Siting of a non-vented drift

There are several factors to take into account in selecting an appropriate site for a non-vented drift on a dry riverbed.

The first is to select a narrow, shallow section of the dry river. Narrow sections are preferred in order to save material and labor costs. It is also critical to conduct a geophysical survey at the riverbed to measure the depth of sand sediment on top of a rock/subsurface formation. The best location for a non-vented drift has shallow sand depth, narrow width, and low riverbanks, and should not be on a river bend.

A second requirement is to select sandy or gravelly rivers. Sand and gravel in the riverbed will store floodwater in the open space between the particles and make it easy to retrieve. Ephemeral rivers that predominantly carry clay or silt are unsuitable for a non-vented drift-sand dam, because the fine material will not hold much extractable water.

Third, it is important to understand hydrology. In selecting the site for a non-vented drift-sand dam, special attention must be given to the amount and periodicity of rainfall in the area, the floodwater within the catchment area, and historical flood levels. A hydrological study of the area and information from the local community are essential.

Fourth, the socioeconomic potential of a non-vented drift needs to be taken into account, including the presence of population, farmland, and the scope for non-agricultural activities. For instance, brickmaking is a rewarding rural economic activity that the presence of water can boost.

Fifth, there is preference for the development of a series of structures in a dry river. Preferably, a series of non-vented drifts and other structures (weirs and sand dams) are built in a dry river. This helps regulate the flow, stabilize the river, and improve floodwater retention across the entire river. None of these structures can be fully connected (as in a cutoff weir) to the river’s bedrock: this will block subsurface flow to the downstream areas and deprive those living there.

Finally, prior assessment needs to be conducted to estimate the sediment load that comes in with a flood. It must be ensured that enough coarse sand and gravel can be accumulated in order to prevent fine particles from being stored upstream of the retaining wall. Fine particles diminish the storage capacity of sand-retaining structures. Walls that are too high trap silt and fine particles and let coarse material pass; therefore, little water can be stored.