Sand dams: a sustainable solution for water scarce regions

Global and regional studies reveal that climate change will adversely affect the availability of surface water resources in the Sub-Saharan Region. The anticipated impacts of climate change include severe droughts and overall drying in most parts. With this in mind, Swaziland is being supported by the United Nations Development Programme (UNDP) and the Special Climate Change Fund (SCCF) to build adaptation capacity through improved water resources management.

UNDP is providing technical support to the government of Swaziland to carry out climate change resilient projects at community levels to help them adapt to climate change. One such scheme is a sand dam pilot project where their effectiveness will be monitored and eventually inform strategic and policy directions across the country. A successful and cost-effective pilot sand dam project will guide the country into a wider national programme to be driven by the Swazi Government. The project is being supported by three government ministries that will eventually explore in more detail should they be successful. The key institutions are: Department of Water Affairs, Ministry of Agriculture and Ministry of Public Works and Transport.

Sand dams were identified as one potential adaptation measure that could be piloted in Swaziland. However there was limited knowledge and capacity to construct sand dams in Swaziland. In August 2013, a UK based consultant known as Excellent Development, which specialises in these structures, was engaged to conduct a national feasibility on sand dams in Swaziland. A successful national feasibility was subsequently completed in December 2013, identifying several potential sites in the Lowveld region of Swaziland where the climate is very dry.

Sand dams

A Sand dam is typical concrete embankment or wall built across an ephemeral (seasonal) stream. During the rainy season, the wall harvest sand and 20-40% of its total volume is water. The water is then abstracted using various methods depending on the intended use by the communities. In addition to working as water harvesting structures sand dams could be combined with road crossings to have greater benefit.

Water is abstracted from the sand dam using two basic methods:

Traditional scoop holes - where water is simply accessed by digging scoop holes from the sand upstream of the dam wall.
Alternatively a filtration gallery can be installed where water is drained from the sand using pipes with small slots into a chamber or shallow well connected with a submersible or hand pump.

A GIS tool and literature review were used for the Swaziland project to determine their feasibility. Indeed for sand dams to be successful, they must meet certain technical conditions:

Dry land climate zones/agro-ecological zones
Sand dams were successfully proposed in areas with dry land climates. The lowveld of Swaziland was found to meet these criteria.
Seasonal rivers
Areas with seasonal/ ephemeral streams were proposed for sand dam construction.
Rainfall
Sand dams were found suitable where there is certain degree of rainfall capable for flooding at certain times. This allows the required amount of sand sediments to be deposited and fill the dam.
Rock type
Sand dams are usually built across the river beds, anchored on the bedrock. To reduce the cost for this structure, a bed rock that outcrops to the surface is preferred.
Soil types
Maps for soil types around the catchment were also used to ensure that the chosen sites have the potential to yield enough sand sediment to fill the dam during the rainy season. Suitability of sediment in the riverbed was determined through sediment sampling to determine particle size distribution and suitability. Particle Size Distribution (PSD) is an indicator of the amount of water a sediment type can hold, and how much water can be abstracted. Sand holds a lot of abstractable water compared to other sediment types, particularly coarse and medium grained sand. Sediment samples were analysed for an indication of the potential performance.

Source

Media

Taxonomy

• Water Resource Mapping
• Development & Management