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

Revision as of 17:48, 6 April 2012

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

• The base of the pond needs to be impermeable, e.g. unfissured rock or clay, in order 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.
• Site in areas of high intensity rainfall, as this leads to high runoff, causing ponds to fill with water rather than water 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.

Construction, operations and maintenance

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

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 de-silting is required.

Maintenance

Siltation is probably the greatest risks for failure with ponds and dams. The idea is to keep silt out in order to reduce the need for subsequent de-silting, and to have de-silting 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 de-silting, 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 de-silting 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 de-silt 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.

• 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.
3. Water in ponds is also prone to cyanobacterial proliferation. Cyanobacteria can be harmful to human health and can cause minor disorders such as headaches, through to 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). It seems that the start of the rainy season is the most likely time for cyanobacterial proliferation. 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

• 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

Field experiences

Private ownership and De-silting
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.

Reference manuals, videos, and links

Acknowledgements

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