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Feasible rain water harvesting technology for Arid and Semi-Arid Lands
Drought is the most serious natural hazard facing Eastern Africa in terms of severity and frequency of occurrence. The most seriously affected areas are Arid and Semi Arid Lands (ASALS) that face frequent reduction of water or moisture to significantly below the normal or expected amount. Pastoralists and agro pastoralists who occupy this vast region barely meet basic water requirements and the problem seems like it will only get worse as the climate continues to change. Consequently they suffer from livelihood losses, hunger, diseases, conflict and internal displacements. The worst affected are women and children who may have to walk all day long in search of water, sometimes over 20km. The travel time to collect water could be used for multitude of other tasks that need to be done around the home.
Due to limited and unreliable rainfall most rivers are ephemeral seasonal sandy bed streams and only experience heavy water run-off for short periods of time after rain. During such periods of high flows, large quantities of sand are transported downstream while others get trapped on the upstream sides of rocks ledges along the stream. Such sand traps form natural aquifers that are capable of providing clean adequate water if well harnessed. Using appropriate technologies this can be exploited for water storage in the form of sand dams. Sand dams are being increasingly used to supplement local water supplies in the face of the growing water insecurity because they can recharge after with rain and flood waters; they are able to recharge even with the unreliable rains that ASALS often have.
During the dry periods pastoralists and agro pastoralists get water for themselves and livestock by scooping into the sand beds of the dry streams at upstream sides of ledges cutting across the channel or by installing pipes near the base of the dam for extraction by gravity. Hand pumps are sometimes used but less often because they are more costly than extracting the water by using gravity. Water in such sites is usually clean for drinking, since it is filtered by the sand, but quite finite and quickly gets depleted. Sand dams are an artificial enhancement of this traditional practice that puts extra water into these sand beds to recharge and store water for use. A concrete wall is constructed across the channel at specific sites to trap and hold back the sand during flooding; this creates an additional sub surface water bank for harvesting. With proper siting the total amount of water available in the sand dams can be over 6000m3. Sand dam technology is not new. In Kenya, it has been used with good outcome in Kitui, Machakos and Samburu districts. Other countries with similar dry environments such as U.S.A, Thailand, Ethiopia and Namibia have also used it in one form or the other.
Rain and flood water harvesting has its roots in ancient technologies but now the technology is being put to use to help mitigate water scarcity in arid and semi-arid lands. The technology has been used in New Mexico and Australia but it is especially being used in Africa, Ethiopia, Tanzania, and Kenya in particular. There are projects going on that are trying to implement this technology in other arid and semi-arid lands as well.
Sand dam: Technical Description
The first step is to carry out a site survey, which involves analysing the geological and physical characteristics of the site, especially the underlying rock structures and soil properties. It is ideal to have an impermeable or less permeable surface beneath the channel so that the water is stored rather than lost downstream. Riverbeds with crystalline rocks and coarse sand have higher yield compared with volcanic rocks. Similarly, river valleys and regions sloping between 1 and 2% are ideal sites for sand dams as they normally give the highest water storage due to the lower gradient of the channel bottom. The lower gradient keeps the water in an area longer because it does not want to flow as quickly downstream. Knowledge of hydrological data is important for estimating the total stream flow, size of river transportation thereby influencing the thickness and height of the wall. The geologic and hydrologic data can also help when calculating the potential storage capacity of the dam. Information on geological and topographical characteristics and even hydrological data can all be sourced from relevant Government departments.
The second step is to obtain the required tools and materials. Essential materials include waterproof Portland cement, nails, binding wire, timber, sand and stones, which are sometimes readily available along the riverbeds. Other equipment includes spade, sledge hammer, shovels, wheelbarrows, power mixer and water containers. Where stones are readily available, masonry sand dams are recommended; otherwise concrete walls are equally strong and durable. To construct sand dam a deep trench is first dug across the valley or stream, reaching the bedrock or other stable layer like clay. (See fig.1&2). To cut costs, local labour should be mobilized and involved in this process. A concrete or masonry wall is then built on the underlying rock bars across the river channels so that it can trap and hold back the sand brought by the river during flooding. This wall can also be made out of blocks and clay, concrete and stone, or stacked flat stones. The height may range between 2 to 6m high depending on the depth of the underlying rock or other stable layer.
At either end of the dam, especially where the valley sides are flat, wing walls may be added at an angle to the main dam to direct and confine the flows of channel as the sand stores water in its pores. Since the natural sorting and deposition of sediments in streams is a function of channel slope and the shape of channel cross section, channel geometry is quite important in sighting the prospective sand dam. While channel slopes may vary in different valleys and regions a slope of between 1 and 2% normally gives the highest water storage. (See Figure 2). The specific storage normally increases at the lower slopes than the higher ones.
After the construction of a sand dam, a new channel cross section is created together with new gentler channel slope immediately upstream of the dam. The modified channel must safely pass the highest expected flood without overflowing the banks and threatening the bank abutments. In addition to rock outcrops for firm foundations, high riverbanks are another desirable feature. Where banks are low the dam has to be raised on either or both sides and wing walls extended beyond the banks in order to direct floodwater and prevent it from cutting around the dam. Once the sand dam wall is finished rocks or gravel are placed on the downstream side of the dam in order to keep the over flow coming over the dam from eroding the downstream side, erosion that close to the dam could compromise the structural integrity of the dam and make it less able to withstand the pressure of the sand and water that are behind the dam.
Through proper sand dam sighting substantial volumes of water of up to 6000m3 would be available for domestic and agricultural use thereby supplimenting the water supply and alleviating some of the water stress experienced in ASALS.
Extent of use
Sand dams are not new in Kenya, over 500 have been constructed there, many in the last 10 years. Traditionally water harvesting at certain points along the dry river has been widely practiced in ASALS with good results but on a smaller scale. Currently a number of sand dams have been constructed to good effect in Kitui, Machakos, and West Pokot. Most of Kenya's sand dams are located in the Kitui district. Practical Action EA through CORDAID funds recently built 3 sand dams along the seasonal Baragoi River in Samburu district and both have yielded good volumes of water for livestock and human use throughout the drought periods. (See figure 2). This technology has also started being implemented in Tansinia and Ethiopia.
Operation and Maintenance.
Once construction is complete further operation costs are negligible. After construction the dam must mature (which can take 2 to 7 years depending on the geologic and hydrologic conditions, as well as other conditions at the location) and then water can start being extracted from the newly created aquifer. The water can be extracted by pipes or pumps. Only the low riverbanks need to be protected against erosion as this might enable floodwater to cut around the dam. And sometimes the channel just beyond the dam needs to be maintained due to erosion. The structure’s lifespan is approximated at 30 years. For the amount of use and the stresses that the dams must endure they last very well; they do not need much maintenance themselves.
This technique is applicable in sandy riverbeds that are seasonally dry but experiences high siltation during water runoffs. Sites with high riverbanks and lower slopes are also desirable. The geology in the location the dam is to be placed has to be considered as well. It is good when there is an impermeable surface or a less permeable surface below the river channel. This will prevent the water from seeping under the dam too much and it gives something that the dam can be placed upon for stability.
Level of Involvement
Sand dam construction is labour intensive and requires community involvement and full participation.
Where as standardized design and wall construction requires specialized advice to ensure it withstand the pressure from water behind it, local materials and skills should be preferred in order to foster community project ownership and to reduce projects costs. Proper consultations and consensus building among the stakeholders are requisite for winning the commitment, participation and contribution of the beneficiaries.
Compared to other water harvesting techniques, Sand dams are environmentally friendly as they control erosion and manage silt deposition within river basins. Its water is clean and of good quality for consumption due to filtering effect of the sand. Sand dams increase moisture infiltration within the soil profile and into the ground water providing both soil and water conservation benefits. These sites can witness quick regeneration of indigenous trees along the riverbanks thereby attracting other biological resources and ecosystems that hitherto faced threats from recurring droughts. Similarly sand dams also allow for small-scale crop production through irrigation that was otherwise not possible. And it has advatages over surface reservoirs, like little to no evaporation, the storage is available for a long time, the stored water is less susceptible to health hazards and pollution, land above the storage can still be used, and the techniques are usually rather socially acceptable.
The technology is labour and physical capital intensive and most local communities cannot implement it without external aid. The sand dam, also takes several years to "mature" or fill in with sand, anywhere from 2 to 7 years. They can also cause higher rates of erosion downstream and sometimes this has to be maintained when it gets too bad. Another disadvantage is that it is difficult to control who, and what, has access to the water in the sand dam. Usually when sand dams are built the community decides that whoever helps build it gets access to the water but this is very hard to control since it is just like a river and anyone can walk right up to it as they please. It is also difficult because cows or other livestock, if not watched as they drink from hand dug holes, could defecate too close or right in those holes used for water extraction. This could lead to contamination of that hand dug hole or of that section of the sand dam.
Development of the Technology
Despite its cultural acceptability, this water harvesting technique has not been widely replicated in other deserving areas probably due to high costs of materials and labour involved, limited technical skills, and that it has not yet been figured out how to up-scale sand dams. NGO's that are involved in constructing sand dams are typically on-site for about 6 months and so only have time to perform a few quick workshops in order to train the community. It has been suggested that if the dam were built in about 3 phases, each phase taking about a year to construct, it would help the technology gain a better base in the community. The dams would have time to "mature" and the people being trained would have a better and more detailed understanding of them. This would help the technology be more widely implemented, except for the extra time that would be required of the people helping the developing community construct the dams.
Water scarcity is perceived as the major bottleneck to development in the ASALS and innovative ways of rainwater harvesting are worthwhile. Sand dam technology that artificially enhances traditional water harvesting knowledge if well sited can bank of up to 6000m3 of clean water for domestic use thereby alleviating water shortages during drought periods. However, the geological characteristics run off patterns of the catchment and sediment transport regime of the river is prerequisite. Simulated river flow series can be used to estimate the size of flows likely to occur over a given period. Unfortunately hydrological data for most seasonal rivers are unavailable thereby rendering many predictions inadequate. For future development of sand dam technology, there is need to monitor rainfall patterns and install river-gauging structures along the streams. And to look at how projects are implemented to make sure the community gets all of the benefits the sand dam has to offer.
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