The Impact Of Drainage And Other Environmental Discharges From Developments

What has SUDS got to do with it?

An overview by independent consulting engineers BT Bell

Everyone is aware of the media reporting of flooding events in south-east England, Cornwall, Cumbria and other parts of the UK. There are constant reminders of the increasing effects of climate change and those in the quarrying industry will undoubtedly be reading about changes to the regulatory frameworks that govern their activities. Dealing with the regulations on site drainage is often referred to by the acronym SUDS. A further change will be the introduction of the European Water Framework Directive, which is likely to place additional restrictions on storm water run-off and water quality and will have an impact on all current and future developments. These constraints will impact upon mineral extraction in general and particularly upon those activities associated with the quarrying sector that interface with the construction industry, such as coating plants and ready-mixed concrete plants. Drainage design and practice is currently the fastest changing aspect of civil engineering.

Planning

Operators applying for planning permission for new development or for an extension to existing permitted development will have noted that the planning officers at their local town hall are having to consult with numerous other bodies before making their recommendation to the planning committee on whether or not to approve the development. The list within the adjacent panel considers documents that may be required in support of a planning application, such as a flood risk assessment approved by the Environment Agency. Not all of the documents are required with every application and the particular requirement needs to be determined on a case-by-case basis in consultation with the planning officer.

Designing for SUDS

Once planning permission has been granted there are still other considerations to include within the design. Principal among these is the requirement to take into account the principles of sustainable drainage systems (SUDS) in respect of the site drainage. The principle of sustainable drainage is to control surface water run-off as ‘close to its origin’ as possible, before it enters a watercourse, and to treat run-off to remove trace pollutants. The control can take the form of one or a combination of the following:

• Preventative measures, eg rainwater recycling, good-practice design.
• Filter strips and swales to mimic natural drainage patterns.
• Filter drains and permeable or porous pavements to allow rainwater and run-off to infiltrate into permeable storage material below ground prior to discharge.
• Infiltration devices to drain water directly into the ground.
• Basins and ponds, lagoons, reed beds etc used as detention or balancing and attenuation devices (fig. 1).

The aim of these devices is to:

• manage environmental impacts at source rather than downstream
• manage water run-off rates
• protect or enhance water quality
• be sympathetic to the environmental setting and needs of the local community
• provide opportunities to create habitats for wildlife in urban watercourses
• encourage natural groundwater recharge where appropriate.

The guidance on SUDS recognizes, however, that there are limitations to the use of infiltration devices where the soil is not very permeable, the water table is shallow, the groundwater under the site may be put at risk, or where infiltration into the ground, particularly if concentrated in a limited area, could adversely affect ground stability.

Latest advice extends the principles of SUDS to bio-retention areas. These are similar to dry swales but rely on enhanced vegetation and filtration to remove pollution, such as oil droplets spilling from truck bodies, within natural features such as reed beds, rather than sending potentially oily water down the drains to discharge into rivers. Such bio-retention areas can be incorporated into site landscaping as a series of shallow depressions and can sometimes be described as ‘diffuse pollution’ control. Control of pollution by the use of vegetation can be regarded as a better solution than using petrol interceptors.

The application of SUDS to concrete plants

Concrete plants, along with other quarrying operations, by their nature produce a certain amount of dust, sand etc. The environmental management of the site will attempt to control this with water sprays and washing down. The resulting ‘grey water’ contains fine sand and cement particles suspended within it. If this were to be discharged off site into the drainage system it would need to be discharged into the foul system as ‘trade effluent’ and would be subject to a discharge licence. However, it is possible, by careful design, to contain all of the ‘trade effluent’ within the site and to recycle this water.

A well-managed concrete plant will attempt, by careful design, to harvest as much rain water as possible along with the grey water, storing this within a recycled water tank. This will be supplemented as necessary for concrete production with fresh, metered water drawn from the mains supply. Careful design of the site can maximize the quantity of rainwater landing on the surface of the ground that can be transferred to the recycled water tank for subsequent use within the concrete mix.

The levels of the paving on the site can be designed such that there is a slight ‘ridge’ or high point across the centre of the yard, with the operations of aggregate deliveries, concrete production and recycling or washing-out located to one side of this ridge line in the production area. In this way the ‘grey’ water can be confined in the area near to where it is produced. The grey water and any rain landing upon the production side of the site will drain into a wedge pit or similar settlement tank or pond. In times of heavy rainfall there will need to be an allowance for ponding water on the ‘dirty’ side of the site until such time as it drains to the wedge pit and the pump can transfer the water from there to available capacity within the water tank.

Rainfall to the clean side of the site can drain to a separate collection chamber from where it can also be pumped back to the recycled water tank. This collection chamber should have provision for a high level overflow such that, when the tank is unable to accept any more, the excess surface water is discharged to a convenient ditch, a soakaway or, as a last resort, a surface water drain.

If there is very heavy rainfall during a period when the site is not operating and producing concrete, water on the dirty water side may exceed the ponding capacity and overflow the central ridge to be collected in the collection chamber. This will only happen in exceptional circumstances when heavy rainfall occurs during a shutdown or holiday period and in such a case the water will be clean, so that it can be treated as normal surface water. The water can then be pumped back to the recycled water tank when capacity allows or it can overflow to the surface water drainage. The wedge pit on the ‘grey water’ side should also be provided with an emergency overflow at the same level as the ridge in the yard to prevent contamination of the clean side by suspended solids from the production area. A trade effluent discharge consent will need to be obtained for this overflow, which should, as noted above, pass to the foul drainage system. A manhole on this system can be used as a sampling chamber to permit water quality monitoring.

A further ridge, higher than that in the centre of the yard, should be provided across the yard entrance to prevent any surface water flowing out of the site on to the public roads.

The main concrete plant, the foundations to the batch cabin and any walkways from the car park to the batch cabin should all be set at a level 100mm above the ridge in the yard such that they will remain above the level of any ponding.

A foul drain will normally be provided to serve the domestic facilities within the batch cabin and connected to the foul drains or to a septic tank. The manhole cover of this drain should be set at the same level as the batch cabin foundations, ie 100mm above the highest yard level, to prevent surface water entering the foul system under normal operating circumstances.

By designing a drainage system in this way site operators will be a long way down the road towards complying with their obligations under SUDS.

A water-management system of this kind has recently been instituted at two new batching plants for Tarmac Northern, one in Barrow and one near Newcastle (fig. 2). The plant at Barrow Docks was not permitted to have any discharges whatsoever into the dock or the estuary. The overflow from the clean water side leads to a soakaway, while the dirty water is wholly contained on site and used within the concrete mix.

Meanwhile, Tarmac’s ready-mixed concrete plant on Brunswick Industrial Estate, on the north side of Newcastle, was commissioned in the summer of 2005. Here, the local water authority, the Environment Agency and the council are all reported to be pleased with the active and controlled management of water.

Future developments

Operators and developers of concrete plants and similar quarry facilities will find that the Environment Agency is taking a much closer interest in site drainage. The Agency considers that storm water is being allowed to run off the land too quickly and that this is leading to the well publicized flooding occurrences that have been reported widely in the media. The Building Regulations now require the principles of Sustainable Urban Drainage (SUDS) to be taken fully into account when considering surface water. The gradual implementation of the European Water Framework Directive (WFD) is likely to increase pressure on operators and developers to consider all aspects of the sustainable use of water. This may, in practice, restrict storm water discharge from any site to the equivalent of ‘rural run-off’, with further limitations on the rate of discharge so that the run-off from a site cannot be added to drainage from other sites and cause any flooding downstream.

The Environment Agency, which has been designated as the ‘competent authority’ for implementing the WFD, will now prosecute any plant operator it discovers who is discharging silt and particularly aggressive alkalis, such as cement, into a watercourse. The Agency considers that alkali pollution has a significantly detrimental effect upon local flora and fauna, and that existing dirty sites have a significant impact on water quality. As a result, operators may well consider a site drainage audit and an environmental assessment of their existing facilities to be a worthwhile investment.