on water discharge through the existing outlet? This was the situation experienced at Bert Henham No.2 Sportsfield in Auckland. The buzz words associated with designing drainage systems for this site were ‘stormwater neutral’, ‘attenuation’ and ‘throttling back’.
‘Stormwater neutral’ is undoubtedly a term that keeps city stormwater engineers happy. In simple terms, it means that a drainage upgrade can proceed as long as it does not result in making the calculated stormwater budget for a specified catchment area worse. So, if the design involves changing the surface contours of a sportsfield by importing fill material so that an improvement in levels is achieved but, at the same time, the improvement reduces the potential of the site to store stormwater in a ‘high rainfall event’, it may not get Resource Consent. Two solutions are possible. The first is to capitalise on any natural soakage potential of a site. Instead of shunting drainage water off site as quickly and efficiently as possible, for example by using conventional slit drains connecting to coarse gravel aggregate overlying swathes of perforated plastic drain pipe, carefully-designed ‘soakaway’ lateral drains without pipe (and with non- conventional backfill) can be installed that allow the groundwater to be recharged instead of to the sea, nearby river, pond or lake.
This was the solution used on this site.
City engineers had refused connection of a conventional sportsturf drainage system into a nearby soakage outlet because of capacity concerns. However, the natural soakage potential of the soil profile was estimated to be sufficiently permeable to allow discharge of the bulk of drainage water collected by the ‘soakaway’ lateral drains into the surrounding subsoil. Therefore, by ‘throttling back’ the flow of drainage water with deep, pipeless laterals and allowing them to discharge predominantly in-situ, we were able to
demonstrate that the capacity concerns of the nearby outlet could be alleviated. However, as an insurance against particularly high rainfall events, a single, under-sized ‘overflow’ main drain was still connected to the existing under- capacity outlet, although we suspect that this insurance main drain will rarely see a drop of water.
Conclusions
I am sure many of you in the UK will experience similar issues over the coming years, especially with the changing weather patterns. Stormwater
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management on sportsfields located in urban areas, and sportsturf drainage design in general, is starting to require a greater level of alternative design in order to achieve acceptable and sustainable drainage solutions.
Circumnavigating restrictions at difficult sites in order to find a drainage solution is the greatest challenge. Early preparation and the requirement to ensure that all affected parties have expressed support for the final methodology allows projects to proceed on schedule.
Dr Richard Gibbs, Manager, Sports Surface Design & Management, New Zealand.
The full article can be read at
www.pitchcare.com
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