ENVIRONMENT INDUSTRY MAGAZINE

Rainwater harvesting is now seen as a means of reducingmains water consumption, something that we are now allunder strong official pressure to do. It is also now recog-nised that rainwater harvesting can at the same time contribute to attenuating flood water flow, i.e. holdingsome storm water that comes off the roof and letting itflow at a controlled, slower, rate to the drain or soakaway.

While flood attenuation has previously required stand-aloneholding tanks, planning applications that include rainwaterharvesting as part of the storm attenuation plan are nowviewed favourably. In many cases it is made a requirement,particularly for flood attenuation. What the planners arelooking out for are ways of preventing drains overflowingduring what seem to be our increasingly frequent veryheavy downpours. With the growing prevalence of hard-standings for parking in front of houses, the situation is

made worse with storm run-off racing to the drains ratherthan filtering through permeable surfaces like lawns andgravel drives. In fact, since October 2008, planning per-mission is now required to lay traditional impermeabledriveways that allow uncontrolled runoff of rainwater fromfront gardens onto roads. If a new driveway or parkingarea exceeding 5 square metres in area is constructedusing, for example, permeable concrete block paving,porous asphalt or gravel, or if the water is otherwise ableto soak into the ground, planning permission is not required.The new rules also apply where existing hardstandingsare being replaced and, despite starting off as provisionsfor built-up areas (SUDS stands for Sustainable UrbanDrainage Systems) are being applied in the country too.

When installing a rainwater system, there are two ways ofdealing with overflow from the storage tank: either excess

Reduce your tap water use and reduce flood water by Marcus Bicknell –Partner, Marketing and Technical, of RainWater Harvesting Ltd

Flood attenuation is very much a priority for planners nowadays.Marcus Bicknell from Rainwaterharvesting.co.uk explains howrainwater harvesting can be used to offset flood threats.

RainWater Harvesting and Flood Attenuation

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ENVIRONMENT INDUSTRY MAGAZINE

water goes to a soakaway or back to the main drain. Forthe homeowner or installer, this choice should normallybe a matter of convenience. The main drain is the easiestsolution as it avoids having to build a soakaway. Howeverconnection to the main drain is not always permissible,making the construction of a soakaway necessary. For theplanner and the water company, a soakaway solution is thebest, as this makes for less rainwater pouring into localdrains. In fact, the new Flood Management Bill removesthe automatic right to connect to main drains, giving localauthorities the power to impose other drainage solutions.

Normally speaking, rainwater for use in the house for WCsand washing machines and for outdoor needs is collectedoff the roof. Filtered and stored underground, the rainwateris clean and fresh enough for these non-potable domesticuses. The rainwater coming off nonporous ground-levelsurfaces risks being polluted by animal droppings, oil orchemical spills, and even after filtering would not normallybe suitable for use in WCs and washing machines. Itcould, however, be used for outdoor purposes.

A normal rainwater harvesting system does, in any case,relieve pressure on main drains because a large volumeof rainwater off the roof is being diverted into the storagetank and thence into the home for use. If, however, thattank is full or nearly full, the excess rainwater in a heavydownpour will overflow.

One solution is to install a larger capacity storage tankthan would be required for recycling the rain. In additionto the overflow siphon at the top of the tank, a secondoverflow is set at a much lower level. Rainwater is storedbelow this in the normal way, but in a heavy downfall theexcess rain filling the top half of the tank exits via a devicethat slows the flow rate to the main drain. This can be afloating drain choke or a metal attenuating flange fitted inthe lower overflow hole in the side of the tank. This flangehas different-sized holes in it and can be set to the flow-rate required, for example, from 1 to 6.5 litres per second.

Alternatively, a length of perforated drain can be put between the tank exit and the main drain to dissipate theexcess rainwater. Both 1200mm diameter concrete pipesand plastic crates are often used to make big attenuationcells.

Calculating the amount of water in a one in a 10-year orone in a 100-year storm is a task for a specialist consultantengineer, but for some developments the amount of

water to hold, temporarily, can be hundreds of thousandsof litres. In the case of a single home, the soakaway forattenuation is often double the capacity of the rainwatertank. In extreme cases, planners may require the total attenuation volume to be big enough to hold one half ofthe annual rainfall coming off the roof. It must also reachbelow any impervious layers so that the water flows awayto ground within a day. It should also be at least 2 metresaway from the storage tank.

Clearly it makes sense to use the rainwater wheneverpossible, so various combinations of storage tanks withattenuation products can be installed depending on thesite.

For further informationvisit: www.skeletank.comor contact SEL Environmental LtdT: 01204 885555E: skeletank@selenvironmental.com

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The Skeletank® is a unique high strengthunderground tank for rainwater harvesting thathas been developed for the residential constructionmarket and can provide a complete sustainabledrainage solution (SuDS) on a house by house basisavoiding shared liabilities.

In addition to storage for rainwater harvesting, theSkeletank® can be used as a soakaway and it can beused to attenuate rainwater where a restricted dischargenotice is in place.

In fact the Skeletank® can simultaneouslysatisfy the requirements of the code for sustainable homes,the building regulations part H, BS 8515-2009 and theconditions of planning as a result of PPS25.

The Skeletank® is a high-strength structure that is intendedto be installed underground but at the shallowest possible depthjust beneath the surface of a driveway, patio or lawns.

When installed each Skeletank® can hold upto 300litres of fluidand because it’s modular you can simply join multiple tankstogether to suit your requirements or budget.