The Southern Regional Flood and Coastal Committee (RFCC) approved a grant of £30,000 in Local Levy funding to support a West Sussex Lead Local Flood Authority-led Partnership Project.
This project explored the feasibility, design challenges and potential benefits of directing rooftop drainage for waterfront developments over the sea wall rather than to traditional underground gravity drainage networks. We have referred to this innovative approach as ‘Over the wall’ drainage.
About the project
The geography of the coastline of West Sussex presents significant challenges for managing storm run-off following heavy rainfall. During winter months, particularly when ground water levels are high, rainfall run-off from the South Downs drains rapidly onto the coastal flood plain. This typically enters public sewers or piped (culverted) watercourses that drain via gravity to outfalls through the sea defences. These outfalls have a tidal flap to prevent sea-water entering into the drains.
When the sea level is higher than the base of the tidal flap, the water in the drains begins to back up. To prevent flooding there needs to be storage to accommodate rainwater that would otherwise drain when the tidal flaps are not closed by high sea-level.
Sea-levels are predicted to rise more than 1m between now and 2115; as a consequence the period when the flap valves opening is restricted and rainwater in the system backs up will increase significantly.
Areas where excess rain water can be stored inside the defended coastline are very limited. With pressure to build new houses targeting some of the few remaining sites with potential in our coastal communities.
The growth of coastal towns over recent decades, in combination with the increased intensity of rain storms caused by climate change, has resulted in storm water flows entering the system at rates that exceed their capacity to drain. This has caused flooding of some coastal towns.
The project partners include:
- West Sussex Lead Local Flood Authority
- Southern Regional Flood and Coastal Committee
- West Sussex County Council
- Adur & Worthing Councils - Engineers and Local Planning Authority
- Shoreham Port Authority
- Hyde Housing
- Southern Water
- Environment Agency
- University of Portsmouth
- Natural England
- Robert Bray Associates
- Ambiental Environmental Assessment
- HOP Consulting Civil and Structural Engineers.
As a result from the project we hope to:
- reduce the risk of surface water flooding for coastal communities in Shoreham and in the longer term for other areas of West Sussex and beyond
- demonstrate what works well and what doesn’t work in terms of ‘over the wall' drainage for waterfront developments
- work with developers in Shoreham Harbour regeneration to trial the ‘over the wall’ approach for design of drainage, along with other sustainable approaches such as rainwater harvesting and re-use
- initiate a wider debate on the possible merits of this novel approach to drainage for waterfront developments by sharing the results of the study to wider audiences
Supporting information
- LLFA presentation (PDF, 2.9MB)
- Robert Bray Associates presentation (PDF, 6.9MB)
- Southern Water letter of support for the project (PDF, 141KB)
- Note for developers of Shoreham Harbour regeneration (PDF, 971KB)
- Shoreham Harbour regeneration: Adur & Worthing Councils
- Shoreham port masterplan: Shoreham port authority