Tracer Study, West Bay

Background

Maintenance activities at West Bay, Dorset, currently involve periodic beach recycling at both West Beach and East Beach (Figure 1), as well as annual dredging of sediment from the outer West Bay Harbour (which is deposited on West Beach). These activities are guided by Beach Management Plans (BMPs) for each site, both of which are in the process of being updated to reflect changes to the coastal defences as a result of the 2019 West Bay Coastal Improvements Scheme.

Figure 1: View of East Beach, West Bay, during a site visit in May 2021

The SCOPAC Sediment Transport Study (STS) produced a map of the principle sediment sources and sediment transport mechanisms (Figure 2). Whilst the SCOPAC STS is a useful overview, it highlights fundamental uncertainties in sediment pathways and budget over this area.

Figure 2: Sediment transport pathways between Lyme Regis and West Bay, taken from the updated SCOPAC Sediment Transport study (SCOPAC STS, 2017)

These BMPs require an up to date understanding of sediment transport pathways to inform future maintenance activities. An improved understanding will allow greater efficiency and effectiveness of the current sediment recycling operations and improve evidence to support the works along the entire West Bay frontage.

Dorset Council and BCP Council have been successful in attracting £86,460 of local levy funding to undertake this study, alongside the £4,000 SCOPAC contribution, £7,500 from Dorset Council and £7,000 from the Environment Agency.

Aims and objectives

The aim of this study was to investigate the local sediment transport processes through a tracer pebble study to refine the current level of understanding of where beach sediment from both East Beach and West Beach moves to, and what the key drivers and rates of movement between the onshore and offshore (nearshore) areas are.

The key project objectives are:

• To successfully prepare, deploy and track Tracer Pebbles through a series of retrieval surveys, the results of which can be applied to infer sediment transport processes;
• Provide evidence to help refine future understanding of the possible sediment transport links between East Beach and West Bay;
• Use data from the recovered tracer pebbles to aid analysis of sediment drift patterns and provide an estimate of drift rates along the West Bay frontage;
• Apply the tracer results to aid subsequent consideration of the influence of the pier structures on sediment movement across the entrance of West Bay Harbour.

Data collected from this tracer study was used alongside well-established methods of volume analysis applied from topographic and bathymetric surveys, as well as synopsis of previous literature to provide an improved understanding of sediment transport pathways along the West Bay shoreline to inform future beach management operations.

For more information on the tracer pebble method, please visit Tracer Studies 

Key findings

• No evidence found to confirm sediment links between East and West Beaches.
• Material appears to be circulated within the groyne bays west of the harbour entrance, suggesting that the rock groyne and stub groyne structures are effective at holding material in the groyne bays.
• Evidence of a connection between East Beach and Burton Freshwater, with tracer pebbles deployed at East Beach detected near Freshwater Beach Holiday Park (Figure 3).
• Wider scale reversals in beach levels indicated by the longer term erosional and accretional patterns along the study frontage may be acting to keep the beach levels generally stable overall. Follow up tracer retrieval surveys and analysis of topographic data will confirm this.
• Material shown to move in a predominantly eastwards direction at Burton Freshwater and appeared to be interrupted and slowed by the outflow of water from the River Bride.
• The net sediment transport direction determined at every deployment site was eastwards. The highest net sediment transport rates were estimated at two of three the deployments on East Beach (approximately 1,204 and 1,942 m/yr) and the deployment at Burton Freshwater (approximately 1,040 m/yr). The net sediment transport rates west of the harbour entrance were generally lower.
• The furthest distance away that a tracer pebble was detected from its original deployment site was approximately 1,475m in just over four months.

Figure 3: General tracer pebble results from deployment site EB1 at East Beach. Each coloured dot represents a different tracer pebble detected, with the colour of the dot corresponding to the survey that it was detected. The underlying topographic difference plot shows the elevation difference between the 31st March 2022 and 22nd May 2023 (red areas indicate erosion and blue areas indicate accretion).

Future recommendations

• Future retrieval surveys of East Beach, Burton Freshwater and the two groyne bays west of the harbour entrance to determine whether any tracer pebbles have bypassed the rock structures and harbour entrance over a longer period.
• An acoustic tracer study to monitor the movement of material within the nearshore zone (subject to the success of a pilot study at Hayling Island).

References

SCOPAC Sediment Transport Study (2017). New Forest District Council 2012 Update of Carter, D., Bray, M., and Hooke, J. 2004 SCOPAC Sediment Transport Study.