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Surface Water Soakaways

The preferred method for dispersing excess surface water under UK legislation is to ground via a surface water soakaway as defined by BRE Digest 365.

The adequate provision for carrying rainwater from the roof of a building or from paved areas is a legal requirement.  A soakaway is the traditional and favoured method for doing this, however, its important that surface water soakaways are correctly designed and installed.

There is a lot of confusion about soakaways and in particular the difference between a drainage field dispersing septic effluent from a septic tank and a soakaway dispersing water from a roof.  In fact, the two are very different.

Surface water soakaways are designed to take advantage of the fact that it doesn’t rain all of the time.  This means that soakaways can be constructed as a pit which requires less space than the trench typical of a foul water drainage field.  As the soakaway pit has a limited surface area in contact with the surrounding soil, it won’t drain all of the water it receives immediately, instead filling up while it’s still raining.  When it stops raining, the water continues to drain until the pit is relatively empty and ready for the next rainfall.

Foul water drainage fields are different.  We do use our foul drains all of the time, so the foul water drainage field needs to disperse all of the septic effluent it receives in any given day so that it’s ready to receive the next day’s flows.  For this reason, a foul water drainage field requires a much larger surface area in contact with surrounding soils and is typically constructed in trenches, not pits.

The size of a surface water soakaway is defined by how much water it will received and on the permeability of the surrounding soil.  How much water will be received is primarily a factor of the size and pitch of the roof area being drained and where in the country it’s located (because it rains more in different places).  How big a soakaway needs to be is then determined by the local permeability (or infiltration rate) of the soils and this needs to be measured accurately to avoid flooding.

The design of surface water soakaways is defined by a methodology described in BRE Digest 365.  This methodology requires a specific infiltration test to determine the infiltration rate of the local soils.  Once this information is known, the design of the soakaway can be completed and can take a wide range of forms including rubble-filled pits, geocellular ‘crates’, precast concrete rings or even open infiltration ponds and ditches.

WCI designs surface water soakaways using industry standard Microdrainage software.  We take account of site-specific requirements including the available space, the use of a property, the local geology and cost.  We are able to support developers and homeowners with the proper methods for infiltration tests or to do the work ourselves.  We produce professional designs and specifications no matter how large or small the application and install specified equipment to a high standard.