In general, it seems that smaller scale dams owned privately might have more chance of success in terms of participation in the construction
and maintenance processes. However, using a self-help method of community organising, larger projects can also be successful.
====Preconstruction====
* Water can be stored as an open reservoir – this can be done behind a stone masonry or concrete dam built directly onto the rock, or behind
an earth dam in a non-rocky area at the base of the rock face (for earth dams, see “Natural ground catchment & open water reservoir” fordetails). Water can also be stored in a covered tank that gets water directly from the rock catchment. * Cracks/fissures should be sealed up with mortar or concrete.* Siltation should be reduced by ensuring that the rock catchment is clear of soil and debris, and maintaining it in this state. * Open water in certain areas can have a high evaporation rate, depending on the climate. Some ways to reduce this mightinclude: Siting the dam to best use the natural topography to get the deepest reservoir possible, resulting in larger volume to surfacearea ratio; Covering the catchment or building tanks to collect the water directly.
* Gutters are needed to direct water on the rock catchment towards the reservoir. They can be made from stone masonry using rocks gathered from the catchment during cleaning. Gutters should almost follow the contour but should slope a minimum of 3%. Gutters should be high enough to direct water, but where runoff velocity is very high, some kind of wall structure is needed to slow velocity of runoff before it reaches the gutters.
====Building the dam====
* Reservoir size (and therefore dam height) can be decided according to water demand (to a certain extent depending on size of catchment), evaporation losses, length of critical period and average rainfall according to the following:
#Determine water requirement (R litres/day)
* Dam wall can be built onto rocks with slope of up to 15%, width of dam base must always be 3/5 of dam height, width of crest should be 30cm, special attention should be paid to the rock-wall base as this has potential for leakage, and the upstream side of the dam wall should be rendered with up to 30mm of mortar. Procedures for dam wall construction are given in certain guides.
* Phased construction might provide a manageable way for users to construct their own dams, whereby each dry season the dam is raised until experience shows that capacity is sufficient for water demand. In this case, start with a dam of 2 metres height, then build on it successively to a total of 5 metres in 3 more stages. The advantage of doing this is that you build according to enthusiasm and seeing how much water is stored.
====Maintenance====
* Cracks/fissures should be sealed up with mortar or concrete.
* Siltation should be reduced by ensuring that the rock catchment is clear of soil and debris, and maintaining it in this state.
* Open water in certain areas can have a high evaporation rate, depending on the climate. Some ways to reduce this might
include: Siting the dam to best use the natural topography to get the deepest reservoir possible, resulting in larger volume to surface
area ratio; Covering the catchment or building tanks to collect the water directly.
====Other considerations====
* Access to finance will help to allow farmers to implement dams.
* Fish can be introduced to eat mosquito larvae, while at the same time providing a source of nutrition.