Driekoppies Dam

Dam Wall Structure-Driekoppies Dam


The structure for the Driekoppies Dam Wall was dictated by the geological conditions.  It consists of a zoned earthfill dam with a 150m long uncontrolled concrete gravity spillway section.

The Spillway has been profiled for an overflow of 3.900m3/s, based on the attenuation of the Regional Maximum Flood hydrograph.  Energy dissipation of the spillway overflow is by means of a row of individual splitters above a continuous step.  Further erosion protection is provided immediately downstream of the spillway section by concrete aprons with baffle blocks.

The outlet works provide for multi-level intakes into a dual 2m diameter pipe system with downstream sleeve valve control.  The selection of the level of intake is by means of butterfly valves, based on water quality monitoring in the reservoir.

The outlet pipe system is sized to release the estimated short term peak downstream demand of 33 cubic metres per second with the water level in the reservoir at a very low, i.e. 27m below full supply level, wigh only 1,5% of the gross storage capacity remainin.  Should emergency emptying of the reservoir be necessary for any reason, it could be done within a period of about 2 months for a full reservoir, with a maximum combined release buy the dual pipe system of about 75m3/s at full supply level

Extensive preparatory work was required to prepare the alluvial plain on the lower left bank for embankemtn construction.  Drilling of a line of thirty eight pressure relief wells at 20m centres along the downstream toe of the embankment was followed by densification of 72 500m2 of low density
cohesionless material by means of dynamic compaction.  An 800m long portion of the cutoff below the relevant part of the embankment, where it passes through the alluvial deposits, was constructed as a grout curtain by means of the tube-a-manchette grouting process.


The Instrumentation System Installed at Driekoppies Dam includes:

1. hydro/meteorological stations;

2. measurement of deformations along the embankment using precise survey
methods;

3.  monitoring of water levels and seepage flows at the downstream toe
drains and the pressure relief wells;

4. geotechnical instrumentation at two sections along the left embankment
(deformations and pressures) and

5 measurement of pressure at the interface between the left embarkment and
the concrete face of the left tongue wall.