Natural ground catchment and Open water reservoir

Natural pond with lining, Ethiopia. Photo: iDE.

Large open water ponds are useful in storing rainwater. Natural depressions (pans) also hold rainwater in a similar way but are not modified or designed. Ponds described in this section include those that are either excavated and/or which might make use of the natural topography, and which in most cases involve an embankment around part of the pond to retain the water (the material for which may have come from excavation works). They come by different names in different countries, but names include johads and “hafirs”. These reservoirs can also be formed in existing seasonal water courses or valleys, in which case they may also be called valley dams, which are essentially the same as gully plugs (check dams). They can have limited to high aquifer recharge capacity – for ponds purposely built to increase groundwater recharge. Ponds are excellent in storing surface water for various uses (e.g. irrigation, livestock), although they may also recharge groundwater. Ponds can be lined as well as unlined.

Suitable conditions

• Base of the pond should be impermeable, e.g. unfissured rock or clay, to save costs and prevent having to find a form of lining.
• Minimize excavation – use natural or man-made topographical features, e.g. borrow pits from road construction, or sloping ground.
• Find areas of high intensity rainfall, which leads to high runoff, so ponds fill with water rather than infiltrating into the soil.
• Build small reservoirs (5-10 ha) in large watersheds – when built with a good spillway, there is no problem and reservoirs fill up quickly. Siting in this case is best determined by proximity to a village, topographical geometry or presence of roads/access. Hydrology comes into play in the design for larger reservoirs (>15 ha). However, when constructing valley dams specifically (those in a seasonal watercourse), the rule of thumb is not to build small reservoirs (below 10,000 m3) in catchments larger than 400 ha (1,000 acres), otherwise the amount of overflow is excessive to the point of creating washed-out spillways.
• In pastoralist areas, it might be good to site ponds in areas where traditionally pasture is used first after the rains. In this way, as much water as possible can be used to cover water demand before it is taken by seepage and evaporation, leaving other sources with less seepage and evaporation, e.g. sand dams, to be used later on in pasture accessed during the dry season.

Advantages

Disadvantages

- Even if using a pond for direct water use, ponds nevertheless recharge into surrounding ground and can recharge wells around the pond so there is continued water after pond dries up.

- Silt up very easily due to lost vegetation cover in catchment area, leading to soil erosion under intense rainfall and high run-off volumes. De-silting takes time and money. - Maintaining dams requires communal effort and communal institutions might not be strong enough - High combination of evaporation and seepage rates means that water in ponds does not last very long, e.g. 4-6 months in India. - Vectors can breed in open water - Microbiological and chemical water quality is likely to not be acceptable for direct consumption - High cost of construction

Resilience to changes in the environment

Drought

Effects of drought: Tend to dry up quickly, especially if unlined; Conflict over water for animals.
Underlying causes of effects: Lack of rainfall; High evaporation rates; High seepage rates through base of pond and through dam; Storage not sufficient for demand – silting up of ponds due to high silt load, high level of work in constructing ponds.
To increase resiliency of WASH system: Reduce evaporation & seepage; Follow proper construction methods; Reduce siltation so there can be more volume capacity; Promote private ownership of ponds, so that de-silting process more likely; Improve access to low-cost loans with long-time repayment conditions so that farmers can replicate technology; Phased construction until capacity is sufficient for water demand.

More information on managing drought: Resilient WASH systems in drought-prone areas.

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: When the community assembles to plan and build the pond, water user groups are recommended to be ethnically homogeneous or female homogeneous as they might work better for longer term sustainability.

Size the pond

Small, natural pond for rainwater harvesting.
Photo: CTA Rural Radio Packs.

Pond size (and therefore dam height) can be decided according to water demand, evaporation & seepage losses, length of critical period and average stream flow according to the following:

• Determine water requirement (R litres/day)
• Estimate area of reservoir (A m2), evaporation & seepage losses (E mm/day) and therefore the volume losses per day (A x E litres/day)
• Estimate length of critical period during which stream flow is less than water requirement & losses (T days) and when water requirements are met by using water from reservoir
• Estimate average stream inflow during critical period (Q litres/day)
• 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 normal 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) / 6.256
• Height of dam will be D plus 1m (for flood level & safety margin)

Build the pond

Research from Ghana suggests that for new dams, any land ownership issues should be solved prior to construction. Smaller scale dams owned privately might have more chance of success in terms of the participation in construction and maintenance processes.

• Small dams tend to fail much more frequently than larger dams, and this seems due to poor siting and design, lack of design, poor construction techniques and lack of maintenance. One example in Sudan demonstrates this where breached dam embankments were attributed to a gross underestimation of the runoff volume, as well as poor overall design. Proper design, construction and maintenance are therefore important. The following are guidelines used for hillside dams maximum 3m high, where water is retained by an embankment. For heights over 3m, other guidelines are available.

1. Material used for the dam wall should be impermeable. It should have a high clay content (55% minimum), as long as cracks do not form which would induce piping and leakage. The following materials are to be avoided: organic material including topsoil and that with roots/stones, decomposing material, material with high mica content, cracking clays, calcitic clays, fine silts, schists and shales, and sodic soils (high sodium concentration). Piping is often a major reason for structural failure of dams and can be recognized by increased seepage rates, discoloured seepage water, sinkholes on or near the embankments and whirlpools in the water.
2. The dam should have a cut-off (minimum 2.5m wide) which locks it into the subsoil foundation.
3. Strip topsoil away from dam foundation since it contains organic matter.
4. Dam material to be laid in 100-200mm deep layers and compacted (with roller or vehicles/animals) when at optimum moisture content (when material can be rolled to pencil thickness without breaking, yet is as wet as possible without clogging roller).
5. During construction, an additional 10% is added to the design to allow for settlement after construction.
6. Upstream slope should be 1 : 3, downstream slope should be 1 : 2.5.
7. Design should prevent overtopping of dam crest. Water level should be 1m less than dam crest, e.g. for 3m high dam, normal water level (known as D) should be 2m high, leaving 0.5m for floodwater level (height of spillway) and at least another 0.5m as a safety margin for water rising due to wind/wave action and wear and tear on the dam crest.
8. The dam crest should be 10% higher at the centre (convex shape) so that in case of catastrophic overtopping, water will escape from the edges which will require less repair work.
9. Crest width to be 3 metres minimum. For dams over 3 metres, width needs to be greater (4 metres minimum). The crest needs to have a slope of 1 : 50 from downstream to upstream side of crest.
10. Dam embankment needs to be protected both upstream and downstream. This can be done by covering with topsoil and planting spreading grasses (e.g. couch, star or Kikuyu grasses) to protect against erosion. In arid and semi-arid areas where grasses may not grow without irrigation, it has been suggested to cover the embankment with graded rocks (riprap) with maximum size of 600mm.
11. Protect upstream slope: a floating timber beam secured 2 metres from dam will do this (needs to be replaced every 10 years), also stone or brush mattress on upstream slope will reduce erosion. Graded rocks (riprap) has been also suggested to protect the upstream slope, with maximum size of 600mm.
12. A rock toe drain will help to collect seepage water (which is inevitable with all earth dams) – this is built up to 1/3rd the dam height with a graded sand/gravel layer separating the dam material from the rock toe (to stop clay particles being washed out).
13. The spillway outlet needs to be made robust enough to resist erosion (see section on siting). It can be made from concrete, but a cheaper way is to use a grassed spillway. If grass will not grow well, riprap (graded rocks) can be used. Velocity not to exceed 2.5 m/s. Spillway inlet widths vary according to the flood flow, but minimum width to be 5.5 metres. The spillway needs to be kept clear from debris as this has resulted in overtopping in the past.
14. The spillway channel should not allow erosion of the dam structure, and ideally should be lined, with walls to channel the water in the right direction. In place of lining, grass again will suffice – short perennial grasses (e.g. Kikuyu grass) planted in contour lines with 30cm spacing will resist erosion, or another way is to build low stone masonry walls at 2 metre spacing which can act as a staircase to slow down floodwater. The end of a lined spillway channel needs to have a cut-off down to solid ground or should terminate on rock, in order to prevent undercutting of the channel. Spillway slope should be 1 : 33.

• The cheapest form of excavation is where oxen are used.
• Phased construction might provide a manageable way for users to construct their own ponds, whereby each dry season the pond is deepened until experience shows that capacity is sufficient for water demand. For hillside dams with a retaining wall, the wall height and thickness will need to be designed though accordingly.
• A large number of small reservoirs designed to hold water have high seepage rates (up to 24mm/day), so this is important to know for design purposes. However, seepage is often disregarded in design calculations as it is difficult to quantify. A field method to determine seepage rates in the bottom of reservoirs has been developed which can be used to assist in design. While in general it may be better to design for extra seepage loss in pond volume, seepage can still be reduced by:

1. Covering the pond base with clay soil and compacting it with vehicles or animals. Addition of powdered anthills or lime is said to make this lining more robust.
2. Large open reservoirs have been lined in the past with natural or artificial liners, but it is expensive and the lining material is prone to damage by cattle and ultraviolet light, not to mention when desilting is required.

Water extraction and treatment

For open water catchments, like from rock surfaces or water stored behind earth dams, direct abstraction (using pumps or pipes) 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.
• Open waters are subject to cyanobacteria, due to too many nutrients in the water. The start of the rainy season is the most likely time for cyanobacterial proliferation. Cyanobacteria can be harmful to human health and can cause minor disorders such as headaches or lethal deterioration of hepatic functions and promotion of liver cancer. However, the impact of cyanobacteria has largely been neglected in developing countries due to lack of expertise and inefficiency of monitoring programmes (if they exist). For open waters 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.

• Microbiological and chemical water quality is likely to not be acceptable for direct consumption.

1. Water is likely to have a high microbial content due to runoff from contaminated land, as well as communal access to the water by humans and animals.
2. Where runoff is from agricultural areas, there is a possibility of pesticides and fertilizers entering the pond water and sediments – some of which have harmful effects for the aquatic environment and human health. Tests in Côte d’Ivoire showed that in pond water originating from runoff via vegetable plots, levels of pyrethroid compounds were significant. Lack of information and awareness, combined with lax legislation means that many different chemicals might be being used in agriculture.

There are several things that can be done to reduce risks by reducing nutrient loads entering the reservoir – such as rehabilitating vegetation in the runoff zone which can use some of the nutrient enriched water before it enters the reservoir.

Maintenance

Siltation is probably the greatest risk of failure with ponds and dams. The idea is to keep silt out in order to reduce the need for subsequent desilting, and to have desilting mechanisms and institutional arrangements that actually work.

1. Keeping a good cover of indigenous grasses in the run-off area seems to prevent silt build-up. Kambiti Farm in Kitui District provides a good example of previously degraded land being managed and where open dams did not silt up due to pasture management. Contour lines with trees or grasses in the runoff area also work.
2. If the inflow channel is defined, silt traps can be tried out to reduce silt load as is done with Charco dams in Tanzania. In this case, stones laid across the channel form mini dams and perennial vegetation can be grown between these mini dams to reduce flow velocity of water, thereby encouraging silt deposition.
3. De-silting will most probably need to be carried out at some stage. There may be more sustainable ways of doing this compared to the usual approach used in the recovery stage of DCM, where this process is often paid for by NGOs and where there is a lack of community will to contribute. While animals seem to be a good option for effective desilting, food-for-work or cash-for-work incentives are commonly still needed to entice communities to improve their own ponds. It is better to train only a few animals for desilting work to save damaging the equipment, but farmers tend not to want to use their animals to work on someone else’s land. This lack of ownership in communal projects is a recurring thread of failure in WASH projects, and should require new and innovative ways to engender ownership and management of facilities. An institutionally-resilient way to de-silt ponds may be to promote ponds on private land, where one landowner has a vested interest to maintain and desilt the pond, thus reducing the need for NGO intervention in the longer run.

Other considerations

• High evaporation rates are common with open water in certain areas, depending on the climate. Evaporation estimates may be higher than the real situation though - land-based pan evaporation measurements usually exceed reservoir evaporation due to the extra energy a pan receives through its sides and bottom. Even so, water lost to evaporation can be considerable. Some ways to reduce this might include:

1. Digging deeper to have a larger volume to surface area ratio. The Charco dam from Tanzania incorporates this through hemispherical design. The problem might be greater levels of investment needed with increased depth. Experience digging reservoirs in Sudan using food-for-work showed that the deeper the dam, the higher the food ratio.
2. Planting trees around the pond will act as a windbreak, thereby reducing evaporation.

• Construct pond during the dry season.
• Fish can be introduced to eat mosquito larvae, while at the same time providing a source of nutrition. Mudfish are a good option as they can survive dry periods in the silt at the base of pond.
• Experience from South Africa indicates that access to finance seems to be important in allowing farmers to implement ponds.

Costs

High cost of construction – in Sudan, a hafir 80m x 60m x 3m deep (14,400m3) for 400 beneficiaries cost US$8,000. The hafir was completed in 3 months with 190 diggers, did not use food for work but spent the money on tools and installation of inlet/outlet.

Field experiences

Private ownership and Desilting
Experience in India seems to support this where the farmer providing the land for the johad (pond) would be the prime beneficiary, of the recharged water on adjacent land, but where the community also benefited. Experience from Somaliland showed an example of a successful balli which was privately owned where the owner sold water to the community – while this might at first seem exploitative, it was one of the ballis that continued to function every year. Experience from Bolivia backs this up, where farm ponds constructed for communal use often encountered problems of ownership and maintenance, whereas individually owned ponds proved a better option. In Zimbabwe, communities using dams commented that it was difficult for even committed members of the community to work on maintenance tasks as there was little return for work that benefited everyone.

Example from Kitui District in 1979 showed that 43 out of 59 open dams were silted up or broken.

Ox-drawn plough for desilting

The use of ox-drawn ploughs and scoops for desilting ponds in rangeland areas is both feasible and inexpensive in Addis Ababa. The method should prove directly beneficial to and applicable by pastoral groups.

Since 1967, an estimated US$50 million has been spent on pond/dam construction in East Africa and most of them are now abandoned due to heavy siltation. In Ethiopia, the siltation rate in the southern rangelands was estimated at 1,000/2,400 cubic metres per pond, per year. Local pastoral groups were unable to excavate more than 15 to 20 cubic metres by hand per year.

According to the International Livestock Center for Africa (ILCA), it appears possible to construct small to medium-sized ponds and to maintain or enlarge existing ones using ox-drawn scoops and ploughs. In an experiment, ILCA researchers used five metal scoops with a capacity of 0.2 cubic metres each. They were made in Ethiopia at a cost of US$150 each. Six ploughs of the local "Matesha" design-a simple ox-drawn aid - were also used.

The oxen worked in pairs for five hours a day. Two pairs were used in two shifts every day. They were fed two kgs daily of a mixture of bran and oilseed cake (noug) and grazed for seven hours. The feeding of concentrates was considered necessary because drought had reduced grass cover to a minimum. All oxen were watered every three days, which is a common practice. Seven ponds were excavated in 81 days. The amount of silt taken away totaled 3,254 cubic metres. The average volume of silt excavated per ox-pair per day was 13 cubic metres in 62 loads. The cost of excavation by scoop totaled US$ 0.30/0.40 per cubic metre. Excavation costs using heavy earth moving machinery were about US$2.00 to US$3.00 per cubic metre.

Manuals, videos, and links

• Small reservoirs tool kit Edited by Marc Andreini, Tonya Schuetz, and Larry Harrington. Small Reservoirs Project. Topics include: intervention planning, storage and hydrology, ecosystems and human health, institutions and economics.

• CTA: Rural Radio Packs. Interviews in mp3 and transcript format about rainwater harvesting techniques.

• IDE. Provides materials like plastic liners and drums for various rainwater harvesting methods. Serves Africa, Asia, and Latin America.

Acknowledgements

• CARE Nederland, Desk Study: Resilient WASH systems in drought-prone areas. October 2010.