Migration of gravel barriers over a consolidating substrate: implications for coastal management

Abstract

Gravel barrier beaches are important geomorphological features that provide a buffer to low lying coastal areas around the world from wave attack. This sheltering effect also promotes the formation of extensive intertidal habitat and lagoons composed of poorly consolidated sediment substrates of varied stratigraphy in the lee area with silt, clay and peat layers. Gravel barriers naturally respond to increases in sea level and wave overtopping of beach material by migrating landwards over the poorly consolidated substrate in their lee. This application of load causes the substrate to consolidate, causing the barrier crest to reduce over time. The magnitude of consolidation and consequent crest lowering will make the barrier vulnerable to further sea level rise and wave overtopping, resulting in acceleration of landward migration and increases in flood risk. Very few global or local studies exist to explore this problem and there is a lack of qualitative data, despite the implications for coastal management. This thesis aims to address this knowledge gap, utilising Hurst Spit as an interesting and important case study of a natural gravel barrier system, maintained in response to storm events in order to preserve its major flood defence purpose. The gravel barrier has migrated landward by 100 metres over the last 60 years in response to a reduction in sediment supply and storm damage. The next phase of sediment recharge is due within the next 5 years, with material due to be placed on the back slope, to attain a wide crest that meets design requirements. This realignment of the back slope will extend onto the poorly consolidated material, causing it to consolidate. Sediment coring confirmed that the substrate material was predominately marine muds, with high water content, but low permeability. The greater the thickness of the poorly consolidated material, the higher the magnitude of consolidation, and the thickness of the substrate was found to be less than the height of the beach overburden. The magnitude of consolidation at Hurst Spit is varied, making some areas more vulnerable to crest lowering. Vulnerability to consolidation was caused by increased substrate thickness, presence of peat and beach aspect in relation to incoming predominate storm waves which cause enhanced overtopping and landward migration of the barrier. The results highlight the need for further understanding of the consolidation of barrier beaches, especially when conducting beach maintenance which essentially realigns barrier beaches over poorly consolidated materials. The process of consolidation coupled with future sea level rise is of great interest for those involved in the management of gravel barriers in the future, especially where large areas of low-lying land and assets are protected.

More information

Identifiers

ORCID for Robert Nicholls: orcid.org/0000-0002-9715-1109

Catalogue record

Export record