J. R. French

DYNAMICS OF SEDIMENTATION IN A TIDE-DOMINATED BACKBARRIER SALT-MARSH, NORFOLK, UK

Abstract Progressive burial of artificial markers over a 5-year period is used to determine the rate and pattern of vertical accretion within a large backbarrier salt marsh on the UK east coast. Over this period, annual accretion varies spatially from 1 to 8 mm yr −1 . The arithmetic mean rate for the whole marsh is 3.9 mm yr −1 . Spatial variability in accretion is a joint function of (1) elevation-dependent inundation frequency and (2) progressive sediment removal from water masses advected across channel margins. Accretion is, therefore, inadequately represented by simple averaging of point measurements. Numerical integration of the ‘accretion surface’ results in a spatial average rate of around 3 mm yr −1 , well below the arithmetic mean rate. Short-term sediment trap deployments show that local and long-range meteorological effects, and remobilisation of sediment deposited within tidal creeks, often mask the expected link between tidal height and sedimentation rate. Retention of sediment on plant surfaces is minimal, with direct settling accounting for approximately 95% of total deposition. Time-extrapolation of weekly sediment trap data, and comparison with the 5-year marker horizon burial, shows that processes associated with ordinary tides can account for long-term accretion over most of the marsh. However, the highest surfaces receive appreciable sediment input only during aperiodic storm events.

Vertical accretion versus elevational adjustment in UK saltmarshes: An evaluation of alternative methodologies

Abstract Simultaneous measurements of vertical accretion from marker horizons and marsh-elevation change from sedimentation-erosion tables (SET) were made in selected marshes along the East Anglian coast of the UK in order to address the following objectives: (1) to ascertain the validity of treating accretion measurements obtained within tidally dominated, minerogenic saltmarshes as equivalent to surface elevation changes; (2) to explore the implications, in terms of physical and biological processes, of discrepancies between separately measured vertical accretion and elevation change within contrasting marsh types. Data were collected from several marsh environments at Scolt Head Island and Stiffkey on the North Norfolk coast and at an experimental managed realignment project near Tollesbury, Essex. Scolt Head Island was selected for its long-term datasets of marsh accretion, Stiffkey for its contrasting open coast-back barrier settings, and Tollesbury for its experimental management, in order to illustrate the potential application of the SET method and evaluate the relationship between vertical accretion and elevation change in a variety of marsh settings. The relationship between vertical accretion and elevation change varied widely among marsh settings of different age and height (within the tidal frame) at Scolt Head Island and Stiffkey. Rates of vertical accretion and elevation change were similar in the older and midheight settings on Scolt Head Island, indicating control of elevation change by surface accretionary processes (e.g. sediment deposition). However, subsurface processes controlled elevation at three of the marsh sites. Spartina Marsh, the youngest and lowest of the back barrier settings at Scolt Head Island, exhibited continuous shallow subsidence (vertical accretion greater than elevation change) over a 4-year period, implying that compaction controls elevation change. In the upper part of Hut Marsh and the interior of the Stiffkey marshes, elevation change exceeded vertical accretion suggesting that subsurface processes (e.g. organic accumulation) controlled elevation in these settings. Surface accretionary processes control elevation change in both the highly dynamic, outer marsh at Stiffkey and the low, restored marsh at Tollesbury. Despite the occurrence of shallow subsidence, all sites gained elevation at an annual rate comparable to that of sea-level rise. In summary, the SET provides the means to critically evaluate the influence of vertical accretion measures on elevation and represents an improved method by which to evaluate the vulnerability of a marsh to sea-level rise.

Assessing seasonal vegetation change in coastal wetlands with airborne remote sensing: an outline methodology

Airborne remote sensing offers high-density, spatially-averaged synoptic samples of radiation reflected from a surface which can be used to infer ecological processes and community composition. Unfortunately, the cost of the overheads in terms of time and resources, and scheduling constraints when deriving quantitative information have limited its use in operational monitoring programs. When the changes in reflected radiation related to the phenomenon under investigation are large, only minimal low-cost processing is required to identify ecosystem modification if high frequency multi-temporal imagery can be combined with expert knowledge. This methodology is likely to be of particular value in coastal environments where access for conventional measurements is restricted and changes in sensitive tidal wetlands may provide an early indicator of changing natural processes and/or human impact. Regularly acquired airborne remotely sensed imagery has been used in a reconnaissance study to identify and map seasonal vegetation patterns on intertidal surfaces in back barrier environments on the North Norfolk coast, England, U.K. The development of an operational system for coastal zone management is discussed.

Characteristics and ‘event-structure’ of near-bed turbulence in a macrotidal saltmarsh channel

Measurements made within a macrotidal marsh channel using a two-component electromagnetic current meter offer a fundamentally different perspective on the nature and range of fluid motions to that provided by conventional impellor-type sensors. Conditional sampling of near-bed streamwise shear stress series reveals a close match between the physical characteristics of turbulence recorded here, and in previous work in contrasting environments using very different instrumentation. However, conceptual models of turbulent structure derived from laboratory experiments require modification for highly unsteady tidal flows. Firstly, the streamwise turbulent shear team is only one of several terms within the Reynolds stress tensor. Within the channel flows studied here, large normal and mixed cross-product stress terms may be important in maintaining fine cohesive sediment suspension during the flood tide, and in the entrainment of sand during the ebb. Secondly, marked variations in near-bed ‘event-structure’ are associated with large-scale mean flow transients. These are characteristic of meso- and macrotidal marsh channels, and exhibit acceleration terms an order of magnitude larger than those commonly reported for marine boundary layers. Temporal lag effects associated with the decay and production of turbulence provide a possible mechanism for opposing residual transports of mud and sand. Furthermore, they result in a breakdown of empirical ‘burst’ scaling relations devised for steady flows. Identification and scaling of intermittent stress phenomena are also complicated by the lack of consensus on appropriate criteria for the discrimination of coherent motions (events) from background noise.

Capturing coastal geomorphological change within regional integrated assessment: an outcome-driven fuzzy logic approach.

Abstract Climate change will have pervasive effects on the worlds coasts, but at broad scales these changes have typically proven difficult to analyse in a systematic manner. This paper explores an outcome-driven deductive methodology for geomorphological analysis that recognises the nonlinearity of coastal morphology and organises current knowledge and understanding using fuzzy logic concepts. Building on recent large-scale coastal investigations and with reference to a case study of the East Anglian coast, U.K., the methodology defines the active coastal system using a flexible generic classification and integrates expert opinion, using the notion of possibility, as a basis for the assessment of potential future geomorphological response to changes in sea level and sediment supply. Preliminary results for the East Anglian coast suggest that the constraining of the active coastal system by sea defences is already having, and will continue to be, a significant influence on coastal evolution irrespective of the rate of sea-level rise. Therefore, significant potential exists to guide future coastal evolution toward preferred outcomes by using this approach as a component of adaptive shoreline management. This methodology could be applied to a wide range of problems both in geomorphology and other subjects.

Coasts and climate: Insights from geomorphology

Geomorphology is increasingly engaged with the connections between coastal behaviour and climate variability and change. While impacts of climate change at the coast are often primarily viewed in terms of landform adjustments to accelerated sea-level rise, geomorphologists are also starting to unlock the subtleties of how coastal processes are forced by a broader suite of climate factors. This progress report highlights three main strands of recent geomorphological research in this vein: the search for a broader suite of climatic signatures in recent coastal deposits; empirical analyses of the linkages between climate variables and contemporary shoreline change; and enhancement of our capability to predictively model climate-driven changes in coastal morphology.

Travelling forelands: complexities in drift and migration patterns

ABSTRACT Burningham, H., French, J.R., 2014. Travelling forelands: complexities in drift and migration patterns. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 102–108, ISSN 0749-0208. Cuspate forelands have been described from a range of shorelines around the world, but in the majority of cases, the foreland maintains a constant position relative to the neighbouring shoreline. Here, we describe the contemporary geomorphology and historical evolution of a small cuspate foreland on the Suffolk coast, UK, which has been migrating northward for several centuries. Benacre Ness, a mixed sand and gravel sedimentary accumulation, is currently located at Kessingland, 5 km to the north of Benacre, from which it gained its name when adjacent to it in the 19th century. The foreland was previously called Covehithe Ness, having been adjacent to Covehithe (3 km south of Benacre) early in the 19th century. Previous sediment transport experiments and modelling studies have demonstrated a net southerly transport direction on this coastline, yet the foreland has continued to migrate northward over several centuries. Local reversals in sediment transport direction and rates are likely responsible for the northward migration of the foreland, but substantial changes in behaviour over the last 400 years suggest a close relationship between foreland dynamics and coastal configuration.

Morphodynamics and Sediment Flux in the Blyth Estuary, Suffolk, UK

Research into the dynamics of estuary morphology has been stimulated by increasing commercial, environmental and legislative pressures and by the accumulated impacts of human intervention (Roman and Nordstrom, 1996; Soulsby, 1997). Of particular concern in the UK is the impact on estuaries of sea-level rise and large-scale interventions associated with dredging and port development, flood defence and habitat restoration. Prediction of estuary response to such changes requires an understanding of present-day processes and their interaction with morphologies that are often shaped by past human activities. As Pye and Allen (2000) note, estuarine research has hitherto been characterised by narrow disciplinary foci, such that research fronts in engineering, geomorphology and Quaternary science have rarely converged. The UK Estuaries Research Programme (EMPHASYS, 2000; French et al., 2002) has advocated a more holistic and interdisciplinary perspective, combining ‘bottom up’ studies of short-term hydrodynamics and sediment movement with ‘top down’ models of larger-scale morphodynamic behaviour, such that the predictive power of physically-based simulation may be realised within a conceptual framework provided by geomorphological analysis of longer-term sedimentary function. Few estuaries have been monitored in the spatial and temporal detail needed for integrated modelling of this kind and there is a need for intensive studies encompassing a greater variety of natural and anthropogenic contexts.

Improving decadal coastal geomorphic predictions: an overview of the iCOASST project

Coastal areas are already at high risk from a range of geohazards. The cumulative effect of human intervention on soft coastlines has frequently left them far from equilibrium under today’s conditions, especially in densely populated areas. Future changes in marine forcing due to climate change reinforce the need to understand and predict processes of change in shoreline position and configuration at management (decadal) scales. The UK-based iCOASST project is developing new and improved methods to predict decadal geomorphic evolution, linked to coastal erosion and flood risk management. This is based on a framework that links several components to develop a system-level understanding of this change. The framework includes: (1) new methods for system-level analysis and mapping of coast, estuary and inner shelf landform behaviour; (2) well validated ‘bottom-up’ hydrodynamic and sediment transport shelf models which can be applied at shelf scales to investigate inner shelf-coastal interactions; and (3) model compositions formed of existing or new ‘reduced complexity models’ of selected coastal landforms and processes that are suitable for multiple decadal length simulations. This will ultimately allow multiple simulations of coastal evolution which can explore uncertainties in future decadal-scale coastal response, including the effects of climate change and management choices. This paper outlines the current state of progress in the iCOASST Project.

Implementation of a 3D ocean model to understand upland lake wind-driven circulation

A community numerical ocean model is used to extend the understanding of wind-driven circulation in small upland lakes. A 3D model of a case study lake (Llyn Conwy, Wales, UK) is calibrated against measured velocity profiles via adjustment of the bottom roughness coefficient. Validation against a separate set of measured velocity profiles confirms the ability of the model to resolve key features of the flow field. Sensitivity analysis shows that the velocity field responds rapidly to changes in the wind forcing. Analysis of the gross circulation using Empirical Orthogonal Functions reveals a persistent two-gyre circulation pattern in the upper half layer of the water column driven by the interaction of wind and bathymetry. At the bottom, the flow is characterised by locally strong currents and analysis of vertical circulation over short time scales shows strong currents in the deepest parts of the lake basin and the responsiveness of the water column to changes in wind speed and direction. Even in small lakes, the assumption of uniform wind stress across the water surface is not always justified and topographic sheltering or other catchment roughness effects give rise to heterogeneity in the wind field. An idealized experiment for the case study lake shows that differences in circulation emerge if the wind stress is allowed to vary across the lake. Energetic wind forcing in upland areas can drive an energetic lake circulation that has important implications for mixing and sediment dynamics. 3D numerical modelling of wind-driven circulation should be more widely used to provide insights into physical limnology to support a wide range of ecological, biogeochemical and palaeoenvironmental studies.