KITUI SAND DAMS

) Pre Hydraulic Design.At this stage the following parameters will have been determined.Height of spillway H s by usingH s max =H b_ - A w / W bHeight of dam H d by usingH d max = H bSo with this it is possible to carry out: -Spillway Design.- Back flow DesignsSpillway designThis involves the determination of: -1 Length of spillway L s2 Gross free board G fAs earlier stated the following suitability conditions prevail always.a) When H f > H b: G f =0 andb) L s =W b; this is because H s = H d = 0.c) When H b > H f , A d > A wMethod I Wetted area Section.Procedure.Using the flood marks and information from the local people, the area of wetted section(A w ) is determined in five points along the river.Note: Minimizing the G f increases the cost effectiveness of the dam. This is possible byminimizing the length of the spillway (L s).Average Wetted Area Section1(A wa ) = ∑ 5 A5 1A = L.waG fLx max= W b∴ G fMin=AwWbExample.When A wa =4m 2 , W b = 10m, L s = 10mGf min≈ 7m.Awa( m=Wb22) 4m= = 6.7m0.6mNB: If the calculated G f is below 30cm the designers should L s but only when G f doesnot become too cost effective.39