Sedimentation–erosion dynamics of abandoned reclamations: the role of waves and tides

Abstract

Areas of reclaimed agricultural land reflooded due to sea wall failure (abandoned reclamations) are a prominent feature of estuaries in south-east England. These abandonments exhibit contrasting environmental dynamics: smaller sites are observed to preferentially re-establish saltmarsh; larger sites more commonly revert to unvegetated tidal flat. This paper focuses on the relative role of waves and tides in determining sediment dynamics at contrasting locations within the extensive abandoned reclamations of the Blyth estuary, Suffolk. Measurements of surface elevation change, made using a `Sedimentation–Erosion Table', show that at the most exposed location (fetch ∼2 km) surface elevation is currently decreasing at an average rate of 10 mm a −1. More sheltered sites (fetch <0.4 km) are experiencing an elevational increase of 7–16 mm a −1. `Mudpod’ rigs, equipped with turbidity and pressure sensors and an electro-magnetic current meter, have been deployed simultaneously at three stations. A two-stage sampling design is utilised: time-averaged data (from 25 s duration, 1 Hz bursts) are acquired at a 5 min sampling interval and high-frequency data (3 min bursts at a frequency of 8 Hz) are obtained every 15 min. The time-averaged data reveal spatial and temporal variations in the relative importance of tidal advection of sediment; resuspension of sediment by waves and tidal currents; and particle settling as mechanisms driving surface elevation change. At the most exposed site, the time-averaged record of suspended sediment concentration displays greater response to variations in local wind speed than to tidal height. This indicates the dominance of wave processes over tidal currents in generating sediment resuspension. The role of locally-generated wind waves is also confirmed by the higher frequency suspended sediment data. The findings of this work have important implications for the engineered restoration of saltmarsh as part of managed re-alignment of sea defences.