to some 6.8 km/hr in July. In March it reaches its maximum at some 6.9 km/hr. The average annual wind speed is 5.1 km/hr. The prevailing wind directions are primarily North and Northeast (Figure 22 and

Relative humidity

Solar radiation

Wind

Figure 23).

reflect very dry or hyper arid climate.

**%**

Fig. 21. Average monthly relative humidity

solar radiation is 477 cal/cm2 per day.

Fig. 22. Prevailing wind direction (Years 1985-2005)

Pan evaporation rates are very high in Riyadh throughout the year (Figure 24). Annual average evaporation has been measured at King Khalid Airport station as 2910 mm. During rainy months of December, January, February, March and April, rainfall exceeds evaporation.

**Evaporation**

Fig. 24. Evaporation (1985-2005)

#### Runoff

Surface runoff in the site was calculated by the Rational Method (U.S. Soil Conservation Service, 1964; Chow et al., 1988; McCuen, 1998; Willson, 1990; McCuen, 1998). The Rational method is used primarily for computing peak flows for small urban and rural watersheds.

Fig. 23. Average monthly wind speed

Assessing Hydrological Elements as Key Issue for Urban Development in Arid Regions 151

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

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

**3.3 Identification of areas prone to hydrological hazards in Riyadh city, Saudi Arabia** 

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

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

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

**3.2.4 Conclusions and recommendations** 

4.7 and 11.73 m3/sec.

any detailed urban design.

**(Faisal K. Zaidi, et al, 2011)** 

good example in the recent times.

by the Najd Fault system, (Powers et al, 1966).

hydrological hazards.

**3.3.1 Introduction** 

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:

$$\mathbf{Qp} = 0.278 \text{ C\*I\*A}$$

where:

Qp = Peak runoff rate (m³/sec) C = Runoff coefficient (dimension less)

I = Rainfall intensity (mm/hr)

A = Drainage area (km²)

The Rational Formula follows the assumption that:

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 time is equal to the time of rise.

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 calculation are shown on Table 4.


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

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


Table 5. Peak runoff assuming high values of rainfall
