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Calculate the emergency spillway length for the following conditions in the East Demerara Water Conservancy, Guyana:
10,000-yr 1-day precipitation P: 428 mm
Hydrologic abstraction: 13 mm
Contributing drainage area A: 582 km2
Time base of the flood hydrograph Tb: 4 days
Dam crest elevation Ecd: 18.288 m
Spillway crest elevation Ecs : 17.526 m
Broad-crested weir discharge coefficient C: 1.4
Effective precipitation Pe = 428 - 13 = 415 mm = 0.415 m
Total volume of water: V = A Pe
V = 582 km2 × (1000 m/km)2 × 0.415 m = 241,530,000 m 3
Flood peak discharge Qp = 2 V / Tb
Qp = [(2 × 241,530,000) / (4 days × 86,400 s/day) ] = 1398 m3/s
Applicable head, including freeboard, H: Ecd - Ecs
H = 18.288 - 17.526 = 0.762 m
Spillway rating:
Q p = C L H1.5
L = Qp /(C H1.5) = 1398 / (1.4 × 0.7621.5 ) = 1500 m.
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