TDP considers surface water drainage alternatives

Rob Barlow of TDP looks at the increasing challenges posed by drainage problems and considers more sustainable alternatives...

As development has continued to increase, the over use of hard, impermeable materials for roads, driveways and car parks has led to drainage problems which have become a major issue, leading to an increase in flooding and environmental contamination, due to excessive surface water run-off into rivers and groundwater.

The problems with surface water drainage have also been exacerbated by changes in the climate, where storms and heavy rainfall now often exert undue pressure on existing flood control and drainage systems and cause localised flooding and water pollution which looks set to increase as weather patterns continue to change.

The Foresight Future Flooding report estimated that currently 80,000 properties are at significant risk from surface water run flooding, causing on average GBP270m of damage each year (Source: Foresight 2004 Government Report).

Hard surfaces can increase surface water run off by up to 50 per cent which results in drainage systems being overloaded very quickly which can lead to localised flooding.

Serious flooding in 2007 affected several parts of the UK causing damage estimated at about GBP3bn (Source: Environment Agency).

This has lead to a demand for effective solutions to counteract the problem, a different strategy for drainage that keeps water on site for longer, prevents pollution and allows storage and use of the water.

Sustainable Urban Drainage Systems (SUDS) offer an alternative to existing methods which are not meeting the needs of development and climate change.

SUDS are designed to deal with surface water run off in a controlled way, to minimise or prevent flooding.

The philosophy behind SUDS is to try to mimic natural systems that use cost effective solutions with low environmental impact to drain away dirty and surface water run-off through collection, storage and cleaning before releasing it back into the environment.

Ideally, SUDS should be easy to look after, hard wearing, attractive in the environment and require little or no energy input.

An effective SUDS design includes measures to prevent pollution, reduce surface water run-off at source and also to offer a range of physical structures designed to receive the run-off.

Source control Source control measures include the replacement of hard, impermeable surfaces with permeable areas which allow excess drainwater to soak into the ground.

The trend for homeowners to convert their front gardens to driveways using paving and tarmac has prompted the rise in impermeable surfaces in urban areas which has exacerbated the problems with surface water drainage during heavy rainfall.

This issue has now been addressed by legislation.

From 1 October 2008 planning laws that allowed homeowners to pave their front garden without planning permission have changed in order to reduce the impact of this type of development on flooding and pollution of the watercourses.

Planning permission is not required if the new driveway features porous paving such as gravel, permeable concrete block paving or porous asphalt or if the rainwater is directed to a lawn or border to drain naturally.

However, if the area to be paved is larger than 5m2 and the intention is to lay a traditional, impermeable surface, planning permission will be required.

A sustainable future Evidence is gathering pace that the traditional approach to drainage is not only damaging the environment but is not sustainable.

As these two projects illustrate, porous landscaping offers an attractive, cost effective alernative to existing methods of dealing with surface water drainage.

Case Study: Porous surfaces are also becoming a key requirement of large developments and a recent 13,000m project located within the historic Syon Park - the Syon Park Waldorf Astoria Hotel - has incorporated permeable surfacing for a car park and emergency access road.

Due to the sensitivity of the new development, the 500 space car park that jointly serves the hotel, garden centre and zoo, had to be visually appealing whilst providing a durable, high quality, porous surface that was safe for intensive pedestrian and vehicle usage.

To satisfy this design criteria, project architect Ettwein Bridges specified gravel filled TDP Netpave 50 as the surfacing solution with white Netpave markers to delineate the parking bays.

In addition, Meinhardt, the structural engineers also specified TDP Netpave 50 as the surfacing solution to the emergency access road which has been designed to withstand trafficking from fully laden fire engines.

Darwin Forest Country Park, set within mature woodland in Derbyshire's Peak District has recently added 2,000m of additional car parking which needed to be in keeping with its natural setting.

The design and build developer, Pinelog, chose to install TDP Porouspave infilled with locally sourced gravel.

In keeping with its environmental ethos, Pinelog's architect, Roger Langham, incorporated a sustainable drainage solution following discussions with the Environment Agency who imposed a limited discharge consent on any development at the site.

Percolation tests confirmed that infiltration was possible and TDP Porouspave was chosen as the most appropriate surfacing solution in terms of performance, aesthetics and cost.

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