Firstly, before describing the principles that underpin the SuDS approach to drainage, I want to dispel a few common myths:
These are a few of the most commonly quoted reasons for not including SuDS on a scheme and there are more (cost is one).
SuDS principles can be applied to any site, it’s just a case of using the right methods and approach for the site conditions you have. That is not to say that the consideration stated, and others, are not important to how a site is drained, as long as they are considered.
As with anything on a site that is being considered for development, redevelopment, sale or purchase, knowing the main characteristics of a site is extremely important.
The following is a summary of the sort of things that need to be considered anyway and have been put into the context of SuDS.
What has the site been used for in the past?
How steep is your site and relationship to other sites?
What lies beneath the site that could influence the type of SuDS that are used?
You will have an idea of what your site is to be but need to start to plan how all the elements of the site can come together to facilitate your scheme.
The following, although SuDS orientated, is applicable to pretty much any site layout planning process.
There will be major drivers outside of the drainage such as highway access and designated sites (archaeological, Site of Special Scientific Interest (SSSI) etc that need to be considered as well).
The outline described below is taken from the very useful CIRIA document “Planning for SuDS - making it happen (C687F)” available free - https://www.ciria.org/ItemDetail?iProductCode=C687F&Category=FREEPUBS
Now you know your site and where everything is going (mostly) its time to look at the drainage (SuDS and all things connected).
This is best done, almost essentially done, before all the site is fully panned as there are elements that SuDS could enhance and combine with that if left until the end could result in double accounting of land uses and inefficiencies in design.
A common argument of SuDS is that it takes up additional land. In fact, the earlier in the process SuDS can be considered the less land it will take and the multiple benefits are better achieved.
If nothing else, the following general principle should be considered:
“Try and deal with drainage where the rain falls. “
Passing water from one place to another requires land and infrastructure. This infrastructure has to be designed and sized etc. to a set of standards. As everyone is acutely aware, standards change, climate change is significantly impacting flood risk from all sources including surface water systems. Partly as result of the capacity of fixed assets.
A pipe of a given size set at a given gradient has a given and fixed capacity. There is then no future proofing or resilience included (unless as a specific design parameter) to offset changes in the requirements of that system over time.
The SuDS process is to look at drainage in terms of what needs to be drained, when, how and where is it best to be drained. This mirrors the drainage hierarchy as promoted in many current guidance documents including the New Planning Policy Framework (https://www.gov.uk/guidance/flood-risk-and-coastal-change#sustainable-drainage-systems) which states:
“Generally, the aim should be to discharge surface run off as high up the following hierarchy of drainage options as reasonably practicable:
For the principle of SuDS there are a few additional up-front considerations that can and should be applied before looking at the hierarchy:
Taking the SuDS approach we can look at each method or component and see how this can be used to achieve the project goals. The diagram below is a possible way to look at the design;
As can be seen, the goal is not to pass all of the rainfall from a site to somewhere off-site, but to loose water throughout the system and only use an off-site route if you have water left over.
This creates more opportunities and flexibility, more resilience (not all eggs in one basket) and a more natural site drainage system – this is SuDS.
