Flood Defence in the Blackwater Estuary, Essex, UK: The Impact of Sedimentological and Geochemical Changes on Salt Marsh Development in the Tollesbury Managed Realignment Site Y.-H. CHANGà, M. D. SCRIMSHAW , C. L. MACLEOD§ and J. N. LESTER *  Environmental Processes and Water Technology Group, T. H. Huxley School of the Environment, Earth Sciences and Engineering, RSM Building, Imperial College of Science, Technology and Medicine, London, SW7 2BP, UK àNational Center for Ocean Research NCOR), P.O. Box 23-13, Taipei 10617, Taiwan, ROC §School of Earth and Environmental Sciences, The University of Greenwich, Medway Campus, Chatham, Kent, UK Recent changes in the UK's coastal defence strategy have resulted in the introduction of Managed Realignment MR), a technique which attempts to establish salt mar- shes on low-lying coastal farmland. This work investigates the impact of MR, in particular on the interactions between sediment movement, changes in heavy metal concentrations and salt marsh development. Pre- and post- inundation samples were collected and analysed between 1995 and 1997. Sediment transport patterns 1996) dem- onstrated that sediment particles were distributed by tides around the site, resulting in a change in the spatial dis- tribution of the metals which was related to the sediment particle size distribution. Despite the presence of some metal contaminants found within the MR site, vegetated salt marsh has developed since 1997. However, heavy metals such as Cu, Mn, Ni, Pb and Zn exhibited relative depletion in the sediment developing with salt marsh in 1997, which is in agreement with data indicating that concentrations of metals within sediments is related to frequency of tidal inundation. During initial development of the site, sediment transport was the main factor con- trolling metal distribution, however, subsequently the frequency of tidal inundation became the most signi®cant factor. Further work may allow for prediction of how future MR sites will develop with respect to redistribution of sediments and subsequent transport of contaminants in the dissolved phase. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: heavy metals; coastal; marine; sediment trans- port; grain size; erosion. Introduction In temperate regions, salt marshes develop in coastal and estuarine areas, where morphological and tidal conditions are favourable, and are vegetated by herbs, grasses or low shrubs Long and Mason, 1983; Adam, 1990). Salt marshes are among the world's most pro- ductive ecosystems Howes et al., 1986) and also act as buer zones which stabilize coastal areas and in many instances protect sea walls from damage by dissipating wave energy Long and Mason, 1983; King and Lester, 1995). The management of coastal areas is therefore of great ecological and economic importance and many countries engage in strategies to re-develop salt marshes and mud¯ats with the dual functions of ¯ood defence and ecological conservation Maltby, 1986; King and Lester, 1995). Due to actual and predicted rises in sea level, some coastal areas in south-east UK are threatened by ¯ood, and increased salt marsh erosion has been extensively reported Greensmith and Tucker, 1965; Harmsworth and Long, 1986; Cundy and Croudace, 1995). This erosion is related to the subsidence of the southern North Sea basin, the eect of which is most pronounced along the Essex coast of south-east England Shennan, 1989). Traditionally, hard engineering solutions have been adopted to protect low-lying land from ¯ooding. However, there is a growing recognition of the role of Marine Pollution Bulletin Vol. 42, No. 6, pp. 470±481, 2001 Ó 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0025-326X/01 $ - see front matter PII: S0025-326X00)00186-7 *Corresponding author. Tel.: +44-0)20-7594-6014; fax: +44-0)20- 7594-6016. E-mail address: j.lester@ic.ac.uk J.N. Lester). 470