Week5

# Week5 - higher flood risk alculation Example Calculation...

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Calculation Example: Las Vegas, NV ite area: 5000 acre, Soil Type B, flat slope Site area: 5000 acre, Soil Type B, flat slope • Rainfall Intensity = 7 in/yr vapotranspiration Rate (E) = 4 5 in/yr Evapotranspiration Rate (E) = 4.5 in/yr • Change in surface water depth Δ d= 0 (arid area) efore development: 82% open land; 18% • Before development: 82% open land; 18% paved roads; fter development: 52% residential; 18% paved • After development: 52% residential; 18% paved roads; 16% open land; 14% parking lots, schools, commercial

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alculation Example Calculation Example • Before: C = 0.82 x 0.08 + 0.18 x 0.71 = 0.19 • After: C = 0.52 x 0.17 + 0.18 x 0.71 + 0.16 x 0.08 + 0.14 x 0.85 = 0.35 • So, development increases the surface runoff coefficient, and therefore the runoff water (lesser water availability

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Unformatted text preview: , higher flood risk) alculation Example Calculation Example Before water budget (annual basis): Rainfall : 7 in/yr x 1 ft/12 in = 0.58 ft/yr Runoff : 0.19 x 0.58 ft/yr = 0.11 ft/yr ET : 4.5 in/yr x 1 ft/12 in = 0.38 ft/yr Infiltration : I = R - Q – ET ( Δ d=0) Before : 0.58-0.11-0.38 =0.09 ft/yr = 1.08 in nnually annually fter 0 58 35 x 0 58) 38= 003 ft/yr = 036 After : 0.58-(0.35 x 0.58)-0.38=-0.003 ft/yr = -0.036 in annually onceptual Solution Conceptual Solution • eed to capture excess surface runoff Need to capture excess surface runoff generated by development. • To break even (no net water loss due to development), you would need to build a 1.08-(-0.036)=1.116 in x 5000 acre = 465 acre-ft reservoir, which would replenish the infiltration water...
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Week5 - higher flood risk alculation Example Calculation...

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