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Civils & Structurals Bioretention - Thinking outside the box


by Alex Stephenson, Director of Hydro International UK Stormwater Division, Chair of the British Water SUDS Group


In response to the requirements of LID, bioretention became more popular as a means of filtering stormwater runoff through an aerobic plant, soil and microbe complex to capture, remove and cycle pollutants through a variety of physical, chemical and biological processes.


HAVE you ever noticed a wasted opportunity in the islands, trees and plants at the side of our roads and car parks? You see a tree pit in a car park, and close by, a surface water drainage channel. They are completely unconnected to each other, and the surface water drain is discharging water with little or no treatment into the sewer network. Combine the two, and you could have a much more sustainable drainage solution that treats pollutants and sedminets, as well as minimising the amount of surface water runoff heading for end- of-pipe systems.


I confess, a career in stormwater management does tend to make me more likely to look out for such things on visits to Sainsbury’s! But the point is clear. Sometimes it takes just a little imagination to achieve a step change in sustainability and engineer in nature’s way.


The missing link in the car park example is bioretention. It’s been a familiar technology for a decade or more but has failed to capture the UK imagination in any significant way. Now new technology innovations pioneered in the US are beginning to change all that.


WATER QUALITY


In many instances, the need to manage the quality as well as quantity of surface water runoff has been seen as something of a ‘nice to have’ in the UK. But the requirements of the European Water Framework Directive are placing increased emphasis on the removal of oils, pollutants and sediments from surface water.


The implementation of the Water Environment and Water Services Act (Scotland), Flood and Water Management Act (England and Wales) and in particular the introduction of new National Standards for Sustainable Drainage Systems (SuDS) will also require developers and highways authorities to incorporate treatment elements in their SuDS systems before they can be approved by the new Local Authority SuDS Approving Bodies.


Legislative requirements for stormwater treatment have been much tougher in the US for decades, driven by the Clean Water Act (1972). Bioretention was first pioneered in the late 1990’s motivated by the principles of Low Impact Development (LID) in the late 1990’s.


LOW IMPACT DEVELOPMENT LID’s aim is to mimic the natural pre-development site hydrology using a variety of techniques. LID shuns conventional end-of-pipe or in-the-pipe structural methods of drainage, and integrates stormwater controls throughout the urban landscape. It sounds very similar to British SuDS principles, but a key difference with LID is a concept that favours distributed at-source landscape features over centralised high-impact ponds and swales.


Traditional bioretention and biofilter systems have tended to be landscaped drainage systems, planted with vegetation, trees and grasses. But such systems take up space, are limited by the soil conditions of the site, and are vulnerable to compaction, eg. by vehicles driving over them. They make


it difficult for specifiers to design against measurable standards and predict consistent results.


As a result an engineered bioretention system, named Filterra®, was developed which worked in a more compact space, with predictable soils, was more easy to link to drainage inlet and outlet systems, easier to design against flow control and treatment parameters and performed consistently whatever the site characteristics. With the number of installed systems in the US nearing 4,000, Hydro International is introducing Filterra® technology to the UK.


A JOINED-UP APPROACH


Current design guidance for highways authorities is encouraging a more integrated approach to improviing urban street scenes with greater emphasis on landscaping and amenity. In England and Wales the Department of Transport’s ‘Manual for Streets’ and the Scottish Governments’ ‘Designing Streets’ aim to provide distinctive, attractive and safe environments through a collaborative design approach that considers all the functions of a street scene, including sustainable drainage, in an integrated way.


The opportunity for SUDS-compliant solutions in the retrofit and improvement of highways in urban areas can be limited by the space available for stormwater treatment technologies and lead to compromises in the level of treatment provided.


By installing bioretention units along the carriageway with plants selected to conform to biodiversity and landscaping strategies, effective treatment can be provided and the streetscape improved.


WIDESPREAD ACCEPTANCE


From the surface, the system looks like a normal tree box, with suitable plants, shrubs or trees protruding through a decorative grating in a typical concrete slab at pavement level. Underneath, it comprises a concrete container with a 75mm mulch, 500 - 1000mm of a unique soil filter medium, an observation/cleanout pipe and an under drain system.


The stormwater runoff drains directly from impervious surfaces through a kerbside inlet, and flows through the mulch, plant and soil filter media. Treated water flows to drainage outflow or watercourse, or onwards for infiltration if soil conditions allow. It can also be integrated with additional storage systems, such as Hydro’s Stormcell® or Stormbloc®, as part of a secondary attenuation or infiltration system, if required.


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