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Introduction

The Lead Local Flood Authority (LLFA) are consulted by the Local Planning Authority (LPA) to assist with the discharge of planning conditions relating to surface water drainage. The LLFA will review the documents submitted and recommend to the LPA whether the condition can be discharged. The LPA will make the final decision.

For all discharge of conditions applications, a cover letter or brief report should be provided to state how the drawings and plans submitted address each part of the condition, this enables the reviewing officer to efficiently and quickly assess the information supplied.

The information submitted to discharge the condition(s) for surface water drainage should not be assumed or preliminary. The LLFA classifies the discharge of conditions stage as detailed design which builds upon the principles agreed at the planning Application stage.

We as LLFA reserve the right to request additional information to that listed below in relation to the management of surface water where queries arise and the information submitted is not suitable. Any additional requirements will be assessed on a site-by-site basis. It should be noted that some planning decision notices may have alternative wording to that presented below.

Major development pre-commencement condition

1. The development hereby permitted shall not commence until details of the final design of a surface water drainage scheme have been submitted to and approved in writing by the planning authority. The final solution should follow the principles set out in the approved drainage strategy. The design must satisfy the SuDS Hierarchy and be compliant with the national Non-Statutory Technical Standards for SuDS, National Planning Policy Framework (NPPF) and Ministerial Statement on SuDS. The required drainage details shall include:

The submitted drainage strategy must follow the principles set out in the approved drainage strategy agreed, where any deviation from the agreed principles occurs clear justification and evidence must be provided.

a) The results of infiltration testing completed in accordance with Building Research Establishment (BRE) Digest: 365 and confirmation of groundwater levels. Where infiltration is proposed confirmation is required of a 1m unsaturated zone from the base of any proposed soakaway to the seasonal high groundwater level and confirmation of half-drain times

Soakage testing in accordance with BRE Digest:365 (or similar approved method if appropriate) should be provided to confirm the suitability of soakaways on site. Locations should be specific to the location of major infiltration features.

If soakaways are proposed, the closest (in distance) set of test results should be used to calculate the size of the soakaway. Alternatively, the slowest infiltration rate should be utilised.

In accordance with Building regulations a 1 metre unsaturated zone between the base of the soakaway and the seasonal high groundwater level (usually February to March) should be provided with the depth to ground water from the base of each soakaway should be clearly stated.

If a 1 metre unsaturated zone is not achievable and soakaways are proposed, thorough justification and an assessment of the risk to groundwater from contamination should be supplied.

If an Intrusive Ground Investigation Report confirms that the ground has no soakage potential (for example clay soils) or there is onsite contamination that cannot be remediated, it is unnecessary to complete soakaway tests, however clear supporting evidence should be provided.

b) Evidence that the receiving watercourse has onward connectivity and capacity to receive flows from the site.

A topographical survey and/or Ordnance survey (OS) Mapping  photographic evidence should be submitted which clearly demonstrates that the receiving watercourse has onward connectivity and is suitable to receive flows from the site. Basic hydraulic calculations could be used to demonstrate that the watercourse has capacity to receive flows if it does not currently receive surface water from the site.

c) Hydraulic calculations to demonstrate the proposed final solution will effectively manage the 1 in 30 (+ climate change allowance) & 1 in 100 (+ climate change allowance) storm events and 10% allowance for urban creep. If infiltration is deemed unfeasible, associated discharge rates and storage volumes shall be provided using a maximum discharge rate of xx l/s (as stated on the decision notice).

Hydraulic calculations e.g. InfoDrainage/Causeway Flow should be provided, specifically the outputs should include the following information:

• Network Design Table
• Summary of Results
• Rainfall Details
• Simulation Criteria
• Time Area Diagram
• Network Details
• Model Details
• Storage Structures for Surface Water Network
  • Cellular Storage
  • Soakaway Structure
  • Tank or Pond
  • Permeable Paving
  • Swale
    • * All where applicable to the design *
  • On-line Control Details

A statement or plan should be provided which confirms the positively drained area of the site to enable a cross check to the calculations. When relevant an additional % should be added to account for urban creep. The additional % should be clearly identified in the calculations and on the catchment areas plan.

Manhole and pipe referencing used within the calculations should enable a clear and easy cross check to be made with the drainage strategy drawings and should reflect the ‘for construction’ invert and cover levels and pipe diameters.

d) Detailed design drawings for all sustainable drainage elements including cross sections and detailed drainage layout plan.

Proposed Drainage Layout at a legible scale, including but not limited to the following information:

• Finalised Finished Floor Levels and proposed external ground levels
• Boundary Levels
• Proposed gullies and/or drainage channels
• Chambers and pipe runs including chamber references, pipe numbers, diameters and gradients (ensuring they match with the supporting calculations)
• Attenuation features/devices including area and depth
• Location of flow control and flow control rate
• Location of any existing drainage infrastructure which is to be retained or removed
• Location of any existing watercourses on or adjacent to the site

A full set of detailed design drawings should be provided, including but are not limited to:

• Attenuation and conveyance features
• Tanks or Crates
• Inspection Chambers
• Silt Traps/Catch-pits
• Gullies
• Channel Drains
• Permeable Paving
• Headwalls
• Flow Control Units
• Green/Blue Roof
• Raingardens
• Inlet/outlet headwalls
  • There should be consideration of any erosion risk. A new point outfall could result in erosion to the opposite bank or immediately downstream of the outfall and any erosion protection necessary should be clearly stated.
• Pond/basin/swale cross sections showing maximum water level
• Any other relevant drainage infrastructure

Manhole Schedule if this information has not been included on the drainage layout.

If lined above ground storage structures and crates/tanks are proposed the applicant should provide details of any mitigation measures required to appropriately deal with groundwater and offset any flotation risk.

e) An exceedance flow routing plan demonstrating no increase in surface water flood risk on or off site. The plan must include proposed levels and flow directions.

This could be a standalone drawing or included on the drainage layout. The drawing should include the following information as a minimum:

• Finalised Finished Floor Levels.
• External levels, including but not limited to: external plot levels, garden levels, road levels, parking areas, and boundary levels.
• Surface water drainage system including private plot drainage.
• Directional arrows or contours indicating where surface water would flow during exceedance events i.e. rainfall events over and above the design criteria (usually 1 in 100 year + climate change event) this should include the flow route off site.
• Areas where surface flooding would occur, including water depths where applicable
• Any existing retained surface water drainage infrastructure
• Existing watercourse on or immediately adjacent to the site

f) Details of drainage management responsibilities and maintenance regimes for all drainage elements.

A standalone report / statement should be produced which details the maintenance requirements and frequency of the works for each drainage element. A statement should confirm who will likely be responsible for the drainage system upon completion of the works (for example Sewerage Undertaker adoption or private management company etc), full details will be required as part of the verification report prior to occupation condition.

If a watercourse is present within or immediately adjacent to the site a clear statement should be made within the document which confirms riparian responsibilities.

A supporting plan should be included which clearly indicates who will be responsible for parts of the surface water drainage network if responsibilities are spilt across the site. i.e. mixture of private and management company responsibility and adoptable drainage.

g)  Details of how surface water will be managed during construction including measures to protect on site and downstream systems prior to the final drainage system being operational. Including details of how existing watercourse on and adjacent to the site will be protected (included where relevant).

This information should link to the Construction Environmental Management Plan (CEMP) if available. A cover letter and drawings should be provided identifying the following where applicable:

• Where site amenity facilities will be located and how foul and surface water will be temporarily disposed of.
• If below ground surface water attenuation or infiltration is proposed, how these areas will be protected from compaction during construction works.
• How silty water from wheel washing and other construction activates will be prevented from entering the surface water drainage system and/or watercourse.
• Where hazardous materials will be stored and how will drainage systems and/or the watercourse be protected from accidental spillage.
• Phasing of the on-site surface water drainage system for example; the drainage system is constructed in advance and used to manage flows during construction works or a temporary interim system is required to manage surface water flows. A phasing plan should be included where applicable.
• Spoil Heaps – anticipated locations, frequency of removal, interception of run off from heap
• Waste disposal – anticipated locations
• Fuel and Mortar Silo – location, procedures in place in case of spillage
• Details of how surface water will be safely managed on site prior to the completion of the surface water drainage scheme.

Major development pre-occupation condition

2) Prior to the first occupation of the development, a verification report must be submitted to and approved by the Local Planning Authority. This must demonstrate that the surface water drainage system has been constructed as per the agreed scheme (or detail any minor variations), confirming any defects have been rectified. Provide the details of any management company. Provide an ‘As-Built’ drainage layout and state the national grid reference of key drainage elements.

The Applicant should be aware that the LLFA are not verifying the construction of the surface water drainage system but confirming the applicant has taken appropriate action to ensure and evidence the surface water drainage system has been constructed as approved.

The Applicant should provide a statement that confirms the drainage system has been constructed and is functioning as intended. By making such a statement the person signing-off the report is taking responsibility for the system being constructed as per the approved design. The following supporting information should always be provided:

• As Built drawings and statement which includes details of minor variations (and confirmation of impact) from approved plans to As-Built plans.
• Easting and northings to national grid reference for major surface water drainage features e.g. attenuation areas and flow control devices.
• Confirmation of who will be responsible for the long-term maintenance of the system including contact details (for example appointed management company or landowner).

The following evidence should be provided where appropriate, for example this would be applicable to large or phased sites.

• Photographs of the installation / construction phase through to completion
• Site delivery notes for main features of drainage system e.g. crates, flow controls
• CCTV Survey
• If faults or obstructions are identified, evidence should be provided to confirm these have been rectified