Water from Small Dams

6.4 Measurements to be taken of a dam siteWhen a suitable site has been found, the following measurements are required to draw adesign and calculate the Bill of Quantities (BQ) and estimate construction costs:1) Bench Mark (BM)Make a bench mark (BM) on a tree or rock, orconcrete a few stones together near one end ofthe proposed dam wall. Mark the BM pointwith white paint to make it visible from adistance. Plot the position of the bench markonto a contour map of 1:50,000 if available,otherwise draw a sketch on graph millimetrepaper to a scale of 1 centimetre being equal to 2metres.From now, all measurements, levels andheights will be taken from this BM and drawnin the sketch.Plan of a dam site with a BM on the leftside from where all measurements,levels and heights are taken.2) Maximum water level (WL)Draw a builder’s line and a long tape measureacross the valley from the BM to the oppositeend of the dam wall as shown in the photo.Insert the ring at the end of another long tapemeasure onto the builder’s line in such a waythat the tape measure can slide across the valley.Then take the vertical measurements from thebuilder’s line to the floor of the valley for every2 metres, which is read on the tape measurehanging on the builder’s line.Also measure the length and depth of themaximum flood that has ever passed throughthe valley. This data shall be used for estimatingthe width and depth of the spillways.The photo shows a builder’s line and along tape measure being drawn over aa narrow point in the valley33