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Surface water and flood risk

SuDs design

Photo of flooded roadedPlanning authorities and the Environment Agency will require you to demonstrate how surface water will be managed as part of your proposed development if you are building a new structure or increasing the developed footprint of your site.

The required document you will need is typically referred to as a Surface Water Drainage Strategy or a Sustainable urban Drainage Systems (SuDS) Report. You may also be required to provide a second stage detailed surface water drainage design report and a Sustainable Drainage Operation and Maintenance Plan when submitting a planning application for major development.

From April 2015 any development of 10+ units (residential, commercial, industrial, and mixed use sites) are required by law to consider the potential use of Sustainable urban Drainage Systems as part of a drainage strategy. You are also likely to need a drainage strategy if your development is located within an area designated as Flood Zones 2 and 3, or within an area identified as having a surface water flooding problem.

We can offer you an integrated approach to deal with the surface water, to protect your development from flooding, prevent pollution, and deliver a controlled flow of clean water that can be used for amenity and wildlife benefits.

Reports provided will:

  • give recommendations for SuDs and provide initial design parameters for drainage components (i.e. soakaways, ponds, swails etc).
  • identify appropriate surface water runoff management options.
  • comply with National Planning Policies for England, Wales, Scotland and Northern Ireland and with the relevant environmental agency guidance and local planning policy.
  • presented as standalone documents or combined as part of a Flood Risk Assessment.

BRE soakaway (soil infiltration rate) tests

Image showing equipment being used to carry out drainage soakaway surveyYou may be considering soakaways as part of a surface water drainage strategy for your development proposal. However, an effective soakaway will require sufficient storage capacity to handle the immediate run-off volume from a storm and also to be constructed within soil of sufficient drainage capacity to allow the stored water to disperse sufficiently before the next rainfall event. In order to achieve this you will need to establish the drainage capacity of the soil.

We can undertake soil infiltration tests that are in accordance with BRE Digest 365.

Typically a test is undertaken within a machine excavated trial pit which is filled with clean gravel, to ensure sidewall stability during the test, before adding a sufficient head of clean water from a rapid filling water tanker or bowser.

The soil infiltration rate is determined by measuring the length of time for 75% of the volume of water to drain away. Three consecutive tests are undertaken within each location, to ensure at least one full test is completed within saturated soil conditions. The calculated soil infiltration rate is used to determine the feasibility of soakaway drainage, and also the design storage volume and number of soakaways required.

Site specific flood risk assessments

Your local Planning Authority will require you to have a Site Specific Flood Risk Assessment (FRA) carried out if your development site lies within a modelled flood zone (flood zone 2 or 3), or in the vicinity of a fluvial or tidal watercourse, or where a development site is over one hectare in size (within flood zone 1).

You may need a Site Specific FRA if your development is less than 1 ha in flood zone 1, including a change of use in development type to a more vulnerable class (e.g. from commercial to residential), where they could be affected by sources of flooding other than rivers and the sea (for example surface water drains, reservoirs). Also, an FRA may be required if your site is situated within flood zone 1 and has critical drainage problems as notified by the Environment Agency.

We recognise that a robust flood risk assessment should be a bespoke document which is specific to your development proposal and that is why we work with civil engineering professionals, to use their experience, to produce an independent report.

The advice given is typically split into stages. The first stage may include a review of available evidence and a site visit to discuss. Some basic spot levels (i.e. not to Ordnance Survey) may be taken to understand the site topography. If the conclusion of this initial stage is that the site is at risk (usually zone 2 or 3), then more detailed flood risk advice is usually required, suitable for proposal to a Local Authority. At this stage the Environment Agency would be further consulted. It is common that at this stage a detailed topographical survey will be required to Ordnance Survey/Environment Agency standards and we can arrange this for you.

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