Selecting potential catchments

A quick scan is used to make a probability map of an area for building sand dams. This will make site selection more specific and thus efficient. The desk study can be performed using digital or analogue data. The result will be a map showing high potential areas. If available, the below mentioned data are a first indication whether a catchment is suitable for building sand storage dams.

Topography map

A topography map gives general information about the catchment, such as locations of villages and roads. The presence of communities (the beneficiaries) in the area in the dry period (nomads or permanently) is the first condition.
A topography map also shows locations of rivers, and the size and general characteristics of the catchment. Rivers may have a maximum width of 25 metres (otherwise other options may be more suitable, such as subsurface dams (Nissen Peterson, 2006)). The catchment should be hilly (see also Digital Elevation Model).

Digital Elevation Model

A Digital Elevation Model (DEM) contains information on the morphology of an area (elevation and slopes). A local drainage direction map can be calculated from it, which will give the drainage pattern (rivers) of the catchment. Furthermore, information on the slopes within a catchment can be derived from a DEM. The most suitable locations for sand dams have a slope gradient between 2 to 4 percent. The particle size of sediments accumulated along streams and in riverbeds (which will also fill the sand dam) is proportional to the slope gradient, whereas the depth and the lateral extent of the river bed aquifer are inversely proportional to the slope gradient. The optimum relation between these two factors is found on the gentle slopes between hills and plains with a gradient ranging from 0.3 to 4 percent (Gezahegne, W., 1986).

Digital elevation data by the Shuttle Radar Topography Mission (SRTM) can be freely downloaded from the internet (http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp). The resolution of the data is rather coarse; 90 metere horizontal.

Geological map and soil data

Catchment geology determines, together with discharge characteristics and slope, the grain size of the sand dam. A geological map can indicate whether a catchment has the potential to produce (coarse) sand. For example, granite hard rock will produce coarse sand while shales will produces fine (clay or silty) material.
For example, the geology of Kenya is available from USGS. This map is part of the open file report 97-470A, version 2.0 2002, scale of 1:5,000,000. The dataset is an interim product of the U.S. Geological Survey’s World Energy Project (WEP) and can be freely downloaded from the internet.

If the riverbed itself contains large stones and boulders, seepage under the dam may occur. When large boulders occur, special care should be taken to site selection. This will need to be checked in the field. Soil data can give information on where to locate sandy areas within a catchment.

Aerial photographs and satellite images

Aerial photographs and satellite images can help locating sandy riverbeds based on the morphology. Aster satellite images can also be used to indicate sandy riverbeds and different types of geology through the reflection (ratio), as has been done by Gijsbertsen (2007). Aster satellite images can be downloaded from http://asterweb.jpl.nasa.gov

Precipitation and evaporation data

The effectiveness of sand storage dams appears to be quite insensitive to precipitation (Borst and de Haas, 2006; Hoogmoed, 2007). However, when locating suitable regions for building sand dams, precipitation will be important because it influences discharge characteristics (base flow) and thus also the availability of coarse grained material in the riverbeds. Higher annual precipitation will mean more storm events. These areas have a higher suitability based on precipitation compared to regions with a lower annual rainfall. Also, to calculate the yield of a sand dam, it is essential to know the climatologically behaviour of an area.
Tropical Rainfall Measuring Mission (TRMM) satellite images contains rainfall data with a spatial resolution of 4.3 km (In the region between 35°N and 35°S). Data is available on the internet on monthly basis (http://neo.sci.gsfc.nasa.gov/Search.html). LocClim also provides rainfall data (http://www.fao.org/sd/2002/EN1203a_en.htm, where the program can be requested for free).

Flood data

Flood data can be used to determine the maximum flood height and thus the minimal height of the riverbanks (see also paragraph 6.2). It also provides information on discharge characteristics of a catchment during a rainfall event.
For example, the river has to be seasonal, although it is very important that base flow occurs since this prevents sand storage dams of filling with fine (unfavourable) sediments. Local communities and water authorities can indicate whether the river dries up immediately after a rainfall event or if the river continues to flow, showing base flow.