Science of the Total Environment 371 (2006) 19–30 www.elsevier.com/locate/scitotenv Biogeochemical value of managed realignment, Humber estuary, UK ⁎ J.E. Andrews a, , D. Burgess 1, R.R. Cave 2, E.G. Coombes a, T.D. Jickells a, D.J. Parkes a, R.K. Turner b a School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK b CSERGE, School of Environmental Sciences, University of East Anglia Norwich NR4 7TJ, UK Received 9 May 2006; received in revised form 10 August 2006; accepted 12 August 2006 Available online 25 September 2006 Abstract We outline a plausible, albeit extreme, managed realignment scenario (‘Extended Deep Green’ scenario) for a large UK estuary to demonstrate the maximum possible biogeochemical effects and economic outcomes of estuarine management decisions. Our interdisciplinary approach aims to better inform the policy process, by combining biogeochemical and socioeconomic components of managed realignment schemes. Adding 7494 ha of new intertidal area to the UK Humber estuary through managed realignment leads to the annual accumulation of a 1.2×105 tof‘new’ sediment and increases the current annual sink of organic C and N, and particle reactive P in the estuary by 150%, 83% and 50%, respectively. The increase in intertidal area should also increase denitrification. However, this positive outcome is offset by the negative effect of enhanced greenhouse gas emissions in new marshes in the low salinity region of the estuary. Short-term microbial reactions decrease the potential benefits of CO2 sequestration through gross organic carbon burial by at least 50%. Net carbon storage is thus most effective where oxidation and denitrification reactions are reduced. In the Humber this translates to wet, saline marshes at the seaward end of estuaries. Cost–benefit analysis (CBA) was used to determine the economic efficiency of the Extended Deep Green managed realignment. When compared to a ‘Hold-the-Line’ future scenario, i.e. the present state/extent of sea defences in the estuary, the CBA shows that managed realignment is cost effective when viewed on N25 year timescales. This is because capital costs are incurred in the first years, whereas the benefits from habitat creation, carbon sequestration and reduced maintenance costs build up over time. Over 50- and 100-year timescales, the Extended Deep Green managed realignment scenario is superior in efficiency terms. The increased sediment accumulation is also likely to enhance storage of contaminant metals. In the case of Cu, a metal that currently causes significant water quality issues, Cu removal due to burial of suspended sediment in realigned areas translates to a value of approximately £1000 a−1 (avoided clean up costs). Although this is not formally included in the CBA it illustrates another likely positive economic outcome of managed realignment. Although we focus on the Humber, the history of reclamation and its biogeochemistry is common to many estuaries in northern Europe. © 2006 Elsevier B.V. All rights reserved. Keywords: Managed realignment; Cost–benefit analysis; Biogeochemistry; Humber estuary; Carbon; Nutrients; Metals ⁎ Corresponding author. Fax: +44 1603 591327. E-mail address: [email protected] (J.E. Andrews). 1 Current Address: Agricultural and Food Economics Division, Agriculture and Food Science Centre, Department of Agriculture and Rural Development (Northern Ireland), Newforge Lane, Belfast BT9 5PX, UK. 2 Current Address: Department of Earth and Ocean Sciences, National University of Ireland, Galway, University Road, Galway, Co. Galway, Rep. of Ireland. 0048-9697/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2006.08.021 20 J.E. Andrews et al. / Science of the Total Environment 371 (2006) 19–30 1. Introduction Habitats Directive to follow a no-net-loss policy within large designated areas (Crooks and Turner, 1999). For over 300 years European management of coastal Our recent geological/geochemical cycles based lowlands and estuaries has been dominated by land research on the Humber estuary on the east coast of reclamation and flood protection, principally through the England (Fig. 1; Andrews et al., 2000; Jickells et al., construction of ‘hard’ sea walls and the drainage of 2000) has highlighted potentially important additional wetlands. However, set against a modern backdrop of issues related to sediment storage, and carbon and nutrient globally rising sea levels and potential for increased cycling in the intertidal and salt marsh zones that are also storminess due to climatic changes (Nicholls and Klein, relevant to managed realignment. In this paper we outline 2005), the cost of maintaining and upgrading existing sea a plausible, albeit extreme, managed realignment scenario defences, many of which have reached the end of their for the UK Humber estuary, to demonstrate the maximum design life, has prompted policy makers to reconsider possible biogeochemical effects and economic outcomes their long-term cost effectiveness. The recent flooding in of estuarine management decisions. Our interdisciplinary New Orleans, for example, has served to highlight the approach aims to better inform the policy process, by vulnerability of large agglomerations of people and combining the biogeochemical and socioeconomic com- economic assets which have continued to expand behind ponents of managed realignment schemes. Although we engineered coastal defences. It has been estimated that focus here on the Humber, the history of reclamation and 23% of the world's population lives near the coast, with a its biogeochemistry is common to many estuaries in density three times higher than the global average (Small northern Europe. Moreover, a number of the issues we and Nicholls, 2003). As we better understand coastal discuss have global relevance and emphasise the virtue of dynamics, hard defences are increasingly considered adopting an ‘ecosystem services’ approach to analysis and unsustainable both from an environmental and economic policy (Daily, 1997). There is concern, for example, that perspective (Crooks et al., 2001). Not only do hard globally increased nutrient fluxes are damaging coastal defences provide a false sense of security and encourage ecosystems (Jickells, 1998). It has been estimated that development immediately behind defences, they prevent 18% of the nitrogen inputs to the Mississippi are lost a natural geomorphic response to rising sea levels, within river catchments (Donner et al., 2002) and the whereby the intertidal zone migrates landward: preven- figure may be as high as 50% for the Rhine (Billen et al., tion of this by sea walls and flood embankments results in 1991). Most of this loss occurs by denitrification in ‘coastal squeeze’ (Pethick, 2001).Forexample,onthe wetlands. In coastal seas in general, and more inshore Essex coastline of the UK, the presence of medieval to waters in particular, wetland nutrient cycling has been 19th century embankments has caused the loss of estimated to represent their most valuable environmental 40,000 ha of saltmarsh (Dixon et al., 1998). service (Constanza et al., 1997). Similarly, wetlands Emphasis is now moving toward a mixed approach to represent the largest component of the global terrestrial coastal lowland management, protecting areas of high organic carbon inventory, with tidal saline wetlands value, whilst allowing coastal processes to proceed rela- storing in excess to 45 Tg C a−1 (Chmura et al., 2003): tively unhindered elsewhere. More flexible “soft engineer- carbon burial in saline wetlands is thus potentially an ing” measures such as managed realignment help effect important sink for atmospheric CO2 (Chmura et al., 2003; this. The term ‘managed realignment’,alsoreferredtoas Choi and Wang, 2004). Decadal-scale carbon burial rates ‘managed retreat’ or ‘coastal setback’ (Reed et al., 1999), in some coastal saline wetlands are higher than millennial- involves deliberately breaching engineered defences to scale rates (Choi and Wang, 2004), possibly a function of allow the coastline to migrate to a new line of defence increased productivity, which may be linked to increased landward of the old one. Managed realignment schemes (anthropogenic) nitrogen fluxes among other factors aim to re-site defences in a manner that not only reduces the (Choi et al., 2001; Choi and Wang, 2004), illustrating a length of defence required, but also increases the area of clear link between nutrient flux and carbon sequestration. intertidal habitat. The driver for managed realignment has Evaluation of managed realignment at the scale of an in most cases been flood defence: the renewed intertidal/ estuary or administrative region using methods such as cost saltmarsh zone acting as a natural sea defence by atten- effectiveness, cost–benefit analysis and/or multi-criteria uating wind wave height and tidal amplitude (Möller et al., assessment is an urgent policy requirement. The appropri- 1999, 2001; Pethick, 2002), providing a sustainable first- ate scale for the analysis is an important factor and em- line of coastal defence. The recreation of intertidal/ phasises the need to move away from a project or scheme- saltmarsh areas also has a biodiversity value and allows based appraisal scale (Bower and Turner, 1998). The task is government compliance with the European Union (EU) not trivial requiring an appreciation of the full functioning J.E. Andrews et al. / Science of the Total Environment 371 (2006) 19 – 30 Fig. 1. Map of the Humber Estuary and Humberside