Topography

Why of importance?

From Gijsbertsen (2007) it is concluded that slope is an important factor in the suitability of a catchment to build sand storage dams.
1. Material sedimenting in sand storage dam. Slope determines flow velocities and thus transporting capacity of coarser material.
2. Size of riverbed aquifer. Together with the height of the sand storage dam, the gradient of the riverbed determines the size of the reservoir in the riverbed.

Also, catchment area upstream of the sand storage dam location is of importance.
1. River baseflow. A larger catchment area equals a larger area contributing to baseflow and thus larger detachment of fine sediments.
2. Sediment availability. The larger the catchment area, the larger the area contributing to the availability of coarse sediments
3. Groundwater flow. The location of the sand storage dam receives groundwater flow from upstream parts of the catchment (in presence of an upstream sand storage dam through groundwater flow around the dam). The larger the contributing area, the larger the volumes of groundwater flow, resulting in larger available volumes and a larger period of availability.

Optimal conditions

The slope should be large enough to generate large enough flow velocities. However, the slope shouldn’t be too large because this will significantly reduce the size of the riverbed aquifer. Additionally, larger slopes generally occur in upstream parts of the catchment. Catchment size shouldn’t be too small since it determines river baseflow, sediment availability and groundwater flow.

Risks

1. Catchment area too small. Little baseflow occurs and little coarse material is available. This will result in sedimentation of fine materials upstream of the sand dam. Sand dam efficiency will be very low.
2. Slope too large. The storage capacity of the sand storage dam is limited.

How to evaluate?

Catchment area and slope can be evaluated through GIS during the suitability study. Generally, the size of the riverbed is an indication of the location within the catchment. However, the parameters determined by the upstream catchment size are easier to determine.
1. Is there enough coarse material available? See what lies in the riverbed at present. If it’s coarse sand, this is an indication that the location might be suitable.
2. Does river baseflow occur? Visit the river a few days after the last rainfall event after which the river starts to flow. If the river still flows after a few days of drought (also in the more upper parts of the catchment!) baseflow occurs.