Sand dams
Practical Action
The second step is to obtain the required tools
and materials. Essential materials include
waterproof Portland cement, nails, binding
wire, timber, sand and stones, which are
sometimes readily available along the
riverbeds. Other equipment includes spade,
sledgehammer, shovels, wheelbarrows, power
mixer and water containers. Where stones are
readily available, masonry sand dams are
recommended; otherwise concrete walls are
equally strong and durable. To construct sand
dam a deep trench is first dug across the valley
Figure 1. Section of the concrete
wall or stream, reaching the bedrock or other stable
layer like clay. (see figure.1&2). To cut costs, local labour should be mobilized and involved in
this process. A concrete or masonry wall is then built on the underlying rock bars across the
river channels so that it can trap and hold back the sand brought by the river during flooding.
The height may range between 2 to 5m high depending on the depth of the underlying rock or
other stable layer.
At either end of the dam especially where the valley sides are flat wing walls may be added at
an angle to the main dam to direct and confine the flows of channel as the sand stores water
in its spores. Since the natural sorting and deposition of sediments in streams is a function of
channel slope and the shape of channel cross section, channel geometry is quite important in
sighting the prospective sand dam. While channel slopes may vary in different valleys and
regions a slope of between 1 and 2% normally gives the highest water storage. (see figure 2).
The specific storage normally increases at the lower slopes than the higher ones.
Figure 2: Structure of sand dam construction
After the construction of a sand dam, a new channel cross section is created together with new
gentler channel slope immediately upstream of the dam. The modified channel must safely
pass the highest expected flood without overflowing the banks and threatening the bank
abutments. In addition to rock outcrops for firm foundations, high riverbanks are another
desirable feature. Where banks are low the dam has to be raised on either or both sides and
wing walls extended beyond the banks in order to direct floodwater and prevent it from cutting
around the dam.
Through proper sand dam sighting substantial volumes of water of up to 6000m3 would be
available for domestic and agricultural use thereby solving the perennial water crisis
experienced in ASALS.
2