By Using Less Tap Water You Can Help Reduce Flood Water
Rainwater harvesting is now looked upon as a way of reducing mains water consumption, which is something that we are now all under strong official pressure to do. Nowadays, it is also recognized that rainwater harvesting can at the same time contribute to attenuating flood water flow, by holding some of the storm water that comes off the roof and letting it flow at a controlled, slower, rate to the drain or soakaway.
Before flood attenuation needed stand-alone holding tanks, planning applications which have rainwater harvesting as part of the storm attenuation plan are now viewed favourably. In many cases it is made a requirement, particularly for flood attenuation. The planners are seeking methods of stopping drains overflowing in what appear to be our more frequent very heavy downpours.
With the growing prevalence of hardstandings for parking in front of houses, the situation is made worse with storm run-off racing to the drains rather than filtering through permeable surfaces like lawns and gravel drives. In fact, since October 2008, planning permission is now required to lay traditional impermeable driveways that allow uncontrolled runoff of rainwater from front gardens onto roads. If a new driveway or parking area exceeding 5 square metres in area is constructed using, for example, permeable concrete block paving, porous asphalt or gravel, or if the water is otherwise able to soak into the ground, planning permission is not required. The new rules also apply where existing hardstandings are being replaced and, despite starting off as provisions for built-up areas (SUDS stands for Sustainable Urban Drainage Systems) are being applied in the country too.
There are two ways of handling overflow from the storage tank when installing a rainwater system; either excess water goes to a soakaway or back to the main drain. For the homeowner or installer, this choice should normally be a matter of convenience. The main drain is the easiest solution as it avoids having to build a soakaway. However connection to the main drain is not always permissible, making the construction of a soakaway necessary. For the planner and the water company, a soakaway solution is the best, as this makes for less rainwater pouring into local drains. In fact, the new Flood Management Bill removes the automatic right to connect to main drains, giving local authorities the power to impose other drainage solutions.
Normally speaking, rainwater for use in the house for WCs and washing machines and for outdoor needs is collected off the roof. Filtered and stored underground, the rainwater is clean and fresh enough for these non-potable domestic uses. The rainwater coming off nonporous ground-level surfaces risks being polluted by animal droppings, oil or chemical spills, and even after filtering would not normally be suitable for use in WCs and washing machines. It could, however, be used for outdoor purposes.
A normal rainwater harvesting system does, in any case, relieve pressure on main drains because a large volume of rainwater off the roof is being diverted into the storage tank and thence into the home for use. However, if that tank is almost full or full, during a heavy rainstorm the extra water will overflow.
One solution is to install a larger capacity storage tank than would be required for recycling the rain. In addition to the overflow siphon at the top of the tank, a second overflow is set at a much lower level. As normal, rainwater is stored below this, although in a heavy downfall the excess rain in the top 50% of the tank leaves via a device that slows the speed which it flows to the main drain. This can be a floating drain choke or a metal attenuating flange fitted in the lower overflow hole in the side of the tank. This flange has a variety of sized holes and has the facility to be set to the necessary flow-rate, e.g. 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 the excess rainwater. Both 1200mm diameter concrete pipes and plastic crates are often used to make big attenuation cells.
Calculating the amount of water in a 10-year or 100-year storm is a task for a specialist consultant engineer, but for some developments the amount of water to hold, temporarily, can be hundreds of thousands of litres. When referring to a single household, the soakaway for attenuation is usually double the capacity of the rainwater tank. In extreme cases, planners may require the total attenuation volume to be big enough to hold one half of the annual rainfall coming off the roof. It must also reach below any impervious layers so that the water flows away to ground within a day. It should also be at least 2 metres away from the storage tank.
Clearly it makes sense to use the rainwater whenever possible, so various combinations of storage tanks with attenuation products can be installed depending on the site.