- 12th October 2020
- Posted by: Miranda Pont
- Category: Projects
Location: Swanage, Dorset Client: Bournemouth, Christchurch and Poole (BCP) Council
We were approached by Bournemouth, Christchurch and Poole (BCP) Council to undertake a study to better understand the impacts of storm events on beach erosion and draw down levels, and support the update of the Swanage Beach Management Plan.
Following a beach nourishment scheme that was implemented in 2005, this new project aimed to use the latest statistical datasets and modelling techniques to answer the following key questions:
- Does the existing beach profile offer target 1 in 300 year design standard, in terms of drawdown level?
- What design standard does the 2005 scheme profile offer in terms of drawn down level?
- How much will this design standard change by 2070?
- How much volume is lost from the beach annually from storm events?
We combined statistical and numerical modelling using the latest datasets and modelling techniques to achieve the project’s aims. Joint probability (JP) extreme design events were determined through the analysis of the Environment Agency’s multivariate extremes nearshore datasets. The XBeach numerical model was then used to estimate the beach profile response under several JP extreme conditions. For each return period considered, a range of JP events were modelled allowing a profile response envelope to be developed and better inform the results.
Swanage beach is backed by a concrete seawall which was explicitly represented in the XBeach model, meaning that the interaction with the structure under different wave and water level conditions was well represented.
The results from our modelling analysis showed that the existing beach profile was providing an adequate level of protection to prevent the toe of the sea wall becoming exposed under extreme conditions. While the previous recharge scheme profile would still be sufficient, the impacts on the rear structure would become critical as sea levels rise. Additionally the range of return periods that we modelled allowed for an annual average erosion loss to be determined.
All of these results and our recommendations helped to inform the Swanage Beach Management Plan, allowing BCP Council to better understand the management requirements for the beach over the next few years.