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Hydraulic design criteria

The report provides detailed guidance to designers to ensure that pervious pavements have sufficient infiltration and storage capacity. Key considerations are:

• The rate of infiltration through the surface must exceed the design maximum rainfall intensity, if surface ponding of rainwater is to be avoided. The report provides typical infiltration rates and discusses the effects of clogging and the likely maintenance regime and how this may be allowed for in design to ensure long term performance of the surfaces.
• The position and design of geotextiles to maximise filtration and minimise the risk of clogging. 
• Guidance is provided on estimating the volume of rainfall and the required storage capacity of the aggregate layers below the car park, including estimation of outflows via pipes or infiltration to the underlying soils. This includes allowance for freezing effects and specification of aggregates. 
• The time to empty is specified in order to minimise the amount of time the subgrade is exposed to traffic loads when saturated, if an impermeable geomembrane is not used.
• The Environment Agency manages flood risk over a catchment and will assess the risk arising from a development when specifying what standard of attenuation may be required at a site. In the areas of highest risk this may equate to a storm return period of 100 years or more being managed by the surface water drainage system within a site. Early discussions are recommended with the relevant drainage authority so that the required return period is known before design work starts.
• The hydrograph is simplified to a constant distribution of intensity over the given duration and all the rainfall is assumed to pass immediately into the pervious pavement system with no attenuation occurring. This is a conservative method and should provide a built-in safety factor to designs.
• Drainage systems are usually designed based on return periods. For example a return period of 10 years means that, on average, the maximum design rainfall event will be exceeded once every 10 years and flooding will occur (the level of flooding can be determined for the given return period). This can also be expressed as an annual probability of exceedance, which for a 10 year return period is 1/10 or 10 per cent (a 10 per cent chance that flooding will occur in any given year).
• The consequences of flooding or surface ponding occurring.

An important consideration with pervious surfaces is that soil, sand and other site materials should be prevented from contaminating the pervious surface during construction, thus reducing its free-draining nature. The report suggests that most clogging of pervious surfaces (particularly porous asphalt) occurs during construction. This can be overcome by educating contractors and workers and explaining how the pervious surface works and the importance of keeping it clean. Construction traffic should be prevented from tracking mud and soil into the pervious surface and proper supervision and inspection of the installation should be budgeted into all projects.

After actual construction works cease the preparation of landscaping should be carefully designed and managed to prevent carelessly deposited top soil, turf and other materials blocking the pervious surface. Runoff from landscaped areas, which is a sources of soil and debris, should be prevented from flowing onto the pervious surface and causing clogging.

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