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The susceptibility of the Wadi Al-Burdi basin to flash floods is high due to the scarcity of stream meanders, steep slopes, and low surface roughness. The estimated peak flow rate for 2–100 year return periods of the main basin in 2010 vary between 63.7 and 150.3 m3/s (Table 6 and Figure 12(a)). The estimated peak flow rate for the same return periods in 2015 varies between 77.4 and 171.7 m3/s (Table 6 and Figure 12(b)). The comparison between the peak flow rates of 2010 and 2015 shows distinctive increasing of the flash flood peaks by 14–22%. This is due to the nearly 8% increase in urbanization activities (Figure 13 and Tables 4 and 6). The model results show rapid urbanization adversely affects the hydrological processes, since the sprawl on the alluvial channels is significant. This reduces the infiltration into the underlying alluvium and increases the runoff and leads to higher flood peaks and volumes even for short duration low intensity rainfall. The flash floods are formed and concentrated within a short time; however, higher precipitation intensity can result in more runoff as the soil cannot absorb the water quickly enough. The high peak of these floods presents a potential threat to both infrastructure and human life in this area.
Table 6

Estimated rainfall, direct runoff and peak flows of the main basin

  5 years10 years25 years50 years100 years
2010 Rainfall max 24 hr (mm) 57.18 70.23 86.76 98.95 111.13 
Total precipitation (m32,098,500 2,577,400 3,184,100 3,631,500 4,078,500 
Total loss (mm) 19.68 21.60 23.48 24.59 25.53 
Direct run off (mm) 37.50 48.63 63.28 74.36 85.60 
Total run off (million m31,376,100 1,784,900 2,322,500 2,729,000 3,141,600 
Peak flow rate (m3/sec) 63.7 83.7 110.1 130.0 150.3 
2015 Total loss (mm) 14.42 15.69 16.92 17.64 18.25 
Direct run off (mm) 42.76 54.54 69.84 81.31 92.88 
Total run off (million m31,569,300 2,001,600 2,563,000 2,983,900 3,408,800 
Peak flow rate (m3/sec) 77.4 99.5 128.3 149.9 171.7 
  5 years10 years25 years50 years100 years
2010 Rainfall max 24 hr (mm) 57.18 70.23 86.76 98.95 111.13 
Total precipitation (m32,098,500 2,577,400 3,184,100 3,631,500 4,078,500 
Total loss (mm) 19.68 21.60 23.48 24.59 25.53 
Direct run off (mm) 37.50 48.63 63.28 74.36 85.60 
Total run off (million m31,376,100 1,784,900 2,322,500 2,729,000 3,141,600 
Peak flow rate (m3/sec) 63.7 83.7 110.1 130.0 150.3 
2015 Total loss (mm) 14.42 15.69 16.92 17.64 18.25 
Direct run off (mm) 42.76 54.54 69.84 81.31 92.88 
Total run off (million m31,569,300 2,001,600 2,563,000 2,983,900 3,408,800 
Peak flow rate (m3/sec) 77.4 99.5 128.3 149.9 171.7 
Figure 12

(a) Estimated peak flows for 5–100 year events Wadi Al-Burdi main basin: (a) before urbanization; (b) after urbanization.

Figure 12

(a) Estimated peak flows for 5–100 year events Wadi Al-Burdi main basin: (a) before urbanization; (b) after urbanization.

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Figure 13

The relation between the flood peak flow before and after urbanization.

Figure 13

The relation between the flood peak flow before and after urbanization.

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