Water from Small Dams

The outline of the base for a dam wall is determined by multiplying the verticalmeasurements from the maximum water line to the ground with the gradient of theupstream and downstream batter. The upstream measurements are taken from theupstream side of the key and the downstream measurements are taken from thedownstream side of the key as shown in the table below.PointDepth fromcentre lineto theground(m)Gradient ofupstreambatter3:1Upstreamlength ofbase fromkey (m)Depth fromcentre lineto theground (m)Gradient ofdownstreambatter2.5:1Downstreamlength ofbase fromkey (m)1 1.2 X 3 = 3.6 1.2 X 2.5 = 3.002 1.7 X 3 = 5.1 1.7 X 2.5 = 4.253 4.0 X 3 = 12.0 4.0 X 2.5 = 10.004 5.1 X 3 = 15.3 5.1 X 2.5 = 12.755 5.5 X 3 = 16.5 5.5 X 2.5 = 13.756 4.8 X 3 = 14.4 4.8 X 2.5 = 12.007 4.5 X 3 = 13.5 4.5 X 2.5 = 11.258 1.8 X 3 = 5.4 1.8 X 2.5 = 4.509 1.1 X 3 = 3.3 1.1 X 2.5 = 2.757.4 Draw-off pipeWhere a draw-off pipe is required, it should be laid when the foundation has beenexcavated. To prevent seepage along the pipe, it is laid in blocks of concrete every 3metres or so.Alternatively, a siphon pipe can be laid over one of the two spillways as shown below.A siphon pipe laid over one of the two spillways. A non-return valve is fitted onto theintake and a watertap onto the outlet. The siphon is started by closing the watertap andfilling the primer with water until all air is out. Then close the primer and open the tap.47