1.0 WHAT IS A SAND DAM
Sand dams are fantastic water resource solutions in dry lands. Climate change is exacerbating water stress worldwide. Rural dryland communities already struggling to meet water demand will face increasing water shortages under climate change (Huang et al., 2017). Water harvesting techniques, such as roof tanks, dams, and rock catchments are a popular way to tackle climate change-driven increases in rural water insecurity. Sand dams are one such type of water harvesting used in drylands.
A sand dam is a reinforced embankment (built of precast concrete) or a masonry wall across a sandy seasonal river that flows during the rainy season and runs during the dry seasons. The walls of the sand dam are built across a dry riverbed. As rain falls, it is collected behind the sand dam wall, where it is naturally filtered and stored by the sand in the river bed. As the river flows, it transports more sand along with it, which collects upstream of the dam and in turn creates more storage capacity for water. The water is stored safely in the sand where it is protected from evaporation.
In good quality sand, the sand dam volume is approximately 35% water (Beimers et al., 2001). Most of this water does not evaporate as it is protected by the sand. Evaporation decreases by 90% at 60 cm below the surface (Borst et al., 2006). In addition to water been protected from evaporation, sand dam also increase the quality of stored water (Quinn et al., 2018a; Graber Neufeld et al., 2020), improve overall ecosystem health (Manzi and Kuria, 2012; Ryan and Elsner, 2016; Eisma and Merwade, 2020), and improve the well-being of the local community (de Bruijn and Rhebergen, 2006; Lasage et al., 2008; Pauw et al., 2008; Maddrell, 2010).
The volume of groundwater available for abstraction and the period in available groundwater are increased efficiently using sand dams.
1.1 CHARACTERISTICS OF SAND DAMS
- Sand dams consist mainly of a reinforced concrete or masonry wall
- Sand dams are built one to five metres high across a sand river
- Sand dams are mainly built on completed subsoil or a bedrock
- Sand dams recharge aquifers beneath it which provide enough water to establish tree and vegetable nurseries.
- Sand dams are easy to maintain with a low maintenance technology and last for many years.
- Sand dams are simple and require low cost for construction.
- Sand dams are mostly constructed in areas of dry lands
- Sand dams are built across a dry riverbed
- Water is stored in the pores of sand
- Sand dams consist of flat wings walls at either end of the dam which at an angle to the main dam which at an angle to the main dam to direct and confine the flows of the channel.
1.2 WHERE IN AFRICA ARE SAND DAMS COMMONLY USED AS A SOLUTION TO WATER PROBLEMS?
The availability of surface water resources in the sub-Saharan region would be adversely affected by global and regional research. Severe droughts and dryness in most places are expected to have the effects of climate change. In the region, such effects are already apparent and include minor streams drying. Current climate change projections in East Africa indicate a trend toward increased rainfall that is concentrated in fewer, more intense events (Bank, 2013).
In East Africa over a decade around 1,000 sand dams have been constructed where they are commonly used. They are most common in Kenya. Also, similar structures are found in Angola and Ethiopia, Namibia, Mozambique, Tanzania, Somaliland, Yemen, Burkina Faso, Mali, NW Cameroon, Sudan, Turkey and Mexico.
The primary function of a sand dam is increasing the water availability by storing water in the riverbed.
Sand dams can be classified according to their construction material (Negassi et al., 2002):
- Stone-masonry dam: A dam constructed of concrete blocks or stones. This kind of dam can be built by local craftsmen. This kind of dam is durable and ideal for any dam height. The dam is cheap when construction materials are available within the dam area.
- Reinforced concrete dam: A dam comprising a thin, reinforced concrete wall. It is a sturdy, relatively costly structure that is ideal for any dam height.
There are four pre-conditions for a suitable sand dam site:
1. A sand dam must be sited on an ephemeral or seasonal river with clearly defined riverbanks
2. The riverbed must be sufficiently impermeable as to retain water
3. The river must have sufficient sandy sediment
4. A sand dam must be sited where the bedrock or a suitable impermeable layer is accessible, usually no more than 3 metres below the existing riverbed surface.
1.3 ADVANTAGES OF SAND DAMS
Sand dams provide key advantages which includes;
- Contamination decrease (by livestock and other animals)
- Inadequate for mosquito breeding and other insects (malaria)
- Filtration of water flowing through the riverbed sand (disinfection)
- Inexpensive structures with a high level of community involvement.
- Sand dams transform the environment as the stored water raises the water table level both upstream and downstream from the dams (Brandsma et al., 2009; Frima et al., 2002). As the aquifer increases in size, wells and boreholes have more water and springs may return to the area.
Sand dams ensure that communities close to their homes receive reliable, safe water throughout the year. The sand dams elevate the local water table, which means it is a particularly effective way of restoring land and allowing for the growth of vegetation.
2.0 USES OF SAND DAMS
- For communities living in isolated, rural locations, the sand dams provide an enhanced, local and reliable water source.
- Sand dams improve access to water for people, livestock and crops in water stressed environments.
- They help transform the local ecology
- Sand dams help to recharge aquifers.
Taking into consideration the amount of constructing a sand dam to a borehole or well, sand dams are cheaper than boreholes or wells. In areas affected with this situation, a short survey was done and it was computed that “A sandy dam typically costs $2,900 – the work of citizens and construction personnel usually costs approximately $30,000 while drilling a single borehole in the area.” By providing their own labour and sourcing building materials locally, rural communities should be able to develop a sand-storage dam at a lower cost than that of establishing a borehole. It is therefore suggested that sand-storage dams are a more economically feasible method of rural water supply than boreholes
In terms of storage capacity, sand dams and surface dams generally follow the same principle. Sand dams are usually employed when the topography gradient is high and when the topographical gradient is low, subsurface dams are employed. The storage capacity of the basin will be limited if the impermeable layer is near the surface. A sand storage dam will expand storage and a subsurface dam will only take advantage of storage capacity below the surface. The storage capacity of a sand storage dam can easily increase when the river is narrow and has high embankments. The embankment offer an excellent probability of anchoring.
Sand dams make use of simple inexpensive technology and therefore the dam can be constructed by local communities mainly with locally available materials. Sand dam construction and use requires far less technological input compared to the borehole construction, operation and maintenance requirements.
According to PRACTICAL ACTION, the water is clean and of good quality for consumption due to the filtering effect of the sand. Concerning sand dams. A number of sand-storage dams have cumulative effects, which will reduce downstream surface and sub-surface flow continuity. In addition to a projected reduction of the local water table levels, a similar impact can be expected for a number of boreholes drilled into alluvial aquifers. Since sand dams are projected to be below the volume of a bore hole established in a well-defined alluvial or secondary aquifer, they are expected to have less environmental impact than those of borehole storage dams.
Although sand-storage darns appear to be the more favourable method of rural water supply, they have two drawbacks which limit their ability to be developed as an alternative to boreholes. They are as follows:
- The first is the amount of time needed to achieve the full storage potential of a sand dam.
- The second concern is that the fixed storage volume of sand darns is likely to be smaller than that of boreholes established in well-defined alluvial or secondary aquifers, and as such may not meet the water requirements of many rural communities.
Areas of drylands in Nigeria are also being affected with insufficient amounts of water for their living. It has been estimated that between 50 % and 75 % of Bauchi, Borno, Gombe, Jigawa, Kano, Katsina, Kebbi, Sokoto, Yobe, and Zamfara States in Nigeria are being affected by desertification. San dams can be constructed in the aforementioned areas for availability of water to enhance the ecosystem of the area.
Contrary to popular belief, West African countries, including those of the Sahel, do not lack water. They are superficial water tables which are refilled, generally during the rainy season, and on the other hand, they are water tables from an ancient base, and lastly, they are deep water tables of sedimentary basins. There are considerable reserves of fresh water stored in these deep-water tables: on a scale of approximately several thousand billion of m3. Therefore, it could be concluded that the status quo of using wells and boreholes in west Africa/Nigeria is well justified.
20 Fun Facts about sand dams
- Sand dams solves seasonal water shortage problems, it also recharges the aquifer
- Sand dams combat desertification through groundwater recharges and the development of sustainable land management opportunities.
- Sand dams mitigate climate change by providing water security and time for climate-smart farming.
- Sand dans help in Conflict reduction in the water-scarce dryland environment by providing availability to water for people and animals.
- Sand dams support catastrophe resilience through the development of a drought buffer to boost food production in vulnerable individuals.
- Sand dams enable shallow wells to produce safe drinking water.
- Below the surface of the sand held behind the wall, 20-40% of its volume is in fact water.
- A sand dam holds 2–10 million litres of water and is the world lowest cost rainwater harvesting solution.
- There are three main ways to abstract water from sand dams; scooping holes, pipe filtration or shallow wells with pumps. The community decides on the most appropriate option for them.
- Sand dam technology is 2,000 years old.
- The average cost of a sand dam that can last as long as 50 years, is about 7,500 USD.
- For settlements of 1,000 in dryland regions of the world, sand dams may collect and store liters of water under the sand for them.
- Sand dams’ success is comparatively easy.
- Sand dams will take between one and two months to get from the start of excavation works to the completion of the sand dam.
- The oldest known sand dam in operation is 100 years old
- It requires virtually zero operation and maintenance cost
- A sand dam does not stop the flow of a river
- Sand dams protect excessive runoff
- In good quality sand, the sand dam volume is approximately 35% water.
- The technique of sand storage dams is not new: storage of rainfall and runoff, including subsurface storage, for beneficial use has been applied since 9,000 BC (HOOGMOED 2009).
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