October 2008 Issue 5
Storm Water Attenuation:-
Tanks or is there something else?
As developers the last thing you want to hear is …………
“attenuation required”……..“limit your discharge”…….“need large storage volume”
We understand that attenuation seems to you just complete over engineering and in reality means cost….cost… cost… and… more cost.
Surface water attenuation is the current buzz word for all regulators without them really understanding the bigger picture; often seeing it as a planning gain only. The real need to have attenuation is to ensure that people downstream of the project are not flooded by your proposals.
If the design principle of attenuation is not challenged or developed as a viable design, then it will result in expensive engineering works, with large underground pipes, tanks and control devices. Worst still, there will be surplus excavated material to move off site and some of this may be contaminated.
In order for contactors and developers to succeed successfully, all areas of the project need looking at to identify cost effective and innovative ways of achieving the development constraints, and this is where we can help you
An alternative solution
We have just completed the design of a new office building for the Primary Care Trust in Colne, Lancashire. The site had difficult ground conditions with contamination, deep soft fill material and a surface water pipe that was too small to accommodate our site surface water.
The new development covered the entire site area with a total impermeable area of almost 20,000 square metres.
The Attenuation Problem
Due to a very limited discharge rate of 20 litres per second the impermeable area created an extensive attenuation system. This could have created a significant costs associated with offsite disposal of the contaminated excavated materials, large storage pipes and major civil engineering works.
To give the contractor that tender winning design, we proposed the following simple solution……………..
The PWA Tender Winning Design
It was evident that all of the surplus excavated material had to be kept on site. Therefore a storage system at finished surface level had to be developed.
To avoid tanks we proposed to store the excess rainfall within the sub-base of a porous paving system, effectively at surface level. We did this by designing a 350mm drainage blanket of crushed single size demolition material to act as a surface water storage blanket. Over this blanket we then specified porous block paving to the car park bays and normal blacktop to the remaining areas.
The Benefits:-
• No exported contaminated material off site
• No expensive in ground attenuation system to construct
• Minimal underground piped drainage
• Sustainable drainage system for higher BREAM assessment
• Environmentally friendly as it provides pre-treatment of surface water as it flows through the clean stone
• No expensive up front drainage to construct, the storage system can be completed at end of project when the car park is built
• No risk of damaging in ground system by construction traffic
We can bring all of these benefits to your tender/development by embracing up to date techniques that combine a practical and a buildable approach.
For further information or assistance please contact Tim Davis on: 01535 633350 or timd@pwaite.co.uk
