SAND DAMS
FEASIBLE RAIN WATER HARVESTING
TECHNOLOGY FOR ARID AND SEMI ARID
LANDS.
Introduction
Drought is the most serious natural hazard facing Eastern Africa in terms of severity and
frequency of occurrence. The most seriously affected areas are Arid and Semi Arid Lands
(ASALS) that face frequent reduction of water or moisture to significantly below the normal or
expected amount Pastoralists and agro pastoralists who occupy this vast region barely meet
basic water requirements. Consequently they suffer from livelihood losses, hunger, diseases,
conflict and internal displacements. The worst affected are women and children who may have
to walk all day long in search of water.
Due to limited and unreliable rainfall most rivers are ephemeral seasonal sandy bed streams
and only experience heavy water run-off for short periods of time after rain. During such
periods of high flows, large quantities of sand are transported downstream while others get
trapped on the upstream sides of rocks ledges along the stream. Such sand traps form natural
aquifers that are capable of providing clean adequate water if well harnessed. Using
appropriate technologies this can be exploited for water storage in the form of sand dams.
During the dry periods pastoralists and agro pastoralists get water for themselves and livestock
by scooping into the sand beds of the dry streams at upstream sides of ledges cutting across
the channel. Water in such sites is usually clean for drinking but quite finite and quickly gets
depleted. Sand dams are an artificial enhancement of this traditional practice that puts extra
water into these sand beds to recharge and store water for use. A concrete wall is constructed
across the channel at specific sites to trap and hold back the sand during flooding; this
creates an additional sub surface water bank for harvesting. With proper siting the total
amount of water available in the sand dams can be over 6000m3. Sand dam technology is not
new. In Kenya, it has been used with good outcome in Kitui, Machakos and Samburu districts.
Other countries with similar dry environments such as U.S.A, Thailand, Ethiopia and Namibia
have also used it in one form or the other.
Sand dam: Technical Description
The first step is to carry out a site survey, which involves analysing the geological and physical
characteristics of the site, especially the underlying rock structures and soil properties.
Riverbeds with crystalline rocks and coarse sand have higher yield compared with volcanic
rocks. Similarly, river valleys and regions sloping between 1 and 2% are ideal sites for sand
dams as they normally give the highest water storage Knowledge of hydrological data is
important for estimating the total stream flow, size of river transportation thereby influencing
the thickness and height if the wall. Information on geological and topographical
characteristics and even hydrological data can all be sourced from relevant Government
departments.
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