Those old enough to remember, will recall that SuDS was first introduced in the 1960s for new builds and in the last two decades has been mandated as part of planning as a method for reducing loading in sewers, preventing pollution, re-charging groundwater, reducing flooding, attenuating flows and creating civic amenities such as water features, etc.
In spite of resistance from the building industry over cost and loss of building land, the support for SuDS is almost universal throughout government circles and planning departments. However, like everything in life there is also a risk attached to the widespread deployment of SuDS in particular when used as part of a drainage scheme for industrial and commercial areas.
In this modern world dominated by risk assessments, it is very disappointing to find that very few industrial and commercial sites, other than those designated under the Control of Major Accident Hazards Regulations 1999 (COMAH) have ever been subject to risk assessments with regard to pollution from spills and firewater. Hence, it comes as no surprise that similarly very few SuDS schemes have been assessed and consequentially very little thought has gone into their design with respect to risk minimisation.
For those not familiar with the term firewater, this is the collective noun for the cocktail of water, fire fighting foam and combustion by-products that is produced during a fire. Just like a chemical spill, firewater can be equally and in many cases more damaging to the environment. During a fire volumes can be enormous often building up in a matter of a few minutes to many tens of thousands litres.
The consequences can be dire, should a major spill or firewater enter a SuDS chamber, swale or pond. The contamination that could result to the soil, sub-soil and groundwater could be so serious that it requires the excavation and removal of the whole SuDS scheme and the surrounding soil. On top of this are the fines issued by bodies such as in England and Wales, the Environment Agency and in Scotland, the Scottish Environmental Protection Agency SEPA. Plus there costs for labour and then the potentially even more significant costs associated with the reinstatement of the land, the groundwater and paying for the long term monitoring thereafter.
Land values could also drop through the floor resulting in litigation from the land’s freeholders. The list goes on and on, driven by the sweeping powers under the Environmental Liability Directive 2004/35/EC which if you have suffered an incident you would have certainly breached. The final shock is from the insurance industry, as often insurance companies turn around and say we are happy to pay for the reinstatement of the building but do not cover the cost of fines, the environmental damage and the associated cost due to the bad PR that follows a public tabloid flogging.
So what should be done when proposing a new SuDS scheme and how about existing ones? There are many good publications and websites worth a visit including British Water www.britishwater.co.uk , CIRIA www.ciria.org , the Environment Agency via the www.gov.uk portal or SEPA www.sepa.org.uk or the www.susdrain.org setup by CIRIA. However, in spite of the volume of publications you will find it virtually impossible to find a document focussing on specifically industrial SuDS, other than references to the use of oil interceptors and various filters to collect and remove diffused pollution from highways and car parks.
Our personal recommendation is to visit the CIRIA website and download their latest document C736 entitled Containment Systems for the Prevention of Pollution and to adapt the guidance and apply it to your SuDS project.
The risk assessment:
The risk assessment should look at things including; what is handled on site, what is stored, what is the risk of fire, is the site fully bunded if so is the capacity just for spills or does it include firewater, are the drains segregated, what is the topology of the land, where will firewater run, etc. Remember that just because the site is not necessarily COMAH registered this does not mean to say that there is no risk!
Take the example of new build destined to become a parcel Distribution Company or DIY store warehouse. You will have pallets and shelves stacked with large quantities of substances that may range from paints, soft furnishings, camping gas, through to pesticides. Ironically, many of these items originated from COMAH registered sites, yet the warehouse will not be registered. Each item on its own has a quantifiable risk, however mixed together in a fire who knows what will result? Probably nobody does in truth and consequentially now you have a risk from both spills and firewater to deal with.
Finally, please don’t forget to look at your sewers. Think about what happens if your sewer block, can they spill over into the SuDS chamber, perhaps consider the installation of a sewer blockage alarm? Also while we are on the subject of sewers, do not assume it is safe to wash / divert spills and firewater down a sewer the results can be equally as disastrous and costly as many have experienced after knocking out the local Sewage Works.
Practical SuDS Protection
As CIRIA C736 explains containment is the best way to deal with spills and firewater. For new builds implementation is usually relatively easy providing it is considered from the onset. Typically it requires the construction of an impervious area with sufficient volume to act as a temporary receptor for the estimated peak volume of any spill or firewater incident with sufficient spare capacity to allow for rain.
However, for legacy SuDS a different approach is required and one where it is often necessary to make the most of existing assets. In our experience many sites have drains that if retrofitted with an automated closure device such as a flap valve can be turned into high capacity temporary storage vessels. In addition SuDS schemes fitted with oil interceptors should have the outlet of the interceptor closed off so the capacity of the storage equals the drain plus the interceptor. If calculations show that additional capacity is still required, consider creating sacrificial areas, within buildings or car parks. These are areas where small bund walls can be constructed to form high capacity containment areas at relatively low cost, subject to topology and providing the drains are fitted with automated closure devices this can be very effective.
Note: The opinions contained in this document are such, and no responsibility whatsoever can be accepted for errors or omissions contained within consequential or otherwise. Please feel free to call, discuss and debate the content of this document openly.
Brain M Back BEng(Hons) CEng FIET MIoD
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