The solution to flooding problems is the use of more sustainable methods of storm water drainage which take into account quantity, quality and social issues. These are now referred to as SUDS (Sustainable Urban Drainage Systems). SUDS are made up of a number of structures, which include:
- Permeable pavements
- Swales and filter strips
- Basins and wetlands
- Infiltration devices
They are more sustainable because they:
- deal with run off close to where it falls;
- manage potential flooding at its source;
- protect or enhance water quality;
- provide a habitat for wildlife in urban areas;
- protect water resources from accidental spills and pollution;
- allow new development in areas where existing sewerage systems are at full capacity, enabling new development within existing areas;
- are sympathetic to the environmental setting and the
- needs of the local community;
- encourage natural groundwater recharge.
The inclusion of a permeable paving surface as part of a SUDS solution will not only increase the likelihood of planning proposals being approved but may also offer the developer a 10-15% cost savings over traditional methods due to a significant reduction in drainage costs (pipes, manholes etc). Machine laying the paving blocks also reduces costs.
Water quality improvement
Permeable paving is very effective at removing pollution from runoff. The pollutants may remain on the surface or may be flushed into the underlying pavement layers where a very high percentage of the pollutants are filtered, trapped or degrade over time. For more detailed information on water quality please refer to the Guide to the design, construction and maintenance of< concrete block permeable pavements - Edition 5 produced by Interpave available for download on www.paving.org.uk
Four Elements of Permeable Paving
There are four elements which make up permeable paving:
- Permeability
- Detention storm water
- Pollution Control
- Structure
Permeability
The Hydropave range of products have permeability rates of 1800 litres/sec/hectare, providing a significant safety factor on the required permeability rates of 180 litres/sec/hectare.
Detention
The sub-base thickness should be designed to detain rain falling throughout 24 hours and must provide at least 30% void space. A 4/20mm coarse open graded crushed rock to BS EN13242:2002, value of 100kN or more must be used. If the designer opts to specify alternative materials, a site trial must be undertaken to ensure that the laying course material does not invade the subbase material.
The laying course and jointing material must be a 6.3-2mm grit to BS EN13242:2002. This has been proven in tests to perform best and will not invade the sub-base material. In ground conditions where water cannot infiltrate into the subgrade, an impermeable membrane must be laid between the subgrade and sub-base and wrapped up the sides to detain the water.
Pollution
As water flows slowly into the sub-base it collides with individual aggregate components and deposits pollutants on their surface. The large surface area of the aggregates will ensure effective filtering of the pollutants which are then broken down by natural microbial action or eventually rest at the bottom.
Structure
The specification of a permeable paving structure depends upon the hydraulic and traffic loading characteristics and upon the properties of the subgrade. Most pavements in the UK will be required to have a water detention capability rather than infiltration because of the subgrade types. It is recommended to design a permeable paving system to store rainfall occurring during 24 hours. For information relating to the BREEAM ratings of concrete and natural stone products please refer to ‘The Green Guide to Specification’ 3rd Edition 2002. Or alternatively visit www.breeam.org
Tobermore have an extensive range of Hydropave Permeabale Paving products. Many of these quality products can be machine laid which adds a further opportunity for increased cost savings.
To see an working illustration of how Hydropave Permeable Paving Systems works CLICK HERE to see a case study feature on a housing development, Kenmare, Co.Kerry.