Riverbank

Why of importance?

The sediment on the riverbanks determines several factors.
1. Stability. The sand storage dam is built between the riverbanks of the river. Dam stability is thus determined for an important part by the characteristics of the sediment of the riverbanks.
2. Prevention around flow. The height of the riverbanks are of importance, since these will prevent overflowing of the river when the sand storage dam is built. If overflowing occurs, the river will change its course, thereby leaving the sand storage dam as a ruin, in addition to extreme erosion. Higher riverbanks will channel floodwater to flow over the spillway of the sand dam.
3. Sand storage dam design. The shape and slope of the riverbanks will determine the manor of sand dam construction (width, height) since the sand storage dam is built in between them.
4. Peak discharge and base flow. The characteristics of the riverbed sediment (coarseness and crust/crack forming) determine the percentage of precipitation resulting in quick (surface) runoff which will infiltrate in the riverbed or contribute to river flow and groundwater recharge. This thus determines the characteristics of the quick and base flow of the river, which both determine the type of material sedimented in the riverbed.
5. Sediment in riverbed. Gijsbertsen (2007) concludes that there is a large correspondence between the grain size on riverbanks and riverbeds. Coarse grained material is not mainly detached from deep into the weathered hardrock incised erosion gullies, but comes mainly from the riverbank surface.

Optimal conditions

Considering building sand storage dams, the stability of the river banks are of major importance. The sediment should be compact which will protect it from erosion by the river. The optimal conditions are silty or clayey riverbanks, which are normally quite resistant to erosion, underlain by hard rock. Also when rock outcrops occur close to the riverbed, this could be suitable for attaching the sand storage dam to it. Secondly an outcrop of this impermeable layer is preferred to build the dam upon. Water and sand behind the outcrop have already accumulated which increases the storage capacity of a sand storage dam.

Also the height of the riverbanks is very important. Flood water should never be allowed to flow over the wing walls and river banks, but it should be channeled to flow over the dam spillway. Another positive effect is that the dam is smaller (in width), reducing material need and thus building costs.

The grain size of the sediments on the riverbanks shows a large correspondence between the grain size on riverbanks. Coarser material on the riverbanks, results in a larger the amount of coarse material available to fill the sand storage dam.

Of somewhat lesser importance is the fact that silty soils have a higher infiltration capacity compared to clayey soils. This will lead to a larger groundwater recharge (and storage) and thus larger base flow and groundwater flow during the dry period.

Concluding from the above, silty soils have the best sedimentological properties. These soils are quite stable and compact, have a higher infiltration capacity than clay and usually contain coarser fractions which will provide coarse material for the sand storage dam.

Risks

Erosion of the riverbanks at the location of the sand storage dam is the largest risk. If this occurs, the river will change it’s course, thereby leaving the sand dam as a ruin.
Low riverbanks increase the risk of around flow.
Also, extreme low infiltration of precipitation is a problem, since limited groundwater recharge will occur, making the functioning of the sand storage dam more problematic.

Opportunities

Construction of wing walls improves stability and decrease susceptibility to erosion.
To decrease erosion and increase infiltration in the riverbanks, land husbandry practices are applied. Cultivation of the riverbanks, planting of Napier grass and creating contour bunds are examples.

How to evaluate?

Check what sediment is present on the riverbanks. If rock outcrops occur on the riverbanks, find out whether this is a boulder or an actual rock outcrop (by probing at several locations around the outcrop, digging a small trench and/or talking to the community). If not, find out if and at what depth basement is present underneath the sediment.
Take into account the steepness of the riverbank. Very steep banks are not preferred, since these are usually instable and will collapse.