Subsurface harvesting systems

The use of subsurface dams in naturally occurring alluvium creates groundwater storage upstream of the dam, raising the water table and preventing evaporation losses. These are structures that reduce or stop the flow of shallow groundwater. A subsurface dam is constructed across a valley in a seasonally dry, sandy riverbed, by digging a trench down to the bedrock or other impervious layer (clay) at the base of the river. The dam, which is placed in the trench, may consist of a wall or screen and covered with excavated material until it is completely concealed. The refill material must be properly compacted. There are two types of groundwater dams: a subsurface dam (which is explained here) and a sand dam. A subsurface dam is the one easier to build of the two. The structure is built inside an existing riverbed, so that after a flooding event, for example, water is increased due to a new and higher water level within the sand.

Contents

Suitable conditions

• Siting requirements are similar to the sand dam.
  
• Subsurface dams should be built where sand volume is already sufficient to store water.
  
• Seasonal rivers in semi-arid regions with permeable sediments and an impervious layer at a shallow depth (maximum 3 to 4m deep).
  
• River valleys with gradients of between 1 and 2% usually enable the highest storage.
  
• Ideal where groundwater flow converges from a large catchment into a narrow passage.
  

Advantages

Disadvantages/limitations

- Subsurface dams are not likely to deteriorate, with little danger of breaching - Construction has long lifespan and require minimal maintenance - To some degree, subsurface water-harvesting systems improve water quality by filtering water as it moves to the drains - The breeding of insects and parasites such as mosquitoes and bilharzia parasites is prevented - Contamination of stored water, by people and animals, is greatly reduced, particularly if well and handpump are provided to abstract water in a hygienic and controlled manner

- Leakage is often difficult to detect - Water losses may occur through cracks in the impermeable layer (it may be possible to drill a well in the fracture zone and utilize the dam as an artificial recharge structure) - Downwater flow will be impacted to communities downstream - The dam may become undermined or eroded owing to faulty design or construction, such as when boulders are not used in the building of the dam - the river may change course - the possible dam construction sites may be too far from the water users

Construction, operations and maintenance

Mechanics of a subsurface dam.

The dam can be constructed of concrete, masonry, blockwork, stone-filled gabions with waterproof membranes such as plastic sheet or clay layer, or stabilised soil.

• Build the crest of the underground dam at least 1 metre beneath the ground surface, so the land does not become water-logged.
• Subsurface dams should have wing walls, just like a sand dam does, where you can pile up rocks so that erosion does not occur.
• Water can be abstracted by a well a short distance upstream of the dam, or by a drain system which collects water from the upstream base of the dam and leads it to the downstream side to a well or gravity-pipe system.
• Where possible, a flushing valve is installed to facilitate cleaning of the subsurface reservoir.

Maintenance

• The well or gravity pipe should be regularly cleaned.
• After each large flood, any damage to the dam should be repaired and the dam protected with large stones if necessary.
• For big repairs, such as when a dam has been undermined by infiltration, or damaged by a flood, many people and heavy machinery may be needed, and specialist technicians should be consulted.

Maintenance can normally be carried out by the users of the system or by a caretaker or watchman. Larger repairs may require skilled labour, which can usually be provided by local craftsmen. In some cases, unskilled labour may be required on a large scale (e.g. for repairing a leaking subsurface dam). The labour may be provided by the users (with or without pay), or by other people who are hired for the purpose.

Users may need to establish a local committee to manage issues, such as controlling or supervising water use, preventing water contamination, carrying out O&M activities, financing O&M, and monitoring how much stored water is still available (a piezometer or auger hole may be installed to allow a caretaker or watchman to estimate how much water is left and decide if rationing has to be introduced). Proper management may also help to prevent social conflict. For O&M tasks at the dam site, a person who lives or farms near this site could be appointed. This person could also be responsible for water alloca- tion and be involved in monitoring activities, if users obtain the water near to, or at, the site. His or her authority should be clear and accepted by all users.

Costs

Initial costs: A 3500 m3 dam costs US$2.40/m3 in Kenya and US$3.90/m3 in Tanzania.

General costs:

• Materials costs and construction: US$1,400
• Feasibility study: US$200
• Technical support: US$275
• Operation and maintenance: low

Field experiences

A subsurface reservoir in Pernambuco, Brazil has an average depth of 4m, is 50m wide, and 500m long, storing about 4,000m3 of water.