Local Standards

The Essex County Council SuDS Local Standards are intended to supplement the national standards through more aspirational criteria relating to hydrology and water quality (local standard 1 and local standard 2). In addition, some of the local standards relate to the design of individual SuDS features.

The Essex SuDS Design Guide outlines 16 local standards in total: 

  • Hydraulics
  • Water quality
  • Green roof design
  • Soakaway design
  • Filter strip design
  • Filter trenches and drain design
  • Swale design
  • Bioretention design
  • Pervious pavement design
  • Geocellular structures design
  • Infiltration basin design
  • Detention basin design
  • Pond design
  • Wetland design
  • Rainwater harvesting design
  • Greywater recycling design

These are outlined in more detail below. For more information and case studies, refer to the Essex SuDS Design Guide (2016).

Local Standard 1: Design for Water Quantity

SuDS must be designed to ensure that developments and occupants are protected from flooding, and that off-site flood risk is not increased. Where possible, SuDS should aim to reduce the overall risk of flooding off-site and to drain (by preference) via infiltration, in accordance with the drainage hierarchy contained in Approved Document H of the Building Regulations.

Runoff rate

Unlike developed areas, greenfield sites generally produce no measurable runoff during small rainfall events (up to 5mm). Receiving streams and rivers are likely to be under greater stress during summer months, with lower available dilution levels reducing their capacity to accommodate polluted inflow. In order to mitigate against this, SuDS should be designed so that runoff does not occur for the first 5mm of any rainfall event for 80% of summer events and 50% of winter events. 

In all cases, including on brownfield sites, runoff should where possible be restricted to the greenfield 1-in-1 year runoff rate during all events up to and including the 1-in-100 year rainfall event with climate change. An alternative approach would be for discharge rates to be limited to a range of greenfield rates, based on the 1-in-1, 1-in-30 and 1-in-100 year storm events. However, the use of this method to restrict discharge rates would also require the inclusion of on-site long-term storage, sized to take account of the increased post-development volumes, discharging at no greater than 2l/s/ha. While the latter is acceptable, it is still this council’s preference that the former approach is used wherever possible. If this is deemed unachievable, evidence must be provided and developers should still seek to achieve no increase in runoff from greenfield sites and a 50% betterment of existing runoff rates on brownfield sites (provided this does not result in a runoff rate less than greenfield). The runoff rate should be calculated based on the area that will be draining via the proposed SuDS and will subsequently be the same area that is used to calculate the required storage (before an allowance for urban creep is applied). Unrestricted rates will only be allowed where the outfall is to a tidal estuary. If a Surface Water Management Plan has been produced for the area, it may set out further advice on allowable runoff rates. 

It should be noted that rates should not be limited to 5l/s on the basis that lower rates may cause blockage. Historically 5l/s was applied to an outlet where Qbar was lower than 5l/s, as most devices would require an outlet orifice size smaller than 50mm, which would increase the susceptibility of blockage and failure. Vortex flow-control devices are now available that can be designed to discharge at 1l/s with a 600mm shallow-design head, and which still provide an orifice of more than 50mm in diameter. In order to further reduce the risk of blockage, drainage systems should be designed in such a way that materials that may cause blockage are removed from the system before they reach the flow restriction. 

Greenfield runoff rates should be calculated using the ICP SuDS method contained in Micro Drainage; otherwise, the IH124 method for a site of 50ha should be applied and reduced down proportionately in accordance with the site size. For brownfield sites, the Modified Rational Method should be used to calculate the peak brownfield rate. Alternatively, runoff from a brownfield site can be estimated using the urbanisation methods within the ReFH2 software. For brownfield sites, at the outline stage of the application process, an estimate can be made based on an assumed rainfall intensity of 50mm/h. However, during detailed drainage design, outfall rates should be expressed in litre/second (l/s) for the 1-in-1 year, 1-in-30 year and 1-in-100 plus climate change events. 

Runoff volume

The aim of long-term storage is to ensure that any volumes leaving the site above the greenfield runoff volume discharge at 2l/s/ha. The same should be achieved for brownfield sites unless this can be shown to make the development unviable. 

Storage volume

When planning the layout of SuDS, sites should take into account topography and make best use of low points for storage. 

The preference is for all rainfall events up to the 1-in-100 plus climate change to be stored within SuDS. However, should this be considered unfeasible, storage should be provided for the 1-in-30 year event, with greater flows managed in suitable exceedance areas. An additional 10% of impermeable area should be accounted for, to cater for urban creep, unless this is not appropriate for the proposed development use. For outfall to a tidal estuary, SuDS should be sized to accommodate storm runoff during times when the outfall is tide-locked. The storage provision should be calculated by modelling a 1-in-100 inclusive of climate change rainfall event and 1-in-20 inclusive of climate change tidal event coinciding. 

Applications should demonstrate how the required storage of surface water will be achieved. If the design event volumes cannot be contained within SuDS, drainage designers must demonstrate how additional flows will be managed through exceedance flow routes, avoiding risk to people or property and with all flows contained on-site. Storage should half-empty within 24 hours wherever possible. If the storage required to achieve this via infiltration or a restricted runoff rate is considered to make the development unviable, a longer half-emptying time may be acceptable. An assessment of the performance of the system and the consequences of consecutive rainfall events occurring should be provided. Subject to agreement with the LLFA, ensuring that drain-down in 24 hours provides room for a subsequent 1-in-10 year event may be considered acceptable. 

Unless sufficient pre-treatment has been provided, certain SuDS features may require the incorporation of a sediment forebay to capture sediment and ensure the feature does not silt-up. This will also ensure that maintenance activities for sediment removal can be more easily undertaken. Sediment forebays should provide an additional 10% attenuation volume to allow for a level of silting-up, thereby ensuring this does not result in a reduction to the available storage volume. 

Safe conveyance routes and overflow flood storage areas must be established and agreed with the SuDS Team for the 1-in-100 year rainfall event with an allowance for climate change. 

If runoff cannot be restricted to the greenfield 1-in-1 year event for all events, long-term storage should be provided to achieve the same result. The aim of long-term storage is to ensure that any volumes leaving the site above the greenfield runoff volume discharge at 2l/s/ha. The same should be achieved for brownfield sites unless this can be shown to make the development unviable. 

On 19 February 2016, central government published ‘Flood risk assessments: climate change allowances’ which provides updated climate change figures that should be used for flood risk assessments and drainage strategies. Key changes include new peak rainfall intensity figures. Essex County Council takes a risk-averse approach to flooding; it is therefore necessary to use the ‘upper end’ figures shown in the table below as a basis for estimates and calculations (estimates based on the 90th centile are likely to be sufficient for 90% of climate change scenarios). It should be noted that climate change allowances for peak river flows have also been adjusted. These may affect the areas on the site that are suitable for use for attenuation storage during a rainfall event and the surcharging of the outfall, and should be taken into account when designing a drainage scheme in areas at risk of fluvial flooding.

Applies across all of England Total potential change anticipated for 2010 to 2039 Total potential change anticipated for 2040 to 2059 Total potential change anticipated for 2060 to 2115
Upper end 10% 20% 40%

Peak rainfall intensity allowance in small and urban catchments (use 1961 to 1990 baseline). 

Further information on the updated climate change allowances can be found online. 

Local Standard 2: Design for Water Quality

The level of pollution found within surface water runoff will depend on the nature of the development from which it arises, the time since the last rainfall event and the duration and intensity of rainfall.

An appropriate ‘train’ of SuDS components must be installed to reduce the risk of pollutant entering watercourses via runoff from developed sites. Interception storage should be used as part of the treatment train to ensure that pollutants are managed at source, which will reduce the risk of them contaminating water bodies. Following the SuDS Management Train hierarchy, a series of drainage techniques should be designed into the development layout. A successful system will see pollution incrementally reduced at each stage.

Treatment options to address pollution issues include:

  • Infiltration, i.e. infiltration basins and soakaways.
  • Filtration, i.e. filter strips, filter trenches and drains.
  • Detention basins and ponds.
  • Permanent ponds.

These options reduce pollution either by filtering out pollutants or by reducing flow rates to encourage deposition of any contaminants. Polluted surface water runoff should not run directly into permanent ponds, so as to protect biodiversity and amenity and prevent maintenance problems caused by heavy silts and oil.

The amount of treatment stages required within the SuDS train will depend on the nature of the site. In most cases a simple indices approach can be applied to pollution risk – this should be based on the approach outlined in the updated SuDS Manual C753. In some cases, a more detailed risk assessment may be needed to assess high-risk sites.

The varied nature of the pollutants that affect development mean that treatment may need to be provided by the use of a range of different features with different treatment properties. When selecting features, it is important to minimise the risk of remobilisation and washout of any pollutants. Although some gullypots and catchpits can trap sediment, their performance is linked closely to the regularity of their maintenance. There is a significant risk of pollutants contained within them being dislodged and washed downstream; for this reason Essex County Council does not consider them an appropriate form of treatment.

Detailed guidance on water quality and treatment stages: chapters 4 and 26 of The SuDS Manual C753 (CIRIA, 2015).

Local Standard 3: Design of Green Roofs

A multi-layered system that covers the roof of a building with vegetation/landscaping/permeable car parking over a drainage layer. These features will not be considered for adoption.

  • Designed for interception storage
  • Minimum roof pitch of 1-in-80, maximum 1-in-3.
  • Multiple outlets to reduce risk from blockages.
  • Lightweight soil and appropriate vegetation.

Local Standard 4: Design of Soakaways

Square or circular excavations, filled with aggregate or lined with brickwork, or pre-cast storage structures surrounded by granular backfill.

  • Should be designed for the 1-in-100 year rainfall event as a minimum.
  • Infiltration testing carried out in accordance with BRE Digest 365.
  • Fill material should provide >30% void space.
  • Base of soakaway at least 1m from groundwater level.
  • Minimum of 5m away from foundations.

Local Standard 5: Design of Filter Strips

Vegetated strips of land designed to accept overland sheet flow.

  • Recommended minimum width of 6m.
  • Runoff must be evenly distributed across the filter strip.
  • Slopes not exceeding 1-in-20, minimum of 1-in-50.

Local Standard 6: Design of Filter Trenches and Drains

Shallow excavations filled with stone to create temporary surface water attenuation. 

  • Excavated trench 1-2m deep filled with stone aggregate.
  • Effective upstream pre-treatment to remove sediment and fine silts.
  • Infiltration should not be used where groundwater is vulnerable or to drain pollution hotspots.
  • Observation wells and/or access points for maintenance of perforated pipe components.

Local Standard 7: Design of Swales

Linear vegetated features in which surface water can be stored or conveyed. Can be designed to allow infiltration where appropriate.

  • Limit velocities during extreme events to 1-2 m/s.
  • Maximum side slopes of 1-in-3 where soil conditions allow.
  • Minimum base width of 0.5m.

Local Standard 8: Design of Bioretention

Shallow landscaped depressions or pre-cast units which rely on engineered soil and vegetation to remove pollution and reduce runoff.

  • Sufficient area to temporarily store the water quality treatment volume.
  • The water quality treatment event should half-drain within 24hrs to provide adequate capacity for multi-event scenarios.
  • Minimum depth to groundwater of 1m if unlined.
  • Overflow/bypass facilities for extreme events.

 

 

Local Standard 9: Design of Pervious Paving

Permeable surface allowing rainwater to infiltrate through into underlying layer where it is temporarily stored.

  • Pervious sub-base to be structurally designed for site purpose.
  • Temporary sub-surface storage must provide infiltration and/or controlled discharge.
  • Geotextile may be specified to provide filtration treatment.
  • Surface infiltration rate should be an order of magnitude greater than the design rainfall intensity.

Local Standard 10: Design of Geocellular Structures

Modular geocellular systems with a high void ratio that can be used to create below-ground infiltration (soakaway) or storage device. 

  • Standard storage design using limiting discharges to determine storage volume.
  • Structural design should be to relevant standards for appropriate surface loadings.
  • Use appropriate geotextile (for infiltration) or geomembrane (for storage).

Local Standard 11: Design of Infiltration Basins

Vegetated depressions designed to store runoff and allow infiltration gradually into the ground. 

  • Pre-treatment is required to remove sediments and fine silts.
  • Infiltration should not be used where groundwater is vulnerable or to drain pollution hotspots.

Local Standard 12: Design of Detention Basins

Surface storage basins that provide attenuation of storm-water runoff and facilitate settling of particulate pollutants. They are normally dry and may also function as a recreational facility.

  • Maximum side slopes of 1-in-4.
  • Bioretention and/or wetland/micropools at outlets for enhanced pollution control.

Local Standard 13: Design of Ponds

Provide storm-water attenuation and treatment. Permanent pools to support aquatic vegetation. Retention time promotes sediment removal. 

  • Permanent pool for water quality treatment and temporary storage volume for flow attenuation.
  • Maximum water depth for open water areas of 1.2m.
  • Maximum side slopes of 1-in-3.

Local Standard 14: Design of Wetlands

Shallow ponds and marshy areas for attenuation and water treatment. Aquatic vegetation and extended detention allow sediments to settle.

  • Shallow, temporary storage for attenuation.
  • Sediment forebay or equivalent upstream pre-treatment.
  • Combination of deep and shallow areas (maximum depth <2m).
  • Length-to-width ratio of greater than 3-in-1, shallow side slopes.

Local Standard 15: Design of Rainwater Harvesting

Rainwater harvesting is the process of collecting and using rainwater. If designed appropriately the systems can be used to reduce the rates and volumes of runoff. For more information see Appendix 1 of the Essex SuDS Design Guide (2016)  

  • Can range from complex district-wide systems to simple household systems linked to a water butt.
  • Most simple rainwater harvesting systems are relatively easy to manage.
  • Rainwater harvesting systems can be combined with greywater recycling systems to form an integrated process.

Local Standard 16: Design of Greywater Recycling

Greywater recycling is the reuse of waste water collected from showers, baths, washbasins, washing machines and kitchen sinks. For more information see Appendix 1 of the Essex SuDS Design Guide (2016)

  • Common features include a tank if storing water, a pump, a distribution system and, where it is needed, some sort of treatment.
  • Greywater stored for any length of time has to be treated, as otherwise it deteriorates rapidly.

Page updated: 9/02/2018


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