SUDS (Sustainable urban drainage systems) have become a serious issue as planners stipulate maximum outfall rates for new developments.
Many commercial manufacturers promote the use of storage cells that store water below the ground surface. However they rarely take into account the natural ability of the soils (even clays) to infiltrate rainfall into the natural ground water system. This is because they are trying to maximise the volume of storage cell sold.
Another popular system is permeable paving but again most manufacturers do not like to take into account infiltration and specify very thick stone bases with a tanked base to make their designs conservative. However this can be very expensive for the developer.
The above systems are both valid and indeed sometimes essential but there is no emphasis on the supplier to minimise the quantity specified in any design advice they provide.
Geoman Ltd assesses the soils present and takes into account the infiltration to minimise stone thickness and storm cell volumes. We have found that most clay soils are capable of infiltrating over 25mm of rainwater per day and this allows a significant reduction in imported stone volume. With a permeable but strong geotextile base the majority of storm water can be infiltrated on site.
Our permeable paving solutions often require only 300mm of stone base suitably reinforced with geotextiles to accommodate wheel loads. The geotextile is designed to support a saturated stone sub-base.
Where there are areas of impermeable surface on sites we usually extend the drainage stone sub-base below this to increase the area of storage under the permeable areas. This also requires basal reinforcement. If permeable surfaces are not acceptable we can use crossfalls and open drains along the impermeable roads that lead to a general stone storage area below surface.
Geoman Ltd provides advice throughout the process and we can also provide independent specialist analysis of the various geotextile, storm cell and permeable paving manufacturers. Advice usually starts with assessment and testing of the soils on site for permeability to compare with the planners stipulations and local rainfall records.