Drainage Strategy Reports
New development or redevelopment sites
The drainage of any new development or redevelopment site is an important planning consideration. A drainage strategy report would aim to satisfy this by carrying out an appraisal of the options for the disposal of the foul and surface water flows and by presenting the details of the most cost effective solution in each case.
Need to consider drainage matters from an early stage
It is suggested that drainage matters are best considered at an early stage in the overall design process, even if not immediately required by planning considerations. Drainage matters can take a considerable time to resolve and can have significant implications for the layout of the site.
Preliminary Drainage Layout
The preliminary drainage layout showing the proposed on and off-site drainage arrangements would be produced based on the overall site layout produced by others. It would show the following:
• routes of the main on-site foul sewers
• sites of any foul pumping stations
• main surface water drainage routes (pipe networks,
• sites of any SUDS site control measures (ponds,
A preliminary estimate of the depth and size of the proposed sewers / drains / channels would be made together with the size and capacity of any foul pumping stations or surface water site control measures (ponds or basins). Items with abnormal cost implications would also be highlighted. Constraints would also be identified, such as for example, an existing public sewer crossing the site which would need to be diverted under section 185 of the Water Industry Act, 1991 (WIA, 1991).
It is anticipated that the completed preliminary drainage layout would then feed back into the overall site layout so that it could be amended accordingly.
Further details of the foul and surface water drainage arrangements are discussed in the following sections.
Foul water drainage
The first stage of the assessment of the options for the foul drainage of a site would be to estimate the foul flow to be discharged into the public sewer system. The position of the existing public sewers in the area would then be investigated and the developer services section of the water company consulted to identify a suitable point of connection.
It may be that a sewer requisition under section 98 (of the WIA, 1991) would be required to allow the off-site foul sewer to cross private land and we can advise on the different payment options for this. Any constraints with significant cost or programme implications (ie crossing Network Rail land) would also be identified.
Surface water drainage
The document 'Preliminary rainfall runoff management for developments' published by the Environment Agency would be an important source document for this work.
Investigation of the options for the surface water drainage would involve an initial consideration of the types of SUDS techniques likely to be applicable at the site. This would be determined in large part by the permeability of the ground and it is important to base this on actual percolation tests if at all possible. Any of the rainfall run-off from the site not disposed of via infiltration into the soil or similar means would need to be attenuated by site control measures. This would be to ensure that the overall run-off from the site was not increased. As part of this overall process the Environment Agency / Council would be consulted.
The means to ensure the long-term maintenance of the surface water drainage arrangements would also need to be considered. In addition, any obvious constraints, such as potential land issues for any off-site surface water drainage, would also be identified.
For a site in England larger than one hectare, the part of a drainage strategy report covering surface water drainage duplicates some of the contents of a Flood Risk Assessment, so there may be advantages in combining the two reports.
We have a strong capability in the design of all types of drainage for new developments, please
for further details.