Leaky dams

Leaky dams are a flooding prevention measure, moderating the flow of water downstream. Barriers are added to a stream/river to prevent soil and silt escaping and allowing water to escape at a slower rate.

History

With the regular flooding of settlements in the UK, a rethink on flood risk management was initiated.[1] One of the proposals put forward was containing flood waters at source to prevent large volumes of water flowing downstream causing soil erosion and flooding of properties.[2]

A variety of solutions was put forward including using naturally occurring composting materials in path of flow to hold valuable fertile silt and dam up waters over many small dams.[3] A leaky dam was added to the arsenal of flood prevention tools.[4] This type of structure is what nature uses for similar events. Logs from fallen trees are placed at intervals down the stream acting as a barrier, holding the silt and small debris back and allowing just the water that overflows the structure to continue the course of the stream.[5]

Benefits

The area surrounding the leaky dams have many benefits including better quality of water to life behind the barrier, prevention of soil erosion, available nutrients for wildlife, stabilisation of river banks, spawning ground for aquatic life, rooting habitat, place of nesting birds, areas of growth for microbes, algae and fungi, efficient temporary storage of water and a slow release of water into surrounding area.[6]

Soil erosion

After a heavy downpour, soil, gravel, silt and valuable nutrients are lost when the rains wash away the top soil. This animation of a side view of a mountain embankment shows how bad soil management contributes to tonnes of topsoil ending up in flood areas and river beds. It costs a fortune to collect the soil from river beds, estuaries and river mouths, decontaminate, transport and physically replace the reclaimed sand.

Examples

Barriertypes

Theses are types of barriers used by organisations in UK to prevent soil erosion and flooding downstream.

The following are possible approaches:[7]

• 
  Barriertypes

One method is to act construct a barrier so when the fallen trees and shrubs are placed behind the barrier the running water and debri is prevented from washing away.[8]This method is more effective than the one below as it holds more water back, has a higher level of overflow and more valuable fertile silt is retained.[9] More vegatation is able to be supported behind the barrier due to the higher level of nutrients captured from debri and silt. Aquatic life is able to populate and continue to multiply with limited inventions. This method shows better results.
Another method is to place fallen logs in a zig zag pattern that would mimic nature in the event of a natural felling. Water isn't dammed up as much with this method but valuable silt is still filtered and not lost downstream. Aquatic life can move between barriers and continue upstream. This method is favoured by environmental groups who claim that it closely resembles nature.
The idea of these dams is to create a method whereby the water can slowly drain into the surrounding water table. Surrounding vegetation is able to absorb more water and nutrients from standing body of water rather than a flowing one The larger the tree, the better the root growth, the more water is absorbed and the more soil is held together. Some trees can absorb hundreds of litres per day, making it a very effective water collection system[10]

Observations

Many county bodies have initiated their own trials and photographic and video recordings show positive results.[11][12][13]

A well known project in Pickering, North Yorkshire, UK showed how effective leaky dams can be when there projected flooding risk assessment dropped from a 25% probability to about 4%.[14]

Artificial leaky dams constructed from injection molded plastic have also been tried in some areas. Some of the benefit derived from this method is the wall doesn't corrode or compost over time requiring less management.[15]

These are deemed soft defenses as opposed to hard defenses that include solid concrete retaining walls that cost a fortune and are unsightly to local people.

It would seem local residents, environmental agencies, local councils and budget managers are more open to a natural flow system that mimics nature and holds large volumes of water back for later draining. The biggest motivator is this method costs a fraction of the hard defenses that professional government agencies have been promoting.

Only time will tell which methods suit different rainfall patterns.

References

1. "UK floods: 'Complete rethink needed' on flood defences". BBC News. Retrieved 2016-04-03.
2. "What next for Water Friendly Farming? - Freshwater Habitats Trust". Freshwater Habitats Trust. Retrieved 2016-04-03.
3. "Damned if we don't - flood defences | Big Issue North". www.bigissuenorth.com. Retrieved 2016-04-03.
4. "lecture of leaky dam".
5. "pdf of forestry site using leaky dams" (PDF).
6. "somerset leaky dam pdf" (PDF).
7. "forestry project with photos" (PDF).
8. "Pickering and District Civic Society - Debris Dams during the high flows of 4 April 2012". www.pickeringcivicsociety.btck.co.uk. Retrieved 2016-04-13.
9. "FWAG SouthWest on Twitter". Twitter. Retrieved 2016-04-03.
10. "pdf of tree statistics" (PDF).
11. "pdf of leaky dam project" (PDF).
12. "FWAG SouthWest on Twitter". Twitter. Retrieved 2016-04-03.
13. Administrator. "Build a Leaky Dam and do Your Bit!". www.wookeyhole.info. Retrieved 2016-04-03.
14. "September | 2015 | The official blog for the North York Moors National Park". northyorkmoorsnationalpark.wordpress.com. Retrieved 2016-04-04.
15. Ltd), Paul Allington (Intelligent Penguin. "Plastic Piling Control Structures". FindTheNeedle. Retrieved 2016-04-04.