In our last post, “What is drainage assessment”, we considered the main factors that could affect drainage on a proposed site that would need initial risk assessment to inform a more detailed approach to the development’s drainage strategy.
Now, we turn to the main components of a detailed drainage assessment, required for major developments (10 dwellings or above) that are required by Local Planning Authorities (LPAs).
Approaches vary by LPA, but a typical drainage assessment form usually defines:
- the site’s characteristics (slope profile, soil geology, existing flood risk)
- the calculated surface water runoff rates and required storage volumes
- where the runoff is to be discharged to
- the sustainable drainage system (SuDS) features proposed
- how the drainage system is going to be managed and maintained
In an application, developers will need to show that the excess runoff will be managed effectively and not cause an increase in flood risk anywhere else, i.e. both to and from the site.
SuDS are now a material planning consideration for major developments of 10 dwellings or more and equivalent non-residential schemes).
Drainage Strategy Components
Councils will usually wish to reduce the need for pre-commencement conditions, but drainage is always viewed as critical to be agreed prior to work beginning on site.
Therefore to speed up the review of proposals, drainage consultants need to design the drainage strategy to an appropriate standard of detail.
These as minimum contain and demonstrate the following points:
- Summarise the proposals within a surface water volumes and flows pro-forma:
- Calculations must adhere to the revised Environment Agency guidance on Climate Change last revised in Feb 2017. This requires attenuation to be provided for Commercial 1 in 100 year plus 20 percent and residential 1 in 100 year plus 40 percent.
- Calculations should be provided to demonstrate water is controlled to green field run off rates, at a variety of return periods.
- A 5l/s rate should not be used as a minimum, as flow control devices have evolved to allow smaller discharge rates.
2. Plans, drawings and specification of SuD’s proposed integrated with landscaping and materials requirements.
It is expected that design, drainage and landscaping consultants coordinate and integrate plans to create the final design.
- Detailed site layout showing SuDS integrated with landscaping proposals. SuDS should contribute to green links and chains as well as accessibility to the Blue Ribbon Network across Hillingdon.
- Topographical survey of the site currently showing above ground ponding, overland flow routes and proposed levels showing changes manage these risks as well as providing exceedance routes.
3. Demonstration of the consideration of the spectrum of SuDS in accordance with the SuDS hierarchy, ideally using a variety of options and clear justification provided where the most sustainable solution cannot be provided.
4. Section 106 may be considered for off-site works where these standards cannot be met.
5. Information on the receptors of the drainage system.
- Where infiltration is proposed a suitable site investigation must be provided including information on groundwater levels, or sizing for a tanked or no infiltration must be assumed.
- Together with necessary consents i.e. statement from any water utilities that there is suitable capacity in the foul and surfaces water networks or if not that suitable upgrades have been agreed.
6. Methods of appropriate water quality control to be provided.
- Management and maintenance plan for all SuDS to be provided.
- To ensure there is ongoing maintenance over the lifetime of the development, a management and maintenance plan will be expected to be provided along with the details of the body legally expected to maintain the system where it is anyone other than a single private land owner.
- This should also contain details of the standard of operation.
7. A condition will be added to planning decisions, or developers will be expected to enter into a legal agreement under S106 of the Town and Country Planning Act 1990 (as amended). An example of an appropriate management plan can be found on Susdrain – Useful frameworks and checklists.
Fitting the Right Design to the Site
An essential first step to the right design is to understand the ground drainage conditions and the site’s suitability for infiltration. GeoSmart’s range of Drainage Assessment Reports, SuDSmart, provides an overview of site suitability for a range of SuDS options.
They can also provide run-off calculations and initial SuDS designs, which can be used as part of the drainage strategy submission to the LPA.
For more information, contact us on 01743 298100 or email firstname.lastname@example.org