Determining the optimum size for a soakaway can be a complex iterative calculation including many factors such as location, soil types, size and shape, maintenance etc.. This is particularly the case where space is lacking and the soakaway size must be minimised. In these circumstances a specialist software package is usually used, such as this Soakaway Design Spreadsheet

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For small simple applications where space is not a major concern, an adequate size for a soakaway can be calculated for any location using this simple design process. An adequate storage size for a soakaway can be determined using only two factors, the size of the catchment area and the water infiltration rate of the surrounding soils. 

Working Out The Size Of Your Catchment Area

The size of the catchment area connected to the soakaway determines the amount of rainfall which will enter the soakaway during a storm. So if it is connected to the downcomers from your roof, the catchment area will be the plan area of the roof, ie the length of your house in metres multiplied by the width. Similarly if the soakaway drains a driveway, the catchment area is the area of the hardstanding. This area can be measured directly with a long tape or can even be estimated reasonably accurately from Google Earth. Note that soft areas such as lawns and soil do not contribute to the catchment area as the rainfall will soak into the soil. Also areas downslope of the soakaway will not contribute, water cannot run uphill!

Calculating The Volume Of Rainfall For Your Soakaway

When the catchment area is known the volume of rainfall entering the soakaway can be calculated. For large commercial soakaways this should be calculated using the Wallingford Procedure. For small soakaways this can be estimated to a satisfactory accuracy by multiplying the catchment area (in m2) by 0.05. This gives the volume of water in m3likely to fall during a typical heavy winter storm event.  This assumes a rainfall intensity of 50mm/m2which will be sufficient where the consequences of flooding will not be serious. At this design level it will overflow for around half an hour, once or twice a year depending on the precise location. 

This required storage volume value can then be modified to take account of the water infiltrating out of the soakaway during the storm. The below table shows the modification factors for different soil types;

Gravel0.1
Sandy Soils0.5
Loam0.8
Clay1.0
Silt1.0

Note that a percolation test is recommended for clay or silty soils as these soil types are not always suitable for a soakaway. A very simple test can be undertaken by digging a 1m deep hole and filling with water. If the hole does not empty by more than half overnight, the soil may not be suitable. 

Calculate The Water Storage Volume

Next simply multiply the required storage volume by the soil infiltration factor to estimate the water storage volume required. Next this value needs to be modified to account for the type of soakaway. Different types have different void arrangements, which means an open soakaway crate system can be smaller overall than a soakaway backfilled with rubble simply because the open crate is able to store a greater volume of water in the same size excavation.

 If soakaway crates are to be used, multiply the storage volume value by 1.1. If the soakaway will consist of a hole backfilled with rubble, multiply the water storage value by 3. This will give the required volume of the excavation in m3

Now the excavation volume has been determined, the soakaway can be installed. The excavation volume includes only the volume below the inflow pipe. This will prevent the pipe from backflowing during a storm event. 

This simple procedure will give you a conservative size of the soakaway you will require. For larger projects or where the size must be optimised, a more complex design procedure should be followed such as BRE 365 or professional help may be required. 

Have you designed your own soakaway for your home garden? Let us know your experiences and suggestions in the comments section below.

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