Over the wall drainage forum

Overview - 17 March 22

This project space has been set up to provide access to the project team members to relevant documentation, to help progress the over the wall drainage approach and achieve the project outcomes.

You have been sent the link as a trusted project partner and you have the ability to access and review the existing documents and upload other material that you consider pertinent to the project and of interest to the project team.  

After protracted delays to the project, I am delighted to report that work has today resumed on this study and West Sussex LLFA is working in partnership with EPG / STRI and Polypipe to review the existing data gathered for the project, complete data collation and analysis and prepare best practice guidance for Over the Wall drainage.

Updates on the key workstreams associated with the project are as follows:

Modelling of surface water flood risk and related reductions in flood risk from adoption of over the wall drainage has now been completed and two reports have just been uploaded to this blog site.

Relative costs of traditional vs. over the wall drainage.  Use of actual costs has kindly been made available by one of the Project Partners and analysis of this can now proceed.

Please share your views and provide your comments using the link provided to the comments form below. There will also be an opportunity to upload a document and share it for others to comment should you wish to.

Your comments will be reviewed and then published on this site for other stakeholders to see (providing appropriate consent to do so has been given).   Publishing your views enables others to make a considered response if they wish to do so. I will also publish shared documentation provided your permission has been given to do so.

All published responses will be made available on the 'landing page' (or title page) for this project.  Published responses may be accessed by scrolling down to the 'Published Responses' section, which is just underneath the 'Comments Form' link near the bottom of the page.  Please click on the Published Responses link to access all published project comments.   Comments can be sorted by date or alphabetical order by clicking on the appropriate link.  The search function can also be used to look for comments with key words.

Stakeholder list:

Adrian.Jackson@environment-Agency.gov.uk

AndrewK@hop.uk.com

BRousell@shoreham-port.co.uk

Bruno.Venturini@wsp.com

Bryan.Bleeker@southernwater.co.uk

Chris.Jones@adur-worthing.gov.uk

Cigolene.Nguyen@southernwater.co.uk

glenn.longley@adur-worthing.gov.uk

jezeph@yahoo.com

kevin@robertbrayassociates.co.uk

kevin.macknay@westsussex.gov.uk

Laura.Gibbons@adur-worthing.gov.uk

Simon.Sharp@southernwater.co.uk

Sophie.Brown@naturalengland.org.uk

stephen.cantwell@adur-worthing.gov.uk

steve.mitchell@port.ac.uk

Tom.Gillham@westsussex.gov.uk

Charlotte.Markey@polypipe.com

Maria.Menga@polypipe.com

johnroberts@epg-ltd.co.uk

lee.swift@environment-agency.gov.uk

Jenny.Hornsby@environment-agency.gov.uk

Lee-Ann.Maritz@polypipe.com

david.burgess@polypipe.com

Project Aim

To explore the feasibility, design challenges and potential benefits of directing rooftop drainage for waterfront developments over the seawall rather than to traditional underground gravity drainage networks the discharges of which are constrained by tidal flaps.

Background to the Study

The geography of the urban coastal strip of West Sussex presents significant issues for surface water management.  During winter months, particularly when ground water levels are high, pluvial run-off from the South Downs drains rapidly onto the coastal flood plain and typically enters culverts that drain via tidally constrained gravity outfalls through sea walls.

Potential flood storage inside the defended coastline is very limited and pressure for housing development is increasingly targeting some of the few remaining sites that offer such capacity.   The expansion of urban areas over recent decades, in combination with the increased intensity of rainfall events associated with climate change, has resulted in storm water flows entering the system at rates that exceed their capacity to drain.  Furthermore, as one looks ahead throughout the lifetime of new developments the constraints on the traditional surface water sewer network will increase as a consequence of sea level rise that will further restrict the period during which existing tidal flaps can operate.

Figure 1 illustrates the problem of constrained surface water discharge in and around Shoreham Harbour.

Figure 1 Plan of Shoreham Harbour showing surface water flood risk.

Figure 1: Plan of Shoreham Harbour showing surface water flood risk

Shoreham Harbour is the subject of a major regeneration forming a key element of the Adur Local Plan and the Joint Area Action Plan https://www.adur-worthing.gov.uk/shoreham-harbour-regeneration/about/

The regeneration offers a unique opportunity to work with developers  through the master planning process to trial an innovative and more sustainable approach to managing rooftop run-off that will form a significant element of the drainage from the development.

Outcomes

• Contribute to reduction in surface water flood risk for coastal communities in defended floodplain;
• Investigate the opportunities, constraints and overall feasibility of ‘over the wall’ drainage for waterfront developments;
• Work with the Kingston Wharf development team in particular to apply this innovative approach to the design of drainage for the scheme.
• Influence developers to adopt more innovative and sustainable design in drainage; it is stressed that the concept of over the wall drainage does not necessarily rule out the scope for rainwater harvesting / re-use where space and feasibility allows; such an in-combination option will be explored as part of the study;
• Assess the relative costs and benefits of traditional vs ‘over the wall’ drainage; and 
• Though publication and dissemination of the Study, initiate a wider debate on the possible merits of a changed approach to drainage of waterfront developments.