'Over the wall' drainage project

Find details of our project to direct rooftop drainage for waterfront developments over the sea wall.

The Southern Regional Flood and Coastal Committee (RFCC) recently approved a grant of £30,000 in Local Levy funding to support a West Sussex Lead Local Flood Authority-led Partnership Project.

This will explore 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.

The project had its launch on 2 March 2020 and was due to run until mid-June 2020. However, in view of the current working restrictions, the Lead Local Flood Authority (LLFA) will seek approval to extend the project for a further three months.

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 in the drainage system to accommodate rainwater that would otherwise drain when the tidal flaps are not closed by high sea-level.

Sea-levels are also predicted to rise by 1.15m between now and 2115 as the climate warms; as a consequence the period when the flap valves are closed 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. The pressure to build new houses is also targeting some of the few remaining sites that could potentially be used.

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 best 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
  • assess the relative costs and benefits of traditional versus ‘over the wall’ drainage
  • 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

Last updated:
15 December 2020
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