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A Planning Guide to Sustainable Drainage Systems

Introduction

Working in co-operation with the Environment Agency, Severn Trent Water Ltd., the Highway Authority and developers, the planning system can play a vital role in preventing the drainage of urban developments from damaging our environment.

Conventional drainage systems have led to many problems including:

Flooding

Traditional systems have aimed to remove rainfall from impervious surfaces as quickly as possible resulting in higher rates of flow for shorter periods and flooding down stream.

Poor water quality in rivers, streams and ground water.

Surface water outfalls contain certain contaminants including oil, organic matter and toxic chemicals. Cross connection of surface and foul sewers can cause serious degrading of water quality.

Lower water table

An increase in impermeable areas caused by development results in less water available for infiltration into the ground, which reduces the volume of water stored in the ground, thereby decreasing ground water levels and the base flow of streams.

Harm to natural habits

The above factors, combined with erosion/deposition, caused by higher flows, and reduction in oxygen levels due to silt blanketing stream life can destroy natural habitats, flora and fauna.

The alternate approach which aims to safeguard the environment for existing and future generations is referred to as Sustainable Drainage Systems (SUDS).

What Are Sustainable Drainage Systems?

These are physical structures built to receive surface water run off from urban developments. e.g. ponds, wetland swamps, pervious surfaces and soakaways. These may provide treatment for water prior to discharge using natural processes of sedimentation, filtration, absorption and biological degradation.

By reducing the quantity of run off, slowing down flow rates to rivers and streams and treating water in a natural way, it will:

  • reduce the risk of flooding
  • improve the quality of water in rivers, streams and ground water
  • protect natural habitats
  • improve the appearance of urban areas as more water will be on the surface rather than underground

How Can The Planning System Help?

The two main rivers in the District (Severn and Stour) flow through at least one of the three towns. It is therefore particularly important that sustainable drainage measures are adopted here. The Development Plan provides the framework for drainage relating to urban developments. The Wyre Forest District Local Plan (Policy D.7) requires that "Wherever practicable and subject to other layout, design and conservation considerations, all development proposals should include infrastructure that directs surface water to sustainable drainage systems rather than to sewer and watercourses. Proposals for schemes in settlements adjoining the Rivers Severn and Stour will be subject to particular scrutiny".

Legal agreements may also be sought in riverside locations to reduce the impact of storm water run-off on flooding. Planning permission will not be granted for development contrary to this policy and the council will wherever practicable promote sustainable drainage measures. Permission will not be granted for urban development in flood plains (without the support of the Environment Agency) or where there is a risk of flooding and the development may accelerate the problem.

Types of Sustainable Drainage Systems

Rainwater Harvesting

A system that collects rain water from where it falls rather than allowing it to drain away. It includes water that is collected within the boundaries of a property, from roofs and surrounding surfaces.

Porous Surfaces

A surface that infiltrates water to the sub-base across the entire surface of the material forming the surface, e.g. grass, gravel, porous concrete and porous asphalt.

Pervious Surfaces

Surfaces that allow inflow of rainwater into the underlying construction or soil.

Contained flooding - it may be acceptable to allow shallow flooding of a car park once or twice a year rather than building a larger drainage system to cater for such events.

Infiltration Trenches

A trench, usually filled with permeable granular material, designed to promote infiltration of surface water to ground.

Infiltration Basins

A dry basin designed to promote infiltration of surface water to the ground.

Filter or French Drains

A linear drain consisting of a trench filled with permeable material, often with a perforated pipe in the base of the trench to assist drainage, to store and conduct water, but may also be designed to permit infiltration.

Swales

A shallow vegetated channel designed to conduct and retain water, but may also permit infiltration; the vegetation filters particulate matter.

Filter Strips

A vegetated area of gently sloping ground designed to drain water evenly off impermeable areas and to filter out silt and other particulates.

Detention Basins

A vegetated depression, normally dry except after storm events, constructed to store water temporarily to attenuate flows. May allow infiltration of water into the ground.

Bioretention Areas

Vegetated areas designed to collect and treat water before discharge via a piped system or infiltration to the ground.

Retention Ponds

These are features where run-off is detained for a sufficient time to allow settlement and possibly biological treatment of some pollutants.

Wetlands

A pond that has a high proportion of emergent vegetation in relation to open water.

What Can The Developer Do?

  • Seek advice before planning a development
  • Understand the drainage of the site and endeavour where possible to replicate the natural drainage regime of the site
  • Do not interfere with the natural surface water drainage
  • Recognise that works in or near a water course or within a floodplain are likely to require the Flood Defence Consent (please contact the Environment Agency for advice regarding obtaining consent)
  • Maintain or restore the natural flow regime of the receiving watercourse
  • Do not sever existing under ground drains
  • Make sure that septic tanks should be appropriately sited away from drains / watercourse

Where appropriate a flood risk assessment should be carried out and submitted with a planning application (see www.pipernetworking.com for guidance on flood risk assessments or contact the Environment Agency for advice).

Consideration should be given to the employment of sustainable drainage systems and they should be clearly described in the application. Soakaway systems should be employed where ever practicable.

What Can An Individual Do?

  • Use less water - this will reduce the amount of water being referred to the river system and could save you money
  • Install showers, low flush toilets spray taps and consider water recycling systems
  • Collect rainwater in butts/pools to reduce flows
  • Employ a qualified plumber, and seek advice to ensure that there are no cross connections between foul and surface water drains (this is an endemic problem)
  • Farmers can create pools/lakes for irrigation as may be required
  • Take sump oil and chemicals to recognised collection points - do not empty into the drains.


Wyre Forest District Council