Developer Pro-Forma for Surface Water Drainage
Northamptonshire County Council Surface Water Guidance for Developers April 2015
Developer Pro-forma for Surface Water Drainage
For all major development a Flood Risk Assessment / Drainage Strategy which includes full consideration of surface water flood risk, drainage and mitigation should be provided. Please see the Surface Water Drainage Guidance for Developers leaflet for more information on what NCC would expect to be included. We advise that developers should complete this form and submit it to the Local Planning Authority, alongside the Flood Risk Assessment/ Drainage Strategy, referencing from where in their submission documents this information is taken. The pro-forma should be considered alongside other supporting SuDS Guidance, but focuses on ensuring flood risk is not made worse elsewhere. This proforma is based upon current industry standard practice.
The top part of the pro-forma includes a section where the developer can state the difference in rates and volumes as a result of the proposed development. The lower sections are provided to demonstrate how discharge rates and volumes will be dealt with on the site in order to not increase flood risk.
The pro-forma includes a section where the developer should identify where the information is demonstrated. If the pro-forma is completed and signed by the developer, this can serve as a summary of the surface water strategy on the site and will allow them to demonstrate that they have complied with the Technical Guidance to the National Planning Policy Framework (NPPF).
Contents
1 Site Details 3
2 Impermeable Area 3
3 Proposed Method to Discharge Surface Water 3
4 Peak Discharge Rates 5
5 Total Discharge Volumes 6
6 Total Attenuation Storage 7
7 Drainage Design 7
8 Evidence 8
1 Site Details
SiteAddress & post code or LPA reference
Grid reference
Is the existing site developed (brownfield) or undeveloped (greenfield)?
Total Site Area (including any proposed open space)
Total Site Area (excluding proposed open space) (ha)[1]
2 Impermeable Area
/ Existing / Proposed / Difference (Proposed-Existing) / Notes for developers & Local Authorities /Impermeable area (ha) / If proposed is greater than existing, then runoff rates and volumes will be increasing.
3 Proposed Method to Discharge Surface Water
Follow the hierarchy below. Evidence that infiltration is not possible must be provided before consideration of discharge to watercourse or surface water/combined sewer.
/ Y/N / Evidence behind reasoning / Notes for developers & Local Authorities /(1) Via infiltration / Please complete table below. / See table below.
(2) To watercourse / State whether it is main river or ordinary watercourse. Watercourse needs to be within the boundary of the site or developer land ownership, otherwise the developer must be able to confirm permission to cross 3rd party land.
(3) To surface water sewer / Confirmation is required from the sewer provider that sufficient capacity exists for this connection.
(4) To combined sewer / Confirmation is required from the sewer provider that sufficient capacity exists for this connection. This option should only be used if no other option is viable.
Combination of above / e.g. part infiltration part discharge to sewer or watercourse. Provide evidence above.
To demonstrate whether infiltration is feasible on the site please complete below:
/ Notes for developers & Local Authorities /State the site’s geology and known Source Protection Zones (SPZ) / Avoid infiltrating in made ground. Infiltration rates are highly variable. Refer to Environment Agency website to identify and source protection zones (SPZ).
Have infiltration rates been obtained by desk study or infiltration test? / Estimating infiltration rates through desk studies is acceptable at most stages of the planning system, as long as an alternative design including attenuation is provided should infiltration rates prove inadequate following site investigations.
What are the infiltration rates and are they suitable? / Infiltration rates should be no lower than 1x10 -6 m/s.
What is the distance between any proposed infiltration device base and the groundwater level? / Need minimum 1m between the base of any infiltration device and the groundwater table to protect groundwater quality and ensure groundwater doesn’t enter infiltration devices and reduce storage/infiltration capacity.
Is the site contaminated? If yes, consult the Environment Agency on whether infiltration is appropriate. / Water should not be infiltrated through land that is contaminated, to protect groundwater quality. The Environment Agency may be able to provide advice in planning consultations for contaminated sites.
Any other evidence why infiltration is not appropriate for the site? / E.g. ground stability issues, history of groundwater flooding
In light of the above, is infiltration feasible? / If infiltration is not feasible the applicant should then consider the options in the next section.
4 Peak Discharge Rates
This is the maximum flow rate at which surface water runoff leaves the site during a particular storm event, without considering the impact of any mitigation such as storage, infiltration or flow control.
/ Existing Rates (l/s) / Potential Rates (l/s) (without mitigation) / Difference (l/s) (Potential- Existing) / Notes for developers & Local Authorities /QBAR / QBAR is approximately equivalent to the 1 in 2 storm event.
Proposed discharge rates (with mitigation) should be no greater than existing rates for all corresponding storm events. Discharging all flow from site at the existing 1 in 100 event would increase flood risk during smaller events.
1 in 30
1in 100[2]
1 in 1002 plus climate change / N/A / To mitigate for climate change the proposed 1 in 1002 +CC will need to be no greater than the existing 1 in 1002 runoff rate. 30% should be added to the peak rainfall intensity for residential development, and 20% for commercial development.
Developers have two choices for limiting surface water discharge rates from the development:
Option 1 Simple – Provide a simple flow restriction to limit the final discharge from site during all events to the existing QBAR rate, or 5l/s/ha (2l/s/ha in Corby). Store and/or infiltrate the additional volume on site. This may require a large volume of storage on site.
Option 2 Complex – Provide a complex flow restriction which varies the final discharge rate from the site depending on the storm event. As a minimum, the QBAR, 1in30 and 1in1002+cc rates should be checked. Store and/or infiltrate the additional volume on site. The flow control is likely to be more expensive but this option will reduce volume of storage required on site.
/ Option / Notes for developers & Local Authorities /Please confirm which option has been chosen
Proposed Discharge Rates
/ Existing Rates (l/s) / Proposed Rates (l/s) (with mitigation) / Notes for developers & Local Authorities /QBAR / QBAR is approximately equivalent to the 1 in 2 storm event.
Proposed discharge rates (with mitigation) should be less than or equal to existing rates for all corresponding storm events, or equal to QBAR for all events if a simple flow restriction is chosen..
1 in 30
1in 1002
1 in 1002 plus climate change / N/A / The proposed 1 in 1002 +CC will need to be no greater than the existing 1 in 1002 runoff rate. 30% should be added to the peak rainfall intensity for residential development, and 20% for commercial development.
5 Total Discharge Volumes
The total volume of surface water runoff leaving the current site, the potential total volume from the developed site (not taking into consideration any mitigation) and the potential volume that can be infiltrated (where appropriate). The difference provides the total volume that will need to be stored on site.
/ Existing volumes of runoff (m3) / Potential volumes of runoff (m3) (without mitigation) / Volumes infiltrated (m3) (where appropriate) / Difference (m3) (Potential - infiltrated - existing) / Notes for developers & Local Authorities /QBAR / QBAR is approximately equivalent to the 1 in 2 storm event.
Proposed discharge volumes (with mitigation) should be no greater than existing volumes for all corresponding storm events. Where simple flow restriction is proposed, all events will be restricted to QBAR.
1 in 30
1in 1002
1 in 1002 plus climate change / To mitigate for climate change the proposed 1 in 1002 +CC volume discharge from site must be no greater than the existing 1 in 1002 volume discharge.
6 Total Attenuation Storage
Attenuation storage is provided to enable the rate of runoff from the site into the receiving water body to be limited to an acceptable rate.
/ Total Storage Volume (m3) / Notes for developers & Local Authorities /Please confirm total volume of storage to be provided on the site. / The applicant at this stage should have an idea of the site characteristics and be able to explain what the storage requirements are on site and how it will be achieved.
7 Drainage Design
Summary of the proposed drainage design for the development.
Design Element / Details/Evidence / Notes for developers & Local Authorities /What drainage systems are proposed? Are they traditional (piped underground) or SuDS (overground, near to source and mimicking natural flowpaths)? Provide a brief description. / SuDS can be adapted for most situations even where infiltration isn’t feasible e.g. impermeable liners beneath vegetated swales or ponds allow treatment but not infiltration. See CIRIA SUDS Manual C697.
If SuDS methods are not proposed, provide evidence why they are deemed inappropriate for this site. / The Written Statement made by The Secretary of State for Communities and Local Government (Mr Eric Pickles) on 18 Dec 2014, states that sustainable drainage systems for the management of run-off should be put in place for all major development, unless demonstrated to be inappropriate. The onus is on the developer to provide evidence why SuDS may be inappropriate.
Confirm that the proposed drainage system can contain the 1 in 30 storm event without any flooding. / This is a requirement of the Sewers for Adoption and is good practice, even where drainage system is not going to be adopted.
Confirm that any flooding between the 1 in 30 & 1 in 1002 plus climate change storm events will be safely contained on site. If there will be any flooding on site, provide details of location. / “Safely” means not causing property flooding or posing a hazard to site users by restricting locations and depths. Guidance on hazards to people can be found in the Environment Agency R&D Technical Report FD2320/TR2
Confirm that exceedance routes have been considered. / Exceedance is any storm event greater than the design (1 in 1002) storm. Where possible, exceedance routes should divert flow away from properties and more vulnerable uses of the site, whilst maintaining access/egress for emergency services.
How are discharge rates from the site being restricted (e.g. hydrobrake) / Hydrobrakes should be used where discharge rates are low, i.e. between 2l/s and 5l/s. Orifices and pipes should not be used below 5l/s as they are prone to blockage.
Confirm the owners/adopters of the entire drainage system. Please list all the owners. / If there are multiple owners then a drawing illustrating exactly what features will be within each owner’s remit must be submitted with this Proforma.
How is the entire drainage system to be maintained? / Poorly maintained drainage can lead to increased flooding problems in the future. Clear details of the maintenance proposals of all elements of the proposed drainage system must be provided. Confirm who will be responsible for the maintenance of each drainage feature. Where possible also provide a maintenance schedule but this may be acceptable to provide as part of Condition.
8 Evidence
Please identify where the details quoted in the sections above were taken from (plans, reports etc). Please also provide relevant drawings and calculations that need to accompany your proforma, in particular exceedence routes and ownership and location of SuDS (maintenance access strips etc).
Pro-forma Section / Document reference where details quoted above are taken from / Page/ Paragraph Number /Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
The above form should be completed using evidence from the Flood Risk Assessment/Drainage Strategy and site plans. It should serve as a summary sheet of the drainage proposals and should clearly show that the proposed rates and volumes of runoff as a result of development will not be increasing.
Acknowledgement
This form is completed using factual information from the Flood Risk Assessment/Drainage Strategy and Site Plans and can be used as a summary of the surface water drainage strategy on this site.
Form Completed By……………………………………………………………………………………......
Qualification of person responsible for signing off this proforma ......
Company……………………………………………………………………………......
On behalf of (Client’s details) ......
Date:……………………………......
1
[1] The Greenfield runoff off rate from the development which is to be used for assessing the requirements for limiting discharge flow rates and attenuation storage from a site should be calculated for the area that forms the drainage network for the site whatever size of site and type of drainage technique. Please refer to the Rainfall Runoff Management document or CIRIA manual for detail on this.
[2] For sites in the Upper Nene catchment this is the 1 in 200 event