Understanding Site-Level Estimation of Climate Change Flood Zones

The National Planning Policy Framework (NPPF) requires all new developments to assess flood risk and ensure that proposals will not increase the number of users at risk of flooding or exacerbate flooding to other properties, either upstream or downstream. However, identifying whether a site is located within a flood zone, particularly Flood Zone 3 plus climate change impacts, can be challenging for town planners, planning authorities, and the general public.

Limitations of Current Flood Mapping

The UK government’s published flood maps, widely used for planning purposes, do not explicitly indicate the extents of Flood Zone 3 with climate change considered. Instead, the Environment Agency and local planning authorities are tasked with developing and publishing these flood extents through Strategic Flood Risk Assessments (SFRAs). Accessing these maps is not always straightforward; they are often difficult to locate on council planning portals and require considerable effort to obtain.

Additionally, in some cases, this data may only be available through requesting detailed datasets (e.g., Product 4 or Product 5) from the Environment Agency. Requests for these datasets, made via the Flood Data for Planning portal, can take up to 20 working days or more, and even then, the data may not always be readily available.

Estimating Flood Zone 3 + Climate Change Extents

When a client requires a high-level flood risk assessment early in the development process, it is often necessary to estimate the potential extent of Flood Zone 3 plus climate change impacts. This can be achieved by using the extent of Flood Zone 2 as a rough proxy.

Flood Zone 2 represents areas that could experience flooding during extreme events (e.g., a 1 in 1,000-year event). It effectively illustrates what flooding might look like with increased rainfall or tidal volumes in a catchment, which can overlap with the potential impacts of climate change. Although the extents of Flood Zone 2 and Flood Zone 3 plus climate change do not always align perfectly, the following guidelines can provide a useful preliminary approach:

1. Assessing Sensitivity to Climate Change Impacts:

   • If a site is not currently located in Flood Zone 3 but falls within Flood Zone 2, this indicates a high likelihood that the site is sensitive to climate change impacts. A detailed flood risk assessment and engagement with the Environment Agency will be necessary to refine understanding of the site’s future flood risk.

2. Comparing Flood Zone Extents:

   • In cases where Flood Zone 3 and Flood Zone 2 extents closely match, the site or area may demonstrate reduced sensitivity to climate change impacts. This is often seen in regions with channelised rivers or steep, well-defined channels, where increased water flows are less likely to exceed the riverbanks.

3. Applying This Approach to Tidal and Surface Water Flooding:

   • For tidal flooding, while the correlation between Flood Zone 2 and Flood Zone 3 plus climate change may not be as precise, a similar estimation method can be applied.

   • For surface water and overland flooding, sensitivity assessments can provide insights into how increased rainfall or runoff might impact a site.

     
     

When to Seek Detailed Data

Where a site is located near the boundary of Flood Zones 2 and 3, or where initial estimates indicate potential sensitivity to climate change impacts, it is critical to:

• Engage with the Environment Agency to obtain detailed flood risk data.

• Conduct a thorough assessment to determine the suitability of the site for development and identify appropriate mitigation measures.

  
  

Key Takeaways

• Flood Zone 2 extents can provide a useful starting point for estimating Flood Zone 3 plus climate change impacts in the absence of detailed data.

• Sites located within Flood Zone 2 but not Flood Zone 3 are often at higher risk under climate change scenarios, necessitating closer evaluation.

• If Flood Zones 2 and 3 align closely, it may indicate that the area is less sensitive to increased flood risks due to climate change.

  
  

How We Can Help

If you are uncertain about the flood risk to your site or require a detailed assessment, we can provide expert guidance and analysis. Our team has extensive experience in navigating flood risk assessments, interpreting data, and ensuring compliance with planning requirements. Contact us to review your site and develop tailored strategies for managing flood risks effectively.