Flood Risk Data Updates from the EA:
In December 2024 the Environment Agency (EA) released new data from their National Flood and Coastal Erosion Risk Assessment (NaFRA) and National Coastal Erosion Risk Map (NCERM), which exposed the alarming reality that 6.3 million properties in England are at risk of flooding.
Of these 6.3 million properties, approximately 2.4 million properties are susceptible to fluvial (river) and coastal flooding. Despite the overall number of properties at risk from fluvial and coastal flooding not increasing from the previous NCERM update in 2017, there has been a dramatic increase in the number of properties situated within the highest risk zone where the likelihood of flooding exceeds the 1 in 30 (3.3% AEP – Annual Exceedance Probability) scenario.
This shift in risk can be attributed to several factors such as climate change and urban creep, with the primary reason being advancements in data collection and modelling to more accurately assess flood risk.
Why are some parts of England more prone to coastal flooding than others?
Some areas in England are more vulnerable to coastal flooding than others due to a combination of local and global factors. Local factors include elements such as the geology of the shoreline, topography and the local bathymetry; while global factors include storm surges brought on by low pressure systems, climate change and sea level rise.
The south coast of England is particularly prone to coastal flooding and erosion due to its geographical location, whereby low-pressure systems moving in from the Atlantic bring strong tidal surges which get funnelled through the English Channel.
Yet, there is another element that makes the south coast of England susceptible to coastal flooding, known as Glaciostatic Rebound.
Glaciostatic Rebound or Isostatic Uplift in which it is also commonly known as, refers to the process whereby the Earths crust gradually rises or sinks from being compressed by the weight of the large ice sheet during the previous ice age.
During the last Ice Age, which ended approximately 10,000 years ago in the UK, the sheer weight of the ice sheet that covered northern England caused the land directly beneath to sink, while the southern part of England was pushed upward. When the ice sheet began to retreat, the land began to adjust, with some areas rising and others sinking, a process known as Glaciostatic Rebound.
In the UK, the effects of the last Ice Age are still being felt today with the land in northern areas of the UK slowly rising by approximately 10 centimetres per century, while southern areas of England are slowly sinking by approximately 5 centimetres per century.
The sinking of southern England and rising land in the northern areas increases the likelihood of coastal flooding along the south coast. Additionally, sea level rise brought on by global warming is further exacerbating the risk of coastal flooding in southern England.
Case Study on Coastal Flood Risk:
FPS Environmental were contacted by a customer who had concerns about their property situated on the south coast of England, which had unfortunately flooded on three separate occasions over the past five years from coastal flooding.
The customer wanted a clearer understanding on the potential flood risk to their home, particularly how climate change may affect their location over the coming years, as well as advice regarding their plans to mitigate the property and whether their mitigation plans could work. This information would help guide their decision on whether it would be worthwhile making a significant investment in protecting their property, which involved tanking the rear wall and additionally building a wall to prevent coastal encroachment from the property rear.
FPS Environmental undertook a ‘Map & Chat’ service with the customer, which comprised a comprehensive desktop assessment of the property in question focusing on the underlying geology, surface water maps, Flood Zones, architectural drawings provided by the customer, LiDAR Data, climate change allowance data as well as predicted sea level rise data for the area. This desktop assessment was then followed up with an hour-long consultation video call with the customer to discuss the desktop assessment findings and answer any additional queries the customer needed to clarify.
Findings on Flood Risk:
FPS Environmental identified that the proposed flood wall intended at the property rear would likely become outflanked and bypassed. This conclusion was achieved through analysing the Flood Zone 3 extents and the EA Surface Water Model data in GIS Software. The mapping and desktop assessment identified the presence of a culverted watercourse located approximately 280m north of the property, which was located topographically higher than the property based on LiDAR analysis. A fluvial and surface water flow path was identified that flowed southward towards the topographically lower property from the north, originating from this culverted area.
FPS Environmental were able to inform the customer of this surface water risk at the front northern section of the property, which at the time was unknown to the customer; and advise that PFR Measures (Property Flood Resilience) should also be considered for the front section of the property.
Additionally, the desktop assessment identified that the property was situated above a Highly Productive Aquifer* overlain by superficial deposits. This posed a high risk of groundwater emergence at the property and a likelihood that any proposed flood wall or rear wall tanking would be bypassed and possibly fail. Advice was then given to the customer regarding potential property flood resilience (PFR) measures to minimise the effects of groundwater emergence within the internal areas at the property.
*An underground rock formation with a high capacity to store water and allow it to move through the rock
Climate change allowance data was also analysed during the desktop assessment, paying particularly close attention to the predicted sea level rise which was one of the main concerns from the client. The property was located within the Southeast District Area of England and was identified to have a cumulative sea level rise of 261mm by the year 2065, and a sea level rise of 1.2m by the year 2125 based off the Higher Central allowance (see Table 1 below):
The predicted sea level rise estimations were alarming news to the customer.
Outcomes:
From the Map & Chat service, the customer was able to gain a clear insight into the risk of flooding at their property, particularly in regard to climate change and the likely effectiveness of their proposed mitigation plans.
FPS Environmental highlighted several significant concerns with the customer’s initial mitigation plans, and it became evident that the proposed flood wall and tanking at the property rear would likely be insufficient to protect the property over its lifetime. Elements such as surface water flow routes, the underlying geology and potential flooding via the partition walls with the adjacent terraced properties all contributed to this assessment. Additionally, factors such as sea level rise estimations suggested that the proposed wall would likely be overtopped in the near future, further compromising the effectiveness of the planned mitigation.
FPS Environmental were able to advise the customer on alternative PFR measures that could be implemented at the property, enabling the customer to have a re-think on their own mitigation intentions, saving them thousands of pounds from implementing expensive measures which would likely not be effective.
If you have any queries or concerns regarding flooding at your own property and need some advice, don’t hesitate to get in touch.
Written by Reece Hewitt, Senior Flood Risk Consultant, MSc BSc (Hons) MCIWEM
Featured Image ID 26854468 @ Ian Woolcock | Dreamstime.com