SuDs which are known as sustainable drainage system use to be known as sustainable urban drainage system. As SuDs are not just developed in urban areas so the word urban is dropped to prevent confusion.
SuDs with in the UK play a huge role in improving drainage system and the environment as it provides a solution to excess runoff water and helps make waste water treatment work more efficient.
SuDs are almost compulsory in most part of the UK subject to if it is required and design complies to standard set out by the planning requirement.
SuDs would be discussed in depth and also the old conventional system would be look into. The following are the main points that would be discussed
Overview of conventional drainage system and reasons why it not sustainable
Conventional drainage system only provided a solution to control flooding during infrequent storm. It is designed to work in a way by draining excess surface water from built up area by underground pipe system. As there is only one pipe the surface water was combined with the sewage water.
When it rains it the flow of the rainfall use to increase in the combine system, until the volume of the flow become too much for the combine pipe to carry on, then the excess sewage was discharged into the river or sea through a system that was called combined sewer overflow also known as CSO. The diagram below illustrates how the CSO works and the understanding of the combined system.
Figure 1 Shows how the CSO work
CSO prevents overloading which would otherwise lead to flooding of properties and waste treatment works. CSO is fitted with telemetry to indicate the operator either it is discharging or when the screen raking mechanism has failed.
In the past CSO was constructed and designed to lower standard that are considered not to be acceptable to standard we require now to tackle the problem with flooding in the present and future. The problems that are associated with the CSO are as follow
Pipe diameter is too small which causes the CSO to discharge more frequently
Discharge is unscreened
Now the other conventional drainage system that is used is the separate system that has a separate pipe for the surface water and also a different pipe for the sewage water.
Home built after 1945 had these system installed as the designer at that time presumed it would be used in the future to tackle flooding issues. One pipe carries the sewer water straight to the waste water treatment and the other pipe carries the surface water directly to the river.
The pipes are often laid side by side and removes the problem associated with CSOâ€™s but the separate system still has problems of its own. Figure 2 Shows an Overflow CSO
These problems can be considered as being too expensive to install. The surface water would not be completely cleaned as it would contain some contaminated substances.
If for some reason the connection of these pipes are connected wrongly as it is possible to do, it will cause the sewage water to be untreated and pass through the system into the rivers which would cause a lot of problems to the environment such as pollution, unpleasant smell and it would create conditions where plants and fish would be unable to survive.
Even if the connections are wrong by 1% then the benefit of using this system is lost and also the conditions of the environment would also be a concern.
There is another conventional drainage method know as the hybrid system. Normally the pipes are separate but the sewage water pipe will still have some input of surface water.
SuDs have a different technique to deal with drainage system in a different way compare to conventional drainage system. SuDs are designed to tackle problems that have been caused by the conventional drainage system and it considers quality, quantity and amenity characteristic all equally. It has an integrated approach which is defined by the SuDs triangle.
Figure 3 Shows the SuDs triangle
SuDs are sustainable as it has a less damaging impact to the environment. The techniques that it offers could be adopted to any new and redeveloped site or even have a site specific design. This would solve the problem that has been linked with surface water drainage.
It promotes good practice and improves the quality of our urban rivers and an attractive urban environment. SuDs has been predicted the future of solving drainage problems as it is more sustainable compare to the conventional system as it is designed to meet number of objectives such as manage flow rates, improve the quality of our water and provide an easy solution for managing the water environment.
The maintenance of SuDs is cheap and it is cost effective to install at new sites and provides an economic infrastructure to the drainage system.
Design of SuDs
The design of SuDs is aimed at the following
Recharge ground water
Improve the environment
The techniques are designed to both control flow and volume at the same time treat surface runoff in stages.
SuDs are used to ensure that runoff is passed through different stages before it is infiltrated into the ground or being released to the water coarse. Runoff volume and flow rate is controlled by its intensity and by storing and infiltrating surface water locally with in each SuDs component. By doing this it reduces the nedd of having large storage facilities at the end of the system.
The runoff is managed in a way that the rain water is returned back to the water environment as close as possible to the source.
SuDs are designed to help the waste water treatment to work more efficient as the quality of the water coming in the site more contaminated. They are wide range of techniques available so SuDs can be designed to fit most development. The design could either be hard surface or soft landscaped. It could either be permeable paving, ponds and dry detention basins.
Figure 4 Shows the Design Philosophy of SuDs
Performance of SuDs
Ponds could either be designed as wet or dry ponds it is designed so that it takes in account that the variations in water level during storm and also it increases in flood storage capacity. The algae and plants of the wet land provide a high quality of filtering and nutrient removal from the water. There is a pipe at the end of the pond which inputs the rain water. Figure 5 Shows how Ponds work
Figure 6 Shows the reduction in peak flow from clay lands pond which receives run off from M8 motorway.
Swales are a long shallow channel and a parallel surface that is vegetated and it drains water evenly of impermeable areas.
Rain water that runs through the vegetated surface slows down and filters the flow by allowing it to spread across the landscape. It is effective as it removes polluting solids by filtering and sedimentation. Figure 7 Shows how Swales work
Pervious surfaces are used in pavement, walkways and parking area as it allows rain water to pass through and get soak into the ground. This prevents water erosion in heavy rain. The surface water could either be drained through gravel or through the gaps of solid paving blocks. Some time the surface clogs up in which case it prevents water going through.
Figure 9 Shows Blocks
Figure 10 Shows Blocks
Figure 8 Shows how Permeable Surface works
There are lot of problems that are linked with retro-fitting SuDs as the whole drainage needs to be re-designed and try to find a cost effective solution and a big enough area close to the source and what options are available to construct SuDs in a built up area where normally there would be less space available. Everything needs to be balanced up to see if it beneficial or not.
There would be a lot of planning objections as member of the general public wont understand the benefit of fitting SuDs and they would not want to be disturbed during construction phase by the noise and delay of traffic going through the roads.
What can be done is to construct swales in car parks and on the side of the roads if there is any space available or in have ponds in local parks which could be an attraction point.
Green roofs can be constructed on the roof of the building if there not any other options as the excess water in the roof could gradually be put back into the drainage system when the weather is dry.
Also there was an issue of maintenance of the SuDs in Scotland as Scottish water did not want the responsibility of maintaining these SuDs but there hold to it legally. So Scottish water has minimum requirement of how SuDs should be constructed then they would take it on to maintain it. Figure 11 Shows a Green Roof
The discussion has given the idea that conventional drainage system is not sustainable and it does not benefit the environment in any way. The technique is old and causes pollution waste water treatment site does not work to its full potential.
SuDs have solved the issue of excess surface water and as the climate is changing it would also solve issues in the future.
SuDs are the most cost effective way in dealing with issues such as flooding and also the environmental issues. The design is based on the triangle of quality, quantity and amenity. The maintenance of SuDs is also not that expensive and SuDs could also be expanded in the future.
The Government is supporting SuDs as they see it as future of dealing problem that have been linked with drainage flooding. They are also grants available to construct. And some parts of the UK they are compulsory to install.
They are lot of construction going on in regards of new site and retro-fitting SuDs in area where there is a problem with the drainage system.
After this literature review on SuDs has made me understand SuDs as I always use to wonder why the construction company constructed a pond in front of my house where they could have constructed atleast 5 more houses. The pond was actually part of the SuDs in preventing flooding for the future.
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