Water from Small Dams

6.5 DesignThe measurements taken from the builder’s line can be recorded as:Measurement no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14Horizontal m 2 2 2 2 2 2 2 2 2 2 2 2 2 2Vertical m 0 2 3 5 6 6 5 4 5 5 4 3 2 0Max. flood level 0 0 0 1 2 2 1 0 1 1 0 0 0 0These measurements were converted into a profile of the riverbed as shown below.BM 1 2 3 4 5 6 7 8 9 10 11 12 13 14A profile drawn according to the measurements taken from the builder’s line drawnacross a narrow part of the valley where the dam wall will be constructed.SpillwaysThe maximum flood level (MAX. FL) is 15 m wide and 2 m at its deepest. That givesa cross section of: 15 m x 2 m / 2 = 15 square metres. The size of the spillways shouldbe the double of that, namely 30 m wide and 1 m deep = 30 square metres.The maximum water level, and the depth and width of the spillways can now bedrawn on the profile as seen below.The sketch shows the maximum water level (MAX. WL), which is 1 m below theground level, and the two spillways having a width of 30 m and depth of 1 m, equal to30 square metres, below the ground level.FreeboardThe next sketch shows the 1.5 m high freeboard, which ensures that floodwater risingabove the MAX. WL will spill over into the spillways without damaging the damwall. The sketch also shows the crest (top of the dam wall) that must be higher in themiddle than at its ends to cater for settling (sinking) of a new dam wall.34