This is something we see frequently in design and for drainage. Permeable paving and gravel above the tanks forms part of the in-water quality element in SuDS. When the water then filters through, rather than coming into the tank with your pipes, if water comes into the tank for your permeable paving and gravel layers, then this gravel will pick up potential pollutants. The water going into the tank is then cleaner and will be cleaner when it’s discharged from the tanks.
It just means a slight change to the install, and means leaving the top of the attenuation tank. You just don’t put an impermeable membrane on the top, you put permeable tanks down and you bring the membrane up to the surface. But it is something we see very often.
