Tracer Pebble 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, 2012)

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, £2,500 from Dorset Council and £3,500 from the Environment Agency.

Aims and objectives

The aim of this proposed study is 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 Beach;
• 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 Bridport Harbour;

Data collected from this tracer study will be 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.

See Tracer Pebble Studies for information on the tracer pebble method, and links to other studies in the region.

Expected Outputs

The output from this study will be an improved understanding of sediment transport pathways along the West Bay shoreline to inform future sediment recycling operations, along with an exploration of pros and cons of different techniques for monitoring sediment movements on the seabed that would be of use in undertaking similar studies along other parts of the coast.

All sediment transport results will be accompanied by hydrodynamic analysis (i.e. wave roses, wave and sea level time series, storm analysis for the local regional monitoring data sets at West Bay) from the Regional Monitoring Programme (www.coastalmonitoring.org) across the study period which would be interpreted alongside and in relation to the morphological changes.