By Andrew Power
Technical Manager, WSP
Welsh Water has developed an innovative and sustainable approach to solving problems of overloaded sewers, called RainScape, it aims to reduce or slow down the surface water entering combined sewers using a range of techniques. They include:
- Basins and planters: Shallow landscaped basins that capture the surface water runoff from roofs and road and store and treat the surface water. The water filters through the plants within the basin, removing contaminants before being released into the soil or the sewer network. These can be small, and fitted to individual downpipes, or they can be larger, and used within road schemes.
- Porous paving: Paving that is designed to allow surface water to pass through it, rather than over it into nearby drains. Porous paving comes in a variety of appealing designs.
- Grass channels: Strips of grass that can be installed in side streets and back alleys to provide a permeable surface for water to soak through.
- Filter strips: Strips of ground where water running off a site can pass through it, allowing some or all of it to soak away. The rest is often directed into a swale or another RainScape system.
- Swales: Long, shallow, landscaped channels that reduce the speed of surface water – cleaning it and where possible allowing it to gradually infiltrate into the soil. Where it is not possible for the water to infiltrate the water into the ground, the swales gradually return the water into the sewer network at a slower rate.
- Geo-cellular storage: Geo-cellular systems can be either as a soakaway or as a storage tank. They can be installed beneath roads and kerbs and help to reduce the speed at which surface water enters the sewer network.
WSP has worked with Welsh Water and local communities to implement RainScape solutions in a range of catchments with a variety of sewerage problems. Four examples of the many schemes that we have been involved with are described below. All have different problems, but the RainScape approach adapts to each of the issues.
The removal of surface water is aimed at reducing the frequency at which downstream combined sewer overflows spill to the local river. The removal also provides capacity in the sewers for the foul flows from a large area further downstream that has been identified for future development
Disconnecting the surface water from the Forge Factory Industrial Estate has the potential to remove nearly 80,000 m2 of impermeable area from the combined system; which removes as much as 630 l/s from a rainfall event that could potentially cause unacceptable flood risk.
Old Caerau Colliery site has inflow and infiltration of surface water runoff into the combined sewer network. Removal of the inflow will potentially reduce peak sewer flows past two properties in the downstream catchment that suffer from flooding.
Surface water removal may also reduce spill frequency at combined sewer overflows that discharge to the local river which has suffered from pollution incidents.
In Hendy a RainScape solution was proposed to reduce the flood risk to properties. The RainScape solution is more expensive than a traditional option of an underground storage tank; but it is better for the long term resilience of the system to future increases in paved area and climate change.
A proposed RainScape scheme in Llannant is being used as part of the solution to an overloaded pumping station. The pumping station could not be upgraded as it would cause flooding further downstream. The solution is to change the road drainage in the upstream and downstream area so that it no longer drains to the combined sewer but instead makes use of RainScape features. This allows the pumping station capacity to be increased so reducing the risk of flooding and also providing capacity for a future development of 61 houses.
RainScape is providing effective sustainable solutions to a wide range of sewerage problems and often offers additional benefits to solving the problem that was initially identified.