So, you have completed your data-gathering exercise, you have your CCTV results, know your flood risk and what will be going on the site, the next step…the drainage strategy.
This week as we continue to look at what drainage documents are required in planning we will be taking a closer look at the drainage strategy, looking at when it is needed, what should be included and why it is important to spend some time getting it right.
What is a drainage strategy?
It may come as no surprise that the drainage strategy is simply an overview of how you are proposing to drain your site. It is usually presented as a drawing but can also include a written description which can either be included as part of the Flood Risk Assessment document or as a separate report.
By considering how your site will drain early there are opportunities to save costs on your projects as well as ensure that your drainage design can become an asset and bring value to your site as opposed to just being something that is buried in the ground.
Do you always need a drainage strategy?
This will generally depend on the requirements of your Local Planning Authority, however, as a rule of thumb, drainage strategies are typically required for major developments which are classed as being:
- 10 dwellings or more; or
- 1 hectare or more; or
- 1000 square metres or more.
It should be noted, however, that even if officially a drainage strategy is not required, it is still important to consider how your site will drain. This is because even small developments can result in flooding issues if an adequate system is not provided.
What information should you include?
What information you will need to provide all depends on the requirements of your local authority as well as the development itself. To provide an overview, however, I have included some of the more common requirements below.
An overview of how water going to be discharged from the site
This is an opportunity to demonstrate how you have considered the hierarchy of discharge and in the first instance, your application will need to show whether there is a possibility to reuse any water on site.
If this is not possible you will need to provide details about the ground conditions and confirm whether infiltration could be an option. It should be noted here that your local Planning Authority will likely require evidence in the form of soakaway tests to back this up and it won’t be enough for your consultant to simply use information from the British Geological Survey or any Borehole data you may have.
If you find that infiltration is not an option you will then need to highlight any watercourses within the site boundary and only if you can prove that all of the above is unfeasible can you connect into a public sewer.
What climate change factor will you be using
We all know the impact climate change is having on our planet and that over recent years there has been a growing trend of dryer summers and warmer wetter winters. As a result, drainage engineers must apply climate change factors on their calculations to take into account the probability of increased rainfall.
The factor used will be dependent on the type of development and its expected design life but can range between 20-40% and any attenuation calculations submitted as part of your application will need to clearly show which factor has been used and why.
Source: Sky News
Confirmation of how much are you planning to discharge
This will be where your consultant will calculate your allowable discharge rate which and for previously undeveloped sites this will be taken as the greenfield rate (or QBar value).
If a site has been developed in the past there could be an opportunity to calculate the rate based on the pre and post-development impermeable areas and provide a betterment which is usually set as a percentage by the local authority.
Once you know how much you will be discharging you will then also need to provide details of the flow control device you will be using, this could include things such as an orifice plate or Hydro-break.
An overview of the drainage options considered
Once you know what your allowable discharge rate will be you can then set out how much attenuation you will require to accommodate water generated from your site.
As part of your drainage strategy, you will need to provide details of where the attenuation will be located on the site and what methods you are going to use. You will also need to demonstrate how you have considered SuDS within your proposal, working your way through the hierarchy, and showing how you have integrated your drainage strategy into your landscape proposal.
How are you going to manage water quality?
Water quality has become increasingly important over recent years and demonstrating how you are ensuring sufficient treatment to the water you are discharging will need to be captured as part of your drainage strategy.
Generally, this is more than just simply including an oil-water interceptor within your design and the CIRIA SuDS Manual (C753) sets out in Table 26.1 the various methods that can be used to show you are providing sufficient treatment through within your drainage strategy.
How are you going to deal with any foul water generated on the site?
With so much focus on the surface water, how you are proposing to deal with the foul can often be forgotten about.
As part of your foul drainage strategy, you will need to outline how much water will be generated by the development and where your offsite connection point is likely to be.
You may also need to complete a pre-development enquiry with your local water authority.
Any third-party correspondents
As indicated in previous posts, how many conditions that end up being attached to your application is heavily influenced by how much you liaise with third parties to ensure your design meets their requirements.
Therefore if you have made the effort to have those early discussions and taken on board any actions it is always good to include the correspondents as part of the planning application as it demonstrates you are being proactive.
A drainage strategy is not only an opportunity to demonstrates to the planning authority that the site drainage can be effectively dealt with, but it is also your chance to make sure that your developing layout meets any drainage requirements.
As such, it is therefore important to spend some time at the start of the project ensuring that this bit is right as getting it wrong could result in water ending up in the wrong place, inefficiencies or in the worst-case flooding of your project.
If you have enjoyed this entry why not check out some of our other posts, or if you have a particular project where you require a drainage strategy why not get in touch using the contact details on our website.