Siting the construction of sand dams
Background information on sand dams
Sand dams are a sustainable solution for regions facing water scarcity, especially as climate change impacts become more pronounced. Here’s why they are beneficial:
- Easy to Build and Maintain: Sand dams are straightforward to construct and require minimal maintenance. They consist of a concrete embankment built across seasonal streams that flow during the rainy season but run dry during the dry season.
- Long-Lasting: Once built, sand dams can last for decades without major refurbishment. Their durability ensures a consistent water supply over an extended period.
- Water Source: Sand dams provide a reliable water source in arid regions. They capture rainwater and store it in the sand, making it accessible throughout the year—even during dry seasons when water is scarce.
- Beneficial for All Income Levels: While sand dams benefit people of all income levels, they are particularly advantageous for low-income households, disadvantaged communities, and women.
- Local Collaboration: These dams are constructed in close collaboration with local communities. The project provides necessary materials like cement and steel, while the community contributes natural materials such as sand and stones.
- Climate Adaptation: Sand dams help communities adapt to climate change by ensuring water availability for both people and livestock. They reduce the time needed to collect water, allowing community members to focus on other activities.
- Ecological Impact: Sand dams raise the water table around them, benefiting natural vegetation and biodiversity dependent on aquatic ecosystems. They also conserve ecosystems by providing a sustainable water supply.
However, several considerations to be taken before constructing a sand dam:
- To identify if a sand dam is a suitable solution, information on the river must be collected, particularly river width and depth to bedrock. The concrete dam wall needs to be anchored onto the bedrock. Excavation must be done until solid bedrock is reached to anchor the wall.
- The upper and middle courses of a river, at least 4km from the head of a valley are the most suitable areas due to high course sediment load and a reasonable riverbed gradient of <5%.
- The construction of a sand dam needs a lot of manpower. It needs to be constructed in coordination with the dry season Jan-Feb or June - Sept.
- It takes a few years for the sand dam to fill up, as a few seasons are needed for the sand to build up (sedimentation process).
- The remote sensing data that can be used needs a resolution suitable to the river width. Sand dams are typically on rivers between 5-50 meters in width.
Construction of a sand dam in the Samburu County
When constructing a sand dam in Samburu County several important factors must be considered to ensure its effectiveness and sustainability:
- Site Selection: Choose a suitable location along a seasonal riverbed or stream. The site should have a consistent flow during the rainy season and dry up during the dry season.
- Assessment of the geology and soil conditions to ensure that the sand and rock layers are suitable for dam construction.
- Hydrological Assessment: Study the local hydrology, including rainfall patterns, runoff, and stream flow. This information helps determine the dam’s capacity and water storage potential.
- Design and Construction: Design the dam’s dimensions based on the expected water flow. The dam should be wide enough to capture sufficient sand and water.
- Construct a concrete wall across the riverbed, reinforced with steel bars. The wall should extend into the riverbanks.
- Create a spillway to allow excess water to flow downstream during heavy rains.
- Use locally available materials (such as sand, stones, and cement) to build the dam.
- Sand Storage: The primary purpose of the dam is to store sand and water. As water flows, it deposits sand behind the dam.
- The sand acts as a natural filter, allowing water to percolate and recharge the groundwater.
- Maintenance and Monitoring: Regularly inspect the dam for signs of erosion, cracks, or damage. Train community members to perform minor repairs and maintenance.
- Long-Term Impact: Consider the broader impact of the sand dam on the ecosystem, vegetation, and wildlife.
- As in the Samburu County most rivers are seasonal, the seasonality of the existing rivers needs to be assessed. Furthermore, the sediment load needs to be assessed; the river must be composed mostly of medium-coarse grained sand and low clay/ silt content
- Cost to create a sand dam in Kenya is about 6000 - 8500 EUR - sometimes provided 50 / 50% by NGO and community - used for materials.
For the construction of sand dams, the elevation of the area (DEM) of interest and the differentiated vegetation index (NDVI) are crucial required data:
- Digital Elevation Model
For the construction of the sand dams several data are required:
- River width: determines the optical data that can be used (due to spatial resolution)
- Sediment load of the river
- Depth to bedrock / rock outcrops in bed and bank: Ideally don’t want to have to excavate >5m deep
- Precipitation data
- Existing scoop holes on the river existing far into the dry season (indicates good water storage already)
- Vegetation on the banks (indicates localized water source)
- Slope of riverbanks (shouldn’t be too shallow)
- Slope of riverbed (ideally 1 - 5 %)
- Stream length
- Catchment area (from DEM)
- Location of faults and fractures
Finally, for the implementation of the construction plan the engagement of the community is needed to find local partners, such as NGOs who are giving out the equipment and to identify community members, that are willing to help in the building of the dam.