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

Areas of reclaimed agricultural land reflooded due to sea wall failure (aba ndoned reclamations) are a prominent feature of estuaries in south-east Eng land. These abandonments exhibit contrasting environmental dynamics: smalle r 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 contra sting locations within the extensive abandoned reclamations of the Blyth es tuary, Suffolk. Measurements of surface elevation change, made using a 'Sed imentation-Erosion Table', show that at the most exposed location (fetch si milar to 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 e levational increase of 7-16 mm a(-1). 'Mudpod' rigs, equipped with turbidit y and pressure sensors and an electromagnetic current meter, have been depl oyed simultaneously at three stations. A two-stage sampling design is utili sed 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 frequenc y 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 s ediment, resuspension of sediment by waves and tidal currents; and particle settling as mechanisms driving surface elevation change. At the most expos ed site, the time-averaged record of suspended sediment concentration displ ays greater response to variations in local wind speed than to tidal height . This indicates the dominance of wave processes over tidal currents in gen erating sediment resuspension. The role of locally-generated wind waves is also confirmed by the higher frequency suspended sediment data. The endings of this work have important implications for the engineered restoration of saltmarsh as part of managed re-alignment of sea defences. (C) 2000 Elsevi er Science Ltd. All rights reserved.