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

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, s0 crt exerted by the moving fluids over the bed, is exceeded erosion will occur. A cohesive strength meter (CSM) and the fallcone 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, sf ¼ 228 kPa) and highly resistant to erosion (s0 crt ¼ 6:23 N m 2). During this period much of the site had been covered by sediment, and saltmarsh plants (Salicornia europaea) 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 ðsf ¼ 11:6 kPaÞ and resistance to erosion ðs0 crt ¼ 2:45 N m 2Þ. 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 ðs0 crt ¼ 1:5 N m 2Þ and had very low shear strengths ðsf ¼ 0:33 kPaÞ. 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 ðsf ¼ 10:8 kPaÞ and stability ðs0 crt ¼ 4:3 N m 2Þ. 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 ðs0 crt ¼ 2:45 N m 2Þ and shear strength ðsf ¼ 25:6 kPaÞ. Undrained sediment shear strength, sf, obtained with the fall-cone apparatus can also provide a useful indication of critical erosion shear stress, s0 crt. Values of s0 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.