What is storm attenuation (SuDS)? A guide for self builders.

When discussing requirements with clients, it soon becomes obvious that only a small minority fundamentally understand what SuDS are and why it is important.

Many describe it as an ‘unnecessary hurdle’ during planning, simply adding to the cost of their build with no tangible benefit.

SuDS (Sustainable drainage systems) prevent surface water flooding by slowing down the rate of water entering the drainage network.
Most systems comprise of some sort of holding area (normally underground tanks) that fills up during a storm. In the tank is method of releasing storm water at a rate the drains can handle without backing up. (A calculated orifice plate, or a vortex flow control.)

So when it rains hard, the tank fills up and lets the water out slowly. (Instead of dumping it all straight into the drains.)

Where systems are most effective is during very heavy storms (so the rate of storm water entering the drainage system would be high.)

In an ideal world, all domestic developments would divert all impermeable surfaces into a soakaway. This is the simplest and cheapest method (and doesn’t put any strain on the drainage system.)
The problem is not all developments can have a soakaway, for example;

  • Sites in clay or high water table areas where a soakaway won’t work.
  • Small or city based sites as soakaways must be a certain distance away from a development.
  • Riverside/ecological sites.

Built areas where a soakaway will be impossible to replace in the future

The question is often asked “Why are they needed, surely a self build isn’t going to make a difference to a drainage network?”.

Surface water flooding is becoming a real issue and any new development will turn permeable land impermeable. Planning authorities are very wary of adding more load to the drainage system that in many cases is already at (or over) capacity.
Climate change is also increasing the strain as storm events are becoming more intense.

When building, if your site is within an area where the drainage system is near capacity or it is widely known a soakaway won’t work in the area a stipulation may be placed on your planning consent.
This stipulation from your local LLFA (lead local flood authority) –part of your local planning office will ask you to provide evidence your development will discharge at a maximum rate. For example;
Your site must not exceed 2 l/s during a 1:100 year storm event +40% to account for climate change.

The water supplier often maintains the drainage system so will act as advisers to the LLFA.
In rural areas drainage is handled by the Internal Drainage Board (IDB) who again will advise the LLFA.

One of the problems with SuDS is the fact every system is bespoke. The calculations are also very complex and normally require a drainage engineer. (Plug #1; Our system is bespoke and but we provide the calculations free of charge.)
Our system (Rain Activ) starts at around £2000 including calculations. Larger roof areas or very limited discharge rates increase costs to around £3000.

Most SuDS system require specialist installation as the tanks require sealing and pressure testing. (Plug #2; Rain Activ use fully assembled shallow dig tanks that can be installed by any groundwork team.)
As you have diggers on site, it is normal to install at the beginning of the build with the foundations.

SuDS and maintenance share a very uneasy relationship. As part of planning consent, a maintenance plan should be submitted to make sure they don’t fail and cause flooding.
(Plug #3 Rain Activ is user maintained with a single check of the self cleaning filter per year. No more than 5 minutes and saves hugely on professional maintenance requirements.)

No, all systems including Rain Activ have no power requirement and therefore no running costs.

Rain Activ has been developed especially for smaller developments and self builders.
We believe we’ve developed a more cost effective solution (during purchase, installation, and maintenance.)
More information on how Rain Activ works and why we think it’s great can be found here;

Rain Activ does not just save costs verses a traditional SuDS system, it can also reduce your water bills. As we filter the water differently to other systems, Rain Activ can be used as a dual purpose system for both SuDS and rainwater harvesting.
The cost to add full domestic rainwater harvesting is as low as £500, reducing water bills by around 50%.