What is a Drainage Assessment?

Every planning application needs a drainage assessment. These vary considerably between Local Planning Authorities (LPAs) and depend on the proposed size of the development, as well as the sensitivity of the local environment and topography to flooding or drainage issues.
A crucial early part of the assessment is the identification of risks and sensitivities that the site may present.

Drainage assessments for developments must usually include foul drainage and surface water drainage assessments.

Foul Drainage

Any type of development that connects to the mains or a non-mains system will need a drainage assessment or statement to confirm the proposed arrangement. This includes householder applications for additional bedrooms / bathrooms.

Surface Water Drainage

Many areas can be affected by surface water flooding in different ways. A rural area will have limited drainage infrastructure, which will leave lanes, fields and village streets vulnerable to surface water flooding. Traditional Development on top of green field environments will reduce natural flood routes and increases the run off volume.

Equally, urban areas with extensive storm sewer infrastructure may be vulnerable to inundation from local heavy rainfall if the capacity cannot meet demand. Adding a development traditionally to the existing sewer network could exacerbate this overloading.

With the Flood and Water Management Act 2010, LPAs now work in conjunction with Lead Local Flood Authorities (LLFAs) to review developments and their impact on drainage.

Many take a pragmatic view on small developments (typically less than 10 dwellings), but above this LPAs and LLFAs will want to see a realistic drainage solution and full testing and run off calculations will be more than likely required.

Flood Risk

Once the level of development has been established, the next step is to see whether the planned development site lies within a higher risk flood zone, as defined by the Environment Agency.

Minor development can cumulatively affect flood risk. There is increasing demand on the capacity of surface water sewers and often the water company will not increase the capacity of the sewer. Again, policy will vary by LPA, but there is an increased emphasis on small scale developments reducing flood risk by ensuring that:

• all hard-surfacing is made permeable.
• all minor development should include plans which shown down pipes are connected to soakways and or other methods of control and not the sewer.
• front garden paving is not considered permitted development, if is completely hard surfaced.

Critical Drainage Areas

The LPA will also make a review on whether the proposed development site lies within a Critical Drainage Area (CDA).

A CDA is defined in the Town and Country Planning (General Development Procedure) (Amendment) (No. 2) (England) Order 2006 as “an area within Flood Zone 1 which has critical drainage problems and which has been notified… [to]…the local planning authority by the Environment Agency”.

They are where man made drainage infrastructure has been identified as at critical risk of failure, resulting in flooding. Such areas can be completely different or similar, to the areas identified by the Environment Agency as at risk of natural watercourse, river and sea flooding.

There can be a strong link to a Water and Sewerage Company’s DG5 register. This is a formal register of properties which have experienced sewer flooding (either internal or external flooding) due to hydraulic overload, or properties which are ‘at risk’ of sewer flooding more frequently than once in 20 years.

They can cover wide areas within both rural and urban environments. In London, for example, much of Chelsea is considered to be within a CDA.

Assessing Drainage

Once the site has been assessed against these sensitivities, a drainage assessment is usually required by the LPA. For major developments, you’ll typically need to:

• complete a drainage assessment form
• provide detailed drainage design plans
• check to see if you need to provide a flood risk assessment 

In the next post, we will assess the components of the drainage assessment and a typical LPA approach with an emphasis on Sustainable Drainage Systems (SuDS) as a standard part of drainage solutions for major developments

Understanding Site Conditions Better

GeoSmart provides architects, planning consultants and their developer clients with a comprehensive range of contaminated land, flood risk and drainage assessments to ascertain conditions ahead of full planning submission.

These can provide essential pre-screening of sites for potential risks, through to providing a forensic approach to risk from surface, river and groundwater flooding and how these could affect site drainage.

For more information contact us on 01743 298100 or email enquiries@geosmartinfo.co.uk