3.1 Design principles
SuDS features should be in line with NPPF, the National Standards for SuDS seven standards and CIRIA SuDS requirements and should follow the four pillars of sustainable systems as outlined in Figure 1. SuDS should follow the drainage hierarchy as set out in the NPPF, National Standards for SuDS and Building Regulations part H3 which states that surface water discharge from developments should follow the below hierarchy which can be seen below and follow the 5 priorities outlines in standard 1 of the National Standards for SuDS (Figure 3):
Collected for non-potable use – Rainwater harvesting is a priority and should be applied in all developments. This ensures that there is a source control procedure in place which follows the SuDS train, allows for a reduction on flows leaving the site and provides an amenity for water reuse which can be utilised for gardens, agriculture and businesses.
Infiltration – The next most sustainable and preferred option is to discharge surface water via infiltration, to determine if the ground is suitable for infiltration, testing should be conducted using an appropriate methodology such BRE Digest 365. NLC has guidance on the requirements for this which can be found here. This guidance follows the BRE Digest 365 which sets out the standard for infiltration testing. Assuming the water drains away at a rate as a minimum of 1×10-6m/s then the test can be considered successful, and infiltration can be used as a viable method for discharge.
Other points to note for infiltration is if there is sufficient space within a development (a minimum of 5m from building foundations and adjacent highways is required and a minimum of 2.5m for boundary lines), location within a source protection zone and groundwater must also be considered when considering soakaways with a minimum of 1m below the base of the soakaway being a necessity for infiltration to be feasible. Hydraulic modelling should be conducted to determine storage capacities required for discharge via infiltration and require a factor of safety input within the calculations. An appropriate factor of safety should be input within any hydraulic modelling for infiltration features, this is usually set at 2, however if the infiltration feature is designed to discharge a large surface area. If this is the case, then the factor of safety may change (factors of safety should align with Tables 25.2 in Section 25 of the CIRIA SuDS Manual 2015). A justification for a factor of safety different to the standard should be include within the drainage strategy/ report.
Where infiltration rates are lower than 1×10-6m/s a hybrid solution may be possible by way of designing small infiltration basins within the development for small storm events and a piped outfall into attenuation which allows for added storage capacity for larger events and would bring half drain times down to allowable amounts.
Suitable watercourse – Where infiltration is not suitable or has failed due to low permeability then discharge of surface water into a watercourse should be the next preferred option (if appropriate). Assessment into the site should be conducted to determine if there are any suitable watercourses within the vicinity that could be suitable for discharge via this method.
North Lincolnshire Council promote that all developments regardless of greenfield or brownfield should attempt to achieve the greenfield runoff rate or the lowest practical discharge rate possible. If this is not achievable then justification will need to be provided to the LLFA within a drainage impact assessment/ surface water management plan.
Watercourses fall under various authorities within North Lincolnshire, and this should be determined with permissions gained before any discharge can be allowed. Within North Lincolnshire all open watercourses which could be suitable for discharge of surface water fall into different categories of ownership or management. It is the developer/ consultant’s responsibility to contact the relevant owner/ manager for permissions. Watercourses either fall into main rivers which are managed by the Environment Agency and may require an environmental permit. Riparian watercourses come under the jurisdiction of the Land Drainage Authority. This can be either Local Authority – North Lincolnshire Council or one of the five the Internal Drainage Boards (IDBs) which operate in the county if they cover the area. These are:
There is a requirement to complete an ordinary watercourse consent form under the provision of section 23 of the Land drainage Act 1991 and it should be returned to LLFAdrainageteam@northlincs.gov.uk. Completion of this form is only required if the watercourse falls outside of an IDB area. The IDB will need to be contacted directly for a copy of an ordinary watercourse consent form.
Surface water sewer – If there are no suitable watercourses present then discharge into a surface water sewer is the next preferred option, surface water sewers are predominantly managed by a Water Company, however some may be managed by private owners, a limited company or the Highway Authority if it only drains the highway. As such it is important to determine the ownership of the surface water sewer of the proposed discharge point.
North Lincolnshire has two Water Companies within its boundaries, and a pre-planning enquiry should be made to the relevant company in the first instance to determine feasibility of the discharge point.
Combined sewer – If there is no other possibility then it may be applicable to discharge into a combined sewer system although this should be sought after all other options have been considered and dismissed.
As with the surface water sewer system, combined sewers are managed by Water Companies and permissions should be granted from them before discharge into their network.
