**3.2.3 Results**

Field observations

GPS locations, types, dimensions of all culverts within the project site were measured (Table 1). The coordinates were measured by a hand held Garmin GPS and are reported in degrees, minutes, and seconds. The culverts in the site include three types: multispan box, double tube and single tube pipe culverts. Table 2 shows locations, and dimensions of existing ponding areas in the site and potential ponding areas.


Table 1. Culvert types and dimensions


Table 2. Pounding and potential pounding areas

Assessing Hydrological Elements as Key Issue for Urban Development in Arid Regions 145

Fig. 16. The four sub drainage systems and their catchments areas.

Fig. 17. Detailed drainage system of the site and its surroundings.

Catchment area and drainage analysis

Catchment area and drainage analysis of the study area were depicted with the help of published topographic maps (sheet 4624-14), SPOT 4 image and DEM analysis.

Analysis of DEM of the area show that the western part topography is land marked by part of Wadi Hanifa Escarpments, high lands occur towards the north east of the study area (Figure 15).The southern part occupies the relatively lower most elevations in the study area. Within the study area the general slope is towards the south (0.02). Drainage analysis of the study area shows four sub dendritic systems that drain towards the south and finally joining Wadi Hanifa (Figure 16).

Fig. 15. Topography and general drainage systems of northwest Riyadh

The site of the present study lies in the downstream of the middle sub catchment. This sub catchment (Figure 4) covers a surface area of about 11.25 km2. The stream length within the site varies from 68.29 to 686.16 m with an average of 396.23 m. Within the site the stream slope varies from 0.0009 to 0.016 with an average of 0.00632 (Table 3).

The site represents the trunk of the sub catchment under consideration. All calculations of surface hydrology are based on the surface area of this catchment (Figure 17).

Catchment area and drainage analysis of the study area were depicted with the help of

Analysis of DEM of the area show that the western part topography is land marked by part of Wadi Hanifa Escarpments, high lands occur towards the north east of the study area (Figure 15).The southern part occupies the relatively lower most elevations in the study area. Within the study area the general slope is towards the south (0.02). Drainage analysis of the study area shows four sub dendritic systems that drain towards the south and finally

published topographic maps (sheet 4624-14), SPOT 4 image and DEM analysis.

Fig. 15. Topography and general drainage systems of northwest Riyadh

slope varies from 0.0009 to 0.016 with an average of 0.00632 (Table 3).

surface hydrology are based on the surface area of this catchment (Figure 17).

The site of the present study lies in the downstream of the middle sub catchment. This sub catchment (Figure 4) covers a surface area of about 11.25 km2. The stream length within the site varies from 68.29 to 686.16 m with an average of 396.23 m. Within the site the stream

The site represents the trunk of the sub catchment under consideration. All calculations of

Catchment area and drainage analysis

joining Wadi Hanifa (Figure 16).

Fig. 16. The four sub drainage systems and their catchments areas.

Fig. 17. Detailed drainage system of the site and its surroundings.

1991.

Fig. 19. Maximum daily total rainfall

**0**

**10**

**20**

**30**

**Daily total rainfall (mm)**

**40**

**50**

**60**

**27.9 28.3**

Air temperature

Assessing Hydrological Elements as Key Issue for Urban Development in Arid Regions 147

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

**Maximum daily rainfall (1985-2005)**

**32.4**

**25.4**

**47.8**

**30**

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.

> **Temperature (1985-2005)** Max Min Mean

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

**Jan Feb Mar Apr Nov Dec**

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

**Degree Centigrade**


Table 3. Sub basin characteristics

Climate

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

#### Rainfall

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