Difference between revisions of "Natural rock catchment and Open water reservoir"

Revision as of 13:08, 29 May 2012

Natural rock catchment on Sullivan Rock, USA.
Photo: Paul Madden.

Rock catchment project in Kitui, Kenya.
Photo: Friends of Kitui.

The technique applied for utilizing rocks for water supply is called rainwater harvesting and the structures built for harvesting rainwater from rocks are called rock catchments. 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 or cracks that would result in a loss of water through seepage.
• 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.
• Masonry dams are more site-specific than tanks and earth dams because:
  

1. Gorges between two rocks can be closed with a straight dam wall.
2. 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.

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.

Construction, operations and maintenance

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 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:

Building a roof for the rock catchment dam to keep it clean and minimise evaporation losses.
Photo: Water from Rock Outcrops. DANIDA.

A rock pool in near Taveta, Kenya.
Photo: Water from Rock Outcrops. DANIDA.

To measure the rock gradient, a spirit-level horizontal on a 100cm length of timber is used. This rock is determined to be too steep.
Photo: Water from Rock Outcrops. DANIDA.

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:

1. Determine water requirement (R litres/day)
2. 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)
3. 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
4. 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).
5. Catchment area will decrease with increasing slope (e.g. 23% fewer square metres if slope is 40%).
6. Calculate effective storage required (S litres) = (R + AxE – Q) x T
7. 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
8. 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

Rubble stones save about 50% of the cost of cement, sand, and ballast.
Photo: Water from Rock Outcrops. DANIDA.

There are 3 types of rock catchment costs to compare:

1. Earthen dams are the easiest and cheapest to construct using manual labor or animal draught (yet vulnerable to seepage, erosion, and evaporation).
2. 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).
3. 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.

Reference manuals, videos, and links

• How to Calculate Runoff Coefficient.
• Friends of Kitui - 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. 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.

Acknowledgments

• CARE Nederland, Desk Study Resilient WASH systems in drought prone areas. October 2010.
• Nissen-Petersen, Erik, Water from Rock Outcrops. Danish International Development Agency (DANIDA), 2006.