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Drainage (SuDS & Highways)

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Kent County Council and Medway Council have a range of statutory duties regarding drainage. As Highway Authorities, we may adopt drainage systems associated with highway drainage.

As Lead Local Flood Authorities, Kent County Council or Medway Council need to be consulted on major development proposals in relation to local flood risk.

Whatever their location, type and scale, your drainage proposals must:  

  • Protect people, property and highway on the development site from flooding; and,
  • Not exacerbate flood risk outside of the development in any part of the catchment, either upstream or downstream.

You should always obtain detailed information about the local drainage system prior to any development. 

This page summarises Kent County Council’s drainage guidance, full technical specifications and requirements are available to download here:

Sustainability (Drainage Systems) – Kent Design Guide

Sustainable Drainage Systems (SuDS) 

These should mimic the natural surface water runoff processes at the site prior to development. This is achieved most effectively when a series of SuDS techniques are applied progressively as part of an interconnected system, or ‘management train’. 

A good management train comprises prevention (preventing runoff by reducing impermeable areas), source control, site control and regional control. These techniques provide several stages at which water can be treated and allowed to infiltrate into the ground.   

Runoff does not need to pass through all stages of the train: it is usually best dealt with locally, as close to the source as possible; wherever possible, it shouldn’t be conveyed elsewhere. 

Planning for SuDS

SuDS must be considered in your development’s masterplan. You should consider the most appropriate
SuDS features for buildings and built form, the street network, and open spaces and landscape areas. This will ensure they’re properly integrated, efficient and cost-effective; an especially important consideration on larger, phased developments.

Design, Submissions and Evaluation Process 

Creating a sustainable drainage design for your development site should run in parallel with the planning process.  Layout, urban design approach, landscaping and open spaces all have direct impacts on surface water drainage. 

Conceptual drainage design forms part of the preliminary planning phase for any site. We cannot overstate the importance of holding pre-application discussions with the Local Planning Authority, Highways Authority and the Lead Local Flood Authority at this stage. As well as avoiding unnecessary design, you may identify other potential drainage solutions, and the Council’s intentions regarding adoption. 

You should also consult other parties that may be involved with delivering the drainage scheme, including  sewerage undertaker, Internal Drainage Board or third-party property owner. 

For larger development proposals, you should submit an outline drainage strategy alongside any planning application. 

Detailed drainage proposals may be submitted with a planning application, or to discharge planning application conditions later in the development process.

Drainage Strategy Requirements:  Your drainage strategy must include information supporting your choice of specific drainage measures, design arrangements with design calculations and construction aspects.

Ground Related information

You should conduct a desk study of publicly–available sources and previous studies to gather as much relevant geo-environmental/geo-technical information as possible about the site and its vicinity. You should also carry out a site reconnaissance visit or walkover survey.

Drainage Design  

Your hydrologic and hydraulic calculations must be based on the design storm criteria set out in the National Planning Policy Framework Practice Guidance and the CIRIA SuDS Manual C753.    

In all cases, your drainage design must include an appropriate allowance to the runoff from the 100-year return period design rainstorm to allow for the predicted effects of climate change. For commercial developments, this may be reduced to 20%; however, you must provide written evidence that the building and its foundations are unlikely to exist after 60+ years. The following aspects of drainage design require your attention in order to design a successful scheme:

Drainage design factors:
Catchment AreasWe don’t set specific catchment area standards for individual drainage measures, but you should aim to limit them, and design your drainage system to reflect the natural catchment. Surface flows should not be directed to other catchments, and pumping systems are a last resort. Your site area should be broken into smaller sub-catchments with controls to maximise treatment and storage capacity. For large phased developments, you should create sub-catchments with appropriate attenuation and discharge controls within each phase. When calculating areas to be drained, you should make allowance for all parts of the highway, including footways, footpaths, paved areas and verges.
Discharge RatesAs a rule, your site’s total discharge should aim to mimic greenfield rates; where calculations show these to be less than 5 litres/second, you may apply a rate of two litres/second. Different rates may apply in certain circumstances, so you should confirm them with Kent County Council and/or the Local Planning Authority. The proposed discharge rate to a receiving watercourse may also need to be agreed in advance with the relevant Internal Drainage Board. Total runoff should be restricted to the theoretical greenfield runoff rates for each of the 1, 30- and 100-year return period storms. Please use the calculation methods specified in Ciria SuDS Manual, Report C753 (2015) or as updated. You should also refer to any relevant Strategic Flood Risk Assessment, Surface Water Management Plan or other adopted flood risk document which may specify specific local discharge rates or requirements. Infiltration measures must avoid concentrating runoff from large impermeable areas into a small discharge area.
AttenuationUnderground attenuation and piped sections should be designed for a minimum of the 30-year rainfall event. Attenuation of events exceeding the piped system’s capacity may be achieved by temporary flooding of open spaces.
Draw Down TimeAny measure designed to manage the 1-in-10 to 1-in-30-year rainfall event should discharge from full to half-full within a reasonable time so as to manage risks associate with a follow-on rainfall event.
Separation DistancesYour design must show the impact of infiltration on any adjacent structures or features.
Relation to HighwayBelow-ground infiltration measures must not be located within the adopted carriageway or junctions. No part of any highway drain, manhole or gully should be positioned less than 3m from any structure without approval from the Highway Authority. Minimum cover to pipes and other below ground structures also applies.
Flow Control DevicesSystems to be adopted by Kent County Council that include weirs, pumps, orifice plates (hydro brakes) and other devices will require approval.
Water QualityYour drainage strategy should include a water quality assessment, using the approach outlined in the Ciria SuDS Manual, Report C753 (2015) Chapter 26. The Simple Index Approach is usually sufficient.
First FlushThe principle of ‘first flush’ is set out in the SuDS Manual. We expect you to demonstrate that the first 5mm of any rainfall event can be contained and disposed of on-site, rather than discharged to any receiving watercourse or surface water sewer.
Oil-Water SeparatorsIn certain circumstances highway drainage systems may require the use of petrol interceptors. These should only be provided following consultation with the statutory Water Authority and the Environment Agency, with the approval of the Drainage Asset Manager.
Pre-Treatment MeasuresYou should install silt traps upstream of any deep-bore or trench soakaway, filter drain or strip, underground storage or attenuation tank. You should also asses other pre-treatment options including (lined) filter trenches, filter strips, grass swales and filter chambers.
Access and MaintenanceYou must provide appropriate access to allow personnel and heavy equipment to perform non-routine maintenance tasks, such as reconstruction or rehabilitation. Maintenance requirements for access routes or pathways, clearly identifying responsible parties or bodies, should be stated within a Maintenance Plan.
Third Party LandYou must demonstrate that off-site surface water drainage works or measures located within private land are appropriately secured and protected. You should consult Kent County Council early in the planning process.
ExceedanceYou must provide a flood exceedance plan to identify locations where flows cannot be contained within the drainage system and will require management. It should show the anticipated course of the surface water, together with predicted depths and speeds, with consideration to people and properties on and off the site and highway.
Pumping stationsThese are to be avoided wherever possible, and will not be adopted by Kent County Council.
Factors of SafetyDesigns submitted for adoption must include a minimum Factor of Safety of 2. If failure would result in more than ‘minor inconvenience’, a Factor of Safety of 10 should be applied to calculations for design purposes.

Construction Considerations  

Drainage construction is a highly technical process: all the requirements are set out in Sustainability (Drainage Systems) linked at the top of this page. As an overview, the principal considerations are:  

  • Planning approval

Construction must comply with the agreed plans, which will be approved under a specific planning reference. We understand that variations may be required, but please note these may require further approval. 

  • Ground conditions 

Unforeseen circumstances, particularly in areas of ground instability, may require changes to design and construction methods.  You should inform our inspecting engineer straightaway if the system is to be adopted by Kent County Council.

  • Contamination 

You must avoid significant impacts to soil, groundwater or surface waters throughout the construction period. In particular, you should prevent any accidental spillage of contaminants, or allowing soil and other material to enter underground pipework, storage chambers or attenuation tanks. 

  • Sediment control 

Among the greatest hazards to any sustainable drainage system, particularly infiltration systems, is silt accumulation during construction. Rectifying it is complex and costly, so you should reduce the risk through regular road sweeping, wheel washes, geotextile filter fabrics on inlet systems, silt fences and other measures. Use tracked equipment where possible and confine traffic to designated routes to minimise soil compaction. 

  • Phasing 

Any major construction must be accompanied by a plan showing the location, size and anticipated start-date for each phase. It should also show the location of haul roads and other designated routes, which must be agreed in advance with the Lead Local Flood Authority or Local Planning Authority. Strategic drainage features must be delivered in a timely manner to serve the development as required. 

  • Consent to begin works 

You may require written confirmation from authorities including the Environment Agency, Internal Drainage Board, sewerage undertake and Kent County Council before starting construction. Planning approval may be conditional on the drainage design responding to details required by them; if so, you’ll need to provide evidence before work begins. 

  • Setting out 

The line and level of the formation, side slopes, drains, carriageways and footways must be carefully set out in accordance with the planning consent. Checks made by the Project Engineer don’t affect your responsibility as developer for ensuring these are correct. 

  • British Standards Specification 

Where materials and methods need to comply with British Standards (BS) you should always use the most recent definition.  

  • Testing of materials  

Before work commences, you must have all your materials tested by an independent UKAS accredited facility. During the works, materials will be selected for testing from batches or consignments on-site. Any materials that fail must be removed; if they’ve already been used in the works, affected sections must be removed and replaced. 

  • Workmanship 

Your materials and workmanship should be the best of their kind. As a County, we’re keen to encourage the use of recycled materials, so these will be accepted provided they meet the minimum requirements of conventional materials.  

  • Excavation of trenches

Excavations are to be carried out to the lines, levels and dimensions shown on the approved drawings, or as directed by the Project Engineer.  

  • Inspection intervals 

For adoption by the Highways Authority, inspections will be conducted at key construction intervals agreed with Kent County Council at the approval stage. The Local Planning Authority may also require inspections as a condition of planning approval.    

  • Photographic record 

We encourage you to take images of ground conditions and installations throughout construction, as this provides valuable reference for future maintenance inspections and performance reviews.   

  • Reinstatement of existing highways 

If you need to install a new drain within the existing adopted highway, you must consult Kent County Council regarding construction details, traffic management and reinstatement. 

  • Keeping the highway clean 

It’s an offence to deposit mud and debris on a public highway, so this must be removed at the earliest opportunity. You should provide facilities to ensure vehicles are cleaned before they leave the site. 

  • Verification reports 

The planning process now often requires a verification report condition to be attached to the permission. This is to ensure the drainage network has been installed in line with the approved design. If the final as–built system varies from the approved design, you’ll need to prove that it still complies with the original general principles, discharge rates and other requirements. 

 

Existing Highway Drainage 

Where you have no alternative for surface water drainage, and the development site drainage is not being adopted, we may consider permitting new connections to the existing highway drainage system. You must not make any such connection without an agreement under Section 115 of the Water Industry Act 1991.  

If your design includes amendments to the existing highway drainage systems, you’ll need to provide: 

  • Topographical survey data, including the location of all drainage assets
  • CCTV survey data, as specified in MCHW Appendix 90/1  
  • Drainage and manhole schedules 
  • Retained height and dimensions of all headwalls  
  • Copies of all consents from the Water Authority and any easements 
  • Construction details for all proposed assets
  • Development Drainage Arrangement, including non-adopted drainage measures 

If the system is to be adopted by the Highways Authority, your must provide a record of the ‘as-constructed’ drainage networks. 

Inspections and Maintenance  

Inspection is a vital part of future maintenance. As-constructed details and associated construction records will provide more comprehensive data, particularly notably in relation to ground conditions.

Inspections will be conducted by the Highways Authority on systems adopted as highway drainage, and by the sewerage undertaker for those adopted as public drainage systems. Inspections for other systems may be specified through a planning condition attached to the approval.

A global review of the system environment should be undertaken to assess overall performance. Internal inspectionsshould also be made where possible; we recognise this may not be feasible for some techniques, including many SuDS solutions.

Highway Drainage Adoption Process 

As Highway Authority, Kent County Council will adopt drainage in relation to the highway under Section 38 and Section 278 processes. The adoption process is managed by our Development Agreements Team, but any Section 115 drainage provision will be confirmed with the Drainage Team, so you’re strongly recommended to consult them early in the planning process.