Flood Risk Assessment
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Flood Risk Assessment and Surface Water Drainage

We offer Flood Risk Assessment and Surface Water Drainage services, including Sur 1 and Sur 2 Code for Sustainable Homes Asssesments, for all types and scales of development throughout the UK where flood risk and surface water drainage information is requested by the Local Planning Authority. Typically, we are contacted by Architects, Planning Consultants, Construction firms, Sustainability Consultants and private individuals who are responsible for collating and submitting the following documentation;

NPPF / SPP7 Flood Risk Assessment

TAN15 Flood Consequence Assessment

Code for Sustainable Homes Sur 1/Sur 2 Surface Water/Flood Risk Assessment

BREEAM Pol13/Pol5/Pol6 Flood Risk Assessment/Minimising Watercourse Pollution Assessment

Surface and Foul Water Drainage Strategy, Layout, and Design

We understand what is important to our clients:

  • Professionalism – Our Flood Risk Engineers are fully qualified, offering expert advice and assistance throughout the Flood Risk Assessment process. We have created an excellent working relationship with the various Environment Agency offices, which allows our Engineers to successfully negotiate with the determining authority.
  • Service – We understand the importance of offering our clients excellent service and aftercare. Our client base is largely built on recommendation and from repeat customers with whom we have forged close working relationships. We endeavour to work within our clients' time frames, and offer a fast track service where this is possible which is often essential at the latter stages of planning submissions.
  • Price – We believe our prices are amongst the best in the market, but should you find a lower comparable quote, ask about our price matching offer.

For more information or to speak to a Flood Risk Engineer without obligation, please contact us.


NPPF / SPP7 Flood Risk Assessment (FRA)


As a client or agent submitting planning applications, you will be aware that your Local Planning Authority may request a Flood Risk Assessment for any site which is shown to lie in the modelled flood plain within the vicinity of a fluvial or tidal watercourse. A Flood Risk Assessment may also be requested in areas of low fluvial or tidal flood risk, where a development site is over one hectare in size, or lies within a critical drainage area.

Without a sufficiently detailed Flood Risk Assessment, the Environment Agency or Scottish Environmental Protection Agency will routinely object to developments in these areas which can lead to subsequent delays and objections to your planning application. To find out whether your proposed development lies within the indicative flood plain for rivers and the sea, please follow the appropriate link to the Environment Agency or Scottish Environmental Agency Flood Map below;

Environment Agency Flood Map

Scottish Environmental Protection Agency Flood Map

Our Flood Risk Engineers are experienced in undertaking Flood Risk Assessments throughout England and Scotland in accordance with PPS25 and SPP7 planning guidance. We have successfully negotiated with the Environment Agency and Scottish Environment Protection Agency on over 98% of our projects, overcoming objections raised and forging close links with their development control teams across the country.

We undertake full Flood Risk Assessments for all scales of development. Where you are unsure if a full survey is required, we can provide a desktop Flood Risk Scoping Study prior to completing a full Flood Risk Assessment. This is fully deductible from any final invoice.

In certain cases the use of hydraulic modelling may be necessary to accurately quantify the flood risk to a site. We are able to produce modelling reports using the ISIS, TUFLOW, MIKE FLOOD, Info Works RS, and HECRAS software as appropriate to accurately determine flood levels, depths, and rates of inundation on a site, both with and without consideration for flood defences, and breaching of these assets.

Our Flood Risk Assessments also quantify the risk of flooding to the site from sources such as surface water flows, sewer flooding, and a rising ground water table beneath the site. A Flood Risk Assessment provides a conceptual drainage strategy which includes calculations of the pre and post development surface water runoff from different storm events on the site, and details suitable methods of managing increased flows. This often incorporates use of Sustainable Urban Drainage Systems (SuDS).


TAN15 Flood Consequence Assessment (FCA)


As a client or agent submitting planning applications in Wales, you will be aware that your local planning authority requests a Flood Consequence Assessment for any site which is shown to lie within the modelled flood plain for a fluvial or tidal watercourse within the vicinity. A Flood Consequence Assessment may also be requested in areas of low fluvial or tidal flood risk, where a development site is over one hectare in size, or lies within a critical drainage area.

Without a sufficiently detailed Flood Consequence Assessment, the Environment Agency Wales will routinely object to developments in the above areas which can lead to subsequent delays and unexpected expenditure. To find out whether your proposed development lies within the indicative flood plain for rivers and the sea please follow the link to the Environment Agency Wales flood map below;

Environment Agency Wales Flood Map

AAH Planning Consultants are experienced in undertaking Flood Consequence Assessment across Wales in accordance with TAN15 planning guidance. We have successfully negotiated with the Environment Agency on over 98% of our projects, and forged close links with their development control teams across the country.

We are able to provide a simple Flood Risk Scoping Study prior to a planning submission, or undertake a full Flood Risk Assessment for any scale of development. In certain situations the use of hydraulic modelling may be necessary to accurately quantify the flood risk to a site. We are able to produce modelling reports using the ISIS, TUFLOW, MIKE FLOOD, InfoWorks RS, and HECRAS software as appropriate to accurately determine flood levels, depths, and inundation rates on a site, both with and without consideration for flood defences, and breaching of these assets.

Our Flood Risk Assessments also quantify the risk of flooding to the site from sources such as surface water flows, sewer flooding, and a rising ground water table beneath the site. The Flood Consequence Assessment also provides a conceptual drainage strategy which includes calculations of the pre and post development surface water runoff for different storm events on the site, and details suitable methods of managing increased flows. This often incorporates use of Sustainable Urban Drainage Systems (SuDS).


Code for Sustainable Homes Sur 1/Sur 2 Surface Water/Flood Risk Assessment


It is increasingly common that the Code for Sustainable Homes (CFSH) Assessment criteria must be satisfied prior to approval of a planning application, or dealt with as a subsequent planning condition. Category 4 of the CFSH Assessment, 'Surface Water Run-off', is divided into three sub categories:

  • Sur 1 Mandatory
  • Sur 1 Non-Mandatory
  • Sur 2

Sur 1 Mandatory and Non-Mandatory relate to the sustainable management of storm water arising on the site. Sur 2 considers the flood risk posed to the site from rivers, the sea, and all other sources. At present there are two versions of the Code for Sustainable Homes Technical Guidance, the version relevant to a project depends on when an assessment was registered with the accreditation body. A summary of the requirements of both guidance documents may be accessed through the links below;

May 2009 Code For Sustainable Homes Technical Guidance

November 2010 Code For Sustainable Homes Technical Guidance

AAH Planning Consultants have experience in producing both Sur 1 and Sur 2 Surface Water and Flood Risk Assessments to meet the objectives of the May 2009, and November 2010 Code For Sustainable Homes (CFSH) guidance. Typically we are approached by Sustainability Consultants, land owners and construction companies to undertake this work. The suitability of the proposal to meet the optional credit criteria is researched prior to tender, so that we are confident in providing the best possible advice, and the most accurate quotation.

May 2009 Code For Sustainable Homes Technical Guidance

The mandatory elements of Sur 1 and Sur 2 must be achieved for all developments by demonstrating that the peak rate of run-off from the site does not increase following development. The proposal must wholly reduce the increased volume of surface water arising from the site as a result of an increased footprint through use of infiltration and/or rainwater harvesting. Where this is not possible, additional surface water must be discharged at an attenuated rate from the site into a watercourse.

Two credits are available for using SuDS to improve water quality of the rainwater discharged, for protecting the quality of the receiving waters by ensuring no discharge to the watercourse for rainfall depths up to 5mm, or to establish agreements for the ownership, long term operation and maintenance of all sustainable drainage elements used.

As part of the Sur 2 criteria, 2 credits are available for developments situated in Zone 1 – low annual probability of flooding (as defined in PPS25 – 'Development and Flood Risk') and where the site specific Flood Risk Assessment indicates that there is low risk of flooding from all sources.

A single credit is available for developments situated in Zones 2 and 3a – medium and high annual probability of flooding, where the finished ground floor level of all habitable parts of dwellings and access routes to the ground level and the site are placed at least 600mm above the design flood level of the flood zone.

The Flood Risk Assessment accompanying the planning application must demonstrate to the satisfaction of the Local Planning Authority and statutory body that the development is appropriately flood resilient and resistant, including safe access and escape routes where required, and that any residual risk can be safely managed.

November 2010 Code For Sustainable Homes Technical Guidance

The mandatory elements of the Sur 1 criteria must be met by all developments undergoing a Code for Sustainable Homes (CFSH) Assessment. This includes a number of sustainable storm water management principles relating to the peak flow runoff from the assessment site, and the volume of runoff discharge following a storm.

If there is no increase in the man-made impermeable area as a result of new development, then the peak rate of run-off criterion does not apply. Where there is an increase in impermeable area, the Sur 1 Assessment must ensure that the peak rate of run-off over the development lifetime, allowing for climate change, will be no greater for the developed site than it was for the pre-development site, with a minimum limiting discharge rate of 5l/s from any one development site.

As with the peak flow runoff rate criteria, if there is no increase in the man-made impermeable area as a result of the new development, then the volume of run-off criteria does not apply. If the developed site would discharge a greater volume of rainwater run-off than the pre-development site for the 100 year 6 hour event, then the post development volume of run-off, allowing for climate change over the development lifetime, should be no greater than it would have been before the development.

The additional predicted volume of run-off must be prevented from leaving the site by using infiltration of other SuDS techniques. If the above cannot be achieved, then the development should reduce the post development peak rate of run-off to the limiting discharge.

The final element of the mandatory Sur 1 criteria requires that developments demonstrate that the flooding of property would not occur in the event of local drainage system failure caused either by extreme rainfall or a lack of maintenance.

Two credits are available under Sur 1, the first credit can be awarded by ensuring there is no discharge from the developed site for rainfall depths up to 5 mm, the second can be awarded by ensuring that the run-off from all hard surfaces receives an appropriate level of treatment to minimise the risk of pollution.

Under the Sur 2 criteria, two credits are available for developments situated in Flood Zone 1, the low risk area, with an accompanying site-specific Flood Risk Assessment which indicates that there is low risk of flooding from all sources.

Alternatively, one credit is available for developments situated in Zones 2 and 3a , medium and high annual probability of flooding, where the finished ground floor level of all habitable parts of dwellings and access routes to the ground level and the site, are placed at least 600mm above the design flood level of the flood zone.

The Flood Risk Assessment accompanying the planning application must demonstrate to the satisfaction of the local planning authority and statutory body that the development is appropriately flood resilient and resistant, including safe access and escape routes where required, and that any residual risk can be safely managed.


BREEAM Pol5/Pol6 Flood Risk Assessment/Minimising Watercourse Pollution


Pol5 forms the 'flood risk' criteria within the BREEAM Assessment for a site. The credit aim is to encourage development in low flood risk areas, or to show that a development takes measures to reduce the impact of flooding on buildings in areas with a medium or high risk of flooding.

Two credits are available where evidence provided demonstrates that the assessed development is located in a zone defined as having a low annual probability of flooding. One credit is available where evidence provided demonstrates that the assessed development is located in a zone defined as having a medium or high annual probability of flooding and the ground level of the building, car parking and access is above the design flood level for the site's location.

One further credit is awarded where evidence provided demonstrates that surface water run-off attenuation measures are specified to minimise the risk of localised flooding, resulting from a loss of flood storage on site due to development.

Pol6, 'Minimising Watercourse Pollution' aims to reduce the potential for silt, heavy metals, chemicals or oil pollution to natural watercourses by surface water run-off from buildings and hard surfaces. One credit is available where evidence provided demonstrates that effective on site treatment such as Sustainable Urban Drainage Systems (SuDS) or oil separators have been specified in areas that are or could be a source of watercourse pollution.

Our BREEAM Flood Risk Assessments quantify the risk of flooding to the site from fluvial/tidal sources, surface water flows, sewer flooding, and a rising ground water table beneath the site. The Flood Risk Assessment also provides a conceptual drainage strategy which provides calculations of the pre and post development surface water runoff for different storm events on the site, and details suitable methods of managing increased flows, which often incorporates use of Sustainable Urban Drainage Systems (SuDS).


Surface Water Drainage Strategy, Layout, and Design


Details of a drainage system on a proposed site may be requested prior to your planning application being approved, or may be required as a planning condition to be subsequently discharged prior to construction beginning on the site.

We are able to design surface water conveyance and attenuation systems for all scales of development. Since the introduction of Planning Policy 25 (PPS25), Sustainable Urban Drainage Systems have become integral to all scales of development within the UK. Routinely, the Local Planning Authority will require the calculated peak flow runoff from the pre and post development site, means of attenuating post development surface water flows to existing discharge rates, and methods of onsite storm water storage.

Recommendations for surface water drainage will be in accordance with the sustainable hierarchy described in PPS25, which states that in order of preference, additional storm water flows should be discharged to soak away, to an unadopted water course, an independent surface water sewer, or as a last resort to a combined sewer.

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