Planning Practice Note:
Surface water, land drainage and flood protection
INTRODUCTION
Surface water, land drainage and flooding have always been matters of concern to LPAs. In recent years, however, there has been a rising rate of flooding incidents, some on a very large scale. The reasons for these are complex and inter-related, but they include:
- an increase in occurrences of very heavy rainfall over short periods. This may be a side effect of global warming.
- raised water tables arising from higher rainfall.
- increases in the speed at which rainfall finds its way into rivers and streams. This can be caused by:
- large new developments (including roads) which have surfaces which do not absorb water;
- changes to agricultural practice which reduce water retention at times of heavy rainfall.
- new development in areas which should be reserved for flood management.
- alterations to drainage patterns and systems (whether designed or accidental) which reduce capacity to handle peak flows.
- poor maintenance of surface water drains.
A leaflet of this type can only deal with the issues in outline.
PLANNING POLICY
Planning policy in South Shropshire seeks to reduce the frequency, scale and impact of flooding both locally and in the river system. This includes contributing to national initiatives to reduce flooding incidents in other Districts downstream. The Government's planning guidance "Development and Flood Risk (PPS25)Dec 2006" recommends a risk-based approach to development.
Planning permission will not be granted where such risks exist. The promotion of best drainage practice in developments will be encouraged. The Environment Agency's (EA) advice will usually be sought.
SITE SELECTION
Planning permission will not, normally, be granted for developments in areas:
- shown on the EA's indicative flood plain map (IFPM). The IFPM shows those areas where it is essential to maintain capacity for flood storage or out-of-bank flows.
You can find the IFPM on the web- The Environment Agency Website - Flood Advice (External link)
- where there is a history of localised flooding.
- where there are indications that the development itself may create flooding problems.
Developers are advised to investigate these issues at an early stage.
SITE INVESTIGATION
Make sure you fully understand the drainage of your site before you prepare any scheme. In dry periods sources of surface water may not be obvious. Ditches that are apparently dry may sometimes be full (or even overflowing). Springs that have been dormant for years can suddenly reappear. Try to obtain information from previous owners and local people.
Natural drainage pattern
Development should not interrupt the surface water drainage of surrounding land. Provision should be made to ensure that existing drainage systems continue to operate effectively and that neighbouring owners are not adversely affected.
Land drains
Make sure you know the whereabouts of any land drains. Problems often occur when these are severed. If septic tank drainage is located close to land drains, pollution of watercourses could occur for which you would be responsible. Land drains may pre-date the subdivision of your property, and you may be responsible for drains that start on your neighbour's land.
Water courses
You need consent from the EA to alter watercourses (including culverting and dredging) or impound water from them.
Flood risk assessment
Applicants are responsible for:
- providing an assessment of whether:
- the development is likely to be affected by flooding;
- it will increase flood risk elsewhere;
- proposing measures to deal with these risks;
- satisfying the LPA that the risks can be managed with the minimum environmental effect, to ensure that the site can be developed and occupied safely.
The EA is happy to advise developers on the scope of flood risk assessments.
USE LESS WATER
Try to design your scheme so that it reduces the amount of mains water you use. This will reduce the amount of water being returned to the river system and save you money!!
In the home
Install devices such as showers, low-flush cisterns and spray taps.
Rainwater collection
Collect rainwater for outdoor use:
- in butts or pools for use in the garden;
- in ponds or lakes for use in agriculture. These usually need planning permission. Please ask for advice.
Water recycling
Consider using a water recycling system such as grey water separation, but remember that this is a specialist area of design.
Water abstraction
If you intend to abstract water from the ground or from a river or stream, you may need a licence from the EA.
THE DESIGN OF SURFACE WATER DRAINAGE
Existing drainage
Make sure your proposal is designed to cater for the existing drainage patterns on the site as well as the new ones you will create with your development.
Separation from foul drains
Surface water should not be discharged into foul drains because this can:
- reduce the effectiveness of the sewage treatment plant;
- lead to overloading of sewers and pollution at times of peak flows;
- result in water being returned to the river system too quickly.
Use of soakaways
Only use soakaways where:
- ground conditions are suitable;
- water tables are low enough that they will work properly;
- they will not receive effluent from septic tank outfalls.
Slow down run-off rates
Try to slow down the rate at which water runs away from your site using:
- ponds, lakes, grass swales or wetland habitats;
- permeable surfaces for car parking and other paved areas where ground conditions are suitable, and the risk of pollution is low;
- French drains (gravel filled trenches) rather than pipes;
- hydrobrakes at pond or reservoir outfalls where the downstream drain capacity is limited.
The EA expects developers to slow run-off to greenfield run-off rates for a 1 in 100 year event.
Outfalls to watercourses
Outfalls to watercourses need consent from the EA and should be designed with headwalls and spillways to avoid erosion.
Avoid pollution
If there is a risk of pollution from oil or chemicals from roads and industrial and parking areas, consider the following remedies:
- silt and oil traps;
- biological filters in layered French drains or hard surface construction.
Reduce maintenance requirements
You can reduce the risk of drain failure through lack of maintenance by:
- making sure pipes are laid at gradients which promote self-cleansing;
- placing traps, rodding points, inspection chambers and manholes where access for cleaning or emptying is easy.
CONTACTS
The Department for Communities and Local Government (DCLG)
- www.communities.gov.uk (External link)
Environment Agency
- The Environment Agency Website - Flood Advice (External link)
South Shropshire District Council
- Land drainage and pollution control advice (01584) 813384 or 813385
- Planning Support (01584) 813000
© Copyright South Shropshire District Council,
Stone House, Corve Street,
Ludlow, Shropshire, SY8 1DG.
Tel: (01584) 813000
Fax: (01584) 813128
e-mail: planning@southshropshire.gov.uk
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Subscribe to changes to this page here.Author: Planning and Building Control. Last Updated: 15/2/2007.
