The principle behind an attenuation tank is that rainwater from a site is collected in the tank and allowed to slowly discharge into the downstream drainage system at a slow rate equal to the rate of rainwater run off before the site was developed. This storage and slow release of rainwater, reduces the risk of flooding in the downstream drainage system, including creeks and rivers.
It is now common practice for local authorities and water companies to restrict the allowable discharge from a development. How this is calculated can vary. A flat rate per hectare is commonly allowed (say 5.0 l/s), or there are various systems based on ground gradients and soil types.
Types of Tanks
All large attenuation tanks are constructed underground. The only exception I can think of is the water butt, where a small area of roof can be stored.
All types of tank are low maintenance and easy to construct, and the principles of how they work are easy to understand by the maintenance gangs when the construction gangs have left the site.
The most common system is a large tank or pipe with a restricted discharge at the bottom end. The tank can be made of large drainage pipework, plastic tank, or concrete and corrugated steel. This has the advantage of being cheep, fairly easy to install, and robust.
No, not the local duck pond, but an excavated and engineered hole in the ground, probably lined with plastic sheeting. Obviously it can be made into an attractive feature on a site, but they are not as cheap as it would look at first sight, and they do use up a lot of the site area, which then becomes unavailable for building.
Yes, small plastic crates, which can easily be handled by one person and which surrounded with a plastic sheet, are used to form a tank. These are becoming more common due to light weight, relatively shallow depth of excavation, and a reduced need for lifting equipment. However, it is more labour intensive.
Road Sub-base Construction
One interesting option is for rainwater to be allowed to percolate through the surfacing and stored within a special sub-base that contains a high proportion of voids. This has no maintenance requirements and involves little extra excavation and cost. The sub-base also filters pollutants out of the rainwater. The weak point is the road surfacing – how to ensure the surface will remain porous over many years of grit and grime.
Restricting the Discharge
Two basic systems are available: use a small hole or vortex (whirlpool).
It is possible to calculate how much water can flow through any size hole for any height of water. (Many are a plate with a hole or a small pipe). So when the local water uuthority has given a discharge rate, it is possible to calculate how small the hole needs to be to comply with the requirements.
A steel fabricated unit with no moving parts. The incoming water is funnelled into a vortex, which has a known discharge rate. The advantage is the manufacturer works out what size of vortex control is needed, and it has a clear opening that is larger than that of a simple hole. Thus there is less chance of a blockage.
There is a bewildering array of different systems available to comply with SUDS requirements. I hope this article will help with selecting methods of on-site storage. There are other methods available such as soakaways, green roofs, swales, etc.
Don’t forget prevention! If you can stop runoff by reducing drained hard surfacing, it reduces costs, takes less time to construct, and is automatically SUDS compliant.