Riverbed sediment

Why of importance?

The grainsize of the sediment in the riverbed determines the pore space of the sediment. This parameter is important to several processes.
1. Storage capacity. The larger the grainsize of the sediment, the larger the storage capacity of the riverbed aquifer.
2. Infiltration speed. A larger grainsize results in a larger infiltration rate. Groundwaterlevels will respond faster on precipitation.
3. Groundwater flow. Groundwater flows faster through aquifers with a larger grainsize. If the grainsize of the riverbed sand is larger, water can be abstracted more easily from wells of scoop holes in the riverbed aquifer. Also, since the riverbed enables faster groundwater flow, this will be the preferent path. Groundwater will thus be concentrated in the riverbed.
4. Contamination. The fact that the water is stored underground results in protection against contamination and evaporation. Additionally, groundwater flow has a positive influence on water quality, since the water is filtered from contamination as is flows through the riverbed. A similar method is used in the dunes of the Netherlands, to filtrate infiltrated surface water (PWN). Increasing grain size diameter results in a larger groundwater flow velocity and thus filtration capacity.

Also the width of the riverbed is of importance, since it determines the dimensions of the sand storage dam.

Optimal conditions

A riverbed consisting of clay or silt is unsuitable for sand storage dams because water will infiltrate very difficult resulting in ponding (open water is very susceptible to contamination). Additional, storage capacity is limited and groundwater flow is very slow making abstraction very difficult.
When the riverbed consists of boulders and pebbles the pores are too large, leaving the aquifer vulnerable to contamination and evaporation.

(Coarse) sand will form the best aquifer, since the storage capacity is large, infiltration rates are high resulting in fast responses of groundwater levels on precipitation, and groundwater flow velocity is quite large. Groundwater can thus be abstracted easily because of high flow velocities and while flowing, the water is filtered. Because the riverbed has a larger pore size diameter, the riverbed will be the preferent path for groundwater flow, concentrating the groundwater in the riverbed.

The width of the riverbed should be no more than 25 meters. When larger, construction costs are too high due to the large amount of material and reinforcement for the dam wall.

Risks

When a sand storage dam is built, one of the risks is it filling with too fine sediments. Causes can be
1. Building the sand storage dam too high (creating a too large barrier) resulting in ponding of water upstream of the sand storage dam in which fine sediment will settle.
2. Building in areas with little baseflow. The importance of baseflow is explained here.
3. Building in areas where little or no coarse sediment is present.

Also, when boulders are present in the riverbed, caution needs to be taken that the sand storage dam is not built on such a boulder because this construction is unstable and impossible to make water tight (see also Bedrock).

Opportunities

To overcome the above, careful siting is very important (see next paragraph). In upstream catchment it is recommended that sand dams are built in stages, since the availability of coarse material is generally limited and baseflow is small or absent. When build in smaller stages a smaller velocity restriction is created, leaving only a small silt layer behind the dam (Gijsbertsen, 2007). This means raising the spillway of the sand dam in stages of eg. 0.30 meter height over the sand level in the riverbed (Nissen-Petersen, 2006).

How to evaluate?

Siting is always very important when building a sand storage dam. This will therefore be elaborately discussed here. However, for this specific physical factor, the following evaluations can be made.
1. Sediment already present in the riverbed is a very good first indicator of the sediment to be expected. If coarse material is abundantly present in the riverbed, this material is likely to sediment behind the sand storage dam.
2. Presence of stones of stony hills in the catchment area are another indicator of the possibility of coarse riverbed material. Rainwater and floods can transport eroded stone particles (which form coarse sand) to the riverbeds and deposit them into the dam reservoirs (Nissen-Petersen, 2006).
3. Presence of weathered rock (granitoides) is also a positive indicator since it is a source of coarse grained sediments (sands).