Large-scale Coastal Behaviour Research Articles

Equilibrium shoreface profiles: A sediment transport approach

Large-scale coastal behaviour models use the shoreface profile of equilibrium as a fundamental morphological unit that is translated in space to simulate coastal response to, for example, sea level oscillations and variability in sediment supply. Despite a longstanding focus on the shoreface profile and its relevance to predicting coastal response to changing environmental conditions, the processes and dynamics involved in shoreface equilibrium are still not fully understood. Here, we apply a process-based empirical sediment transport model, combined with morphodynamic principles to provide new insight into equilibrium shoreface profile development and shape. The quantitative model is based on field measurements and shows that a balance between onshore sediment transport due to oscillatory wave motion, and offshore sediment transport due to gravity, produces profile shapes that are similar to natural shoreface profiles from the Danish North Sea coast and the southeast Australian coast. Model predictions of equilibrium profile response to the effects of climate change, such as rising sea level, or changing wave climates, are examined. Implementation of the model is straightforward; there is no tuning or calibration and computation times are short. It is therefore easily implemented with repeated iterations to manage uncertainty.

Directional wave climate and power variability along the Southeast Australian shelf

Variability in the modal wave climate is a key process driving large-scale coastal behaviour on moderate- to high-energy sandy coastlines, and is strongly related to variability in synoptic climate drivers. On sub-tropical coasts, shifts in the sub-tropical ridge (STR) modulate the seasonal occurrence of different wave types. However, in semi-enclosed seas, isolating directional wave climates and synoptic drivers is hindered by a complex mixed sea-swell environment. Here we present a directional wave climate typology for the Tasman Sea based on a combined statistical-synoptic approach using mid-shelf wave buoy observations along the Southeast Australian Shelf (SEAS). Five synoptic-scale wave climates exist during winter, and six during summer. These can be clustered into easterly (Tradewind), south-easterly (Tasman Sea) and southerly (Southern Ocean) wave types, each with distinct wave power signatures. We show that a southerly shift in the STR and trade-wind zone, consistent with an observed poleward expansion of the tropics, forces an increase in the total wave energy flux in winter for the central New South Wales shelf of 1.9GJm–1 wave-crest-length for 1° southerly shift in the STR, and a reduction of similar magnitude (approximately 1.8GJm–1) during summer. In both seasons there is an anti-clockwise rotation of wave power towards the east and south-east at the expense of southerly waves. Reduced obliquity of constructive wave power would promote a general disruption to northward alongshore sediment transport, with the cross-shore component becoming increasingly prevalent. Results are of global relevance to sub-tropical east coasts where the modal wave climate is influenced by the position of the zonal STR.

20th century sediment budget trends on the Western Gulf of Lions shoreface (France): An application of an integrated method for the study of sediment coastal reservoirs

This paper presents a shoreface sediment budget established for the 20th century (1895–1984–2009) along the microtidal wave-dominated coast of the western Gulf of Lions (Languedoc-Roussillon, Mediterranean Sea, SE France). The implementation of a diachronic bathymetric approach, coupled with the definition of sand reservoirs (upper sand unit — USU) by very high-resolution seismic surveys and the results of LiDAR investigations, offers a new means of defining precisely the magnitude and change trends of the sediment budget. The aim of this study is to link the Large Scale Coastal Behaviour (LSCB) of the littoral prism (expressed in terms of shoreface sediment budget, shoreface sediment volume and spatial distribution pattern of cells) to climatic change, river sediment input to the coast, longshore sediment transport distribution, impact of hard coastal defence structures and artificial beach nourishment. The results show a significant reduction of the volume of the western Gulf of Lions littoral prism over 114years (−26.1±4.6×106m3). From 1895 to 1984, the overall budget is slightly positive, with a volume estimated at 4.1±3.5×106m3. For 1984–2009, however, the estimated sediment budgets clearly indicate that erosion is dominant over the last 25years, with a volume loss of −30.2±4.2×106m3. In relation to the long-term sediment budget and longshore drift pattern, the long-term trend of the USU volume distribution displays strong spatio-temporal contrasts linked to longshore sediment drift, spatial distribution of fluvial sediment inputs and hard engineering structures. Locally, the sedimentary reservoir is significantly eroded within a century (−80% of USU), since the initial amount present was low and not sustainable. The emphasis is on the importance of considering the volume changes of available sediment reservoirs rather than their losses and gains. Erosion of the Languedoc-Roussillon shoreface is likely to continue in the future due to the “natural” decrease of river sediment input and the sand removal for human purposes. Consequently the littoral sand prism results in sedimentary reservoirs that are gradually being used up.

Assimilation of Airborne Imagery with a Wave Model for Bathymetric Estimation

One of the most useful survey methods in nearshore studies is airborne light detection and ranging (LIDAR), which is able to densely sample topographic and shallow bathymetric elevation data over large geographic regions. Airborne LIDAR bathymetry systems are dependent on water clarity, but in the surf zone sediment and air bubbles entrained in the water column by wave breaking attenuate the laser pulse and compromise the LIDAR's ability to retrieve accurate bottom elevations. Data assimilation techniques can improve the ability of LIDAR systems to estimate bathymetry inside the surf zone. The assimilation methods are based on comparing pixel intensity patterns (scaled by offshore wave energy flux) extracted from time-averaged airborne imagery with dissipation profiles produced by a simple wave-energy transformation model. The subaerial topography and the offshore bathymetry are assumed known and an initial featureless bathymetry is assumed in the surf zone (where the data are missing). Differences between modeled dissipation and observed image pixel intensity patterns can be minimized by incrementally modifying the bathymetry. Final assimilated bathymetry estimates are compared with surveyed bathymetric data collected at the U.S. Army Corps of Engineers Field Research Facility in Duck, NC using traditional surveying methods. Analysis of data from three aerial overflights produced average root mean square differences between assimilated and surveyed bathymetry of 25–35 cm, similar to results from land-based systems. This methodology can be used to improve LIDAR-derived profiles where large gaps exist because of surf that attenuates the laser pulses, and allow for more complete evaluation of large-scale coastal behavior that includes profile evolution within the surf zone.

Delta lobe degradation and hurricane impacts governing large-scale coastal behavior, South-central Louisiana, USA

A large deficit in the coastal sediment budget, high rates of relative sea-level rise (~0.9 cm/year), and storm-induced current and wave erosion are forcing barrier shoreface retreat along the periphery of the Mississippi River delta plain. Additionally, conversion of interior wetlands to open water has increased the bay tidal prism, resulting in degradation of barrier islands due to inlet widening, formation of new inlets, and sediment sequestra- tion at ebb-tidal deltas. Single-beam bathymetric surveys along a 165-km stretch of south-central Louisiana barrier coast, from Raccoon Point in Terrebonne Parish to Sandy Point in Plaquemines Parish, were conducted in 2006. These data, combined with historical bathymetry from three time periods (dating to the 1880s), provide a series of digital elevation models that were used to calculate sediment volumetric changes and determine long-term erosional-depositional trends. Dominant patterns during the 125-year period include (1) erosion of ~1.6×10 9 m 3 from the shoreface, forcing up to 3 km of shoreface retreat, (2) sediment deposition in coastal bights and at ebb-tidal deltas, and (3) a combined increase in tidal inlet cross- sectional area from ~41,400 m 2 to ~139,500 m 2 . Bathy- metric and shoreline change datasets separated by shorter time periods (sub-annual) demonstrate that these long-term trends are driven by processes associated with major hurricane impacts, and that rates of shoreface erosion are an order of magnitude greater during active hurricane seasons compared to long-term trends.

Connecting large-scale coastal behaviour with coastal management of the Rhône delta

The aim of this paper is to connect the Large Scale Coastal Behaviour (LSCB) of the Rhône delta (shoreface sediment budget, river sediment input to the beaches, climatic change) with the impact and efficiency of hard engineering coastal structures. The analysis of the 1895 to 1974 bathymetric maps as well as 2D modelling of the effect of wave blocking on longshore transport allows us to draw up a conceptual model of the LSCB of the Rhône delta. The river sand input, settled in the mouth area (prodeltaic lobe), favours the advance of adjacent beaches. There is however a very weak alongshore sand feeding of the non-adjacent beaches farther off the mouth. After a mouth shift, the prodelta is eroded by aggressive waves and the sand is moved alongshore to build spits. This conceptual model suggests that there is a “timeshift” between the input of river sediments to the sea and the build up of a beach (nonadjacent to the mouth). Nowadays, as the river channels are controlled by dykes and human interventions, a river shift is not possible. It thus appears unlikely that the river sediments can supply the beaches of the Rhône delta coast. Under these conditions, we must expect that the problems of erosion will continue at Saintes-Maries-de-la-Mer and on the Faraman shore, in areas with chronic erosion where the shoreline retreat has been partially stopped by hard engineering practices in the 1980s. Therefore, these artificially stabilised sectors remain potentially under threat because of profile steepening and downdrift erosion evidenced in this paper by bathymetric profile measurements. In the long-term (1905 to 2003), the temporal analysis of the storm surges and the sea level show very weak but reliable increasing trends. Thus, these climatic agents will be more aggressive on the beaches and on the coastal structures calling their efficiency into question. We also evidence that the hard engineering structures were built in a favourable climatic context during the 1980s meanwhile the storm surges and the sea-level rise are stronger since the 1990s. Regarding to the LSCB of the Rhône delta, and the impact of hard engineering coastal structures, we suggest that classical hard coastal protections are not the best option to protect the coast.

The sediment response of a dissipative beach to variations in wave climate

Using wave and wind data, topographic survey data, and a robust video record, we have quantified the Large Scale Coastal Behavior (LSCB) of Agate Beach, a dissipative end member beach in the Pacific Northwest USA. Nine years of study (1992–2001) contain important observations of the intra-and interannual sediment response under variations in wave climate. In order to describe the variability of the nearshore system, wave forcing and beach response were modeled as annually periodic functions superimposed upon long-term trends. The amplitudes of the seasonal periodicity in significant wave heights ( A H s =0.94 m±0.06), dominant wave period ( A T p =2.1 s±0.1), and mean wave direction ( A θ =12.3°±2.0) exhibit larger variability than the long-term trends observed within a year ( β H s =6.7 cm/year±2.6, β T p =0.15 s/year±0.04, β θ =3°S/year±1.0). Assuming alongshore transport of sediment at Agate Beach is mostly wave-driven, and that the northward flux of sediment goes to zero at the northern end of the beach, the observed long-term increase in significant wave heights ( β H s ) and change to a more southerly approach in wave direction ( β θ ) (coincident with the 1997–1998 El Niño/1998–1999 La Niña sequence) result in net accretion along Agate Beach. Beach survey data collected by RTK-GPS and lidar quantify the increase in subaerial sediment volume, Δ V b=1.84×10 5 m 3. This change in volume corresponds to a mean bed elevation increase of 0.21 m over the 8.7×10 5 m 2 area of study. The amplitude of annual variation in sediment volume was A V b =7.85×10 4 m 3±2.13×10 4 m 3. Video image analysis (1992–2001) also quantifies a long-term offshore migration of the outer sand bar ( β OB x =11.0 m/year±0.8) coincident with the increase in subaerial beach sediment volume at the northern end of the Newport littoral cell. This result suggests accretion of sediments in a much wider cross-shore region than observed within the beach survey record. Similar to the signature of beach volume variations, the cross-shore position of the outer sand bar also varies with season ( A Oβ x =114.9 m±4.2). The agreement between long-term trends in wave statistics and morphology suggests a directly forced beach response. An empirical orthogonal function (EOF) analysis of the topographic survey record at Agate Beach resolves two distinct eigen-modes of variance associated with seasonal patterns of sediment flux. The first mode (34%) is related to the summer growth of a dune field that is limited to elevations above MHW, z=1.076 m. Analysis of concurrent wind field measurements shows this mode of variance is well correlated with aeolian processes. The second mode (21%) is wave-driven, and corresponds to the seasonal behavior of the beach surface below MHW. Observations show the MHW elevation serves as a transitional zone between dune related and wave-driven processes that affect the seasonal evolution of Agate Beach.

Coastline evolution at different time scales – examples from the Pomeranian Bight, southern Baltic Sea

Sedimentological and morphological changes on the upper and lower shoreface during relatively stable sea-level highstand conditions have been investigated in the Pomeranian Bight, southern Baltic Sea, at time scales ranging from storm events to millennia. In order to cover that variety of time scales, different methods have been applied. Seasonal variations in the morphology of the upper shoreface were measured accurately using the tracer stick method. The ratio of breaking waves and energy dissipation due to wave breaking are the main forces controlling redeposition on the upper shoreface with the depth of disturbance up to three times the net change. The impact of single storm events can be observed from sidescan sonar mosaics to remain on the decadal scale. Aerial photographs covering the upper shoreface show that the location of gates, channel-like systems where water masses move offshore created during storm events, also remain stable over decades. Sedimentological and geomorphological variations and changes on the lower shoreface are only measurable on the century to millennium scale because the main driving forces are long-lasting processes like sea-level fluctuations or neotectonics. Data on these scales have much more uncertainty in their relationship to forcing functions than data at shorter time scales. Because the effects of coastal processes active on different time scales can interact, comprehensive understanding of large-scale coastal behavior requires investigations from short events to long-term processes.

Predicting coastal evolution using a sediment budget approach: a case study from southern England

Despite the fact that coastal evolution occurs over a range of temporal and spatial scales, most existing modelling tools for predicting coastal evolution are founded on the science of the small-scale coastal processes. This paper discusses the role of a variety of approaches to predicting coastal evolution and considers that the ability to predict long-term (e.g. 50–100 years plus) larger-scale changes in coastal systems is of paramount importance to coastal managers if the ultimate aim of developing long-term sustainable management of the coast and its resources is to be achievable. The advantages of a sediment budgetary approach to predicting coastal evolution are discussed and it is suggested that such an approach is better suited to predicting long-term and large-scale coastal behaviour than existing hydrodynamic and physical models. The procedure for developing a conceptual sediment budget model for predicting coastal evolution is outlined, based upon the identification of littoral cells, and consideration of sediment sources, throughputs and sinks. This modelling procedure is then applied to a “real case” coastal unit located in central southern England, namely Poole Bay, in order to exemplify its benefits and the type of results achievable. From the model, it is suggested that the most appropriate strategy for Poole Bay is one of continued beach replenishment, with maintenance of the existing seawall and groyne defences.

Simulation of large-scale coastal change using a morphological behaviour model

Quantitative simulation of large-scale coastal behaviour (LSCB) is possible using a model based only on principles of sand-mass conservation and geometric rules for shoreface and active sand-body morphology. Dynamic processes are subsumed into a set of parameters that define the geometry of the active cross-shore profile which in turn can represent an entire coastal cell. The parameters, together with a set of local behaviour rules governing adjustments at discrete points along the profile, provide a simple formulation that is constrained by sediment mass balance to yield emergent, numerical solutions for profile kinematics. The emergent qualities of this shoreface-translation model are exemplified in simulation experiments on changes to sand deposits on the southeastern Australian coast and shelf during the post-glacial marine transgression. In another example, the model is used to simulate LSCB over engineering time scales at an idiosyncratic site. Both examples illustrate the over-riding importance of small residual sand movements that accumulate through time to cause LSCB. In comparison to gross sand movements, the residuals are too small to measure directly or predict on the basis of available theory. Exploratory (inverse) simulation provides a quantitative method for side stepping these limitations. The shoreface-translation model therefore is used as a tool to explore possible forms of coastal behaviour through an approach that incorporates available data along with theoretical considerations. Validation is not strictly possible for this type of inverse approach, so the degree to which a simulation reproduces observed features in nature is a measure of its success.

Longshore sand waves at Southampton Beach, New York: observation and numerical simulation of their movement

This paper describes measurements and numerical simulations of the movement of longshore sand waves observed at Southampton Beach, Long Island, New York. These large morphologic features are created by periodic opening of a small inlet and subsequent welding of its ebb shoal to the beach. Longshore sand waves are wave-like forms that maintain identity while moving along the shore and represent a simple type of collective sediment movement or largescale coastal behavior. Despite being described as early as 1939, few measurements of longshore sand waves have been made, and their dynamic behavior is not well understood. Eleven longshore sand waves were identified along Southampton Beach and have an average length of 0.75 km and an amplitude of 40 m. Migration speed was greatest under winter wave action and reached an average 1.09 km year−1 and yearly average speed including summer calms was a much lower 0.35 km year−1. During a 16-month observation period, the longshore sand waves did not disperse during their steady migration westward in the direction of predominant wave incidence, and longer waves moved faster than those with smaller wavelengths. A numerical model of shoreline change including both standard particulate longshore sand transport and a component describing migration of a longshore sand wave is developed in which the velocity of the wave is related to the calculated longshore discharge of water. The model shows wave refraction to be an important mechanism contributing to the longevity of longshore sand waves, acting to retard the otherwise expected diffusion. Model calculations of the migration of a single longshore sand wave agree quantitatively with the observations.

Extracting decadal morphological behaviour from high-resolution, long-term bathymetric surveys along the Holland coast using eigenfunction analysis

The understanding and prediction of coastal behaviour over a time span of decades and alongshore stretches of tens of kilometres is important in coastal management. The controlling variables for coastal behaviour on these scales are yet unknown. It is hypothesised that regions exhibiting different large-scale coastal behaviour (LSCB-regions) are controlled by different hydrodynamic forcings, or by different sedimentological or morphological constraints, or by some combination of these factors.The objective of this study is to elaborate a method that effectively quantifies large-scale morphological changes from a large bathymetric data set (large temporal and spatial extent combined with a high resolution), such to define LSCB-regions. The method of analysis consists of a combination of the empirical eigenfunction technique and a moving window approach.The method has been applied on a data set of over 14,000 nearshore profiles. The data set consists of yearly soundings of the nearshore profile along a coastal stretch of 115 km length (the Holland coast). The alongshore profile spacing is generally 250 m. All profiles surveyed in the period 1963–1990 are included in the analysis.Application of the method on the Dutch data set reveals that LSCB-regions indeed can be identified. The sizes of the LSCB-regions are 5 km, 15 km, 20 km, 32 km, and 42 km. Differences in behaviour of the ‘secondary’ morphology (i.e. morphology other than the mean profile shape, e.g. nearshore bars) are most pronounced. The change in behaviour between LSCB-regions generally occurs over distances as short as about 2 km. The boundaries between the regions are therefore surprisingly sharp.In general, no clear long-term trends in the development of the shape of the sub-aqueous profile have been observed along the Holland coast. Along certain stretches, trends in cross-shore shifting of the sub-aqueous profile occurred. The average shape of the sub-aqueous profile seems to have no relation with the trend in cross-shore shifting of the shoreline (the +1 m-contour). The (sub)decadal shoreline fluctuations, however, are possibly related to the largescale dynamics of multiple bar systems.

Prediction of cross-shore sediment transport at different spatial and temporal scales

Coastal sediment transport and associated morphological change can be examined at many spatial and temporal scales. These scales range from instantaneous movement of single grains on a time scale of the local turbulence to seasonal and long-term movement of large sand bodies such as longshore bars and tidal deltas. Researchers have begun to recognize that a first-principles (microscale) approach to calculating long-term and large-scale (megascale) coastal change may not be possible or even adequate. Therefore, new approaches are being sought to advance quantitative understanding and prediction of large-scale coastal behavior. The present paper opens with a discussion of compatible spatial and temporal scales for calculating sediment transport and morphology change. Motivation is given for the need to calculate morphologic change at scales greater than the typical one of microscale. Example calculations are made at different scales, showing the assumptions and approximations of each. The main conclusion is that calculations at different scales can be related and reconciled if limitations in prediction of initial and boundary conditions and in the fluid forcing are recognized.

La cartographie prospective de l’évolution du trait de côte : un instrument indispensable pour l’aménagement des espaces littoraux

The processes at work in coastal waters interact and shape the shore. In the last decades, important progress has been made in the analysis of nearshore dynamics and the resulting forms. However, if instrumentation, quantification, and modelling are very helpful for the understanding of short term and local effects, we must admit that our knowledge is still insufficient to use this approach for the prediction of large scale coastal behaviour with the required confidence. Large scale coastal behaviour is defined as changes over a period of several tens of years and over a longshore distance corresponding to a coastal segment considered as an independant morpho-sedimentary unit The interest in large scale coastal behaviour is increasing because it is of utmost importance for management to know which long term trend has to be expected in shoreline evolution. Such kind of approach implies analysis of maps, charts, and aerial photographs. The information has an empirical character which cannot only be used to define historical tendencies but can also be used directly to forecast future changes. According to such a predictions model, setback lines may be established taking into account the expected life time of buildings. Doing so, the development of a retreating coast will not be soon followed by the well know "erosion problem" necessitating more and more elaborate, expensive, and too often unsightly defensive structures.

Shoreface translation model: Computer simulation of coastal-sand-body response to sea level rise

The response of coastal sand bodies to sea level rise can be modelled using computer simulation of large scale coastal behaviour based only on principles of sand-mass conservation and geometric rules for shoreface and barrier morphology. The model simulates horizontal and vertical translation of coastal sand bodies over the pre-existing coastal substrate which undergoes reworking as a consequence. This produces changes in position of the coastline as well as reconfiguration of the backshore and inner-continental shelf morphology and stratigraphy. Application of the model is illustrated with examples from mineral exploration of the continental shelf and assessment of coastal erosion risks in the face of sea-level rise due to the enhanced greenhouse effect.

Mathematical modelling and large-scale coastal behaviour

The paper addresses the possible role of mathematical models in the prediction of large-scale coastal behaviour (LSCB). Various large-scale model concepts are considered, and the research needed for their further development is identified. The overall conclusion is, that mathematical models offer good perspectives to the quantitative prediction of large-scale coastal dynamics, especially if they are supported by laboratory and field measurements.

Mathematical modelling and large-scale coastal behaviour

The paper addresses the possible role of mathematical models in the analysis of large-scale coastal behaviour (LSCB). The constituent processes (waves, currents, sediment transport) and their interaction with the bottom topography changes (morphodynamics) are considered at various space and time scales. The overall conclusion is, that mathematical modelling has much to add to the analysis and interpretation of large-scale coastal dynamics, especially if it can be integrated with laboratory and field measurements.