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Surface water disposal and soakaway design
Climate Change is having a significant impact on building design, because of this traditional methods of surface water disposal have had to be updated.
Good design of soakaways etc. must take into account the area of the surface to be drained, the permeability of the sub-soil and have storage capacity for a 1 in 10 year storm (a 1 in 10 year storm is usually preceded and followed by heavy and persistent rain which also must be designed for).
For an accurate soakaway design a percolation test should be carried out and calculations prepared, guidance of this can be obtained from BRE Digest 365 Soakaway design and See Approved Document H
However, generally for domestic property’s certain rules of thumb can be adopted (these rules of thumb are taken to give a design which safely meets the requirement, a standard less than given in the rule of thumb must be justified by calculation)
Soakaways and infiltration drainage Rules of Thumb
Soakaway design principles
When calculating the rate of soakage it is assumed that
- the bottom of the soakaway will silt up quickly and give no soakage;
- 50% of the sides of the soakaway will be available for soakage at any time.
When constructing a soakaway it should be ensured that
- infiltration drainage should not be built within 5m of any building, road or unstable land;
- ground water should NOT reach the bottom of the soakaway at ANY time;
- the drainage capacity of the ground must not be overloaded;
- there should be NO contamination within the run off which could result in pollution of the ground water.
General design considerations
- Generally soakaways should be constructed using long trenches of honeycomb masonry to maximise soakage through the sides of the trench and cubic capacity. If the soakaway is to be hardcore filled it will need to be tripled in size to give the appropriate storage capacity.
- Surface water storage tanks can be used along with a 1m3 soakaway, however if this used the drainage capacity of the ground could become overloaded.
- Soakaways should have rodding points. This has the benefit of access for cleaning, use as a water reservoir during the summer for the garden and if necessary can be pumped out in extreme situations.
- Surface water drainage must be provided with inspection chambers etc at changes of direction and in all respects should be treated in a similar manner to foul water drainage.
- In areas with a very high water table it should be considered that non return valves be provided at the bottom of rainwater down pipes. If flooding occurs the water will not flood out at the building.
- Projects on very small sites may be unbuildable if no provision can be made for surface water disposal.
- In high ground water areas ANY part of the trench that fills with water must be disregarded for soakage. Areas near flood plains, rivers etc could have ground water very near or at ground level. In these instances a herringbone layout of pipework can be used for water storage discharging to a very broad flat soakage area.
Creative design ideas
- Holding tanks
- Open topped storage tanks to encourage evaporation. E.g. ponds, shallow wells filled with cobbles, pervious paving 5m from the building
- Garden irrigation system
- Grey water recycling tanks. Tank suitable for 4 bed property fully installed and plumbed £4.5k
Reuse of existing soakaway(s)
If an existing surface water soakaway is to be reused it will need to be investigated. The existing soakaway will only be considered to be suitable for reuse if it meets the criteria laid out in the above paragraph.
08 January 2009
