Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/425132
18 WET NEWS DECEMBER 2014 When a council was keen for a SuDs to be included in the planning of a new housing development, the site's compact size was an issue. The CPSA's Stuart Crisp explains. The challenge: Accommodating climate change Sections of large diameter precast concrete pipes were used to construct underground attenuation tanks F armlands is a new devel- opment of 105 houses in Markfield, a quiet com- muter village nestled in the Charnwood Forest deep in the Leicestershire countryside. As part of the scheme's planning approval process the county council's planners were keen for the project to incorporate a Sus- tainable Drainage System (SuDS) to cope with the run-off from a 1-in-100-year storm. In addition, to future-proof the scheme the planners required the SuDS solution to have sufficient additional capac- ity to enable it to cope with a predicted 30% increase in run- off caused by the impact of cli- mate change over the develop- ment's lifetime. The challenge for the scheme's developer, Jel- son Homes, was to incorporate a comprehensive SuDS solution into the compact site cost effectively. The principle behind a SuDS scheme is that it is designed to mimic the natural drainage characteristics of the land by dealing with rain where it falls, as far as is practicable. In the past residential developments, such as the Farmlands develop- ment, would oŒen feature large areas of impermeable surfaces for car parking, paving and hard standing that were drained by conventional schemes designed to carry run-off from these areas to an outfall as quickly as possible. When compared to the unde- veloped site, these impermeable features could result in large volumes of additional water flowing over land or into a watercourse increasing the risk of flooding. By contrast, SuDS set out to alleviate the problem by using engineered and land- scaped features to limit the rate of run-off. SuDS systems also have the additional advantage that they can also help deal with pollutants in the run-off. Land space There are two main categories of SuDS solutions: hard and soŒ. SoŒ SuDS include landscaped features such as swales, filter strips, infiltration basins and detention ponds. These take up valuable land space but they have the advantage of providing the wider benefits of amenity and biodiversity for a develop- ment. They may also, initially, be cheaper to create. By contrast, hard SuDS solu- tions treat run-off through the use of permeable surfaces and below-ground proprietary engi- neered products such as precast concrete soakaways, attenua- tion tanks and treatment cham- bers. They can provide a perma- nent underground solution capable of functioning reliably THE CONCEPT • The storage-based solution provides attenuation for runoff from the site's upper catchment area • Two precast concrete storage tanks buried beneath the development's access road • The largest tank comprises 62 precast concrete box culvert sections • There is a second, smaller capacity 70m 3 attenuation tank, formed from 1.2m diameter precast concrete pipes • Runoff is released, via a headwall, into a small natural SuDS NEED TO KNOW 1 The SuDS solution has to have sufficient additional capacity to cope with a predicted 30% increase in runoff 2 Soft SuDS include landscaped features – swales, filter strips, infiltration basins and detention ponds 3 Hard SuDS solutions treat run-off through the use of permeable surfaces and below-ground proprietary engineered products ─ precast concrete soakaways, attenuation tanks and treatment chambers THE VERDICT "Developable land can be freed up by using hard SuDS solutions, while also enhancing the landscaping by including soft SuDS elements as part of the combination". Kevin Pryor, Edwards & Edwards • INSIGHT TANKS & STORAGE over the life of a development. For Farmlands, Jelson Homes worked with specialist SuDS consultants Edwards & Edwards to develop a SuDS solution to manage the surface water run- off within the boundaries of this gently sloping site. The initial proposal favoured permeable paving, water butts for homes and a predominantly 'soŒ' SuDS solution based on landscaped features for water retention, including a large dry balancing pond all squeezed into the confines of the development. As the scheme progressed through planning, however, it was decided that an area of woodland, previously destined for development, should be retained. This meant the remainder of the site had to be developed more intensively so an alternative SuDS solution had to be found.