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 ....................................................................................................................................................................................2
2
Impermeable Area .........................................................................................................................................................................2
3
Proposed Method to Discharge Surface Water .............................................................................................................................2
4
Peak Discharge Rates ...................................................................................................................................................................4
5
Total Discharge Volumes ...............................................................................................................................................................5
6
Total Attenuation Storage ..............................................................................................................................................................6
7
Drainage Design ............................................................................................................................................................................6
8
Evidence ........................................................................................................................................................................................7
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Northamptonshire County Council Surface Water Guidance for Developers
April 2015
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
Existing
Proposed
Difference
(Proposed-Existing)
Impermeable
area (ha)
Notes for developers & Local Authorities
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
(1) Via
infiltration
(2) To
watercourse
(3) To surface
water sewer
Evidence behind reasoning
Notes for developers & Local Authorities
Please complete table below.
See table below.
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.
Confirmation is required from the sewer provider that sufficient capacity
exists for this connection.
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.
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Northamptonshire County Council Surface Water Guidance for Developers
Y/N
Evidence behind reasoning
(4) To combined
sewer
Combination of
above
April 2015
Notes for developers & Local Authorities
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.
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:
State the site’s geology and
known Source Protection Zones
(SPZ)
Have infiltration rates been
obtained by desk study or
infiltration test?
What are the infiltration rates
and are they suitable?
What is the distance between
any proposed infiltration device
base and the groundwater level?
Is the site contaminated? If yes,
consult the Environment Agency
on whether infiltration is
appropriate.
Any other evidence why
infiltration is not appropriate for
the site?
In light of the above, is
infiltration feasible?
Notes for developers & Local Authorities
Avoid infiltrating in made ground. Infiltration rates are highly variable. Refer
to Environment Agency website to identify and source protection zones
(SPZ).
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.
Infiltration rates should be no lower than 1x10 -6 m/s.
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.
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.
E.g. ground stability issues, history of groundwater flooding
If infiltration is not feasible the applicant should then consider the options
in the next section.
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Northamptonshire County Council Surface Water Guidance for Developers
April 2015
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)
(PotentialExisting)
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.
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.
QBAR
1 in 30
1in 1002
1 in 1002 plus
climate
change
Notes for developers & Local Authorities
N/A
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
2
For sites in the Upper Nene catchment this is the 1 in 200 event
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Northamptonshire County Council Surface Water Guidance for Developers
April 2015
Proposed Discharge Rates
Existing Rates (l/s)
Proposed Rates (l/s)
(with mitigation)
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..
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.
QBAR
1 in 30
1in 1002
1 in 1002 plus
climate change
Notes for developers & Local Authorities
N/A
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)
QBAR
1 in 30
1in 1002
1 in 1002 plus
climate
change
Potential
volumes of
runoff (m3)
(without
mitigation)
Volumes
infiltrated (m3)
(where
appropriate)
Difference (m3)
(Potential infiltrated existing)
Notes for developers & Local Authorities
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.
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.
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Northamptonshire County Council Surface Water Guidance for Developers
April 2015
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)
Please confirm total volume of
storage to be provided on the site.
Notes for developers & Local Authorities
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
What drainage systems are
proposed? Are they traditional
(piped underground) or SuDS
(overground, near to source and
mimicking natural flowpaths)?
Provide a brief description.
If SuDS methods are not proposed,
provide evidence why they are
deemed inappropriate for this site.
Confirm that the proposed drainage
system can contain the 1 in 30
storm event without any flooding.
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.
Details/Evidence
Notes for developers & Local Authorities
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.
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.
This is a requirement of the Sewers for Adoption and is good
practice, even where drainage system is not going to be
adopted.
“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
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Northamptonshire County Council Surface Water Guidance for Developers
Design Element
Confirm that exceedance routes
have been considered.
How are discharge rates from the
site being restricted (e.g.
hydrobrake)
Confirm the owners/adopters of the
entire drainage system. Please list
all the owners.
How is the entire drainage system
to be maintained?
Details/Evidence
April 2015
Notes for developers & Local Authorities
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.
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.
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.
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
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Northamptonshire County Council Surface Water Guidance for Developers
April 2015
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:……………………………............................
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