If there is an aggregate sub-base, these can provide water quality treatment. There should be good compaction and appropriate geo-textiles especially for areas accessible to heavy vehicles.
Permeable pavements reduce the need for deep excavations thereby providing a cost benefit. This system reduces the run-off rates and peak flow. The overall benefit is that it removes pollutants and holds water so that it does enter the main drainage. A lot of water in the main drainage would either need pumping or treating thereby using energy (Wild et al. 2002).
They are continuous vegetated drainage systems which convey or store water while allowing filtration when appropriate. Usually, they are the equivalent of roadside gullies or drainage pipes in conventional drainage systems. However, swales have gentle gradient so that water moves at low velocity. The sediments in storm water run-off can, therefore, settle out.
The advantage of swales is that it has vegetation which absorbs carbon dioxide at the roadside. The designers may include an under-drain system in the swale. Swales require little capital and any pollution or blockage is visible.
4.6. Filter Strips
These are vegetated linear pieces of land that accept surface run-off as sheet flow. They are usually between a hard surface and a receiving watercourse, say a stream. The vegetation filter the run-off by allowing pollutants to settle and water to infiltrate.
4.7. Filter Trenches
These are shallow trenches that contain stones resulting in some void space. The trenches convey storm water downstream in SUDS and also filter the water. The trenches also allow infiltration and should not contain untreated drainage. The main contribution to the overall scheme is the slowing down of surface run-off.
4.8. Bio-Retention Areas
Bio-retention areas are that use engineered soils and vegetation as part of landscaping in a shallow basin. They remove pollutants and decrease run-off. They are ideal for frequent rain events. Construction of these features should occur at the end of development in order to reduce erosion. The precautions in the construction include not tearing the geo-textile, proper testing of the soil, and avoiding compaction (Roesner et al. 2001).
4.9. Infiltration Basins
These depressions that contain vegetation store storm water and let it infiltrate gradually underground. There is risk of destruction and failure due to deposition of sediments during construction. Therefore, construction of this feature should occur when the site is stable.
4.10. Detention Basins
These surface basins store storm water in them thereby attenuating surface run-off by providing flow control. There should be healthy vegetation. Therefore, the soil on the sides should be porous and of sufficient fertility and depth. Careful preparation would prevent erosion damage while ensuring the basin retain surface run-off.
Wetlands are shallow and marshy areas that have aquatic vegetation cover. They have ecological benefits since they detain flows for long periods, allowing the settling of sediments and removal of contaminants through aerobic decomposition. The soils should be fertile, porous, and deep to allow vegetation to grow.
5. Cost-Benefit Analysis
How do these methods fit in to the scheme of reducing carbon emissions? The green roofs and other vegetation that are part of the SUDS system absorb carbon dioxide. Water harvesting and re-use minimises the energy requirements for pumping and purification. Natural methods of removing pollutant also minimise the need for energy use (Metcalf & Eddy 2007).
The various methods of conveyance in SUDS slow down surface water run-off and its integration into the sewers (Villarreal et al. 2004). This prevents overloading of the sewers thereby reducing energy requirements for pumping and treatment. Therefore, the whole SUDS treatment train decreases carbon emission either by absorption through the vegetative cover, or minimising energy use.
There are a number of issues that arise in Sustainable Urban Drainage Systems. Are they really sustainable given that they require regular maintenance? Granted, the systems require clear ownership and responsibility for operation and maintenance as well as guidelines for future adoption. Eventually, SUDS are cheaper than conventional systems and fit in the bigger picture of a green urban space. Further, the re-use of resources and energy savings pay off in a few years.
Floods destroy infrastructure and result in huge losses both for the individuals and the public. SUDS mitigate flood risk and ensure sustainability even in the wake of uncertain future climate patterns (Mays 2007). Further, the restoration or introduction of biodiversity is essential, especially in modern urban spaces.
The Code for Sustainable Homes is holistic in its approach to sustainability. Coed Darcy should achieve level 4 compliance with the code. There are a number of areas that SUDS can contribute to the overall rating. The assessment criteria will inform the designers so as to achieve the target rating. Given that this is a sustainable village, the compliance to the code has financial implications for the developers.
The first step is to appoint appropriately qualified personnel. Performance of flood risk assessment should prelude the design process and there should be provision for allowances for changes in climate. The peak run-off and volume run-off must be follow the methodology laid out in the SUDS Manual, CIRIA C697 (2007).
The SUDS will contribute credits for efficient energy use and CO2 emissions reduction. It will also score under the item for water use, surface water run-off, pollution reduction, and ecology.
It is clear from the foregoing discussions that SUDS are part of a wider scheme for sustainability of urban spaces. The major aim is to control surface water run-off, yet in the process also reduces energy use, encourages re-use and waste management, improves the ecosystem, increases the aesthetic value of the spaces, and significantly reduces carbon emissions.
The National Planning Policy Framework has combined 44 documents into a single report that covers diverse sectors and issues. This demonstrates the interdependent nature of sustainability issues. Designers have to plan SUDS within the wider context of the development, bearing in mind other policy documents and major aspects like cost.
In conclusion, the SUDS will be instrumental in ensuring the sustainability of Coed Darcy. Their implementation shall start right from the design stage up to the post-construction stage.
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