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 1 Site Details

Site Address & 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

Notes for developers & Existing Difference (Proposed-Existing) Local Authorities Impermeable If proposed is greater than area (ha) 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 Please complete table below. See table below. infiltration State whether it is main river or ordinary watercourse. Watercourse needs (2) To to be within the boundary of the site or developer land ownership, watercourse otherwise the developer must be able to confirm permission to cross 3rd party land.

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. Northamptonshire County Council Surface Water Guidance for Developers April 2015

Y/N Evidence behind reasoning Notes for developers & Local Authorities (3) To surface Confirmation is required from the sewer provider that sufficient capacity water sewer exists for this connection. Confirmation is required from the sewer provider that sufficient capacity (4) To combined exists for this connection. This option should only be used if no other sewer option is viable. Combination of e.g. part infiltration part discharge to sewer or watercourse. Provide above 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 Avoid infiltrating in made ground. Infiltration rates are highly variable. Refer known Source Protection Zones to Environment Agency website to identify and source protection zones (SPZ) (SPZ). Estimating infiltration rates through desk studies is acceptable at most Have infiltration rates been stages of the planning system, as long as an alternative design including obtained by desk study or attenuation is provided should infiltration rates prove inadequate following infiltration test? site investigations. What are the infiltration rates Infiltration rates should be no lower than 1x10 -6 m/s. and are they suitable? What is the distance between Need minimum 1m between the base of any infiltration device and the any proposed infiltration device groundwater table to protect groundwater quality and ensure groundwater base and the groundwater level? doesn’t enter infiltration devices and reduce storage/infiltration capacity. Is the site contaminated? If yes, Water should not be infiltrated through land that is contaminated, to protect consult the Environment Agency groundwater quality. The Environment Agency may be able to provide on whether infiltration is advice in planning consultations for contaminated sites. appropriate. Any other evidence why infiltration is not appropriate for E.g. ground stability issues, history of groundwater flooding the site? In light of the above, is If infiltration is not feasible the applicant should then consider the options infiltration feasible? in the next section.

3 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.

Notes for developers & Existing Rates (l/s) Difference (l/s) (Potential- Existing) Local Authorities QBAR QBAR is approximately equivalent to the 1 in 2 1 in 30 storm event. Proposed discharge rates (with mitigation) should be no greater than existing rates for all corresponding 1in 1002 storm events. Discharging all flow from site at the existing 1 in 100 event would increase flood risk during smaller events. To mitigate for climate change the proposed 1 in 1002 +CC will need to be no greater than the existing 1 1 in 1002 plus in 1002 runoff rate. 30% climate N/A should be added to the change 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.

2 For sites in the Upper Nene catchment this is the 1 in 200 event Northamptonshire County Council Surface Water Guidance for Developers April 2015

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

Proposed Rates (l/s) Existing Rates (l/s) Notes for developers & Local Authorities (with mitigation) QBAR QBAR is approximately equivalent to the 1 in 2 storm event. Proposed discharge rates (with mitigation) should be less than or equal to 1 in 30 existing rates for all corresponding storm events, or equal to QBAR for all 1in 1002 events if a simple flow restriction is chosen.. The proposed 1 in 1002 +CC will need to be no greater than the existing 1 in 1 in 1002 plus N/A 1002 runoff rate. 30% should be added to the peak rainfall intensity for climate change 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.

Potential Volumes Existing volumes of infiltrated (m3) Notes for developers volumes of runoff (m3) Difference (m3) (Potential - infiltrated - existing) (where & Local Authorities runoff (m3) (without appropriate) mitigation) QBAR

5 Potential Volumes Existing volumes of infiltrated (m3) Notes for developers volumes of runoff (m3) Difference (m3) (Potential - infiltrated - existing) (where & Local Authorities runoff (m3) (without appropriate) mitigation) QBAR is 1 in 30 approximately equivalent to the 1 in 2 storm event. Proposed discharge volumes (with mitigation) should be no greater than 1in 1002 existing volumes for all corresponding storm events. Where simple flow restriction is proposed, all events will be restricted to QBAR. To mitigate for climate change the proposed 1 in 1002 +CC volume 1 in 1002 plus discharge from site climate must be no greater change 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. Northamptonshire County Council Surface Water Guidance for Developers April 2015

Total Storage Volume (m3) Notes for developers & Local Authorities The applicant at this stage should have an idea of the site Please confirm total volume of characteristics and be able to explain what the storage storage to be provided on the site. 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 SuDS can be adapted for most situations even where infiltration (piped underground) or SuDS isn’t feasible e.g. impermeable liners beneath vegetated swales (overground, near to source and or ponds allow treatment but not infiltration. See CIRIA SUDS mimicking natural flowpaths)? Manual C697. Provide a brief description. The Written Statement made by The Secretary of State for Communities and Local Government (Mr Eric Pickles) on 18 Dec If SuDS methods are not proposed, 2014, states that sustainable drainage systems for the provide evidence why they are management of run-off should be put in place for all major deemed inappropriate for this site. 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 This is a requirement of the Sewers for Adoption and is good system can contain the 1 in 30 practice, even where drainage system is not going to be storm event without any flooding. adopted. Confirm that any flooding between the 1 in 30 & 1 in 1002 plus climate “Safely” means not causing property flooding or posing a hazard change storm events will be safely to site users by restricting locations and depths. Guidance on contained on site. If there will be hazards to people can be found in the Environment Agency R&D any flooding on site, provide Technical Report FD2320/TR2 details of location. Exceedance is any storm event greater than the design (1 in Confirm that exceedance routes 1002) storm. Where possible, exceedance routes should divert have been considered. flow away from properties and more vulnerable uses of the site, whilst maintaining access/egress for emergency services.

7 Design Element Details/Evidence Notes for developers & Local Authorities How are discharge rates from the Hydrobrakes should be used where discharge rates are low, i.e. site being restricted (e.g. between 2l/s and 5l/s. Orifices and pipes should not be used hydrobrake) below 5l/s as they are prone to blockage. Confirm the owners/adopters of the If there are multiple owners then a drawing illustrating exactly entire drainage system. Please list what features will be within each owner’s remit must be all the owners. submitted with this Proforma. 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 How is the entire drainage system be provided. Confirm who will be responsible for the to be maintained? 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).

Page/ Paragraph Pro-forma Section Document reference where details quoted above are taken from 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. Northamptonshire County Council Surface Water Guidance for Developers April 2015

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 ......