We have answered the most commonly asked questions relating to Flood Risk Assessments.
What is a Flood Risk Assessment?
A Flood Risk Assessment is a report that identifies the main flood risks to a development site, such as a plot of land for new housing. If required, a Flood Risk Assessment will suggest mitigation measures to reduce the potential impact of flooding. Flood Risk Assessments are generally submitted as part of a planning application as a standalone document but can be incorporated into other documents, such as drainage strategies.
Why do we need Flood Risk Assessments?
In the UK, over 5 million properties are at risk from flooding, including from;
- Surface Water;
- Tidal / Coastal; and
Any new development within a flood risk area is likely to require a Flood Risk Assessment to be submitted as part of its planning application. In addition to assessing the likelihood of flooding on a site, our Flood Risk Assessments identify appropriate mitigation measures to ensure that a development is safe from flooding.
Do I need a Flood Risk Assessment for my Development?
Depending on the location of your site you may require a flood risk assessment to be submitted with your planning application. The information below provides a guide as to when a Flood Risk Assessment is required. Alternatively, call our Hydrology team who would be happy to discuss your site requirements further.
If your site is over one hectare in size, or is situated in Flood Zone 2 or Flood Zone 3, you will need to submit a Flood Risk Assessment as part of your planning application.
The Environment Agency has an online tool to help identify whether your site is in a Flood Zone (click here for the Environment Agency Flood Map).
If your site is within an area classified as a medium or high likelihood of flooding, you will need to submit a Flood Risk Assessment as part of your planning application.
The Scottish Environmental Protection Agency (SEPA) has an online tool to help identify whether your site has a medium or high likelihood of flooding (click here for the SEPA Flood Map).
Your development will require a Flood Consequence Assessment if located in Flood Zones C1 and C2.
For flood zone B, your site levels will need to be checked against extreme flood extents which are mapped by Natural Resources Wales. To check flood extents click here for the Natural Resources Wales Flood Map.
The Flood Maps described above are produced using a flood model known as JFLOW. As with all computer models there is a certain degree of imprecision and generalisation, and flood defences are not taken into consideration. As such, Flood Map’s should be used as a tool to guide whether a Flood Risk Assessment is required for your site.
Please get in contact and we would be pleased to advise whether a Flood Risk Assessment is required.
What does a Flood Risk Assessment involve?
- Site work/walkovers;
- Infiltration Testing;
- Initial Site Appraisal;
- Flood Modelling;
- Flood Mitigation;
- Drainage Advice;
- Surface Water Attenuation; and
- Sustainable Drainage Systems (SuDS) Advice and Sizing.
How much does a Flood Risk Assessment cost?
Costs for Flood Risk Assessments are dependent on the size of the site, complexity of the development and likelihood of flooding. In order to provide you with the most competitive quote, it is important for us to gain as much information about your development at an early stage. Flood risk mitigation can then be designed in the development from the outset.
Please contact the Arcus Flood Risk Team for a Flood Risk Assessment cost.
If my site is at risk of flooding, what mitigation measures are available?
Reducing the risk of flooding at a site can be complicated, but mitigation measures include;
- Compensatory Flood Storage;
- Flood Water Pumps;
- Undercrofting (floodable void under your site);
- Flood proofing up to a specified height; and
Our hydrologists have significant experience specifying practicable solutions for a range of developments. We work closely with relevant consultees and local authorities to ensure that flood risks are minimised while seeking betterment where possible.
Why does climate change affect flood risk?
97% of climate experts agree humans are causing climate change.
Projections of future climate change suggest that more frequent, high intensity rainfall and prolonged duration could be expected. This type of event caused the widespread floods of 2015.
In order to ensure that developments are safeguarded against these changes, a percentage uplift is applied to calculations used within Flood Risk Assessments. The amount of uplift varies depending on the type and proposed lifetime of the development. For example, a permanent residential site (lifespan 100+ years) would have a greater climate change allowance applied to flood calculations than a more temporary wind farm development (lifespan approx. 25 years).
Percentage uplift varies from 5 % to 40 %. You can find out more information here. Alternatively, call our Hydrology team to discuss your site: 01904 715470.
What experience does Arcus have in undertaking Flood Risk Assessment?
Our hydrologists have experience of undertaking Flood Risk Assessments for a variety of types and scales of projects from small scale extensions through to large scale commercial developments, such as wind farms and electricity substations.
Arcus adopts the following approach to Flood Risk Assessments:
- Obtain previous flood records for fluvial, pluvial, coastal and sewers;
- Review the Flood Map;
- Review Biennial Reports, Strategic Flood Risk Assessments and Flood Models; and
- Review available topographical information to ascertain most likely source of flooding.
Consultation with Utilities providers such as Scottish Water, Yorkshire Water, Severn Trent, Northumbrian Water, Thames Water, Welsh Water etc;
Consultation with the Environment Agency, SEPA, NRW and local Councils
Site visits to verify the Desk Study and appraise flood risk issues
Hydraulic Modelling using Flood Modeller (if required):
- Surface water flooding using data from the Flood Estimation Handbook (FEH) 2013; and
- River flood levels derived from 1D – 2D linked model.
Hydraulic Modelling for minor open channels using Micro Drainage – IH124 method (if required)
Validation of modelled flood levels obtained from Strategic Flood Risk Assessments
Production of a Flood Risk Assessment
Suggest mitigation (if required)