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Flooding 7: Coastal flooding at Chiswell

strip of sandy land with sea on one side and fresh water on the other; housing in foreground, industry behind

Suitable for A level

Chesil Bank, in Dorset, is a barrier beach constructed of shingle. The beach stretches 29 km from West Bay in the west to Chiswell at the beach’s eastern end, where the beach is connected to the rocks of the Isle of Purbeck. The beach evolved as sea levels rose by eustatic adjustment during the first part of the Flandrian interglacial period from about 12,000 years before present (years BP) to 6000 years BP. During this period, shingle was combed up from the sea floor of Lyme Bay and thrown forwards by breaking waves to form a barrier beach. Over time this barrier advanced forwards (towards land) until it reached its current position. Chesil currently receives little or no new sediment supply so is considered a relict feature of the landscape.

The village of Chiswell is located at the eastern end of Chesil Bank where the beach connects with Portland Bill. Chiswell is vulnerable to flooding during storm events and suffered flood and storm damage in 1979 and 2014. Like all beaches, Chesil Bank is in a state of dynamic equilibrium so the shape of its profile varies depending on wave direction and energy. During storm conditions, large waves crash onto the upper part of the beach profile. Some water percolates through the shingle and escapes on the northern side of the ridge. This can cause flooding of the causeway that connects Chiswell to Wyke Regis. Storms are capable of scouring shingle from the upper part of the beach profile, leaving Chiswell and the causeway vulnerable to flooding during the next storm surge. Reprofiling of the beach after storm damage is, therefore, a strategy used to maintain the beach as a defensive structure. In addition, a stepped concrete revetment, curved sea wall and gabions are used to help protect Chiswell from flooding.

Specification requirements

An understanding of coastal management strategies is a requirement of all A level specifications.

Download the specification audit.

Learning objectives

This resource can be used to meet one or more of the following objectives:

  • create a case study of a coastal management scheme that involves hard and soft engineering
  • use quantitative data sources to investigate the relationship between wind speed, wind direction, atmospheric pressure and wave height.

Download the Assessment Objectives.

Activities

Students can investigate the way in which Chiswell has changed over time by comparing the 1888-1913 six inch map to a modern satellite image here. Doing this will engage students with specialist concepts of place and mitigating risk.

  • How has Chiswell changed since 1913?
  • What new land uses have been developed in places that might be at risk of flooding?

In order to understand the flood defences at Chiswell it is recommended that students have access to the booklet published by Dorset Coast Forum and watch the video below created by the Environment Agency. The video covers a broad range of issues relating to coastal processes, vulnerability, flood defences and flood warnings. Brief teacher notes listing the main themes covered in the video, and their timings, are available here.

Chiswell from The Geographical Association on Vimeo.

Reprofiling is used to maintain the height and profile of the upper beach profile. This Environment Agency document assesses reprofiling as a strategy that needs to be used in conjunction with beach nourishment. Go to Figure 1.1 on page 12 of the pdf and follow the hyperlink to read section 5.4 Beach Nourishment.

  • How do beaches form an important natural defence against flooding?
  • Why are beaches described as dynamic features and why does this matter to places that are at risk of coastal floods?
  • Why is it significant that Chesil Bank is a relict landform?
  • What is reprofiling and how is it achieved? What are the potential strengths and limitations of this strategy?

In order to understand the wider context of coastal flooding, students should understand the processes involved in a storm surge. This NOAA resource (National Oceanic and Atmospheric Administration) describes the factors that contribute to the height of a storm surge such as the one that battered Chiswell in 2014. Extremely low atmospheric pressure is one factor, but other factors are also important:

  • How might the angle of fetch and storm intensity affect the risk of flooding at Chiswell?

The direction of the prevailing wind and the amount of fetch are critical factors in determining the size of the waves that approach Chesil Bank. The wind rose represents wind strength and direction at Weymouth. Go to the end of the webpage to find the wind rose diagram. In order to investigate the relationship between wave height, wind speed and wind direction, students could analyse live and archived quantitative data that is available for Chesil. Wave height is represented graphically on the Channel Coast website. The dialogue box near the top of the screen should be used to select the required range of dates. This data could be compared with data on wind speed and direction and air pressure at sea level which is available on the Magic Seaweed website. By selecting data for the same range of dates at each site, students could investigate the possible relationships between wave height and wind speed, wind direction and air pressure.

 

Other lessons in this series:

Flood risk and flood management: Introduction
Flooding 1: Causes of river floods
Flooding 2: Investigating the effects of river floods
Flooding 3: Response to floods: Oxford Case Study
Flooding 4: Managing the upper drainage basin
Flooding 5: Sustainable Drainage Systems (SuDS)
Flooding 6: Managing river floods – exploring the role of the Environment Agency
Flooding 8: Managed realignment
Flooding 9: The role of the Environment Agency in coastal management and the development of shoreline management plans

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