The temporal and spatial variations in the erosion thresholds of intertidal cohesive sediment, with a focus on clay mineralogy

Abstract

Temporal and spatial variations in intertidal sediment stability are fundamentally governed by the intricate physicochemical properties of cohesive sediment. To further our knowledge of the geomorphological development of these systems and their response to a changing climate, a greater understanding of this relationship between sediment properties and temporal and spatial variations in stability is required. Clay mineralogy is a key sediment property that has received little attention within estuarine erosion studies. The differing characteristic properties of the clay mineral groups are known to be a significant factor in erosion resistance in terrestrial sediment studies. Despite this knowledge there has been little consideration in assessing the significance of this within intertidal settings. Temporal and spatial variations in the surface sediment strength were measured using a Cohesive Strength Meter (CSM) alongside bulk sediment property analysis at 14 sites within four UK estuaries. The influence of clay mineralogy was assessed through a series of CSM laboratory experiments using reconstructed sediments with varying ratios of kaolinite and smectite clays. Multiple regression identified wet bulk density and organic matter (R2 0.76) and particle size, mud content, dry bulk density and porosity (R2 0.84) as the key controls on stability variations within two field study sites. Bed elevation change was identified as a further driver of variations in stability, with strong negative Spearman’s correlations of -0.727 and -0.645 at one study site and a positive correlation of 0.80 observed at a further site. The laboratory study results identify clear differences in erosion thresholds based on clay mineralogy under marine shearing fluids. The greatest erosion thresholds were observed in sediments containing the highest concentrations of smectite. Erosion thresholds reduced in a linear trend (R2 0.97) with decreasing smectite to kaolinite ratio. The use of fresh water as the shearing fluid resulted in a considerable decline in the erosion thresholds of the smectite rich sediments, highlighting the susceptibility of this clay mineral to change in water chemistry. The results of this study contribute essential information on the influence of sediment properties on sediment stability, bridging a crucial gap in the knowledge of cohesive sediment stability within intertidal locations. Furthermore the identification of a relationship between clay mineralogy and sediment stability may lead to improved identification of estuaries that are more likely to be susceptible to erosion under a changing climate.

Date of Award	Apr 2017
Original language	English
Awarding Institution	University of Brighton