Urban Drainage System Sustainable Urban

Urban Drainage System

SUSTAINABLE URBAN DRAINAGE SYSTEMS

Human activity is the major cause of the surge in green house gas releases (IPCC 2007). These gases are a contributing factor to the climate change that has different implications in different regions. For instance, hot dry summers and colder wetter winters are more likely than before. In the United Kingdom, there is likely to be a surge in the frequency of rain and higher rainfall intensity. There are likely to be flooding events that have adverse effects on the ecosystem, the economy, and the society (IPCC 2007).

The urban spaces have a history of annihilating the ecosystem that existed before they came to be. Biodiversity is lost, waste increase, the air quality falls, and impermeable surfaces increase (Heaney 2007). This is unsustainable now and in the future. Sustainability here refers to the ability of the environment to provide for needs of the occupants, and still be viable for the future generations. Therefore, most of the countries and cities within the European Union (EU) are insisting on sustainability, especially as pertains to infrastructure, energy use, waste management, human behaviour.

Sustainable Urban Design is especially important for green or 'smart' cities (Hill 2007). In the United Kingdom, there is the institutional capacity necessary for planning, governance, and coordination of design and management systems for new and existing developments. One important aspect of sustainable design is the Sustainable Urban Drainage Systems (SUDS) (CIRIA 2000). These are control systems and management practices that drain water in a way that is sustainable than the conventional systems. Surface run-off may have devastating consequences on the environment. The overarching objective of SUDS is, therefore, to decrease the amount of run-off.

2. Coed Darcy

Coed Darcy is a planned urban village in Neath Port Talbot county borough in Wales. The village is land 5.3 square kilometres in area and will have about 4000 homes. There will also be a Hospital, primary schools, and a secondary school. An oil refinery had previously occupied the site until the late nineties. St. Modwen Properties, a regeneration specialist, is the developer of the site and will apply the organisation's vast resources this regeneration project.

Land reclamation, rehabilitation, and redevelopment are common in avoiding land dereliction due to mining activities. The area of Coed Darcy will now be a functional area with amenities, a community, and biodiversity that is in sharp contrast to the face of the land after the mining activities. Oil pollutes the land and makes it not conducive to plant and animal life. However, regeneration connotes removal of the pollutants and giving the land a new face.

One way of protecting the biodiversity and ensuring sustainability is by having Sustainable Urban Drainage Systems. The benefits of such a system are numerous, and include reduction of the risk of floods, proper use of water, prevention of pollution of the environment, and reduction of the net carbon emissions in the homes within the village.

3. Approach and Uncertainty

The Coed Darcy regeneration project is massive. The general aspect I will discuss here is Sustainable Urban Drainage Systems in relation to carbon emissions. A warmer globe interferes with the water cycle resulting in a higher risk of floods. Further, the increase in the area of impermeable surfaces means that there will be heavy run-off volumes and high peak rates. The reservoirs would not be able to handle these large quantities of surface water run-off in order to prevent floods (Ahern 2007).

Developments apply SUDS schemes of varying size and complexity in order to ensure sustainable urban spaces and to comply with local and national policy. Coed Darcy will require an elaborate SUDS scheme since it has about four thousand homes. The advantage of this site is that it had no prior developments thus giving designers freedom to fit the SUDS to the natural set up of the area. While the overall objective is to reduce surface run-off, there are stringent policies in the United Kingdom that set high targets for designers.

The Planning Policy Guidance Statement 25 on Development and Flood Risk (National Planning Policy Framework replaces this report) stipulates that development sites should have storm attenuation ponds that can accommodate run-off for 1:100-year storm while still leaving a 20 to 30 per cent allowance for climate change. It is clear that this significantly reduces run-off from a site by limiting the discharge resulting in lesser water in sewers and watercourses.

Sustainable Urban Drainage Systems manage water quality, increase water quality, and ensure biodiversity (Gardiner 1991). There are many techniques in SUDS and the designers will combine them since this is a large development. For source control the designers will use green roofs, rain water harvesting, grey water recycling, and permeable paving (CIRIA 2002). These techniques reduce run-off at the source by enhancing absorption, evapotranspiration or use and reuse of water.

The designers should incorporate Swales, filter strips, and filter trenches as conveyance methods. This should lead to bio-retention areas, infiltration basins, detention basins, and wetlands. Each of the techniques has specific functions and contributes to the overall goals of slowing drainage down, keeping water above the ground longer, and removing pollutants and sediments.

The designers should also ensure that the developers protect the SUDS during construction (Jefferies 2003). Further, there should be a way to cope with flows that exceed the drainage capacity. Such a failsafe should take in to account the site topography, permeability, and flood risk assessment.

The residents who will live in the developed site are the eventual customers. Their preferences will affect how the SUDS techniques integrate into the village (Apostolaki et al. 2003). For instance, some residents may be more concerned about the aesthetics than the practical value of the SUDS while others may see the SUDS as obstacles to children, the aged, and the handicapped (Apostolaki et al. 2002). Designers should take in to consideration such minor points that may affect the value of the development.

Finally, the building materials should be recyclable, local,

4. SUDS Techniques

4.0 Overview

The design of the SUDS must first take into consideration the flood risk assessment, the rainfall estimates, and the catchment descriptor. The topography of the site will play a big role in the layout of the SUDS since the drainage mimics nature by making use of gravity. The permeability of the soil and infiltration of ground water must inform the use of infiltration techniques in the site (Macdonald 2003). This will ensure that the efficiency of the techniques is known before hand.

Other issues that are essential to the designers are storm event design and climate change. The designers should estimate and design for 1:100-year storm event, even though the 1:30 is the standard. They should also provide for climate change in the design, say a tolerance of about 30 per cent. The issues of pollutants, water treatment, and sewer connections are also essential to the design of SUDS. The system should apply natural processes for quality improvement of the water.

If the system fails or has overflows, there should be a failsafe mechanism so as to ensure the infrastructure and amenities remain intact (Novotny & Brown 2007). The overall design should be such that the water leaving the site after development is not greater than that prior to the development. This statement is achievable in case of retrofitting. However, it is a tall order for a new development of the magnitude of Coed Darcy since it will contain four thousand homes.

4.1 Green Roofs

These have several layers that cover the roof with vegetation cover on top of a drainage layer. Waterproofing is essential to ensure it last long without destroying the roof. The green roofs retain precipitation thereby reducing run-off volumes and attenuating flows at peak events.

Green roofs absorb carbon dioxide and remove other pollutants thereby improving the quality of the air. They provide thermal insulation by reducing heat emission and/or absorption. This reduces cooling and/or heating energy necessary, which translates to lower energy consumption. The absorption of carbon dioxide and the use of less energy reduce carbon emission to the environment (Novotny 2007).

4.2. Rain Water Harvesting

Continuous guttering, filters, and rainwater storage tanks make up the system for rainwater harvesting from roofs and other hard surfaces on buildings. These systems significantly reduce run-off for small rain events. They also decrease peak flow. The system reduces the demand for water from the mains. The system requires little maintenance and has a short return period. Rain water harvesting replaces water from long distances when it rains. This reduces the energy use and thereby contributing to reduction in carbon emissions.

4.3. Grey Water Recycling

Grey water recycling involves using water from sinks and showers for other functions instead of letting run into the sewer with the black water. This requires sufficient planning of the plumbing and storage as well as the eventual uses. A common application of grey water is irrigation of flower gardens and flashing down of toilets (Anon 2008).

4.4. Permeable Paving

Pervious paving allow rainwater to infiltrate into the layers inside where it can stay temporarily before it infiltrates to the ground or discharges in to a watercourse or a drainage system. 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).

4.5. Swales

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.

4.11. Wetland

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).