Sustainable Drainage

Modern sustainable building and water conservation policies require adequate application of engineering concepts in order to alleviate problems related to frequent flooding. Sustainable drainage systems (SUDS) and water management strategies are therefore immensely important in addressing problems which affect formulation of urban designs (Environment Agency 2006, p.3). Built-up sites in urban centers must be drained thoroughly to remove excessive surface water which may otherwise lead to fast floods. However, traditional drainage systems have often failed in addressing water management problems. For instance, diverting fast flowing water to drainage pipes can lead to abrupt rises in water levels which eventually cause flooding (Environment Agency 2006, p.3). In addition, traditional systems have persistently promoted contamination of watercourses. SUDS have emerged as the best drainage solutions in recent past. This paper explores the underlying problem of flooding, regulations enacted to address this problem and utilization of SUDS techniques in improving drainage systems.

The underlying problem of flooding to urban areas

The 2007 flooding disaster in the UK undoubtedly exemplified the extent of damage that can be caused by excessive flooding in urban centers (Great Britain, Parliament 2008, p.484). According to the Environment Agency in the UK, more than half of the victims of the vicious floods in 2007 were affected because of insufficient drainage of run-off water (Great Britain, Parliament 2008, p.484). This occurrence overwhelmed the capability of the existing drainage systems, thereby causing floods.

More than 60,000 properties in the coastal towns were damaged, amounting to huge losses (Great Britain, Parliament 2008, p.484). In addition, the Environment Agency has estimated that approximately 80,000 properties in different cities and towns are at risk of being affected by surface water flooding (Bregulla, Powell & Yu 2010, p.7). Another report by the Department of Trade and Industry (DTI) indicates that properties of approximately £270 million in worth are at risk of being damaged by surface water floods in various urban areas of the UK (Bregulla, Powell & Yu 2010, p.7).

These urban flooding statistics in the UK indicate surface water run-off. One of the major causes of flooding in the urban areas is increase in levels of surface water (Bregulla, Powell & Yu 2010, p.7). This causes surface water flooding, which is further promoted by the lack of reliable surface water drainage systems in the cities and towns. The flow of overland surface water overwhelms the conventional drainage networks, thereby causing floods in the urban centers (Bregulla, Powell & Yu 2010, p.7). As such, the government of the UK has developed a national plan to manage surface water run-offs in urban areas.

Dubbed the ‘water strategy’, this plan is intended to enable the country to achieve sustainable drainage by the year 2030 in virtually all flood-risk areas (Bregulla, Powell & Yu 2010, p.8). Approximately 62% of new growth points in the UK are prone to water quality disasters (Bregulla, Powell & Yu 2010, p.12). Another 80% of such sites are susceptible to surface water flooding (Bregulla, Powell & Yu 2010, p.12). The adoption of SUDS has been cited by the Environment Agency as the most suitable solution.

Recent regulations on the issue of flooding and drainage systems

The Flood and Water Management Act of 2010 is one of the recent regulations that have been enacted to address the problem of flooding and drainage systems (Legislation England and Wales 2011). This Act is intended to help people in both the urban and rural areas to manage drainage systems. Reservoir safety has been incorporated into this act as well (Legislation England and Wales 2011). Adoption of SUDS by local and urban councils has been stipulated as the most reliable solution to alleviating drainage bursts and flooding risks. This Act has also established a SUDS approving body (SAB), which will be in charge of assessing and approving any proposed SUDS projects in the country (Legislation England and Wales 2011). Enforcement of this Act in England and Wales will considerably improve flood risk management, especially on the aspect of surface water flooding in urban areas (Legislation England and Wales 2011). The Act also gives the government the mandate to add or remove water uses from the stipulated list of uses. This will lead to efficiency in regulating water flow into the drainage and sewerage lines.

The Flood Risk Regulations of 2009 also serves an important role in addressing the problem of flooding in the UK (Legislation England and Wales 2011). The Environment Agency has been designated as the lead authority in these regulations, whose objectives are to prepare flood risk maps, flood management plans and risk assessments (Lampe, Barrett & Ballard 2004, p.3-4). The problem of flooding and drainage inefficiencies is also tackled through the Code for Sustainable Homes, which was drafted in 2008 (Legislation England and Wales 2011).This Code is used as the national standard against which design and construction aspects are assessed. One of the major design categories which are incorporated into the Code is surface water run-off (Lampe, Barrett & Ballard 2004, p.3-4).

Relevant hard and soft SUDS concepts

SUDS are basically used to manage run-off surface water using development techniques that imitate the natural drainage. Sustainable drainage systems are aimed at controlling surface water floods, preventing water pollution and recharging groundwater (Great Britain: Department for Communities and Local Government  2010, p.34). In addition, SUDS projects also enhance the quality of environment, especially through infiltration and water cleaning. The flow rate of surface water is minimized before it is released to watercourses (Great Britain: Department for Communities and Local Government 2010, p.34).  

There are several SUDS techniques used to minimize the risk of flooding in urban areas. These include infiltration devices, filter strips, permeable surfaces, basins and water ponds (Barton, Grant & Guise 2003, p.156). It is prudent to refer these methods as design options, since they require reliable engineering skills and specifications in order to achieve efficiency in controlling surface water (Barton, Grant & Guise 2003, p.156). These techniques are used to achieve pipe drainage, water source control, site control and storm control.

Filter strips and swales

Filter strips are sloping areas towards which surface water can be directed (Barton, Grant & Guise 2003, p.156). These sloppy areas are often vegetated with grass and shrub plants in order to facilitate filtration of silt and myriad other impurities from water (Barton, Grant & Guise 2003, p.156). In addition, these strips are specifically designed to allow even drainage of water from soaked areas, thereby reducing the risk of flooding in urban areas. The presence of vegetation on the filter strips helps in easing the flow rate of runoff water (Environment Agency 2006, p.6). The entire process of draining water from impermeable surfaces on the ground works by imitating the natural drainage network.

Swales are grassed depressions which are used to collect runoff water from the water source to a storage system (Environment Agency 2006, p.6). The discharge system is used as the storage mechanism, which eventually eases water flow in the drainage networks. However, swales are just temporary storage systems that are used for tertiary containment (Environment Agency 2006, p.6). These shallow ditches also collect water from roadside kerbs and offlets. The role of vegetation in swales is mainly to remove liquid pollutants from water through biological mechanisms (Bregulla, Powell & Yu 2010, p.19). In addition, they slow down that rate of runoff flow, thereby facilitating optimisation of sedimentation and infiltration of the present pollutants. In a typical drainage pattern containing a filter strip and a swale, surface water runs from the overland source (such as road), drains through a filter strip and eventually enters into a swale, as shown in Figure 1.

Figure 1.Cross-sectional view of a filter strip and a swale (Bregulla, Powell and Yu 2010, p.24)

Green roofs and bio-retention systems

Green roofs are important structures in controlling surface water flooding. The main aim of a green roof is to reduce the discharge of rainwater from the roof, thereby decreasing the amount of runoff water that causes flooding (Butler and Davie, 2009, p.522). These roofs are usually multilayered and mostly consist of vegetative layers. A typical green roof consists of a vegetative layer, growth medium and soils, which are layered over a waterproof membrane (Hordeski 2010, p.114).

In some other cases, garden containers can be placed on the green roofs in order to minimise water flow from the roofs. For intensive green roofs, shrubs and small trees can be planted to increase absorption efficiency of the vegetative roof (Hordeski 2010, p.114). These eco-roofs delay or minimise runoff during heavy downpours. As a result, they reduce the level of surface water which could cause floods in urban areas. Recycling of gray water can also be achieved through adoption of green roofs (Bregulla, Powell & Yu 2010, p.19). This is because the vegetative medium and soil layers can be watered using water that has already been used for other purposes.

Green roofs can retain approximately 40% of rainfall water in winter (Hordeski 2010, p.114). This is a considerable amount of water, which would otherwise be discharged as runoff and eventually causing surface water floods (Bregulla, Powell & Yu 2010, p.19). Bio-retention systems can be defined as vegetated areas which are specifically designed to hold surface runoff and to allow water treatment processes to take place before the eventual discharge of the water to piped drainage networks (Bregulla, Powell & Yu 2010, p.19). A diagrammatic illustration of a green roof is shown in Figure 2. site control facilities (Bregulla, Powell & Yu 2010, p.19). These facilities are very suitable in providing amenity advantages and wildlife habitat. Ponds should be dug to depths of 1m to 1.5 m (Environment Agency 2006, p.8). This encourages oxygenation to take place. However, deeper ponds (2.5 m) can also be used to hold runoff water from the overland sources.

In most cases, ponds are permanently wet and are designed to harvest storm water (Bregulla, Powell & Yu 2010, p.19). Similar to wetlands in water holding concepts, ponds can be planted with aquatic plants to improve biological breakdown of pollutants (Butler & Davies 2009, p.522).  These water bodies can be used to store water runoff during heavy rains. Therefore, they expand flood-storage capacity in urban drainage systems. They also reduce the flow rate of surface water, which is an important factor in alleviating problems associated with drainage systems in the urban centres (Butler & Davies 2009, p.529).

On the other hand, water basins are often dry, except during heavy rains (Environment Agency 2006, p.6). These basins offer temporary water storage for runoffs, thereby reducing peak flows which may otherwise cause surface water floods. Water basins are also planted with grass to facilitate sedimentation of water pollutants (Barton, Grant & Guise 2003, p.157). In addition, these storage facilities hold runoff long enough to allow a natural water cleaning process to take place. This direct infiltration of pollutants into the soil ensures that the urban drainage system provides both flood management and water quality improvement (Bregulla, Powell & Yu 2010, p.20).

Infiltration devices

Infiltration devices can be referred to as structures which are designed to drain water from the overland sources into the soil (Butler & Davies 2009, p.529). They can be constructed in the form of infiltration trenches and soakaways. In most cases, infiltration structures are filled with granular substances which serve as transient reservoirs (Bregulla, Powell & Yu 2010, p.20). These granular substances allow a slow infiltration process to take place and they allow temporary storage of surface water runoff (Bregulla, Powell & Yu 2010, p.20).

Excessive solid waste in the drainage pattern can be reduced by using sump pits and gullies. Infiltration structures therefore improve water quality, since they allow gradual removal of water pollutants before runoff is discharged into piped systems (Bregulla, Powell & Yu 2010, p.20). The concept used in soakaways is based on the ability of soil to absorb ground water from the infiltration trenches (Barton, Grant & Guise 2003, p.157). These devices also recharge aquifers, thereby improving the water quality in the drainage systems.  

Hard engineering concepts in SUDS

The use of filter strips, basins, ponds, swales and infiltration devices is termed as application of ‘soft’ engineering options. However, SUDS also take into consideration the aspect of hard engineering. In hard engineering, several control measures in the urban drainage systems are used (Ferrier & Jenkins 2009, p.435). Such design options include construction of storage tanks, interceptor tanks and permeable pavements (Ferrier & Jenkins 2009, p.435). In addition, the conventional piped systems are products of hard engineering.

Storage tanks in the drainage systems are used to modulate water-flow rates downstream. This form of storage ensures that the capacity of underground pipe network in the drainage system is not strained (Ferrier & Jenkins 2009, p.435). On the other hand, interceptor tunnels are used to ease pressure on the main sewer and combined drainage lines (Ferrier & Jenkins 2009, p.435). Thus, an interceptor tunnel reduces the risk of bursting of the main pipe network, thereby leveraging the capacity of the entire drainage system (Ferrier and Jenkins, 2009, p.435). This is achieved through reduction of the flow rate inside the drainage pipes.

The construction of piped inlets and outlets in swales, basins and ponds also relates to hard engineering (Butler & Davies 2009, p.533). These piped inlets and outlets are designed to provide a reliable flow distribution for surface water. For instance, road kerbs are designed with overflow arrangements to control runoff levels during heavy rainfall (Great Britain: Department for Communities and Local Government 2010, p.34).

The use of permeable surfaces is also another immensely important concept in sustainable drainage in urban areas in the UK. In permeable pavements rainwater is slowly absorbed by porous materials used in the construction of the pavement (Bregulla, Powell & Yu 2010, p.20). The pavement surfaces are made of porous concrete slabs, soft stones and ground asphalt. The technique of using permeable surfaces and materials can be used in alleviating various drainage problems encountered when developing property in urban centers (Bregulla, Powell & Yu 2010, p.20).

One of the applications of this SUDS concept is the construction of car parks and traffic light grounds. The park surfaces can be made of permeable slabs, which absorb voluminous amounts of water during heavy downpours (Bregulla, Powell & Yu 2010, p.20). In addition, gravel surfaces in traffic structures can be constructed using absorbent asphalt and crushed stones (Barton, Grant & Guise 2003, p.157). Some of the advantages of permeable pavements include low maintenance costs; they also do not require additional land for operation and significantly reduce the flow rate of runoff water from sources.

A bedding layer in the permeable pavement is layered with a geo-textile membrane and a plastic mesh structure to increase absorption of water from the surface (Butler & Davies 2009, p.528). A silt trap is also used to avoid clogging of the upper layers in the permeable pavement (Butler & Davies 2009, p.528). A diagrammatical representation of the permeable pavement concept is shown in Figure 3.

Surface water flooding in the UK has been identified as one of the major disasters that affect people in the urban areas. This type of flooding is caused by surface runoffs during rainy seasons. In order to avoid loss of lives and property, the UK government has embarked on a strategic plan to establish SUDS, which are viable drainage systems. SUDS take into consideration the quality of water, natural groundwater recharge, control of the water-flow rates and also protection of the environment. There are various structures and design options which are incorporated into SUDS projects. These include filter drains, filter strips, ponds, basins, permeable pavements, green roofs and also infiltration trenches. These projects can be used to effectively eradicate the risk of flooding in urban areas. However, the conventional drainage systems which rely on hard engineering can also be used. These methods include piped networks, large storage tanks and interceptor tunnels. Establishment of SUDS project by urban councils is the most suitable idea in alleviating inherent problems associated with flooding.