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Herrington Consulting has significant experience in developing and appraising surface water management strategies. Whilst Planning Policy does not require a detailed drainage design to be included, a Flood Risk Assessment (FRA) must demonstrate that the development does not exacerbate the risk of flooding elsewhere and in order to appraise the impact of the proposed development, a Surface Water Management Strategy (SWMS) is required to be included within the FRA.

Herrington Consulting also works closely with developers and Code Assessors to provide Code for Sustainable Homes reports for the Sur 1 and Sur 2 assessments and BREEAM Pol 3 & Pol 5 Flood Risk & Surface Water elements.

The Sur 1 assessment, for example, aims to ensure that the design of the surface water drainage for housing developments avoids, reduces and delays the discharge of rainfall run-off to watercourses and public sewers using Sustainable Drainage Systems (SuDS). The objective is to protect receiving waters from pollution and minimise the risk of flooding and other environmental damage.



Surface Water Management Strategy

Whilst a site may be shown by the Environment Agency's/SEPA's flood zone maps to be within a low risk zone (Zone 1); an area defined by the flood maps as having a low annual probability of flooding from the sea or a main river - the risk of flooding from other sources such as groundwater, overland flow, sewers etc, may still be present.

For sites that exceed 1 ha in area or, are identified by the planning authority as being a site with problematic drainage or localised flooding issues, Planning Policy requires that a Flood Risk Assessment (FRA) is undertaken so that such risks can be identified and mitigated. Of equal importance is the requirement to ensure that the proposed development does not exacerbate the risk of flooding elsewhere.

In order to appraise the impact of a new scheme, a Surface Water Management Strategy (SWMS) can be developed as either a stand-alone study, or be included as part of a Flood Risk Assessment. In most cases the SWMS is based on a desk-top study that tests the application of the preferred hierarchical approach to surface water discharge, i.e. via infiltration, connection to a watercourse or to a public sewer. Planning Policy also promotes the use of Sustainable Drainage Systems (SuDS) and therefore the SWMS will investigate the appropriateness of incorporating a range of SuDS elements within the surface water drainage system.

Herrington Consulting has range of numerical models that assist in the development of SWMS, including FEH, ReFEH and Micro Drainage Source Control. For complex sites direct rainfall 2D overland flow path and flood route modelling can also be undertaken using Surface-water Modelling System & TuFLOW.

If the preferred method of surface water discharge is via infiltration then it is likely that the SWMS will need to be supported by site-specific soakage tests. We would not normally undertake these ourselves, however, we can advise on appropriate methodologies if required.

To prepare the surface water management strategy we will often require topographic information of the site. Once again we would not normally undertake a topographic survey ourselves, however, we can provide you with a quotation for a surveyor to undertake a survey with the appropriate level of detail.

SuDS

Urban drainage is moving away from the conventional thinking of designing for flooding, to balancing the impact of urban drainage on flood control, quality management and amenity. Surface water drainage methods that take account of quantity, quality and amenity issues are collectively referred to as Sustainable Drainage Systems (SUDS). These systems are more sustainable than conventional drainage methods because they:

  • Manage runoff flowrates, reducing the impact of urbanisation on flooding
  • Protect or enhance water quality
  • Are sympathetic to the environmental setting and the needs of the local community
  • Provide a habitat for wildlife in urban watercourses
  • Encourage natural groundwater recharge (where appropriate).

These systems do this by dealing with runoff close to where the rain falls, managing potential pollution at its source and protecting water resources from point pollution. The underlying philosophy of SuDS is to replicate as closely as possible the natural drainage from a site before development and to treat runoff to remove pollutants, thereby reducing the impact on receiving watercourses.

Until now, the use of SuDS has been fairly limited and usually only considered by developers when conventional drainage options are not suitable or if trying to create a 'green' development. However, this is changing as SuDS and surface water management forms a key component of the Flood and Water Management Act 2010. The Act implements many of the recommendations identified through the Pitt Review of 2008, following the devastating floods during the summer of 2007. In December 2011, DEFRA released the Draft National SuDS Standards for consultation.

The National Standards are designed to provide a structure for the new SuDS Approving Bodies (SABs) to make judgment on the acceptability of drainage schemes for developments and redevelopments. They are an essential part of the Government's proposals to encourage widespread SuDS adoption in England and Wales and prevent the automatic connection of surface water drains to the public sewer network.

The Flood and Water Management Act 2010 requires new developments and redevelopments in England and Wales to have drainage plans for surface runoff approved by the SAB where the construction work would have drainage implications. Once the National Standards have been approved, this will come into force across the both England and Wales.

By talking to us you can ensure that solutions to overcome difficult drainage problems are not overlooked and opportunities to make economic savings and social and environmental enhancements are not missed.

Herrington Consulting is a specialist consulting practice providing bespoke solutions that help our clients to reduce the risk of flooding, from both urban and natural sources. The services we offer range from providing advice on the suitability of particular systems to detailed design. Regardless of the size of development, we can add value, so why not contact us today for a free of charge, no obligation quotation.



Code for Sustainable Homes/BREEAM

The Code for Sustainable Homes is intended as a single national standard to guide industry in the design and construction of sustainable homes. It was introduced to drive a step-change in sustainable home building practice. The Code is used by home designers and builders as a guide to development, and by home-buyers to assist in their choice of home.

Although Herrington Consulting does not provide a full Code for Sustainable Homes (CfSH) assessment service per se, as a consultant that specialises in flood risk management and hydraulic modelling we are ideally placed to provide the reports required to demonstrate compliance with the two elements of Category 4: Surface Water Runoff Assessment (Sur1 and Sur2).

Sur 1 - The aim of Sur 1 is to ensure that the design of the surface water drainage for housing developments avoids, reduces and delays the discharge of rainfall run-off to watercourses and public sewers using Sustainable Drainage Systems (SuDS). The objective being to protect receiving waters from pollution and to minimise the risk of flooding and other environmental damage.

The assessments that we undertake include the following:

  • Calculation of pre and post development impermeable areas
  • Calculation of peak rates of runoff in accordance with the requirements of the CfSH
  • Calculation of pre and post development volume of runoff
  • If the proposed development results in additional runoff, then infiltration techniques are examined to prevent this leaving the site.
  • If infiltration is not appropriate due to soil types, contamination, space restrictions etc, then methods for limiting the post development peak discharge to the 'limiting discharge' will be examined.
  • Examination of the impacts of local drainage system failure
  • Advise on the appropriate use of SuDS
  • Recommendations for a surface water management strategy that will meet the mandatory elements of Sur 1 and where possible achieve the 2 additional credits for water quality.

Sur 2 - The aim of this element of the Code is to promote housing development in low flood risk areas, or to take measures to reduce the impact of flooding on houses built in areas with a medium or high risk of flooding. The first stage of this assessment is to identify which of the Environment Agency's flood zones the site is located within. If located within Flood Zone 1; an area defined by the Environment Agency's flood maps as having a low annual probability of flooding, it will be necessary to demonstrate that the risk of flooding to the site is low for all sources of flooding. This will be undertaken as part of a site specific Flood Risk Assessment (FRA) that examines the risks of flooding from the following sources:

  • Sea/tidal
  • Fluvial (main river)
  • Ordinary and man-made watercourses
  • Overland flow/surface water (pluvial)
  • Groundwater
  • Sewers
  • Reservoirs

Providing an FRA can demonstrate that the risk of flooding from all of the above sources is low, then 2 credits are available. This is a bespoke assessment and is not a 'Generic' on-line Flood Report.

Herrington Consulting is a specialist in flood risk management and therefore all of our FRAs are based on a professional appraisal of risk. The 'generic' on-line Flood Reports do not cover all sources of flood risk and they are not detailed enough to appraise flood risk to a level of detail required to satisfy the specific requirements set out in Planning Policy documents such as PPS25.

If the site is located within Zone 2 or 3 (defined as an area that could be affected by flooding, either from rivers or the sea if there were no flood defences), then it will be necessary to demonstrate that finished ground floor levels of all habitable parts of all dwellings are placed at least 600 mm above the design flood level. Furthermore, all access/egress routes to/from the ground level of the site should also be located above the flood level. Development in Zones 2 and 3 not only requires a more detailed assessment of flood risk in order to satisfy the requirements of Sur2, it is also likely that an FRA (that is fully compliant with the specific requirements of PPS25) will be required at the planning application stage.

BRE Environmental Assessment Method BREEAM

BREEAM sets the standard for best practice in sustainable building design, construction and operation and has become one of the most comprehensive and widely recognised measures of a building's environmental performance.

Herrington Consulting can provide detailed reports that fulfil the Flood Risk and Surface Water Runoff Criteria required for a BREEAM Assessment, enabling BREEAM assessors to gain additional credits for these elements.

We work closely with both developers and Code Assessors to ensure that the maximum number of credits can be achieved. For a free, no obligation quotation please use the Contacts page to supply us with the information for your specific project.