Approaches to manage surface water that take account of water quantity (flooding), water quality (pollution) biodiversity (wildlife and plants) and amenity are collectively referred to as Sustainable Urban Drainage Systems (SUDS). Such drainage systems not only help in preventing floods, but also improve water quality. In addition they can enhance the physical environment and wildlife habitats in urban areas.
A key characteristic of many artificial urban drainage systems, as compared with natural systems, is a more rapid build-up of flows and higher peaks, causing an increase in flood risk. It is possible to return the catchment response to a more natural state by using more natural methods of drainage. These use the infiltration and storage properties of semi-natural devices such as infiltration trenches and swales (both discussed below) or ponds, all of which slow down the catchment response, reducing the peak outflow and thus lowering the flood risk.
SUDS devices are most effective in combination, in the form of a ‘management train’. Wherever possible, stormwater should be managed in small, cost-effective landscape features located within small sub-catchments, rather than being conveyed to and managed in large systems at the bottom of drainage areas. Water should be conveyed elsewhere only if it cannot be dealt with locally.
Like all drainage systems, SUDS are designed to provide capacity for a storm event of a particular frequency. For more extreme events, exceedance flows are likely to be generated and must be carried by the major drainage system.
Many SUDS devices are based on infiltration to the ground, the risk of groundwater pollution is an important consideration, especially where surface runoff is likely to be polluted and the groundwater is used for drinking supplies. The design of a permeable pavement system, for example, can be adjusted to allow infiltration, or not, in order to account for this (discussed below).
The main types of SUDS devices, all of which are discussed later in this chapter, can be listed as:
- Inlet control
- Infiltration devices
- Vegetated surfaces
- Permeable paving
- Filter drains
- Infiltration basins
- Detention ponds
- Retention ponds
- Constructed wetlands.
Inlet control devices provide storage close to the point where the rainfall is first collected. Rooftop ponding uses the storage potential of flat roofs; as this creates an additional load there is an increased need for water tightness, as well as good maintenance of outlet control devices. A green roof is a planted area that provides storage, encourages evapo-transpiration and improves water quality. A water store, consisting of a water butt or a tank near to ground level, can store rainwater and make it available for garden use, though some outflow must be assured to provide capacity for subsequent rainfall.
Instead of connection to the drainage system, water collected from roofs can be diverted at the bottom of the downpipe to infiltrate in nearby stable pervious areas. Paved area ponding, to accommodate heavy rainfall, can be achieved by restricting inflow to the piped drainage system, thereby reducing flood risk downstream.
Detention basins are storage facilities formed from the landscape with controlled outflow. They store stormwater temporarily, and are dry between storms.
Retention ponds provide storage within a permanent body of water. They allow natural treatment of the water and provide environmental and amenity benefits.
The benefits of SUDS can be realized in existing urban areas by retrofitting. Its challenges tend to be associated with availability of space, and the difficulty of adapting existing systems. Local application of inlet control may be the most feasible approach.
Infiltration and permeability of urban areas
Urbanization affects the natural water cycle. When rain falls, some water returns to the atmosphere (through evaporation or transpiration by plants); some infiltrates the surface and becomes groundwater; and some runs off the surface. Since urbanization increases the proportion of the surface that is impermeable, it results in more surface runoff and reduced infiltration. As we have seen, surface runoff finds its way to a watercourse far quicker than groundwater and therefore increases flood risk, and if the surface runoff is conveyed via a piped drainage system the effect is even more pronounced.
Increasing infiltration via improved permeability in urban areas can reduce flood risk, but in many cities the opposite is occurring. The increasing densification of towns and cities implies that every space is utilized to the maximum for the use of urban dwellers. This leads to an increase in hard surfaces and a decrease in permeability of any open space left after the construction of buildings. An example of this is the paving of front gardens in the UK to allow for parking spaces: in one part of London, 68 percent of the area of front gardens is now hard-surfaced, and the figure is rising. Leisure and recreational uses also tend to involve impermeable surfaces. Cost-cutting measures designed to limit the regular maintenance of green spaces can also lead to the concreting or de-greening of spaces.
A major characteristic of most SUDS systems (as discussed above) is to increase permeability and therefore infiltration. The use of SUDS is promoted in the UK via formal Building Regulations, which state that ‘surface water drainage should discharge to a soakaway or other infiltration system where practicable’. Planning guidance in England, specifically related to development and flood risk, also strongly favors the use of SUDS in new developments. Measures like these have the effect of in-creasing infiltration, and are steps in the right direction in terms of preventing flood risk from increasing as a result of urbanization.
These include soakaways and infiltration trenches. A soakaway is an underground structure, typically circular in plan, which facilitates infiltration into the ground. An infiltration trench is a linear excavation, usually stone-filled, achieving the same aim with a greater area of exposure to the ground. These devices are only suitable in ground with suitable infiltration properties, positioned above the level of the water table at any time of year. Filter drains are perforated or porous pipes laid in a trench containing granular fill and are typically located in the verge of a road to collect water from the road surface and carry it away. Infiltration basins are open depressions in the ground which collect water and allow it to be absorbed gradually.
Swales are grass-lined channels which allow the infiltration, storage and conveyance of stormwater. Small swales can run beside local roads, large swales beside major roads, and swales may also form landscaped channels for conveyance of stormwater. Filter strips are gently sloping areas of vegetat-ed land. Swales and filter strips delay and reduce stormwater peaks, and trap pollutants and silts.
Permeable paving creates a surface that allows infiltration, either because it is porous, or because specific openings have been provided (for example, the spaces between paving blocks). The most common applications are for car parks, but lightly trafficked roads and driveways are also suitable. The sub-base provides storage for rainwater, typically in the voids between granular particles. The collected water may then be allowed to infiltrate into the ground; alternatively, where it is important to protect groundwater from pollution, the base and sides may be sealed, and water flows to a piped outlet, but far more slowly than it would in a piped system.
The only serious restriction on infiltration in urban areas is where there may be a risk of polluting groundwater that is used as a water resource.
A significant solution to the problem of increased and more rapid rainfall runoff is by using the system of development permitting. Guidelines need to be issued giving examples of how urban design can maximize infiltration into groundwater. Permits are then only authorized if such appropriate measures are included in the construction works. On a wider level, urban area administrations need to draw up a land use management and zoning plan, which recognizes the need for open spaces that can act as temporary rainfall storage, as well as being an urban recreational amenity.
Some agricultural practices reduce infiltration, and these increase flood risk to urban areas down-stream. Adapted agricultural practice to reverse these effects includes conservation tillage, ploughless cultivation and avoiding bare soil. The preservation and extension of existing wetlands and forests in the upstream areas of a catchment enhances infiltration, and in addition reduces runoff through evapo-transpiration. Primary forests with broad-leafed trees are much more effective in reducing runoff than planted pine species.