Onshore Migration Research Articles

Onshore Migration of Emerged Ridge and Ponded Runnel

AbstractRidge-runnel features can comprise large volumes of sand, and their migration can have a significant influence on the coastal sediment budget and beach recovery after a storm. Numerical modeling of ridge-runnel migration is challenging partly because of the strong interactions between the hydrodynamics and morphology and partly because of limited available field and laboratory measurements. This study investigated the effect of water ponding and runnel drainage on onshore ridge migration in an experiment in a sand flume where detailed free‐surface and velocity measurements were taken in the ponded water zone of the runnel and in the intermittently wet and dry zone on the ridge crest. The test scenario with a drained runnel showed a ridge migration speed five times larger than the scenario in which water and sediment could only exit the runnel as offshore return flow over the ridge. The time-averaged numerical model CSHORE was modified to include the ponded water effect in the sediment transport fo...

Numerical Modeling of Breaking Waves and Cross-Shore Currents on Barred Beaches

A process-based numerical model has been used to study nearshore hydrodynamics on barred beaches. A laboratory experiment with an offshore bar migration case followed by an onshore bar migration is simulated. A new mechanism is incorporated in the model accounting for the breaking-wave persistence improving model performance for wave properties, particularly in the bar-trough region. A cross-shore variation of the breaking-wave parameter is introduced to the dissipation model. The effects of the surface shape parameter and a roller lag on radiation stresses and mean water-level predictions are investigated and found to improve the mean water-level predictions. The cross-shore variation of the momentum balance is presented to illustrate the variation of forcing for the undertow current. The persistence length and the roller lag mechanisms are shown to be important for predictions of the undertow currents on barred beaches and the current predictions are improved once both methods are used.

Energetics of swimming to shore in the puerulus stage of a spiny lobster: Can a postlarval lobster afford the cost of crossing the continental shelf?

Nonfeeding postlarval pueruli of spiny lobsters migrate tens of kilometers across the continental shelf to settle in coastal waters. A model that analyzes hydrodynamic forces during swimming in the puerulus of Jasus edwardsii is described. The model calculates the speed at which forward propulsion balances drag. Calculated speeds agree with observed puerulus behavior. The computed mechanical work is converted to metabolic energy consumption using an assumed efficiency. Values concur with reported estimates of the utilization of lipid energy reserves in pueruli. For biochemical energy reserves reported for pueruli collected 20 km off the east coast of New Zealand, the model suggests that this distance and durations of 5 days active swimming are the approximate limits to endurance. Sustained swimming exceeding 15 cm s− 1 will likely exhaust energy reserves before an animal can reach the coast, whereas swimming at less than 5–7 cm s− 1 is inefficient because of the overhead of nonswimming, inactive metabolism. Successful onshore migration of this species is potentially limited by the animals' energy reserves. Reduced energy reserves at the outset due to prior poor feeding, or delays encountered en route due to unfavorable currents, could lead to exceeding the stored reserves of the pueruli, and death. Potentially, relatively small shifts in coastal ocean climate conditions could generate marked changes in recruitment to important spiny lobster fisheries, as has recently been observed in many coastal populations of spiny lobsters.

A behavior-oriented dynamic model for sandbar migration and 2DH evolution

[1] A nonlinear model is developed to study the time‐dependent relationship between the alongshore variability of a sandbar, a(t), and alongshore‐averaged sandbar position, xc(t). Sediment transport equations are derived from energetics‐based formulations. A link between this continuous physical representation and a parametric form describing the migration of sandbars of constant shape is established through a simple transformation of variables. The model is driven by offshore wave conditions. The parametric equations are dynamically coupled such that changes in one term (i.e., xc) drive changes in the other (i.e., a(t)). The model is tested on 566 days of data from Palm Beach, New South Wales, Australia. Using weighted nonlinear least squares to estimate best fit model coefficients, the model explained 49% and 41% of the variance in measured xc and a(t), respectively. Comparisons against a 1‐D horizontal (1DH) version of the model showed significant improvements when the 2DH terms were included (1DH and 2DH Brier skill scores were −0.12 and 0.42, respectively). Onshore bar migration was not predicted in the 1DH model, while the 2DH model correctly predicted onshore migration in the presence of 2DH morphology and allowed the bar to remain closer to shore for a given amount of breaking, providing an important hysteresis to the system. The model is consistent with observations that active bar migration occurs under breaking waves with onshore migration occurring at timescales of days to weeks and increasing 2DH morphology, while offshore migration occurs rapidly under high waves and coincides with a reduction in 2DH morphology. Citation: Splinter, K. D., R. A. Holman, and N. G. Plant (2011), Abehavior‐orienteddynamicmodelforsandbarmigrationand2DH evolution, J. Geophys. Res., 116, C01020, doi:10.1029/2010JC006382.

Negotiating State Logic

This article examines the way in which onshore asylum seekers negotiate state logic and harness resources to campaign for their right and opportunity to stay on in the host country permanently. This paper is based on a multi-method study of the case of about 45,000 Chinese nationals, mostly students, no less than 50 percent of whom were over-stayers, living in Australia who sought to stay after the Tiananmen incident of 1989. The Chinese students, as they were called in Australia, succeeded in the end in obtaining residence through pursuing intensive lobbying activities over a period of four or so years from June 1989 to November 1993, and became the largest onshore migration intake in Australian history.BR Regardless of the causality in the relationship, both humanitarian and refugee immigration programs in Australia in 1989 were undergoing the most far-reaching changes, aimed at controlling the increased numbers of illegal entrants and onshore asylum seekers, including the Chinese students. In such a culture of control (Cronin 1993), the Migration Act of Australia had been amended 11 times between 1989 and 1992, and the Regulations governing this country"s immigration programs had seen 58 amendments during the same period. Similarly, during the period covered by this study, more than six major changes to the humanitarian and refugee programs were made, toughening the residence terms for all asylum seekers who would only get four-year permits rather than immediate permanent residence, as was the case at the time. In fact, within the first 12 months after the Tiananmen incident, in the first set of policy responses to the student issue, the Australian government had extended the students" visas unconditionally four times, all of which indicated nothing more than temporary protection similar to the US government position on the Chinese students living in America (Frelick and Kohnen 1995). Despite such an unfavorable situation, the Chinese students prevailed. Active and strategic lobbying delivered residence to not only those who arrived in Australia before the event, but also to tens of thousands of others who arrived afterwards.BR This study is concerned with how the state logic was challenged and broken, and how the migrant logic was formulated and implemented to negotiate the state logic, with a particular focus on the interaction between the two types of logic. The discussion considers three key aspects of the interaction. The first section looks at how the students acquired a number of clues from government policies and created their own logic. This is followed by an analysis of how the changing stance of the Australian government towards onshore asylum seekers led the students to form an organized approach while their temporary protection was repeatedly renewed. Also examined are the organized political activities carried out by the students in order to deal with the politically fundamental issues behind the state logic and to capitalise on the permissiveness of the local polities. The analysis attempts to convey a picture of the typical social experiences of onshore asylum seekers and how they turned the impossible into the possible.

The influence of nearshore sand bank dynamics on shoreline evolution in a macrotidal coastal environment, Calais, northern France

Analyses of shoreline and bathymetry change near Calais, northern coast of France, showed that shoreline evolution during the 20th century was strongly related with shoreface and nearshore bathymetry variations. Coastal erosion generally corresponds to areas of nearshore seabed lowering while shoreline progradation is essentially associated with areas of seafloor aggradation, notably east of Calais where an extensive sand flat experienced seaward shoreline displacement up to more than 300 m between 1949 and 2000. Mapping of bathymetry changes since 1911 revealed that significant variation in nearshore morphology was caused by the onshore and alongshore migration of a prominent tidal sand bank that eventually welded to the shore. Comparison of bathymetry data showed that the volume of the bank increased by about 10×10 7 m 3 during the 20th century, indicating that the bank was acting as a sediment sink for some of the sand transiting alongshore in the coastal zone. Several lines of evidence show that the bank also represented a major sediment source for the prograding tidal flat, supplying significant amounts of sand to the accreting upper beach. Simulation of wave propagation using the SWAN wave model ( Booij et al., 1999) suggests that the onshore movement of the sand bank resulted in a decrease of wave energy in the nearshore zone, leading to more dissipative conditions. Such conditions would have increased nearshore sediment supply, favoring aeolian dune development on the upper beach and shoreline progradation. Our results suggest that the onshore migration of nearshore sand banks may represent one of the most important, and possibly the primary mechanism responsible for supplying marine sand to beaches and coastal dunes in this macrotidal coastal environment.

Mechanisms controlling crescentic bar amplitude

The formation of crescentic bars from self‐organization of an initially straight shore‐parallel bar for shore‐normal incident waves is simulated with a two‐dimensional horizontal morphodynamical model. The aim is to investigate the mechanisms behind the saturation process defined as the transition between the linear regime (maximum and constant growth of the crescentic pattern) and the saturated state (negligible growth). The global properties of the morphodynamical patterns over the whole computational domain are studied (“global analysis”). In particular, consideration of the balance of the potential energy of the emerging bar gives its growth rate from the difference between a production term (related to the positive feedback leading to the instability) and a damping term (from the gravity‐driven downslope transport). The production is approximately proportional to the average over the domain of the cross‐shore flow velocity times the bed level perturbation. The damping is essential for the onset of the saturation, but it remains constant while the production decreases. Thus, it is notable that the saturation occurs because of a weakening of the instability mechanism rather than an increase of the damping. A reason for the saturation of the crescentic bar growth is the change in bar shape from its initial stage rather than the growth in amplitude itself. This change is mainly characterized by the narrowing of the rip channels, the onshore migration of the crests, and the change in the mean beach profile due to alongshore variability. These properties agree with observations of mature rip channel systems in nature.

Relative Importance of Vessel-Generated and Wind Waves to Salt Marsh Erosion in a Restricted Fetch Environment

Erosion of a salt marsh at Fort Pulaski National Monument threatens a pier of historical significance along the North Channel of the Savannah River, the main shipping channel for the Port of Savannah, Georgia. The erosion was initiated by the alongshore and onshore migration of a supratidal oyster shell ridge that exposed a marsh scarp that is susceptible to lateral retreat in response to both wind-generated and vessel-generated waves undercutting the upper-marsh surface. An instrumented field study was completed between October 2007 and February 2008 to examine the relative importance of wind-generated and vessel-generated waves to the retreat of the scarp by 0.21 m along the instrumented transect. On average, there were 14 sailings by container ships per day and up to twice as many sailings by pilot boats ferrying the harbor pilots to container ships offshore, but only those vessels that sailed within 1 m of the high-tide water level had waves capable of reaching the scarp. The vessel-generated waves accounted for ~5% of the cumulative wave energy over the study, but because of their larger height and longer period, they accounted for almost 25% of the cumulative wave force. It is the locally generated wind waves that account for most of the wave force acting on the exposed scarp and which are largely responsible for the observed retreat. Waves generated during both frontal and tropical storms tend to be associated with storm tides that maintain water levels at or near the elevation of the scarp for several days. Now that the shell ridge has largely migrated past the site and the areas of greater retreat historically are protected by a wedge of sand, it is argued that an increase in vessel traffic and/or the use of larger, post-Panamax ships will not significantly accelerate the retreat of the marsh.

Synchronization of transport and supply in beach-dune interaction

This review considers the role of nearshore processes and morphological change as a flexible and dynamic constraint on the supply and transport of sediment between beach and dune. It is argued that the lack of information in this regard remains a central barrier to the development of a theory of beach-dune interaction that can be translated across scales and between field sites. Existing beach-dune models do not consider how and when sediment gets transferred to the backshore where it becomes available for transport by wind. Rather, existing models largely ascribe regional variations in dune morphology to fixed constraints on beach slope and sediment budget, without explicit recognition of processes involved. Recent (process-scale) transport studies have shown that the transfer of sediment is both spatially variable and temporally intermittent as a result of transport limitations across the beachface. While these studies have identified varied controls on sediment transport and exchange, there remains a limited capacity to predict the evolution of beach-dune systems, largely because the beachface tends to be viewed as a static transport surface without regard to supply or to the changing limits to transport. Following storm erosion, dune recovery first requires that the beach recovers through the onshore migration and welding of nearshore bars, followed by accretion in the backshore to create a supply of sediment for transport by the wind. The dependence of dune recovery on the synchronization of transport events with the recovery of sediment supply in the backshore creates a strong asymmetry in dune recovery that makes barrier island susceptible to widespread erosion and breaching if a change in the frequency or grouping of storm events is capable of resetting the bar system (offshore) before the next extreme storm.

Seasonal-scale nearshore morphological evolution: Field observations and numerical modeling

A coupled waves–currents-bathymetric evolution model (DELFT-3D) is compared with field measurements to test hypotheses regarding the processes responsible for alongshore varying nearshore morphological changes at seasonal time scales. A 2001 field experiment, along the beaches adjacent to Grays Harbor, Washington, USA, captured the transition between the high-energy erosive conditions of winter and the low-energy beach-building conditions typical of summer. The experiment documented shoreline progradation on the order of 10–20 m and on average approximately 70 m of onshore sandbar migration during a four-month period. Significant alongshore variability was observed in the morphological response of the sandbar over a 4 km reach of coast with sandbar movement ranging from 20 m of offshore migration to over 175 m of onshore bar migration, the largest seasonal-scale onshore migration event observed in a natural setting. Both observations and model results suggest that, in the case investigated here, alongshore variations in initial bathymetry are primarily responsible for the observed alongshore variable morphological changes. Alongshore varying incident hydrodynamic forcing, occasionally significant in this region due to a tidal inlet and associated ebb-tidal delta, was relatively minor during the study period and appears to play an insignificant role in the observed alongshore variability in sandbar behavior at kilometer-scale. The role of fully three-dimensional cell circulation patterns in explaining the observed morphological variability also appears to be minor, at least in the case investigated here.

Berm formation and dynamics on a gently sloping beach; the effect of water level and swash overtopping

AbstractBerm formation and morphological development of the beach face have been observed during a neap–neap tidal cycle on the gently sloping and accreting beach at Vejers, Denmark. During the field campaign, an intertidal bar migrated onshore and stabilized as a berm on the foreshore. A new intertidal bar occurred on the lower beach face, migrated onshore on the rising tide and finally merged with the pre‐existing berm. As the tide continued to rise, the new berm translated further onshore as an intertidal bar to the uppermost part of the foreshore. The sediment transport during the berm transition was onshore directed in the upper swash and offshore directed in the lower swash. This berm development can be described through both the neap‐berm, ridge‐and‐runnel and berm‐ridge development concepts proposed by Hine (Sedimentology 1979; 26: 333–351), and all three stages were observed during only three tidal cycles. The main factors controlling this fast transformation were the gentle slope of the cross‐shore profile, rapid water level translation rates, substantial swash overtopping of the berm, and low infiltration rates. Despite the onshore migration of intertidal bars and berm formation, no net foreshore accretion took place during the field campaign. This was largely due to the formation of rip channels with strong rip currents cutting through the intertidal bars and the berm, which acted as a sediment drain in the profile. Copyright © 2009 John Wiley & Sons, Ltd.

Daily to interannual cross-shore sandbar migration: Observations from a multiple sandbar system

The most conspicuous characteristic of the cross-shore behaviour of nearshore multiple sandbar systems in sea-dominated settings is interannual net offshore migration (NOM), while that of similar systems in swell-dominated environments appears to be the on/offshore migration in response to individual wave events. Here we analyse an approximately 7.5-year data set (July 1999–February 2007) of daily 10-min time-exposure video images of the swell-dominated double-barred northern Gold Coast, Australia and show that events with wave heights exceeding three to four times the annual average can trigger NOM behaviour similar to that observed elsewhere. During four such events, we observed how the outer sandbar migrated some 75–100 m further offshore than usual and then decayed during slow onshore migration in the subsequent low-energy weeks. The next, moderately high wave event caused the ( ≈ 20 – 30 m / day ) offshore migration of the inner bar to become the new outer bar and the onshore birth of a new bar. Between the large wave events both the outer and the inner bars responded primarily to the seasonal signal in wave height, whereby both bars were located further offshore during the higher (lower) energy winter (summer) months. We suspect that bar size (height and width) may control whether a multiple sandbar system exhibits episodic NOM (as observed here) or interannual NOM. In contrast to interannual NOM, episodic NOM is steered by the order of storm events.

Field observations of an evolving rip current on a meso-macrotidal well-developed inner bar and rip morphology

The Aquitanian Coast (France) is a high-energy meso-macrotidal environment exhibiting a highly variable double sandbar system. The inner and the outer bar generally exhibit a bar and rip morphology and persistent crescentic patterns, respectively. In June 2007, an intense five-day field experiment was carried out at Biscarrosse Beach. A large array of sensors was deployed on a well-developed southward-oriented bar and rip morphology. Daily topographic surveys were carried out together with video imaging to investigate beach morphodynamic evolution. During the experiment, offshore significant wave height ranged from 0.5 to 3 m, with a persistent shore-normal angle. This paper identifies two types of behavior of an observed rip current: (1) for low-energy waves, the rip current is active only between low and mid tide with maximum mean rip current velocity reaching 0.8 m/s for an offshore significant wave height ( H s ) lower than 1 m; (2) for high-energy waves ( H s ≈ 2.5–3 m), the rip current was active over the whole tide cycle with the presence of persistent intense offshore-directed flows between mid and high tide. For both low and high-energy waves, very low-frequency pulsations (15–30 min) of the mean currents are observed on both feeder and rip channels. A persistent slow shoreward migration of the sandbar was observed during the experiment while no significant alongshore migration of the system was measured. Onshore migration during the high-energy waves can be explained by different sediment transport processes such as flow velocity skewness, wave asymmetry or bed ventilation. High-frequency local measurements of the bed evolution show the presence of significant (in the order of 10 cm) fluctuations (in the order of 1 h). These fluctuations, observed for both low- and high-energy waves, are thought to be ripples and megaripples, respectively and may play an important but still poorly understood role in the larger scale morphodynamics. The present dataset improves the knowledge of rip dynamics as well as the morphological response of strongly alongshore non-uniform meso-macrotidal beaches.

Lobbying to Stay: the Chinese Students’ Campaign to Stay in Australia

AbstractThis paper examines the way in which onshore asylum seekers lobby to stay in the host country permanently, and how they interact with local institutions. This study is based on a multi‐method case study of about 45,000 Chinese nationals in Australia, mostly students, who sought to stay after the so‐called political disturbance of 1989 in China. The students succeeded in obtaining residence by pursuing intensive lobbying activities over a period of about four years from June 1989 to November 1993, becoming the largest onshore migration intake in Australian history. This paper is concerned with how the students and their organizations negotiated the changing stance of the Australian government towards asylum seekers, and the way in which the students harnessed resources in the campaign. It examines the strategies adopted by the students, the roles of the main local institutions in the issue, and analyses the permissiveness of local polities.

Bimodal behavior of the seasonal upwelling off the northeastern coast of Taiwan

Observations over the outer shelf and shelf break off the northeastern coast of Taiwan indicate a curious seasonal variability of upwelling. At deeper levels 100 m below the surface, upwelling is most intense in summer but weaker in winter. Nearer the surface at approximately 30 m below the surface, the opposite is true and the upwelling is stronger in winter than in summer. Results from a high‐resolution numerical model together with observations and simple Ekman models are used to explain the phenomenon. It is shown that the upwelling at deeper levels (∼100 m) is primarily induced by offshore (summer) and onshore (winter) migrations of the Kuroshio, while monsoonal change in the wind stress curl, positive in winter and negative in summer, is responsible for the reversal in the seasonal variation of the upwelling near the surface (∼30 m). This mechanism reconciles previous upwelling data.

Short term morphodynamics of an intertidal bar on megatidal ebb delta

The morphodynamics of an intertidal bar located on an ebb delta of a megatidal inlet system in Normandy, France, was examined during four short experiments under low to high energy wave conditions and spring or neap tide contexts. Although there have been numerous studies on ebb-tidal bar morphology and on the processes affecting such bars, these concern only microtidal or mesotidal settings, and are mainly based on observations. No detailed work involving hydrodynamic conditions embracing the whole tidal cycle has been carried out so far on the mechanisms of onshore migration of these coastal accumulation features, especially along macro and megatidal coasts. In this study, the morphological response of the bar was coupled with the residence times and intensities of each hydrodynamic process (swash, surf, shoaling waves) over a representative bar cross-shore profile, and sediment transport patterns deduced from fluorescent tracer. The results show that the bar migrated exclusively shoreward during moderate to storm conditions (Hs > 0.7 m at the bottom of the bar seaward slope). During low energy wave conditions, no bar movement was observed. Swash action was not the dominant process, mainly due to its duration, which did not exceed 8 min in the course of a semi-diurnal tidal cycle. The movement of sediments and the bar morphology is induced mainly by surf processes and their associated currents. The increase in the significant wave height disturbed the general mean current behaviour, which was parallel to the bar crest during low energy conditions. Under surf conditions, mean flows are directed onshore, with an absence of bed return flow; this allows sediment to be transported towards the bar crest causing a landward bar migration. An offshore-directed cross-shore current (not favourable for bar migration) is recorded with larger water depths and no breaking waves (shoaling conditions). This study also highlights the influence of the tidal water level fluctuations in such a large tidal range setting, which induce a rapid shift in processes (such as bed shear and sediment re-working) across the shore-normal profile. These conditions explain the relative low rate of migration of the bar over the delta platform (about 33 m/year). This rate is much less than those reported from micro-mesotidal environments. Calculations of bed shear stress also show that the mean currents alone are not responsible for onshore bar movement, notwithstanding the megatidal character of the field site.

Response of high-energy, macrotidal beaches to seasonal changes in wave conditions: examples from North Cornwall, UK

An experimental wave farm ('Wave Hub') will be installed 20 km off the north coast of Cornwall in 2010. To assess its potential impacts on the nearshore wave climate and beach morphology, an extensive beach monitoring programme has been established. This paper will discuss some preliminary results with specific emphasis on the seasonal morphological response. The North coast of Cornwall is exposed to spring tidal ranges of 5-6 m and a 10% exceedance significant wave height of over 3 m, producing a highly energetic environment. Described as intermediate-dissipative, the beaches exhibit boundary classification features with dominant low tide bar/rip morphology. The sensitivity of such beach types to small variations in seasonal wave conditions can lead to changes in the morphological response of these beach systems which has implications for beach safety during high visitor periods. Seasonal and storm-induced morphological responses are presented for four high-energy, macrotidal beaches. Monthly 3D beach morphology was mapped using differential GPS (DGPS), and supplemented at two sites using daily ARGUS video images. In addition, nearshore wave conditions and tidal data have been combined to provide comprehensive storm and sea-level analysis. Seasonal trends are evident for all sites with clear low water bar morphology dominant in early spring. During summer wave conditions, rip currents are prevalent within the defined bar features prior to onshore migration of bars produces 2D beach profiles with pronounced berm growth.

Behavior exceeds physical forcing in the diel horizontal migration of the midwater sound-scattering layer in Hawaiian waters

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 365:91-101 (2008) - DOI: https://doi.org/10.3354/meps07491 Behavior exceeds physical forcing in the diel horizontal migration of the midwater sound-scattering layer in Hawaiian waters Margaret A. McManus1,*, Kelly J. Benoit-Bird2, C. Brock Woodson1 1University of Hawaii at Manoa, Department of Oceanography, 1000 Pope Road, Honolulu, Hawaii 96822, USA 2College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin Bldg, Corvallis, Oregon 97331, USA *Email: mamc@hawaii.edu ABSTRACT: The mesopelagic sound-scattering layer around the Hawaiian Islands undergoes diel vertical migrations. This migration also includes a significant diel horizontal onshore–offshore component. Before this study, the mechanisms underlying the horizontal movement of the mesopelagic layer had not been investigated. We took an interdisciplinary approach to evaluate relationships between the mesopelagic layer and physical oceanographic processes in the water column. The study area extended west of the leeward coast of Oahu, Hawaii. We found that the nightly inshore movement of micronekton occurs very predictably each day; onshore migration brings animals nearshore between 20:00 and 20:13 h and offshore migration commences between 03:30 and 03:45 h during the months of April and May. Simultaneous observations of nearshore physical flow and micronekton distribution indicate that the observed nightly inshore movement of micronekton in the sound-scattering layer is not directly due to physical processes in the nearshore environment. Rather, this nightly inshore movement of micronekton can be attributed primarily to micronekton swimming behavior. This behavior may be cued by changes in sunlight, but that cue is strongly mediated by a factor tied to the lunar phase, potentially moonlight. Horizontal migration velocities exceeded the maximum across-slope physical flow by 5 to 6-fold. Thus, typical across-slope flows likely represent a relatively small additional cost or benefit to onshore-offshore swimming, depending on flow direction. We suggest that energy acquisition via increased prey availability nearshore far exceeds the high energetic cost of onshore and offshore migrations. KEY WORDS: Mesopelagic layer · Diel migration · Physical processes Full text in pdf format PreviousNextCite this article as: McManus MA, Benoit-Bird KJ, Brock Woodson C (2008) Behavior exceeds physical forcing in the diel horizontal migration of the midwater sound-scattering layer in Hawaiian waters. Mar Ecol Prog Ser 365:91-101. https://doi.org/10.3354/meps07491 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 365. Online publication date: August 18, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Onshore sandbar migration at Tairua Beach (New Zealand): Numerical simulations and field measurements

We observed the onshore migration (3.5 m/day) of a nearshore sandbar at Tairua Beach, New Zealand during 4 days of low-energy wave conditions. The morphological observations, together with concurrent measurements of waves and suspended sediment concentrations, were used to test a coupled, wave-averaged, cross-shore model. Because of the coarse bed material and the relatively low-energy conditions, the contribution of the suspended transport to the total transport was predicted and observed to be negligible. The model predicted the bar to move onshore because of the feedback between near-bed wave skewness, bedload, and the sandbar under weakly to non-breaking conditions at high tide. The predicted bathymetric evolution contrasts, however, with the observations that the bar migrated onshore predominantly at low tide. Also, the model flattened the bar, while in the observations the sandbar retained its steep landward-facing flank. A comparison between available observations and numerical simulations suggests that onshore propagating surf zone bores in very shallow water (< 0.25 m) may have been responsible for most of the observed bar behaviour. These processes are missing from the applied model and, given that the observed conditions can be considered typical of very shallow sandbars, highlight a priority for further field study and model development. The possibility that the excess water transported by the bores across the bar was channelled alongshore to near-by rip-channels further implies that traditional cross-shore measures to judge the applicability of a cross-shore morphodynamic model may be misleading.

Coupled barrier island–resort model: 2. Tests and predictions along Ocean City and Assateague Island National Seashore, Maryland

The fate of coastlines and their human settlements under the effects of global climate change will depend critically on the nonlinear dynamics of and feedbacks between shoreline processes and human agency. This hypothesis is explored on the barrier island coastline of Ocean City and Assateague Island National Seashore, Maryland, using a model‐coupling natural coastal processes, including erosion, accretion, island overwash, alongshore sediment transport, dune growth and migration, inlet migration and ebb tidal delta growth to economics of tourist resort development through storm damage and beach and dune replenishment. Initiating the model in 1845, the RMS difference between model and measurements of the shoreline position in 2001 is 84.97 m compared to a net onshore migration of 472.2 m and the RMS difference between modeled and measured hotel room density in 2001 is 2950 rooms km−1 compared to a net gain of 28,824 rooms km−1. Simulations to year 3400 for a rate of sea level rise of 3.5 mm a−1 show a steady state barrier island position 158 m further offshore and 0.54 m lower in elevation compared to its natural counterpart. Changing the rate of sea level rise to 10.5 mm a−1 increases these differences to 288 m and 0.76 m. Changing storminess by increasing the standard deviation of storm size 50% diminishes coupling between resorts and barriers, bringing the natural and coupled attractors into near coincidence. These results suggest that predicted increases in the rate of sea level rise will lead to enhanced vulnerability for Ocean City.