Water Portal / Rainwater Harvesting / Groundwater recharge / Leaky dams

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Leaky dam. Partners voor Water (2009). Re-hydrating the earth by sustainable, small-scale sub-surface water retention techniques, Vietnam.

A floodwater harvesting technique, these are permeable structures built across seasonal riverbeds which retain flash flood water that has a high silt load. The idea is to retain the high-energy floods and stimulate settlement of suspended sediment behind the dam. Water with a lower sediment load is then available to leak through the dam and infiltrate the downstream riverbed which is not blocked by sediment deposits. They have been proven to be able to recharge local aquifers. Leaky dams are excellent in semi-arid areas since they greatly reduce evaporation, in regards to storing water.

Suitable conditions

Climate change & Natural disasters
 
 
 

Good practice is similar to that for a sand dam.

  • Ensure they are not built in an area where water will bypass the structure. Riverbanks should be equal in height and tall enough (height of dam + height of flood +10%), and dam should not be constructed near the bend in a river.
  • Site where river is narrower, so that construction is cheaper.
Advantages Disadvantages/limitations
- Can assist recharge of shallow wells

- Can reduce salinity in groundwater
- Reduces velocity of water, which means less erosion & sediment transport
- Since most of the sediment settles out behind the dam, water is free to infiltrate in a sediment-free area downstream. If the main aim is to recharge groundwater, then leaky dams are preferred over gully bunds or check dams

- They can silt up and will need maintenance

- Possible similar issue as with gully plugs, namely unclear land tenure can result in ownership of the structure
- Can be expensive: a 5-step dam, 4.9 metres high, costs around $26,000 to construct


Construction, operations and maintenance

Leaky dam. Balochistan, Pakistan. Facing downstream.

Similar construction as sand dams:

  • Construct wing walls to avoid erosion around edges.
  • Spillway is designed for river flow, and therefore varies according to the site.
  • Avoid downstream erosion of dam base by making protective slab made from more rock-filled gabions.
  • Timing is important: dams should be built during the dry season, but don’t build dams too close to the rains in order to avoid dam being washed away.

The dam can be made from rocks of various sizes (boulders, cobbles, stones, large gravel) that are found in and around the riverbed. In Pakistan, rocks of 200mm diameter were used. In Balochistan, the rocks are held within wire mesh nets that are built in 5 steps to total height of 4.9 metres. The exact height of the dam will vary and depend on the riverbed topography. A reinforced concrete cut-off 1.5 metres deep with 2 metre toes on both sides of the dam ensure stability. To prevent the water behind the dam percolating too quickly, it helps to put adjustable sheets filled with small size gravel on the upstream side of the dam body (helps in cases of drought).

Water extraction

  • Water that infiltrates is used as soil moisture for crops cultivated after a rainfall event.
  • In Balochistan, Pakistan, pipes have been added on top of the 2nd and 4th steps which discharge surplus water downstream.
  • Water can be used via shallow wells and boreholes in the area.

Costs

Costs vary, but a 5-step dam, 4.9 metres high, could cost around US$26,000 to construct.

Field experiences

A leaky dam constructed in Balochistan has a catchment area of 1.79 km2 with a storage capacity of 11,000m3.

Reference manuals, videos, and links

Acknowledgements

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