Validation Requirements for Flood Risk and Surface Water Drainage
The following document highlights the Flood Risk and Surface Water drainage information that the Development Control Section within Leeds City Council Flood
Risk Management requires applicants to submit in relation to the various stages of the planning application process. This information should be in accordance
with our Minimum Development Control Standards for Flood Risk which can be accessed via the following link:
https://www.leeds.gov.uk/docs/Minimum%20development%20control%20standards%20for%20flood%20risk.pdf
If applicants have any concerns or require additional guidance, they should e mail [email protected]ov.uk with the title Validation Requirements – Pre Application
Advice and FRM will seek to respond within five working days.
Minor Developments and Change of use requirements:
If the development is for a simple change of use, or minor i.e. less than 250m
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, then a plan
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showing the existing/proposed drainage connections would be
required along with the relevant approval. Please refer to our Minimum Development Control Standards.
If your development site is in either Flood Zone 2 or 3 of the Environment Agency Flood Risk Map for Planning, then the applicant will need to submit a Flood
Risk Statement outlining the risks and proposed mitigation measures. For further information please refer to the EA’s standing advice:
https://www.gov.uk/guidance/flood-risk-assessment-standing-advice
Large Minor and Major Developments
Developments outside of the criteria above will require a Surface Water Drainage Assessment to be provided. In addition, for sites within Flood Zone 2 or 3 at risk
of flooding from rivers or the sea, in Flood Zone 1 if the development introduces a more vulnerable land sue and is subject to others sources of flooding, or in
Flood Zone 1and development is greater than 1 ha in size, then a Flood Risk Assessment will also be required which should include details of the proposed surface
water management strategy.
The Table below outlines what information should be submitted for each stage of the application process. This information should be provided to enable
Development Control to review the application in full and hopefully provide a faster decision. Where the information stated is not provided, then there is the
possibility that the application may not be validated and it is the applicant’s responsibility to ensure the relevant information is submitted in a suitable format.
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For small developments where less than 250m2 of drained area and no attenuation required, then only a drainage plan showing pipe sizes, cover and invert levels and outfall
location is required.
Documents/Information required
Pre
Planning
Outline
Planning
Full
Planning
Reserved
Matters
Condition
Discharge
Site Location
Site Location Plan at a suitable scale showing the boundary and area of the application site, together
with the OS grid reference and post code.
Flood Risk Assessment
Required for all sites that fall into the following categories:
- Flood Zone 1 at risk of flooding from rivers or the sea that are 1 ha in size or greater,
- Flood Zones 2 and 3 at risk of flooding from rivers or the sea,
- land which has been identified by the Environment Agency as having critical drainage problems;
- land identified in a strategic flood risk assessment as being at increased flood risk in future;
- land that may be subject to other sources of flooding (including surface water), where its
development would introduce a more vulnerable use.
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Existing Site Characteristics
Details of how the site currently drains. This can be an OS/LIDAR contoured map or where available, a
site specific topographic survey. The plan should show all existing watercourses and current overland
flow arrows.
a. Location of nearest drainage assets – The location of any existing watercourses and private
and public sewers or highway drains within or adjacent to the site. Where details of any
existing drainage systems and areas drained are available, then these should also be shown
on the plan.
b. Sources of existing flood risk – An initial review of existing known flood risks should be
provided based on information available from GOV.UK flood maps and other publicly
available sources (e.g. river modelling data).
c. Details of existing ground conditions – Information regarding existing ground conditions
should be provided from site specific ground investigation reports and supported
by publicly available information, such as the BGS database to assist in reviewing
the use of SuDS infiltration features. .
d. Groundwater and Aquifer Protection- A review should be undertaken from the BGS
database and EA and DEFRA Magic Map in relation to the existing ground water profile and
the presence of any groundwater source protection zones.
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NPPF July 2021 Section 167, Note 55
e. Details of existing and historic site use which may identify possible contamination issues
which may influence the SuDS drainage design.
f. Preliminary calculations of existing pre-development run off rate – An assessment should
be provided of the existing permeable and impermeable areas (shown on a plan) together
with a preliminary assessment based on the guidance set out within the Leeds CC Minimum
Development Control Standards for Flood Risk of the existing discharge rate.
g. Details and condition assessment of any existing structures on or adjacent the site, such as
culverts, which a SuDS drainage strategy may rely on.
Proposed Development Characteristics
a. Type of Development – General description of proposed development together with an
indicative masterplan layout.
b. Area of Development – Details of total site area proposed for development which
post development impermeable areas calculated (and shown on a plan). It will
also include an indication of how the various sub-catchments of the development
site will be drained.
c. Proposed Site Levels – Preliminary post development site ground levels shown on a plan.
d. Details of any specific Flood Risk Mitigation measures proposed – Indicate if raised
Finished Floor Levels (FFLs), flood compensatory storage or flood resilience/resistance
measures are required. This should account for all sources of flooding but also any
flood levels within the proposed SuDS drainage scheme.
e. Details of any, amenity, ecological and bio-diversity mitigation measures proposed –
Identify proposed open spaces and areas to be used for SUDs storage and conveyance
features.
f. Details of any proposed structures on or adjacent the site, such as culverts for site access.
Indicative Proposed Drainage Strategy
a. Review of SuDS options – Undertake a review based on the information available to define
and identify the SuDS options that can be incorporated into the emerging site masterplan.
b. For all Major applications a separate Conceptual SUDS Design Assessment should be
undertaken to define the relevant SUDS design criteria to be adopted in draining the
development and how it shall integrate with the proposed bio diversity proposals for the
development. Also, the assessment shall include the reasoning for either including or
excluding the associated SUDS features in accordance with the requirements of the NPPF
and criteria set out within Paras 082 to 085 of PPG (Flood Risk & Coastal Change).
c. Define the future climate change allowance factors to be adopted as agreed with the LLFA
and based on the lifetime of the development which should be identified.
d. Provide a conceptual surface water drainage plan with preliminary sizing, design calculations
and location of on-site attenuation features. Note a review of the proposed levels and site
topography shall be undertaken at this stage to verify if any proposed SuDS such as Swales,
detention basins or attenuation ponds can be practically constructed.
e. Confirm the proposed post development discharge rate and volumes of surface water–
Provide initial calculations and assessments to justify the basis of the post development
discharge rate and volumes to be agreed with the LLFA.
f. Where the use of infiltration is proposed, then unless site permeability testing to BRE 365
or equivalent has been undertaken to demonstrate that infiltration is viable, an
alternative drainage strategy to either a watercourse or sewer system shall be defined
and the point of connection or outfall and proposed discharge rates defined.
g. Where discharges from the development site outfalls to an existing private drainage system
evidence must be provided to demonstrate that the private drainage system has the
capacity to accept the additional flows and that the private drainage system also has a final
positive outfall to a watercourse, balancing pond, public sewer or highway drain.
h. Future Maintenance Requirements – Identify areas of drainage that are to be adopted and
areas that are to remain private and define the mechanism for inspection and maintenance
of all private drainage features over the lifetime of the development. A maintenance
schedule and items to be maintained shown on a plan will be supplied.
i. Identify how exceedance events in excess of the design storm will be dealt with - Identify
future or existing overland flow paths for events in excess of the design storm event.
j. Provide details of any phasing of the development and how the drainage strategy will
incorporate these requirements.
Detailed Drainage Assessment
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The Drainage Assessment shall include the following:
a. A drawing showing the pre and post development impermeable areas, suitably annotated that
relate to any drainage calculations.
b. Calculations and any supporting survey/existing drainage record drawings and CCTV
investigations to justify and demonstrate the existing and proposed discharge rate.
Calculations of discharge runoff rate and volumes plus detailed information on sizing of
SuDS drainage features will be provided.
c. A drainage layout plan including SuDS features any associated manholes including cover and
invert levels, proposed site levels, pipe sizes and gradients, all on-line controls, on and off line
storage structures, water quality mitigation and outfall details.
d. Full details of all proposed attenuation features and flow controls.
e. Information (Micro Drainage or similar) shall be shown on a plan including all numbered
nodes and drainage network model and details.
f. A summary of results showing all the modelling criteria and summary network results for critical
50% AEP, 3.33% AEP and 1% AEP plus required climate change (CC) storm events showing
maximum water level, flow and velocity. A plan will be supplied to show the extent and depth of
any surface flooding anticipated. Comment will also be provided to show how freeboard has
been provided to any finished ground floor levels of buildings or essential infrastructure (e.g.
plant rooms).
g. Urban creep should be incorporated into the drainage design ensuring that the appropriate
allowance is included for the lifetime of the development. Refer to Table 8 in Leeds CC Minimum
Development Control Standards for Flood Risk.
h. Plan showing overland exceedance routes in the event of a failure of the drainage system or
storm event in excess of the 1% AEP plus required climate change storm event.
i. An assessment of water quality mitigation proposed for the surface water flows in accordance
with Section 26 of the SUDS Manual (C753) can be achieved during all phases of the
development. The Simplified Index Approach may be appropriate for residential or
commercial developments depending on anticipated type and number of traffic movements.
j. Where relevant written confirmation from Yorkshire Water as to acceptance and agreed point of
connection to the public sewer system for the proposed foul and surface water flows.
k. Where the off-site drainage works cross third party land, then written approval shall be provided
from the land owner or alternatively if a sewer requisition under Section 98 of the Water
industry Act 1991 is to be considered, then written acceptance from Yorkshire Water confirming
their acceptance that a requisitioned sewer shall be provided.
l. Where the surface water is to discharge to a watercourse outside of the applicants ownership,
then written confirmation as to acceptance of the proposed discharge rate from the riparian
owner of the watercourse shall be provided.
m. Copies of the Site Investigation report and infiltration testing results (BRE 365 or equivalent)
which demonstrates the viability of designing an infiltration based drainage system.
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n. Where infiltration and the use of soakaways is to be adopted then full supporting calculations
and drainage plans shall be provided as set out above.
o. Details of which body will be responsible for vesting and maintenance of individual aspects of
the drainage proposals including a management statement to outline the management goals
and means of securing continuity of payment from all future owners of the site for the required
maintenance.
p. Description of inspection and maintenance schedule including plan showing all access points,
easements and outfalls.
q. Where required for major developments or phasing of minor developments, a plan showing
each development plot (e.g. a development block of houses) which shows the allocation of
surface water volume storage and discharge rate given to that plot as part of a wider overall
drainage/SuDS strategy.
r. A timetable for implementation of the drainage works including an assessment of any phasing of
the development and how surface water run-off will be controlled during the construction
process to prevent an increase of flood risk or pollution.
Reserved Matters Application where the provision of the Drainage/SUDS information is relevant and
impacts on the proposed layout
a. Drainage Plans showing the proposed foul and surface water drainage layout together with
sufficient drainage design calculations to determine the size and location of the proposed SUDS
features.
b. Preliminary detailed layout plans of each SUDS feature to demonstrate that the finished site
ground levels will allow the physical construction of each feature.
c. Typical sections of each SUDs feature.
d. A Drainage Assessment incorporating all of the above requirements.
e. Plan and cross sections and all supporting calculations in respect of any flood storage areas or
compensatory flood measures as determined within the outline planning conditions
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In respect to full planning applications, please note that it is now not acceptable to leave the details of the drainage design, including details of the proposed SuDS measures to a
future planning condition. It is now required to provide the necessary drainage design and supporting calculations and investigations to demonstrate that the proposed development
will comply both with the Leeds CC Minimum Development Control Standards for Flood Risk and also not increase the flood risk to any area outside of the application site, as part of
the full planning application.
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Where Soakaways are proposed then permeability testing to BRE 365 or equivalent must be undertaken and the supporting calculations provided.
