SuDS the principle
SuDs are physical structures built to receive surface runoff, normally in the form of infiltration or attenuation solutions. They provide treatment of surfacewater by sedimentation, ltration, absorption and bio-degradation.
Permeable paving is expected to absorb 60mm/hr in the UK, all three MetroPave grids exceed these requirements.
Managing the water once it has passed through the grids is an integral part of making a long lasting, good performing SuDs compliant system.
Generally there are 3 ways to manage the surface runoff:
Where the existing sub-grade is free draining and allows the water to pass through with minimal pooling, a full infiltration system is suitable for the area. This allows the rain to lter down through the grids and sub-base into the existing ground, almost the same as before the grids were installed.
No water is discharged into the drainage or sewer systems making the area completely independent and the most cost effective way to make a structural and SUDs compliant area.
Where the sub-grade can absorb most of the rainfall but has a bit of run off, a partial infiltration system is used to get as much of the rain to infiltrate into the local area but has a drain pipe installed within the subbase to stop the rain water filling all the void. This will allow the excess rain water to be redirected to a secondary storage system such as a dedicated attenuation system or swale etc.
Where the existing sub-grades permeability is very poor in areas with lots of clay or is heavily polluted, the water must be totally captured and treated. This system uses an impermeable geotextile placed on top of the sub-grade to seal and separate it from the sub-base and form a storage tank out of the subbase, an outlet must be installed to allow excess water to escape to water courses, sewer, drainage systems or further attenuation treatment areas.