Fetch-limited Environments Research Articles

Relative energy and perceived impact of vessel-generated waves in fetch-limited environments

Houser C, Carswell A, Chittle B, Johnson G, Caschera A, Smith A. 2024. Relative energy and perceived impact of vessel-generated waves in fetch-limited environments. Lake Reserv Manag. 40:285–302 Vessel-generated waves (or wakes) are perceived as an important stressor in both marine and freshwater systems, and they are an increasingly important focus of coastal management. Recent studies suggest that wakes account for a significant portion of the total wave energy in fetch-limited environments and in turn have the potential to resuspend sediment, erode shorelines, degrade water quality, damage infrastructure, and disrupt aquatic ecosystems, in addition to being considered a safety hazard and a nuisance. To assess the relative contribution of wakes to the total wave energy, monitoring was completed at 32 sites across 7 fetch-limited lakes in Ontario, Canada between 2020 and 2023. The percent wake energy was inversely related to the average fetch length, with deviations to the relationship dependent on whether the site was open, an embayment or along a narrow section of the lake. A survey of property owners and residents suggests that the perceived impact of wakes also varies with the fetch length. Shoreline erosion and degraded water quality were identified as primary impacts of wakes in lakes with the smallest average fetch, but very few respondents provided direct evidence of boat wake impacts. Most respondents described vessel operation, experience of the vessel operator and conflict over different boating uses as the primary issues associated with wakes. While boat wakes may contribute a significant portion of the wave energy in fetch-limited lakes, further research is required to determine when and where they may represent a physical and ecological stressor rather than being just a nuisance.

Wave setup estimation at regional scale: Empirical and modeling-based multi-approach analysis in the Mediterranean Sea

Wave setup is a physical process that induces a temporal increase of the mean water level due to wave dissipation by bottom friction and breaking in the surf zone, extending over tens to hundreds of meters in the cross-shore direction. Wave setup contribution to coastal sea level solely induced by wind and atmospheric effects can increase by more than 100% under extreme events and conditions favoring its formation. It is therefore crucial to consider this phenomenon when assessing sea-level-related coastal hazards. Previous studies estimated the wave setup effect by means of numerical modeling and empirical formulations at regional and global scale. Such analyses require either high computational capacity to implement high-resolution numerical models over large domains, and/or accurate information on coastal morphological features from global or regional databases. Although the Mediterranean Sea is a fetch-limited environment, waves generated from extra-tropical cyclones are powerful enough for wave setup to develop, and subsequently for a potential significant wave setup contribution to extreme coastal sea level. Through the use of both numerical and empirical methods, we investigate the uncertainty associated to wave setup representation on the frequency and magnitude of coastal extreme sea levels occurring on sandy beaches in the Mediterranean Sea. Wave setup values are compared at beach scale between process-based modeling and empirical approaches, showing highly variable results. We also quantify the impact of wave setup on return levels of coastal sea level extremes using reconstructed sea levels. We employ various methods to calculate the wave setup component. Results show high spatial dispersion, with clear differences between the numerical and empirical approaches, especially in regions prone to the development of energetic waves. The total inter-method dispersion of 100-year return levels is often higher than 30 cm for average values of 62.4 cm. We emphasize the important limitations related to wave setup modeling (i.e., its underestimation) at large scale, and call for caution when applying empirical formulations (generally developed from local studies) at regional to global scale, which can lead to unrealistic wave setup values.

Morphosedimentary response of a fluvio-estuarine beach to interannual variability in landfast ice dynamics

Landfast ice plays a significant role in the morphodynamics of cold coasts and accelerating anthropogenic pressures on coastal sedimentary environments are increasing the complexity of its distribution and behavior. However, understanding its interannual variability and geomorphic role along the wide range of inhabited and uninhabited coastal environments remains a challenge. In fact, the difficulty in obtaining periodic high-resolution field data on its physical properties and their temporal evolution is often a major limitation in developing knowledge on landfast ice processes. Here, we present an approach for improving the monitoring of landfast ice dynamics and its geomorphic impact on sedimentary systems by combining (1) topographic datasets from UAS surveys, (2) hydrometeorological conditions during the freezing season, and (3) observations from time-lapse video. This study was conducted over two contrasting freezing seasons at the urban beach of Tibbits Cove in the fluvial estuary of the St. Lawrence River in a context of a low energy wave environment. Results demonstrate the great ability of the approach for (1) measuring interannual variations in ice thickness, surface ruggedness and volume with high accuracy and (2) constraining geomorphic changes due to ice action. These results also reveal the existence of a strong relationship between the severity of the freezing season and the response of landfast ice to hydrometeorological events in a fetch-limited environment. Consequently, a distinct geomorphic response of the beach was also observed following the opposite winter conditions of 2021 and 2022, where ice processes exported almost three times more sediment during a cold winter than during a mild winter. The overall trend in sediment budgets remained negative following two winters, suggesting an important role for landfast ice in the long-term evolution of the beach. Observations also suggest the prevalence of wave-independent sediment entrainment processes such as basal adfreezing in fetch-limited environments. Other ice-related processes, including ice wallowing and gouging, show a negligible influence on the sediment budget of the system and appear to be less powerful than those described in open coastal environments. Ultimately, expanding the use of this approach could greatly enhance our knowledge of the behavior, processes and products of coastal ice in a constantly changing climate, while informing decision-makers of its important influence on the evolution of geosystems in cold regions.

Potential of Transplanted Seagrass Meadows on Wave Attenuation in a Fetch-Limited Environment

In this paper, the effectiveness of transplanted (either created or restored) seagrass meadows as a coastal protection measure is assessed through a five-step methodology. The analysis is focused on a stretch of the Catalan coast (NW Mediterranean) which is a fetch-limited environment. The results show that even considering conservative values for the meadow parameters (plant diameter, meadow density and canopy height), significant reductions of the annual average wave heights reaching the beach may be obtained, reducing flooding and erosion risks. Therefore, the investment in the conservation and restauration of seagrass meadows for protecting coastal areas from erosion and flooding is a measure that must be considered, due to the multiple benefits that they provide including ecosystem services. In addition, the proposed methodology may be a useful tool for coastal managers to help them in the design of seagrass meadows for coastal protection.

Shallow lacustrine versus open ocean coastal clastic deposits: Morphosedimentary diagnostic indicators and interpretation

Lacustrine coasts, especially those associated with shallow highly fluctuating lakes, do not evolve in exactly the same way as marine coasts. Climate variability can significantly modify the morphology of the water body, affecting the lake surface area, the available fetch, and the water depth, resulting in possible changes to wave generation and propagation. To look more closely at the nature of coastal sediments generated under such specific environments, we selected examples of both shallow lacustrine and open ocean beach deposits. The first location was Gallocanta Lake, a shallow body of water in the Iberian Chain in central Spain; and the second was Mazagón Beach, in the Gulf of Cádiz, a mesotidal sandy beach exposed to Atlantic waves and tides. Both deposits are formed by beach and ridge and runnel systems. The sedimentary structures of the two deposits were compared, together with a genetic scheme for these. In both cases, applying wave propagation models facilitated the interpretation of the sedimentary processes responsible for their genesis. Although the general morphosedimentary characteristics of the two deposits are roughly similar, a more detailed analysis revealed significant differences, some of which could be used as diagnostic indicators of shallow lacustrine environments in geoarchives. Effective fetch seemed not to be a determining factor for explaining the textural differences between the two deposits. Local factors such as wind strength and direction, and approaching wave refraction patterns are important in controlling the final energy reaching both types of coasts, and, hence, the resulting geometry of the clastic coastal deposits. The type and maturity of the available sediment also influence the final textural properties of the deposits. Open ocean beaches receive sporadic high energetic storms; these partially erode the sedimentary record and reduce the preservation potential of coastal deposits compared with those from shallow lake beaches. Comparing lacustrine deposits with marine analogues and identifying similarities and differences between the two coastal environments may help to infer the main factors underlying the formation and development of coastal deposits in non-tidal, fetch-limited environments.

Measuring Dimensions of Ship-generated Waves in Guadalquivir Estuary Banks (SW Spain): Hydrodynamic Considerations

Morales, J.A.; Sedrati, M.; Boulmirate, O.; Borrego, J., and Carro, B.M., 2020. Measuring dimensions of ship-generated waves in Guadalquivir Estuary banks (SW Spain): Hydrodynamic considerations. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 568–572. Coconut Creek (Florida), ISSN 0749-0208.Estuaries have always been very important to communities, as they often represent the economic centers of many countries. The estuarine banks, as fetch-limited environments, might be highly affected by some anthropogenic actions. Ship-generated waves are starting to be the subject of several studies in order to understand their impact on coastal environments. These waves might be the leading factor to erosion in estuarine channel margins when traffic is frequent. Guadalquivir is one of the most important estuaries in Spain. It connects an important number of commercial ships to the port of Seville. Along the estuary, bank erosion is highly pronounced which is starting to worry both, landowners and ecologists, since a part of the estuarine channel crosses the marine sector of Donana natural park. Last years, port of Seville intended to dredge the channel to get the entrance of bigger ships to the port. Ecologists argue that the magnitude of the waves generated by the ships is proportional to the size of the ship. So, the entrance of bigger ships will increase the erosion along the margins. The aim of this paper, is to obtain preliminary data to verify the relevance of this theory through a hydrodynamic survey using wave gauges on the banks and an ADCP in the bottom of the channel to control the modification of waves and currents caused by the ships.

Multivariate statistical modelling of future marine storms

Extreme events, such as wave-storms, need to be characterized for coastal infrastructure design purposes. Such description should contain information on both the univariate behaviour and the joint-dependence of storm-variables. These two aspects have been here addressed through generalized Pareto distributions and hierarchical Archimedean copulas. A non-stationary model has been used to highlight the relationship between these extreme events and non-stationary climate. It has been applied to a Representative Concentration Pathway 8.5 Climate-Change scenario, for a fetch-limited environment (Catalan Coast). In the non-stationary model, all considered variables decrease in time, except for storm-duration at the northern part of the Catalan Coast. The joint distribution of storm variables presents cyclical fluctuations, with a stronger influence of climate dynamics than of climate itself.

The morphosedimentary behaviour of a headland–beach system: Quantifying sediment transport using fluorescent tracers

Embayed beaches occurring along rocky coasts with headlands controlling hydrosedimentary processes are considered to be constrained sedimentary systems with little longshore sediment transfer between them. Such beaches are likely to narrow under rising sea level and a deficit of fluvial sediment, and because beaches act to dissipate wave energy, cliff erosion is expected to increase. Knowledge of hydrosedimentary processes in fetch-limited environments such as embayed beaches is therefore a fundamental management tool in erosion-prone coastal areas such as the Algarve region (southern Portugal). The main objective of this work is to quantify longshore sediment transport (LST) using a sedimentary tracer and compare the results with the ones derived via empirical formulae from several authors. To achieve this, we establish the spatial and temporal patterns of stained sediment, and relate these to marine climate data over six tidal cycles for three adjacent embayed beaches. The values of displaced sand estimated using tracer and scaled to mixed layer depth were compared with the results of five well-known LST formulae to assess the applicability of each model to mesotidal environments. Of the LST formulae used, the CERC (2013) shows to be the most accurate in our study area.This study demonstrates the efficacy of using tracers for tracking and quantifying the sediment transport and as a basis for assessing the viability of different LST formulae for representing transport in these types of environments. The work also shows the importance of the morphological control of hydrosedimentary processes, including the disruption of longshore drift and the sediment transfer promoted by shore platforms.

Dynamics of rip currents associated with groynes — field measurements, modelling and implications for beach safety

Rip currents can occur around groynes and other coastal structures (e.g. breakwaters and geological headlands), which provide a boundary to the wave-induced flow field and present a hazard to water users worldwide. Shore-normal groynes are common along drift-aligned coasts. In northern Europe these are often low-energy, fetch-limited environments that are dominated by locally generated wind waves. The presence of boundary-controlled rip currents due to groynes is supported anecdotally in literature, but few field measurements of their presence and dynamics exist. This study provides new quantitative scientific understanding of the dynamics of boundary-controlled rip currents within this setting. A 10-day field experiment at Boscombe beach on the south coast of England measured rip currents and nearshore hydrodynamics around an impermeable groyne field. Observations from both fixed instruments (wave, tide and current metres) and GPS-drifters confirmed the presence and characteristics of hazardous topographic rip currents around a groyne structure during a number of oblique wave events and drift reversals. The strongest offshore-directed rip velocities of up to 1ms−1 (10-min average) and 2ms−1 (instantaneous) were measured on the updrift side of the groyne where the longshore current generated within the embayment was deflected offshore. These strong rip flows were measured under relatively small wave heights (Hs<1m) and presented a significant bathing hazard. A calibrated and validated numerical model (XBeach) was used, in support of measured data, to explore the key environmental controls on rip behaviour across a range of groyne configurations and wave conditions not observed in the field. Key outcomes were that: 1) upstream deflected rip flows were found to be strongly correlated to the alongshore thrust due to wave forcing; 2) the groyne length relative to surf zone width strongly controlled the offshore extent of rip flows, with a significant increase in surf zone exits above a relative groyne length (length/surf width) of 1.25; and 3) rip flows increase up to a relative groyne spacing (spacing/length) of around 4–6, above which alongshore currents are fully developed. These findings have been distilled into some key principals that are relevant to the assessment of boundary-controlled rip hazard on beaches.

Managing flood risk in fetch-limited environments

Carrasco, A.R., Ferreira, Ó., Matias, A, 2013. Managing flood risk in fetch-limited environmentsThere is little unanimity concerning the environmental modifications potentially forced by the acceleration of global climate change in the coming century, or the future actions required to work towards a sustainable development of natural habitats in the coastal zone. A simple framework to assess the potential implications of flooding in fetch-limited coastal environments (with no relevant wave setup), experiencing both coastal and river flooding, was recently developed. It identifies tide, storm surge, sea level rise and river discharge as the main sources of flooding hazards. Hazard maps differentiate distinctive zones of flooding: the constantly flooded zone, the frequently flooded zone, and the infrequently flooded zone.The framework prioritizes site-based local management, and identifies alternative tools for sustainable coastal resources, to face non-desired flood effects. It was applied to a morphologically complex (tidal channels network, salt marshes, dunes) occupied backbarrier coastal stretch located in Culatra Island, South Portugal (max. fetch ~ 4 km). The obtained inundation levels predicted for 1, 10 and 100 year-return periods were 2.02 m, 2.39 m, and 2.84 m above MSL, respectively. The high-, moderate-, and low-risk zones include about 34 m2, 1,073 m2 and 31,821 m2 of occupied area, respectively. Besides houses, flood impacts also affect other infrastructures (e.g., walkways and recreational structures). Several best practices principles and guidelines were proposed for the three risk zones, namely the adaptation to uses with ‘added value', which enhances the local ecological values and strengths economic activities. This study contributes to the method validation at a complex morphological area and enriches the understanding about the potential of flood in fetch-limited conditions, praising the perspective of ‘living with climate change'.

Mesoscale geomorphic change on low energy barrier islands in Chesapeake Bay, U.S.A.

This paper presents an analysis of decadal (mesoscale) geomorphic change on sandy barrier islands in the fetch-limited environment of Chesapeake Bay. Low energy barrier islands exist in two settings: on the fringe of marshes and in open water and this analysis shows the various types of barrier island to be genetically related. Barrier islands that face the dominant wind and wave direction (E or W) retreat via barrier translation, preserving the barrier island volume. Those that exist in re-entrants are dominated by longshore transport processes, are strongly affected by sediment supply and are subject to disintegration.Marsh fringe barrier islands are perched on or draped over the surface of the underlying marsh. They migrate landwards via barrier translation during periodic high water events accompanied by large waves (hurricanes and northeasters). The underlying marsh surface erodes under all water levels and the rate of retreat of the barrier island and underlying marsh may take place at different rates, leading to various configurations from perched barrier islands several metres landward of the marsh edge, to barrier islands that have a sandy shoreface extending into the subtidal zone. The coastal configuration during landward retreat of marsh fringe barrier islands is subject to strong local control exerted by the underyling marsh topography. As erosion of marsh promontories occurs and marsh creeks are intersected and bypassed, the configuration is subject to rapid change.Periodic sediment influxes cause spits to develop at re-entrants in the marsh. The spits are initiated as extensions of adjacent marsh fringe barrier islands, but as the sediment volume is finite, the initial drift-aligned spits become sediment-starved and begin to develop a series of swash-aligned cells as they strive for morphodynamic equilibrium. The individual cells are stretched until breaches form in the barrier islands, creating inlets with tidal deltas. At this stage the low energy barrier islands closely resemble open ocean barrier islands. Continued reworking leads to widening of the inlets with consequent loss of constriction of tidal flow. The tidal deltas are, thus, no longer maintained and ultimately the island system disintegrates through inlet widening and is transformed to subtidal shoals. Barrier islands at various stages in this evolutionary cycle can be observed around the bay.Mid-bay barrier islands are affected by wave processes from both sides. This helps maintain the barrier island form and enables barrier islands to persist as sediment is exchanged between both sides of the island.Rates of barrier island translation are extremely high (up to 30m/year over a 12year period). This is attributed to the low volume of sand, which facilitates complete rollover in short periods. Accelerated sea level rise is likely to hasten the translation rates of marsh fringe barrier islands. The rapid disintegration of most spits compared to the persistence of marsh fringe barrier islands points to a reliance on the marsh as a stabilising point. If the marshes are overstepped by rising sea level as appears to be happening, the complete disintegration of the barrier islands is highly likely.

Natural and human-induced coastal dynamics at a back-barrier beach

This study contributes to the understanding of very low-energy fetch-limited environments by reporting the evolution of a back-barrier beach (Ancão Peninsula, southern Portugal). It considers two timescales: a large-scale evolution for the past 60years based on aerial photograph analysis, and a small-scale beach evolution based on monthly topographic surveys performed during three years of monitoring. Each timescale revealed a different rate of evolution, the first reporting a modified beach response-type (from human activities), and the second reporting a natural beach response-type. Human activities caused significant changes in the back-barrier shore, whereas changes under natural forcing were much smaller, were less influential on the area's evolution, and were not sufficient to counteract or mask the consequences of human activities. The findings of the study should contribute to a better understanding about the large- and small- scale changes in other back-barriers characterised by similar very low-energy conditions.

Onshore Bar Migration in a Fetch/Limited Strong Wind Bay

WONG, H and STEPHENSON, W., 2011. Onshore Bar Migration in a Fetch/Limited Strong Wing Bay. In: Micallef, A. (ed.), MCRR3-2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 458-469. Grosseto, Tuscany, Italy, ISSN 0749-0208.This paper investigates bar migration in a fetch limited strong wind bay (Seaford Beach SE Australia). Onshore bar migration on open ocean beaches is dependant on long period swell wave and occurs during low energy conditions. However, in a fetch limited environment such waves are absent and most studies suggest that during low energy conditions the beach remains in an arrested state (Jackson et al. 2002). This paper spans a three month study period (15th March – 8th June 2008). Measurement of near shore morphology and wind data were collected. The two bars at Seaford behaved very differently; both onshore and offshore migration was detected during the three recorded storm events. The paper found that, firstly, wave breaking was incomplete in the middle bar during event 1 — the inner bar migrated offshore. Secondly wave breaking in the middle bar was almost complete during event 2 and the inner bar migrated onshore. Thirdly when wave breaking was complete in the middle bar during event 3 — the inner bar remained arrested on some profiles but bar height increased on others. The wave steepness threshold for onshore migration of the inner bar was parameterized s 0.041 – 0.042. Beach sediment modeling in fetch limited strong wind bays remains understudied. Our results should help to improve the predictive capability of beach sediment budgets in fetch limited strong wind bays.

Sediment Resuspension by Vessel-Generated Waves along the Savannah River, Georgia

Vessel-generated waves have the potential to erode the shoreline where there is regular commercial and recreational boat traffic in otherwise low-energy estuarine and fetch-limited environments. Whereas it is well established that vessel-generated waves are capable of resuspending significant quantities of bottom and bank sediment, there are limited field data showing how and when sediment is resuspended by the wake and the direction of net transport. This paper presents the results of an instrumented field study to measure sediment resuspension and transport in response to wakes generated by supercritical pilot boats and subcritical container ships at the mouth of the Savannah River in Georgia. Suspended sediment concentrations increase with the turbulent kinetic energy of the supercritical pilot-boat wake. The amount of sediment resuspended dependent on the available supply of sediment on the upper-foreshore. Whereas sediment is transported landward by the individual waves of the group, net transport is offshore in response to a low-frequency oscillation similar to a second-order group-forced current. In some cases, the direction of net transport is reinforced or reversed depending on the timing of the pilot-boat wake with the seiche forced by a passing container ship. In contrast to the pilot boats, sediment transport by subcritical container ships tends to be landward, but can also be weakly offshore depending on the timing of the wave group with the low-frequency drawdown and surge.

Circulation and suspended sediment transport in a coral reef lagoon: The south-west lagoon of New Caledonia

The south-west lagoon of New Caledonia is a wide semi-open coral reef lagoon bounded by an intertidal barrier reef and bisected by numerous deep inlets. This paper synthesizes findings from the 2000–2008 French National Program EC2CO-PNEC relative to the circulation and the transport of suspended particles in this lagoon. Numerical model development (hydrodynamic, fine suspended sediment transport, wind-wave, small-scale atmospheric circulation) allowed the determination of circulation patterns in the lagoon and the charting of residence time, the later of which has been recently used in a series of ecological studies. Topical studies based on field measurements permitted the parameterisation of wave set-up induced by the swell breaking on the reef barrier and the validation of a wind-wave model in a fetch-limited environment. The analysis of spatial and temporal variability of suspended matter concentration over short and long time-scales, the measurement of grain size distribution and the density of suspended matter (1.27kgl−1), and the estimation of erodibility of heterogeneous (sand/mud, terrigenous/biogenic) soft bottoms was also conducted. Aggregates were shown to be more abundant near or around reefs and a possible biological influence on this aggregation is discussed. Optical measurements enabled the quantification of suspended matter either in situ (monochromatic measurements) or remotely (surface spectral measurements and satellite observations) and provided indirect calibration and validation of a suspended sediment transport model. The processes that warrant further investigation in order to improve our knowledge of circulation and suspended sediment transport in the New Caledonia lagoon as well as in other coral reef areas are discussed, as are the relevance and reliability of the numerical models for this endeavour.

Beach morphodynamics in a strong-wind bay: a low-energy environment?

The morphodynamic behaviour of a multibarred beach in a fetch-limited, strong-wind bay (Seaford Beach, SE Australia) was examined during both high- and low-energy conditions, and considered in the context of a definition of low-energy provided in the literature. Measurements of nearshore waves, currents, and morphology revealed a bimodal behaviour. Under initial low-energy conditions, the beach exhibited a “low-tide terrace” state, and waves and currents were of very low magnitude. During subsequent high-energy conditions, the beach demonstrated dynamic behaviour through the formation of a transitional “transverse bar and rip-rhythmic bar and beach,” and migration of the middle bar, with the morphology remaining in an arrested high-energy state during intervening low-energy periods. Although broadly conforming to the morphodynamic model, the beach did exhibit some distinct characteristics attributable to its fetch-limited location; limited progression through the morphodynamic model; and the importance of wind direction and magnitude in governing morphodynamic behaviour. Furthermore, rip currents were not significant in driving beach change through intermediate states. The presence of infragravity energy in the storm wave spectra; a dissipative, multibarred surf zone; dynamic inner and middle bars; and the attainment of a “transitional transverse bar and rip-rhythmic bar and beach” state during rising wave conditions, underline Seaford Beach as “bimodal”, exhibiting process and morphologic features of both higher- and lower-energy beaches. As an example of a beach in a strong-wind bay, Seaford, illustrates that not all fetch-limited beaches are low-energy. Furthermore, the presence of infragravity energy in a highly fetch-limited environment indicates that infragravity energy may occur commonly in fetch-limited environments that are subject to periodic strong winds; a process that has remained largely unrecognised.

‘Low energy’ sandy beaches in marine and estuarine environments: a review

This review was undertaken to identify locations where low energy beaches may occur and their diagnostic forms and process controls, including waves, tides and water levels. Examples are drawn from the sheltered coastline of Western Australia near Perth and fetch-limited estuarine environments on the northeast coast of the United States. We suggest that the term low energy be used in locations where: (1) non-storm significant wave heights are minimal (e.g. <0.25 m); (2) significant wave heights during strong onshore winds are low (e.g. <0.50 m); (3) beachface widths are narrow (e.g. <20 m in microtidal environments); and (4) morphologic features include those inherited from higher energy events. Micro-topographic features can persist in the swash zone of low energy beaches under non-storm wave conditions. There is little evidence of cyclic cross-shore sediment exchange. Bars, excepting transverse forms, located seaward of low still-water level do not appear to be part of the sediment exchange system with the foreshore. Developing a better definition of the term low energy requires understanding the occurrence and duration of morphological characteristics and the type, magnitude and frequency of hydrodynamic controls that are responsible for these characteristics. Efforts also should be directed toward: (1) discriminating between processes generated within basins (in true fetch-limited environments) and processes generated outside basins (that affect sheltered environments); (2) identifying the relative contributions of tide- and surge-related water level fluctuations on low energy beach shape; and (3) estimating thresholds for beach change.

Response of the Seabed to Storm-Generated Combined Flows on a Sandy Arctic Shoreface, Canadian Beaufort Sea

ABSTRACT Wave and current measurements in 3.4 and 5.0 m water depths on a sandy shoreface of the Canadian Beaufort Sea showed that a down-welling circulation, accompanied by strong offshore-flowing near-bottom currents, develops during storm surges induced by northwesterly winds. Because fine to medium sand prevails on the shoreface, a significant sediment load is probably moved down the shoreface by such currents during major storms. A sidescan sonar profile recorded in 4.4 m water depth, the day after a storm with seaward-directed currents with mean velocity. up to 0.49 ms-1, revealed the presence of narrow bands of large ripples (0.7-0.8 m wavelength) with crests almost parallel to shore. On the basis of theoretical calculations relating the wavelength of wave-formed ripples to w ve orbital diameter, the observed ripples were probably formed or reactivated during a storm in response to bed-orbital motion of surface gravity waves. On the sonograph, the ripple bands were separated by less reflective areas interpreted as current-parallel sand patches formed by seaward-flowing bottom currents. Cores taken on the shoreface at the end of the wave and current measurement period showed the presence of thick graded sand beds, which are thought to be the products of rapid deposition of sand from suspension during the waning phase of a recent storm. Such beds may he the modern analogues of graded sand beds in ancient wave-dominated sequences and could indicate waning storm sedimentation in shallow fetch-limited environments.

On the determination of breaking surface wave distributions using ambient sound

When surface gravity waves break, they generate an acoustic signal which can be used to derive information about their properties and distribution. Observations of sound formed in this way have been obtained at 1, 10, and 40 m below the surface in the fetch limited environment of Georgia Strait, British Columbia. Interpretation of the results is aided by a model of randomly distributed sound sources on the ocean surface. It is shown that the signal variability is governed by the number density, or mean spacing of the breakers, by the tendency for breakers to repeat at half the wave frequency, and by the group structure of the wave field. Calculated wave field properties reveal several features which can be related to changes in the wind, including variations in wave period, mean breaker spacing, and the average acoustic strength of individual breakers. Bubble clouds caused by the breaking waves can also be detected, and there is evidence that they contribute to the trapping of sound in a shallow layer close to the surface.