Attenuation Water Storage Volume and Outflow Rates

A simple, but helpful fact for calculating the volume of rain coming from a roof is based on the method of measuring rainfall rates using a rain gauge, i.e. the precipitation (rainfall) in millimetres in height during a certain period.

This is equivalent to litres per square metre that is falling on the surface.

Hence, if you know the area of the roof, and the rainfall rate (from slight to storm rainfall periods), you can calculate the volume of rain in litres that will come from the roof under the different conditions of rainfall. Obviously, the main value of a SuDS planter is during storm rainfall periods when the drain and sewer systems are in danger of becoming overwhelmed.

The storage volume and retention and subsequent outflow rates will vary with the dimension of the planter.

The SuDS Manual states that typically the surface area of a bioretention feature would be 2-4% of the overall site area being drained, to prevent rapid clogging of the bioretention surface. Based on this guidance, the surface area of any planter (or combination of planters) should not be less than 2-4% of the roof area it is draining.

The recommended surface area of the planter is based on the height being our standard 900mm. Where possible, the size of the planter should reflect this recommendation.

We offer a range of standard planter sizes to use in projects to attenuate rainfall from roof systems connected via the roof drain downpipes.

These sizes are based on large, but manageable and movable sized units using a fork lift. This offers practical sized unit options to be used in combination to achieve the recommendation catchment areas stated above.

As well as the standard sizes, we can also construct the SuDS planter to bespoke requirements, but recommend a maximum length of 2500mm.

For larger areas, we advise installing our modular linked SuDS planters that can be fed by the same drain downpipe connected via the top of the planters.

Water storage values of our standard SuDS planters

Below is a table of the attenuation storage volume of our standard size SuDS planters which can be made using either Larch or GRP. Values are given for both construction materials:
 

Attenuation Storage Volume for Standard SuDS Planters Larch and GRP Options
Larch Option Size (mm) L x W x H	Attenuation Storage Volume (Litres)	GRP Option Size (mm) L x W x H	Attenuation  Storage Volume (Litres)
1000 x 650 x 900	247.19	1000 x 600 x 900	341.55
1200 x 650 x 900	303.37	1200 x 600 x 900	413.14
1500 x 650 x 900	387.64	1500 x 600 x 900	519.99
1800 x 650 x 900	471.91	1800 x 600 x 900	626.84
2000 x 650 x 900	528.09	2000 x 600 x 900	698.07
NOTE: Storage volumes are based on planting depth of 200mm. We recommend use plants in 2 to 5 litre sized pots.

Outflow and Infiltration Rates

 

The standard SuDS planter is drained via a perforated drainage pipe running along the bottom of the planter. A number of factors influence the drainage rate including the drainage pipe diameter, soil mix and therefore infiltration rate. The impact of plants and soil conditions prior to any particular rainfall event will also be important.

 

Outflow Rate – Larch and GRP Options

15 to 20 litres/hour Depending on head of water in storage

 

To optimize infiltration we recommended using an open, free draining peat free soil mix of 15% loam soil or John Inness No 3 with 85% Peat free Multipurpose compost.

If planted with shallow, light rooting plants, the outflow rate ranges from 15 to 20 litres per hour depending on height of head of water. We recommend light rooting planting, as the infiltration rate will be impaired by the formation of a dense root network from more mature shrub-like plants. Annual planting that can significantly enhance the BNG value of your SuDS planters can be replenished each year. Hence they are ideal for creating wildflower pollinators and long summer flowering plants with a minimum rooting system that can be cultivated annually to retain a free draining soil/compost mix.