Southwest Environmental Limited

Sometimes if it is discovered that you project lies within flood zone 2 or 3, it will be stipulated by you local planning authority that you may need a Flood Risk Assessment, although this is perceived by most as an annoyance, and its certainly can be that, a flood risk assessment can contain useful information such as finished floor levels, and material choices.

Flood risk maps that have been made by the environment agency and their consultant partners, are based on fluvial and marine flooding only, and do not take in to account surface water flooding (separate maps see below) and sewer flooding where large volumes of water can be transferred from one area to another through the sewer network.

So a flood risk assessment, based on available data is by no means fool proof, but more often than not will prove that your site is low risk (hopefully) or a bit of a nightmare.

Zones

 Depending on what flood plains you are in will depend on what you can build as outlined by PPS 25.

However, chances are if you are building in the flood plan area, you will have taken advice from your LPA or the EA.

I you have a worry about anything give us a buzz and we will try to explain, you will get charged for a report but not for informal chit chat.

In the case of the project near the map area above, even though the site was situated within the flood zone 2, it was allowed to go ahead, for the following reasons:

1 - The type of project matched established uses for flood zone area 2 uses. That was good.

2 - There were questions over the resolution of the mapping, it appeared the site was on a raised area of ground not accounted for in the maps.

Historic Flooding

Historic Flood extent maps (left) and surface flooding maps are also used to compile data and flood risk for a site.

Key Parts of Flood Risk Assessment

1 - Exception Test

2 - Sequential Test

3 - Surface Water Management Plan

4 - Safe Refuge Designation

5 - Scoping or Screening

Surface Water Flooding

If you have any questions regarding discharge consents whether existing or proposed then please contact SWEL, and we will offer you (of course) a free of charge initial consultation.

The below section was written by a machine.

Technical Portfolio: Proof of Work in Flood Risk Assessment

The below section was written by a machine.

Technical Guidance: Climate Change Allowances in Flood Risk

Adjusting flood zones for climate change is a mandatory requirement for any robust Flood Risk Assessment (FRA). Current Environment Agency guidance requires developers to account for increased peak river flows and sea level rise over the anticipated lifetime of a development, ensuring that new infrastructure remains resilient to future environmental shifts.

Applying Peak Flow Allowances

Climate change allowances vary depending on the river basin district and the vulnerability of the proposed land use. Technical adjustments typically involve:

• Hydraulic Modelling: Using data from flood zone modelling to simulate increased flow rates, often ranging from 20% to over 45% for high-vulnerability residential sites.
• Vertical Offsets: Establishing finished floor levels (FFL) that incorporate a freeboard above the predicted 1-in-100-year plus climate change flood level.
• Design Life Planning: Assessing impacts for a 100-year horizon for residential developments and 60 years for commercial projects.

Synergy with Drainage and Infrastructure

Increased rainfall intensities also dictate the design of Sustainable Drainage Systems (SuDS). By accurately mapping future flood extents using flood depth GIS mapping, we can ensure that surface water management features are not situated within functional floodplains.

• Resilient Management: Integrating climate data into Surface Water Management Plans (SWMP) to prevent downstream flooding.
• Safe Refuge: Ensuring that Flood Warning and Evacuation Plans remain viable under future sea-level scenarios.
• Environmental Compliance: Aligning climate adaptation with broader Environmental Impact Assessments (EIA) to support sustainable planning outcomes.

Technical Authority and Experience

With 15 years of industry experience and 900+ projects delivered, Southwest Environmental Limited provides the expert authority needed to navigate climate change legislation. Our consultants hold professional accreditations (IES, IAQM) and provide evidence-based analysis to satisfy Lead Local Flood Authorities across the UK. Whether you are managing nutrient neutrality or seeking pre-application advice, we provide the technical clarity required for long-term project success.