The final detailed surface water drainage design submitted for planning approval is expected to be detailed enough to ensure that it can be constructed without any need for interpretation.
Where SuDS are going to be adopted, the adoption body and their standards should be referenced. It is expected that the more stringent requirements are followed in terms of the NSfS, our guidance and adoption standards.
If the adoption requirements conflict with the NSfS or our guidance, then we suggest that this is highlighted within pre-application advice process. It is important that such conflicts are noted within the drainage statement. This reduces the likelihood of delays in decision making.
The presence of existing trees and hedging can have a significant impact on the area where SuDS features and associated infrastructure can be located. This is because tree roots can damage liners, pipes and other infrastructure. This can cause an increase in flood risk, due to structural damage and potential failure of the network itself. For example, if such damage allows groundwater to infiltrate into a system that is designed to be sealed due to high groundwater levels, then this can increase flood risk as it represents an unquantifiable volume of groundwater that is consuming storage that is designed for surface water only.
Landscaping and drainage design should be coordinated to avoid conflict between them. Drainage and landscaping plans are expected to correspond with each other. The potential future growth of proposed trees should be considered and ideally shown on the plans. Where conflict is unavoidable, mitigation measures such as lined tree pits or root barriers may be used. However, it is important that these are demonstrated to adequately protect the SuDS and that they do not inhibit the growth potential of any trees or hedging.
All easements to rivers, watercourses and utilities should be shown on drainage layouts. In the case of rivers and watercourses, these should also be shown on the site plans.
Where there are restrictions as to what may be constructed within these easements, that should be referenced on the plans and in the drainage statement.
Designers are guided to our Land Drainage Byelaws which detail the legal requirement to observe a 3m easement from all ordinary watercourses.
We do not stipulate a minimum maintenance easement on open SuDS features. However, the onus is on the applicant to be able to demonstrate that SuDS can be safely maintained. It is suggested that a minimum 3m clear working strip from the top of bank usually satisfies this requirement. If access appears to be restricted by the layout, then detailed maintenance plans will be expected to be submitted prior to determination of a full or reserved matters application.
Where a development is likely to be phased this will need to be considered in 2 ways which are:
For major development, verification of construction conditions are linked to the occupation of phases. Therefore, it is important that the phasing is approved and linked to the verification. Each phase should include a constructed outfall and downstream network. Phases may link into earlier ones, but must not rely on infrastructure or drainage features which are to be constructed later.
Where a phased approach is necessary flow controls may need to be adjusted during the construction of the development to ensure that flood risk is not increased. This includes adjustment for temporary drainage during the construction phase.
Phasing plans may need to be submitted for different elements of control. For example, the flow control phasing plans and consideration may relate to larger parcels of major development. Whereas the verification phasing may relate to much smaller phases within each of these parcels, representing phases of occupation.
Our guidance is still in development relating to surface water management in construction. However, applicants are guided to Chapter 31 of The SuDS Manual C753 for detailed guidance on this topic. Silt management and control of dewatering are of critical importance to ensure that flood risk is not increased, and that watercourses and groundwater are not polluted during the construction of a development.