With global warming increasing flood risk, projects are turning to sustainable urban drainage systems for solutions. Here, Simon Rawlinson of Davis Langdon considers the options and costs.
Floods have unfortunately become regular news events in the UK. A combination of increasing development at a local scale and growing intensity of storm events has increased pressure on the existing infrastructure. Although floods and the surcharging of sewage systems represent the greatest risks associated with traditional surface water drainage strategies, they are not the only sources of problems. Excess surface water run-off can wash pollutants into natural watercourses, and the rapid removal of surface water through drains interrupts the normal cycle of infiltration that sustains groundwater supplies. Furthermore, peak river flows during floods cause erosion and deposition of silts and solids in rivers and streams, which can affect river bed morphology and ecology.
With hosepipe bans scheduled for large parts of the UK later this spring, the perception of sustainable urban drainage systems (SUDS), designed to alleviate these problems, may be shifting from leftfield to centre ground.
Principles of sustainable urban drainage systems
Sustainable urban drainage is an approach to surface water drainage that aims to make use of rainwater as a water supply, reduce the volume and surface velocity of run-off and control pollution, with the overall intention of reducing a site's contribution to stormwater discharge.
If the objective of traditional piped drainage is to remove water from a site and to direct it to a piped outfall as quickly as possible, then the aim of SUDS is the opposite, retaining water safely on site, and where possible, cleaning and making it available for re-use.
In addition to controlling surface water run-off, elements of SUDS such as swales (see opposite) can also make a positive contribution to habitat diversity, amenity and the richness of landscaping on a scheme. Some commercial developments may even be able to exploit a "pond premium" from their SUDS features.
Planning policy guidance note 25, which deals with development and flood risk, includes a reference to SUDS, promoting a teamwork approach to encourage the incorporation of sustainable drainage in developments. PPG 25 restates the principle that surface water run-off should be controlled as close to its source as possible - preferably using SUDS.
There are four principles of SUDS design that help to determine not only how an individual SUDS solution should be selected, but also how surface water discharge should be most effectively managed:
Prevention The prevention of contamination of water sources through surface water run-off - for example, the oils and heavy metal contaminates that get deposited on roads and hard standings.
Reduction of pollution is a key objective of SUDS, particularly as much surface water outflow is managed as clean water and discharged directly into streams and rivers.
Types of pollution that SUDS seek to control include:
- Petrol and diesel spillage
- Heavy metal particles
- Nitrates and other organic compounds
- Road salt
- Grit, silt and other soil particles.
SUDS systems can either act as a filter medium, or as a barrier to prevent further transport of the pollutant. This can be a very important consideration if there is risk of contamination of groundwater supplies in the area.
Source control Management of surface water as close as possible to the point where rain turns to run-off. Source control is mostly concerned with controlling the size and frequency of downstream flooding, but also has an impact on habitat and groundwater management.
Strategies for source control are based on a number of broad options. Final selection depends upon factors including surface water flow, soil conditions, available drainage capacity, and extent of pollution risk and so on. The three generic approaches that can be adopted are:
- Retention through 100% infiltration. Following this approach, a site has no surface water drainage outfall and all disposal occurs locally via the sub-grade.
- Retention through partial infiltration.
Some site discharge is permitted either into drains or water courses following insitu cleansing of the water through permeable paving and so on.
- Detention. Recycling of water for use on site, storage of water for subsequent controlled disposal, avoidance of contamination of protected groundwater and prevention of the spread of contamination from affected sites.
Approaches are often used in combination to achieve the best balance between on site infiltration, storage and dispersal, and the piped disposal of a limited quantity of surface run-off.
Site control Management of run-off from different sources on a site such as roofs, paving and roads.
Regional control Management of run-off from different sites - for example, collection of water in a jointly developed balancing pond.
These principles demonstrate that successful SUDS solutions need a broad-based strategy to achieve objectives of water re-use, pollution control and reduction of downstream discharge levels, and that it is inappropriate to apply single-point solutions to solve these complex watercourse problems.
Permeable pavings Permeable pavings are the most familiar SUDS system and have very well established markets in Europe. With permeable pavings, load-bearing paviors with designed-in outsized joints are laid over a bed and a graded, single-size granular sub-base. Using crushed rock, the capacity of the reservoir is about 30% of volume. Typically sub-base depths range from 150 to 250 mm, depending upon loadings, the designed rainfall events and the type of sub-soil. Permeable pavings can be used as part of both retention and detention systems. Advantages include:
- Use of a buried infiltration medium, so no extra land take is needed
- Load-bearing surface, so traffic and water management functions can be combined
- Works as an effective filter medium for many forms of pollutant.
Use of permeable pavings can be restricted by factors such as ground water levels, sub-soil structure, location of abstraction wells, requirements for pollution control and so on. Where disposal of water by infiltration is not possible, because of the height of the water table or risk of contamination, permeable pavings installed in combination with an impermeable membrane can be used as a reservoir to regulate surface water flows off site.
Soakaways and infiltration trenches These are used to facilitate dispersal of surface water run-off. Both are below-ground features filled with crushed rock, and rely upon the long-term permeability of surrounding soil for effective operation. Their location may constrain development on the site by increasing soil moisture content or by reducing the bearing capacity of the soil. The performance of soakaways degrades over time as they become clogged with silt and access for inspection and maintenance is required.
Swales and infiltration basins are vegetated surface features that drain, filter and disperse surface water. A swale is a grassland depression that directs water to a dispersal or storage system. Swales are shallow and wide and are dry during normal conditions. During storms they provide temporary storage as well as dispersal. Infiltration basins are similar in construction but are designed as the terminal point of a SUDS system. Swales can provide economic and easy to maintain drainage for highways, car parks and other areas of extensive paving.
Land drainage systems such as perforated pipes or mole drains are designed to provide a line of least resistance to direct surface water to a drainage dispersal point such as a soakaway or leachfield. Land drains are used in conjunction with some insitu storage systems to maintain drainage flow velocities so that the storage cells don't become clogged by silt.
On-site attenuation and storage is used where it is not possible to disperse surface water through infiltration, and where the peak flow rate from a site needs to be restricted. Typical sites that might benefit from this approach are heavily contaminated urban sites, where infiltration might leach out pollutants, or sites where existing drainage infrastructure imposes limits on peak flow rates. There are three components to the system:
- A collector network - typically land drains or permeable pavings
- A flow control device - a valve that limits the flow from the system to the capacity of the downstream infrastructure. Vortex flow control devices are a common, non-mechanical technology used to switch flows in below-ground drainage systems at a defined output level.
- A storage medium - typical alternatives include "geocellular systems" or large diameter concrete pipes. Because of the low load bearing of geocellular systems compared with, say, spun concrete pipes, use of these systems may have an impact on the subsequent use of parts of the site used as a reservoir.
These systems are typically used in combination to provide a solution that addresses the surface water loads, the constraints of the site and the opportunities to create landscape features using swales.
Other technologies that can contribute to SUDS systems include petrol interceptors and green roofs, which retain water within the roof construction.
Objections to SUDS and reasons for low take-up
Although there is a strong case for the widespread adoption of SUDS, the take-up of systems to date in the UK has not been very extensive. By contrast, in Germany the market for permeable pavings alone is claimed to total 18 million m² a year. So what factors hold back the take-up of SUDS in the UK and what can be done to address these?
- The need for widely accepted design criteria - rules of thumb for the calculation of permeable paving sub-base thickness and so on - so that designers and clients can be confident that system capacities are sufficient for peak storm events
- Restrictions imposed by groundwater levels, proposed land use and potential future development
- Concerns about maintenance requirements - piped systems are perceived by owners to have minimal and predictable maintenance requirements
- The perception of the risk of flooding by downstream users caused by doubts over the effectiveness of an upstream SUDS
- Preferences of the managers of adopted systems for conventional drainage solutions
- The split of responsibility for the maintenance of a SUDS system between the water authority for the below-ground drainage and the landlord for the surface elements such as pavings, soakways and swales
- Perception of health and safety risks due to expanses of standing water associated with swales and infiltration basins in populated areas such as housing developments, schools and so on.
SUDS cost drivers
The following factors will have a direct impact on the selection of a SUDS system and its specification, and as a result, the cost of the solution. In essence, the key drivers are the volume of water that needs to be stored and the system options that can be used on a particular site, although a number of other issues need to be considered. These include:
The surface area that requires treatment
The volume and velocity of anticipated water flows
The frequency of the design event - such as a 50- or 100-year storm
The capacity of the drainage system beyond the site boundary
The use of the site, such as:
- Natural surface ratio
- Density of development
- Presence of polluting activity
Ground conditions, such as:
- Depth of water table
- Permeability of soil
- Load bearing capacity
Requirements for insitu decontamination.
- Axle loads on paviours
- Maintenance requirements.