In situ measurements of erosion shear stress and geotechnical shear strength of the intertidal sediments of the experimental managed realignment scheme at Tollesbury, Essex, UK

摘要

Managed realignment is one of several 'soft' engineering options which may reduce the costs of coastal defence, provide a more 'natural' response to the problem of rising sea levels and at the same time deliver environmental, specifically nature conservation, benefits. The success of this technique depends on the ability of the soils and sediments within the site to resist the erosive action of waves and tidal currents and allow sediment accretion to occur, at least at a rate equal to mean sea-level rise. Once a critical shear stress, τ_(0 crt) exerted by the moving fluids over the bed, is exceeded erosion will occur. A cohesive strength meter (CSM) and the fall-cone method were used to gather data, in situ on the strength and stability of sediments from an experimental managed realignment site and an adjacent, established saltmarsh in south-east England. Following six years of regular tidal cover, the underlying agricultural soil appeared both very strong (mean surface shear strength, τ_f = 228 kPa) and highly resistant to erosion (τ_(0 crt) = 6.23 N m~(-2)). During this period much of the site had been covered by sediment, and saltmarsh plants (Salicornia europuea) had become established above the mean high water neap tide (MHWN) level. Above MHWN level (tidal cover time < 15%) sediments had greater bulk densities and lower water contents which resulted in a moderate shear strength (τ_f =11.6 kPa) and resistance to erosion (τ_(0 crt) = 2.45 N m~(-2)). Below MHWN, where sediment accretion rates were greatest, poor consolidation resulted in very high water contents and low bulk densities. These areas were at the highest potential risk of erosion (τ_(0 crt) = 1.5 N m~(-2)) and had very low shear strengths (τ_f = 0.33 kPa). Where sediment exceeded 25 cm depth, gullies formed allowing their banks and adjacent margins to drain faster than the surrounding sediment. This led to a significant increase in bed strength (τ_f = 10.8 kPa) and stability (τ_(0 crt) = 4.3 N m~(-2)). These gullies were probably the early stages of the complex creek patterns characteristic of the adjacent, established saltmarsh. The established saltmarsh was rich in plants and had a well-developed (aggregated) soil structure. These soils had a moderate to high resistance to erosion (τ_(0 crt) = 2.45 N m~(-2)) and shear strength (τ_f = 25.6 kPa). Undrained sediment shear strength, τ_f, obtained with the fall-cone apparatus can also provide a useful indication of critical erosion shear stress, τ_(0 crt). Values of τ_(0 crt), measured across this site were all relatively large compared with computed bed stresses arising from locally generated waves. Thus this experimental managed realignment site was found to be primarily depositional and was thus successfully achieving the twin aims of protecting the coast from erosion and extending a rich ecosystem.