Natural rock catchment and Open water reservoir
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Rock catchment dams are developed from a rock outcrop to catch and concentrate rainwater runoff into a storage structure for productive use. In arid and semi-arid lands with large rock outcrops, a lot of runoff is generated after rains. Through development of rock surface into a catchment, the runoff can be harvested and stored for domestic and livestock use to alleviate water shortages.
These are naturally occurring catchments of bare rock that have high runoff coefficients (around 0.9). Water can be stored as an open reservoir behind a retaining structure, with storage capacities ranging from 20 – 4,000 m3, or can be stored directly in a covered storage tank that collects water directly from the catchment. The capacity of rocks to supply water is significant, because a rock surface of 1 hectare (10,000 square metres = 2.47 acres) can provide 1,000 cubic metres (1 million litres) from every 100 millimetre of rain. Even low and erratic showers can supply large volumes of water provided the rock area is sufficiently large enough to compensate for the low rainfall.
The Kitui district in Kenya has over 400 rock catchment tanks and dams.
Suitable conditions
- The rock that makes up the catchment should be bare and free of vegetation/soil. It should have no fractures, cracks, or soil pockets that would result in a loss of water through seepage.
- The dam should be built in a site that has a high depth to surface ratio (it is more deep than it is wide) so as to minimize evaporation losses.
- Site the dams for rock catchments to maximize the natural topography – to get the best storage volume, build on gorges between rocks, rock pools, and rock shelves. Make dams on the lower side of existing rock pools.
- Catchment may be enclosed by stone gutters (a stone wall built with rough stone/hardcore, joined with mortar)
- There should be no severe soil erosion in the catchment area
- Masonry dams (gravity or pumped) are more site-specific than tanks and earth dams because:
- Gorges between two rocks can be closed with a straight dam wall.
- Rock pools and rock shelves, the latter being nearly flat areas on rock outcrops, can be surrounded with two-winged or three-winged dam walls.
- Other requirements: availability of materials such as sand, hardcore, and ballast locally; skilled person/mason; labour; a means of transporting materials
| Advantages | Disadvantages |
|---|---|
| - High runoff coefficient is similar to roof catchments, even small showers produce water - Minimal seepage |
- Not many sites suitable - If building tanks that store water directly, storage capacity is limited compared to an open reservoir |
Resilience to changes in the environment
Drought
Effects of drought: Tend to dry up quickly.
Underlying causes of effects: Lack of rainfall.
To increase resiliency of WASH system: Site on rock that is bare and free of vegetation/soil with no fractures, so whatever amount of rain does fall, it will have a greater chance of being collected.
More information on managing drought: Resilient WASH systems in drought-prone areas.
Construction, operations and maintenance
Design and construction tips
- Build the dam in stages if it means it will be more solid
- An out-take, gravity pipe and water tap point should be constructed to abstract water downstream from the dam
- The downstream side of the dam should be protected against erosion in case the dam overflows
- The size of the dam and catchment area should be consistent with the available labour force
- The shape of the reservoir created by the dam should minimize evaporation losses
- Local construction materials should be used and relied on
- The catchment area should be protected against pollution
General advice on cement: A common cause of cracks in structures and linings (e.g. in tanks, dams, waterways, wells) is errors in mixing and applying the cement. First of all, it is important that only pure ingredients are used: clean water, clean sand, clean rocks. The materials have to be mixed very thoroughly. Secondly, the amount of water during mixing needs to be minimal: the concrete or cement needs to be just workable, on the dry side even, and not fluid. Thirdly, it is essential that during curing the cement or concrete is kept moist at all times, for at least a week. Structures should be covered with plastic, large leaves or other materials during the curing period, and kept wet regularly.
Specific advice:
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. Water can also be stored in a covered tank that gets water directly from the rock catchment.
- 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)
- Estimate area of reservoir (A m2), evaporation losses from reservoir (E mm/day) and therefore the volume losses per day (A x E litres/day)
- Estimate length of critical period during which water entering the catchment is less than water requirement & losses (T days), in other words, when water requirements are met by using water from reservoir
- Estimate average rainfall entering the catchment reservoir during critical period (Q litres/day) – this should be rainfall x catchment area x runoff coefficient (usually taken as 0.9).
- Catchment area will decrease with increasing slope (e.g. 23% fewer square metres if slope is 40%).
- Calculate effective storage required (S litres) = (R + AxE – Q) x T
- Site should be then surveyed to estimate area (A m2) of reservoir for different values of water level (D) – this will give reservoir capacity which should be greater than storage required (S) to allow for a safety margin. Reservoir capacity can be estimated by the following: (length x width x maximum depth) / 6.175
- Height of dam will be D plus 1m (for flood level & safety margin).
- 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.
Water extraction
For open water catchments, like from rock surfaces or stored behind earth dams, direct abstraction (using a pump or pipe) works well. Abstraction method should minimize disturbance of the settled water, thus reducing treatment requirements later.
- Direct abstraction is one option, via a bank-mounted pump (Small and efficient motor pumps or Handpumps) which uses a floating intake to reduce sediment intake. An outlet pipe and strainer through the dam wall to the downstream side is another option, but these have potential problems of weakening the dam wall. In addition, piping will have to be secured externally when traversing rocks, so care has to be taken to secure pipes with anchor posts.
- With preventive methods to reduce turbidity (silt trap, extraction method) the water is still turbid & contaminated and will require treatment.
- For direct abstraction, promotion of household water treatment is advocated. Choice of household water treatment technology should be based on efficiency of removing contaminants present in the water. For open water that may be prone to cyanobacterial blooms, a Concrete Biosand Filter is a good choice due to its ability to remove cyanobacterial toxins. Other technologies however may be more suitable for mobile communities (e.g. Sodis or a Ceramic pot filter, depending on turbidity levels). For reservoirs near urban environments or where the runoff area has intensive agriculture practised in its vicinity, diversification of water resources is a good idea to provide alternatives for direct drinking purposes. Strengthening controls and restrictions on use of illegal substances will also help.
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.
Costs
- Range of scale: a range from 20 Ksh per m3 (USD 0.27), for a reservoir volume of 5000 m3 (a valley dam constructed by oxen), to 100 Ksh per m3 (1.37 USD), for a reservoir storage volume of 100 m3
There are 3 types of rock catchment costs to compare:
- Earthen dams are the easiest and cheapest to construct using manual labor or animal draught (yet vulnerable to seepage, erosion, and evaporation).
- Rubble stone masonry dams are cheap to build and maintain (but have large evaporation losses yet no seepage. Evaporation can be prevented by building a roof).
- Water tanks are the most expensive to build (but have no seepage or evaporation losses, only limited amount of water by the size of the tank).
Field Experiences
In Kenya, the tradition of Harambee (to pull together) on communal self-help projects has greatly assisted many villages with the affordable implementation of rock catchment dams. In the semi-arid Machakos and Kitui in Kenya, the Wakamba communities have organized themselves into self-help groups, called Mwethya, many decades ago. They know that by working together they can cope with the harsh climatic conditions. This self-help spirit, combined with lack of fresh groundwater, has been the main drive in the construction of the hundreds of rock catchments built in that region for the last 50 years.
Akvo RSR projects
The following RSR project(s) include rock catchment or other open water reservoirs.
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 Raising awareness on rainwater harvesting |
Manuals, videos and links
- How to Calculate Runoff Coefficient. Rain is a normal phenomenon for many mid-latitude areas. It creates runoff, a term for water movement. A runoff coefficient is a number that relates the rainfall rate and runoff rate. Using the runoff coefficient, scientists and hydrologists can calculate how much water passes over a given area per second. There are two ways you can find the runoff coefficient. One is by using the Rational Method, and the other is by using a runoff coefficient table.
- Friends of Kitui - Open reservoir, water projects.
- Water from Rock Outcrops: A handbook for engineers and technicians on site investigations, designs, construction and maintenance of rock catchment tanks and dams or (alternative link). A full project guide from start to finish, on building a dam in a natural rock catchment. Includes building options, how to measure and calculate, how to organise the community, how to fund raise and make a proposal, costs, and more.
- Water for Arid Land. Lots of manuals for technical methods.
- Water from Small Dams: A handbook for technicians, farmers and others on site investigations, designs, cost estimates, construction and maintenance of small earth dams. Erik Nissen-Petersen for Danish International Development Assistance (Danida), 2006.
Acknowledgments
- CARE Nederland, Desk Study: Resilient WASH systems in drought-prone areas. October 2010.
- Nissen-Petersen, Erik, Water from Rock Outcrops: A handbook for engineers and technicians on site investigations, designs, construction and maintenance of rock catchment tanks and dams or (alternative link). Danish International Development Agency (DANIDA), 2006.
- Profit from Storage: the value of 3R. RAIN, et al.
- RiPPLE (Research-inspired Policy and Practice Learning in Ethiopia). Rock Catchment Dams.
