Electromagnetic Current Meters Research Articles

Influence of landfast ice on the hydrography and circulation of the Baltic Sea coastal zone

The influence of landfast ice on hydrography and circulation is examined in Santala Bay, adjacent to the Hanko Peninsula, Gulf of Finland. Three-dimensional electromagnetic current meters and conductivity-temperature-depth (CTD) sensors were deployed in winters 1999–2000 and 2000–2001 during the Finnish-Japanese ‘Hanko 9012’ experiment. In each winter, data collection started one month before the initial ice formation and lasted until one month after the ice had melted completely. Temperature and salinity are compared with long-term data from the Tvärminne Zoological Station, also located on the Hanko Peninsula. The water temperature was 2°C less than the long-term average. Ice formation and melting show up in the salinity evolution of the water body, which makes salinity a good indicator of ice formation and breakup in Santala Bay. The circulation under the ice became weaker by almost 1cms−1.

MEASUREMENTS OF SURFACE KINEMATICS AND TEMPERATURE IN THE SURF AND SWASH ZONES USING INFRARED IMAGE VELOCIMETRY

High intensity air bubbles generated in the surf zone and the thinning of swash flow make velocity measurements particularly challenging in coastal areas. These facts have led the need for a new measurement technique to quantify the surf and swash flow dynamics. Here, we tested infrared image techniques to measure the surface temperature and then to derive the velocity fields using cross-correlation algorithm for large-scale solitary waves breaking in the surf and swash zones. From the comparison with unspiked electromagnetic current meter (EMCM) data and previous validation, it is suggested that the infrared image velocimetry (IRIV) is satisfactory to quantify the surface turbulent flow in the surf and swash zones. The data obtained in the experiment provides a new description of surface thermal structure and kinematics for solitary breaking waves. Two-dimensional organized streaks of temperature structures are evident on the water surface behind the head of rebounding jet. Wavenumber spectrum analysis shows that the directionality of these thermal signatures evolves with time. Evolution of vorticity on the water surface during the run-up and run-down process of the solitary broken wave is discussed.

Quantifying riverine surface currents from time sequences of thermal infrared imagery

River surface currents are quantified from thermal and visible band imagery using two methods. One method utilizes time stacks of pixel intensity to estimate the streamwise velocity at multiple locations. The other method uses particle image velocimetry to solve for optimal two‐dimensional pixel displacements between successive frames. Field validation was carried out on the Wolf River, a small coastal plain river near Landon, Mississippi, United States, on 26–27 May 2010 by collecting imagery in association with in situ velocities sampled using electromagnetic current meters deployed 0.1 m below the river surface. Comparisons are made between mean in situ velocities and image‐derived velocities from 23 thermal and 6 visible‐band image sequences (5 min length) during daylight and darkness conditions. The thermal signal was a small apparent temperature contrast induced by turbulent mixing of a thin layer of cooler water near the river surface with underlying warmer water. The visible‐band signal was foam on the water surface. For thermal imagery, streamwise velocities derived from the pixel time stack and particle image velocimetry technique were generally highly correlated to mean streamwise current meter velocities during darkness (r2 typically greater than 0.9) and early morning daylight (r2 typically greater than 0.83). Streamwise velocities from the pixel time stack technique had high correlation for visible‐band imagery during early morning daylight hours with respect to mean current meter velocities (r2 > 0.86). Streamwise velocities for the particle image velocimetry technique for visible‐band imagery had weaker correlations with only three out of six correlations performed having an r2 exceeding 0.6.

Hydrodynamics and associated sediment transport over coastal wetlands in Quanzhou Bay, China

Coastal salt marshes represent an important coastal wetland system. In order to understand the differences between boundary layer parameters of vegetated and unvegetated areas as well as the mechanisms of sediment transport, several electromagnetic current meters (AEM HR, products of Alec Electronics Co. Ltd.) were deployed in coastal wetlands in Quanzhou Bay, China, to measure current velocity. During the low tide phase, the surficial sediment was collected at 10 m intervals. In situ measurements show that the current velocities on the bare flat were much higher than those in the Spartina alterniflora marsh. Current velocity also varied with distance from marsh edge and plant canopy height and diameter. Around 63% of the velocity profiles in the tidal creek can be described by a logarithmic equation. Over the bare flat and Spartina alterniflora marsh, a logarithmic profile almost occurs during the flood tide phase. Sediment analysis shows that mean grain size was 6.7 Φ along the marsh edge, and surface sediments were transported from bare flat to marsh; the tidal creeks may change this sediment transport pattern. The hydrodynamics at early flood tide and late ebb tide phases determined the net transport direction within the study area.

Effects of turbulence on the transport of individual particles as bedload in a gravel‐bed river

AbstractThis study investigates the association between mean and turbulent flow variables and the movement of individual particles in a gravel‐bed river. The experimental design implemented in the Eaton‐North River (Québec, Canada) is based on the simultaneous observations at a high temporal resolution of both particle movements as bedload using an underwater video camera and of the streamwise and vertical flow velocity components using a vertical array of three electromagnetic current meters (ECMs). The frequency and distance of displacement of particles larger than 20 mm that were sliding or rolling on the bed were measured from a 10 minutes long film. Mean and turbulent flow properties obtained for periods without sediment transport are compared to those when particles were sliding and rolling. When particles are sliding, weak differences are present for the mean streamwise velocity and normal vertical stresses. Instantaneous Reynolds shear stresses are significantly lower for sliding events which was not expected but could be explained by the important dominance of Quadrant 3 events (inward interactions). When particles are rolling, only the vertical normal stresses show a weak difference from those observed in the absence of transport but they tend to occur when Quadrant 2 (ejections) dominate the flow field. For both sliding and rolling particles, vertical and/or streamwise fluid accelerations show high magnitude values when compared to periods without transport. For sliding particles, streamwise acceleration is mostly negative and combines most of the time with a positive vertical acceleration. For rolling particles, streamwise and vertical acceleration are predominantly of opposite sign. These results suggest that fluid acceleration or deceleration in the streamwise and vertical directions is affecting the pressure field around particles. In this study, fluid acceleration seems to play a more important role than Reynolds shear stress or normal stresses for bedload movements. Copyright © 2010 John Wiley & Sons, Ltd.

Validation of volume continuity method for estimation of cross-shore swash flow velocity

The use of flow volume continuity to estimate depth-averaged cross-shore swash flow velocity is validated using appropriate field data. Swash surface and beach face elevation were measured by a network of ultrasonic altimeters mounted through the swash zone of a sandy beach. Estimates of cross-shore flow velocity derived from these data compare well with concurrent measurements obtained from a vertical array of four electromagnetic current meters (EMCMs). Significantly, the continuity technique allows swash velocity to be estimated throughout the full swash cycle. This is not possible using fixed current meters and the new measurements indicate that swash velocity is more negatively skewed than previously observed.

Observations and analyses of floc size and floc settling velocity in coastal salt marsh of Luoyuan Bay, Fujian Province, China

In coastal environments, fine-grain sediments often aggregate into large and porous flocs. Electro-Magnetic Current Meters (EMCM) and Laser In Situ Scattering and Transmissometry (LISST-ST) have been deployed within a Spartina alterniflora marsh of the Luoyuan Bay in Fujian Province, China, to measure the current velocity, the floc size and the settling velocity between 15 and 22 January 2008. During the observations, the near-bed water was collected in order to obtain the suspended sediment concentration (SSC) and constituent grain size. Data show that: (1) the nearbed current velocities vary from 0.1 to 5.6 cm/s in the central Spartina alterniflora marsh and 0.1–12.5 cm/s at the edge; (2) the SSCs vary from 47 to 188 mg/dm3. The mean grain size of constituent grains varies from 7.0 to 9.6 µm, and the mean floc sizes (MFS) vary from 30.4 to 69.4 µm. The relationship between the mean floc size and settling velocity can be described as: ws=adb, in which ws is the floc settling velocity (mm/s), a and b are coefficients. The floc settling velocity varies from 0.17 to 0.32 mm/s, with a mean value of 0.26 mm/s, and the floc settling velocity during the flood tide is higher than that during the ebb tide. The current velocity and the SSC are the main factors controlling the flocculation processes and the floc settling velocity.

Net sediment transport and morphological change in the swash zone of a high-energy sandy beach from swash event to tidal cycle time scales

A field experiment was conducted on a high energy sandy beach (Truc Vert, France) to examine swash zone processes and shoreline erosion–accretion in response to changing wave and tide conditions. Beachface morphological changes were measured at the swash-by-swash timescale using an array of ultrasonic bed-level sensors and at the tidal timescale through total station surveys. Hydrodynamic and sediment dynamic data were collected using arrays of pressure transducers, electromagnetic current meters, optical backscatter sensors and total load sediment traps. Data were collected for 16 consecutive days (33 tides) over a spring–neap–spring tidal cycle with offshore significant wave heights of 1–4 m and periods of 5–12 s. The swash motion was predominantly at infragravity wave frequencies. Over the measurement period, the upper beach experienced phases of accretion, erosion and stability. Trapped sediment loads for the uprush and backwash were predicted reasonably well by cross-shore velocity skewness, but the prediction of net transport over individual swash events by velocity-based models was more problematic, particularly lower in the swash zone where swash interactions were more prevalent. Net sediment fluxes over individual swash events often exceeded 100 kg per unit meter beach width and such rates were only one order of magnitude less than the net transport typically experienced across the high tide swash zone over complete tidal cycles. Upper beach erosion–accretion at the tidal timescale was reasonably well predicted by the Dean criterion based on offshore wave steepness and dimensionless fall velocity, but the beach gradient also played a significant role in controlling the occurrence of erosion and accretion.

The Generation of Internal Wave with Large Amplitude Associated with intrusion of "Kyucho" in Summer

The observation using mooring system (Electromagnetic current meters, Temperature-Salinity meters and Fluorescence-Turbidity meter) at the set net and mounted Acoustic Doppler Current Profiler (ADCP) on the Suruga-wan ferry were performed in the inner part of the Suruga Bay to monitoring of current variation. The vertical profile of a strong current, the maximum current about 30cm/s, caused by the middle layer intrusion of the Kuroshio warm water was observed with ADCP mounted on the bottom of ferry boat in the summer 2008. After the middle layer intrusion of the Kuroshio water, the internal wave with large amplitude was occurred. The maximum current speed was about 80 cm/s at the eastern part of the bay and the period was half a day. The Kuroshio warm water changed vertical density profile and amplified internal wave.

Sediment dynamic responses of coastal salt marsh to typhoon “KAEMI” in Quanzhou Bay, Fujian Province, China

In order to understand the mechanisms of coastal protection by salt marshes during typhoon events, in situ measurements of water level, tidal current speed and direction, and suspended sediment concentration (SSC) were carried out using Electromagnetic Current Meter (EMCM, AEM HR), miniature pressure sensor (MkV/D) and Seapoint Turbidity Meter (STM) sensor on a tidal flat in Quanzhou Bay, during the period when the typhoon “KAEMI” was passing through the region. The analysis of the data obtained shows that the near-bed current speed within the Spartina alterniflora marsh was generally below 5 cm s−1, which was apparently smaller than on the adjacent bare flat (i.e. 5–30 cm s−1). The change in the near-bed current speed in response to the typhoon event was not significant within the S. alterniflora marsh, but the current direction was influenced by the typhoon. The effect of the typhoon on the SSC was highly significant, with the SSC reaching 13 to 19 times the values on the bare flat or within the marsh under fair weather conditions; the near-bed SSC within the marsh was higher than on the bare flat, after the typhoon landed. The near-bed suspended sediment fluxes within the marsh and on the bare flat during the typhoon event were both enhanced, i.e., 4 times larger than under fair weather conditions. During the ebb, the bottom shear stress on the bare flat exceeded the critical shear stress for sediment motion for most of the ebb duration of the tide and, therefore, the bed sediment was eroding, with the erosion flux after the typhoon landed being around 2 to 3 times the value associated with fair weather conditions. In contrast, within the S. alterniflora marsh, the bottom shear stress was mostly lower than the critical shear stress for sediment motion, or lower than the critical shear stress for the maintenance of suspension; hence, the marsh surface was dominated by settling processes, with a settling flux during the typhoon being 3 to 6 times compared with the fair weather situations. The settling flux during the ebb was up to twice the magnitude for the flood. The comparison of sediment dynamic processes between the S. alterniflora marsh and the bare flat demonstrates that during the typhoon event the bare flat was dominated by erosion, whilst the presence of S. alterniflora favored the accretion of suspended sediment.

MEDIÇÕES DE CORRENTES E CURVA VAzÃO-MARÉ NA BAíA DE PARANAGUÁ, PR

O presente trabalho traz informações sobre coleta de dados de correntes realizadas na Baía de Paranaguá,PR, Brasil, utilizando correntômetros eletromagnéticos (S4) e de efeito Doppler (ADCP e ADP), assim como doestudo dos sedimentos em suspensão observados na coluna d’água. A comparação dos resultados das correntesobservadas, por todos os instrumentos, mostrou que estes são muito equivalentes neste tipo de observação. Poroutro lado, a vazão líquida obtida com base nos dados dos correntômetros acústicos resultaram completamentecompatíveis com o prisma de maré (vazão de maré). Os dados de vazão assim obtidos permitiram a elaboraçãodo equivalente estuarino das curvas de vazão, das quais, conhecido o nível, é possível calcular a vazão numadada seção. As análises de material particulado em suspensão foram utilizadas, também, para tentar correlacionaro MPS com a intensidade do eco do ADCP, observando-se que é possível utilizar esta intensidade paraestimativas do MPS. Desde o ponto de vista dinâmico, a principal característica observada na circulação daBaía de Paranaguá refere-se à heterogeneidade lateral das seções, onde apareceram claramente sistemas decirculação residual, em que as áreas da seção são usadas principalmente pelas correntes de enchente, sendooutras para as correntes de vazante. Esta circulação preferencial pode explicar as diferentes condições lateraisnos canais, onde na área de preponderância das correntes de enchente observam-se condições biológicas esedimentológicas com características mais “marinhas”, sendo que nas áreas de vazante, ocorre o contrário,com fauna e sedimentos menos marinhos e mais continentais ou interiores.

Bar Morphodynamic Behaviour on the Ebb Delta of a Macrotidal Inlet (Normandy, France)

The purpose of this paper is to describe the morphodynamics of bars located on an ebb delta of a macrotidal inlet system. Although there have been numerous studies on the morphology and physical processes affecting such bars, these concern only microtidal and mesotidal settings. The west Cotentin coast (Normandy, France) is a fine example of a macrotidal coast with one of the largest tidal ranges in the world. The mean spring tidal range is up to 11 m. Fieldwork was undertaken during one month and involved the deployment of an electromagnetic current meter, a pressure sensor, and differential global positioning system surveys to measure topographic changes. Along the western coast of Cotentin, in spite of the very large tidal inlets, bars are generally small (2 m high and 250 m long) compared with other tidal environments. During the campaign, the bar migrated exclusively landward over a distance of 4 m. Its volume did not change significantly. This slow migration rate is attributed to the large tidal range. In a macrotidal setting, the tidal water level fluctuations control wave height during storms, the duration of emergence and flooding periods, and the intensity and direction of longshore tidal currents. Field observations showed that bar migration rate is correlated with wave activity, whereas volume variations are controlled both by mean tidal currents and wave conditions. These results indicate a morphodynamic behaviour strongly related to specific local macrotidal hydrodynamic conditions, the tidal prism influence being less important.

Characteristics of coastal-trapped waves induced by typhoon along the southeast coast of Honshu, Japan

Mooring observations using ADCP, electromagnetic current meters and thermometers were performed to clarify the vertical and horizontal structure of coastal-trapped waves (CTWs) on continental shelf and slope on the eastern side of Sagami Bay, Japan, in August and September 2003. A strong inflow associated with CTW caused by Typhoon 0315 (CTW15) was observed with remarkable downwelling. The maximum current due to CTW15 was over 100 cm s−1, confined to the upper layer shallower than 90 m. The CTW (CTW10) induced by Typhoon 0310, was associated with the coastal upwelling and maximum outflow was 33 cm s−1; the currents were extended near the bottom at 230 m depth. Remarkable discrepancies were found between the current structures of CTWs. CTW15 was explained by superposing the second CTW mode on the first CTW mode, whereas CTW10 was explained by the first CTW mode. The generation and propagation processes of both CTWs were reproduced by numerical experiments using a three-dimensional level model. The model results indicated that the difference of modal characteristics between CTW15 and CTW10 already exists in the CTW generation region and are due to difference of the wind direction, i.e., the typhoon’s path.

Field observations of nearshore bar formation

AbstractThe formation of an inner nearshore bar was observed during a high‐energy event at the sandy beach of Vejers, Denmark. The bar accreted in situ during surf zone conditions and the growth of the bar was associated with the development of a trough landward of the bar. Measurements of hydrodynamics and sediment fluxes were obtained from electromagnetic current meters and optical backscatter sensors. These process measurements showed that a divergence in sediment transport occurred at the location of the developing trough, and observed gradients in cross‐shore net sediment flux were consistent with the morphological development. The main cause for the flux gradients were cross‐shore gradients in offshore‐directed mean current (undertow) speed which depended upon local relative wave height and local bed slope. Copyright © 2007 John Wiley & Sons, Ltd.

Measuring water velocity in highly turbulent flows: field tests of an electromagnetic current meter (ECM) and an acoustic Doppler velocimeter (ADV)

AbstractTwo field tests were completed to compare the performance of an electromagnetic current meter (ECM) with that of an acoustic Doppler velocimeter (ADV) in gravel‐bed rivers. Research was particularly motivated by the need to measure flow properties in highly energetic turbulent flows. Measurements were made at two field sites, one at moderate velocities (up to 70 cm/s) and with moderate turbulence intensities (10–20% of mean flow), and the other in an area of non‐uniform flow that included locations with fast mean velocities (up to 1.75 m/s) and high turbulent intensities (up to 50% of mean flow). Comparison of means, standard deviations, turbulent kinetic energy and Reynolds shear stress confirm the general agreement between the ECMs and ADVs. The general agreement is subject to limitations associated with the sample volume and frequency response of the instruments, and only applies within restricted velocity (up to ≈1.25 m/s) and turbulence intensity ranges (up to ≈0·125 m/s). At higher turbulence intensities, spectral analysis showed anomalous behavior of the ADV signal, especially in the vertical velocity component. Quadrant analysis of the Reynolds stress suggests that these problems occur predominantly in quadrants 1 and 3. Errors in ADV measurements were estimated using four different methods: one that utilized the characteristic noise floor in spectral plots, one based on internal ADV measurements of signal correlation and two techniques that aggregate errors related to various sub‐factors. Estimates were divergent at high flows. Techniques that rely on sub‐factors appeared to underestimate the impact of high turbulence on signal quality. The key conclusion for future field applications is that the older ECM technology provides the more reliable estimates of flow parameters in high turbulence. Copyright © 2007 John Wiley & Sons, Ltd.

Cross‐shore sediment transport on natural beaches and its relation to sandbar migration patterns: 1. Field observations and derivation of a transport parameterization

Cross‐shore sediment transport processes are investigated with measurements of horizontal velocity and sediment suspension obtained with electromagnetic current meters and optical backscatter sensors on five different beaches across Europe. Data were gathered under a wide variety of hydrodynamic and morphological conditions spanning the swash, surf, and shoaling zones. Results show that the near bed velocity moments, normalized by the local energy level (〈ut2〉n), have consistent shapes (shape functions) when plotted against normalized cross‐shore depth (h/hb). According to the energetics approach, near bed velocity moments are good descriptors of sediment transport processes, consequently, the shape functions describe and quantify the cross‐shore distribution of the most relevant cross‐shore sediment transport processes. The pattern consists of net onshore transport in the swash zone, offshore transport in the surf zone, and onshore transport in the shoaling zone thereby producing divergence of sediment in the inner surf/swash zone and convergence in the breaker zone. In consequence, the shape function supports the breakpoint hypothesis for sandbar generation. This behavior is a product of the balance between multiple opposing mechanisms including undertow, coupling between mean flows and short (long) wave stirring, short (long) wave skewness, and long wave coupling with short wave variance. The cross‐shore structure of the measured and normalized cross‐shore sediment fluxes is consistent with the velocity moment shape functions, showing the robustness of the approach. Apart from the shortcomings bound to the energetics approach, the present parameterization is also limited due to the difficulty of defining with confidence a breaker depth (hb) in bar‐trough systems.

FLOW RESISTANCE AND TURBULENT STRUCTURE IN OPEN CHANNEL FLOW WITH SUBMERGED VEGETATION

Vegetation colonies are often observed to grow in stripe pattern or patches within many stream channel. In this paper, the influence that interaction of longitudinal vegetation zones gave to the flow resistance in an open-channel was examined experimentally. Laboratory experiments were conducted using a two-component electromagnetic current-meter to clarify the momentum transport between main flow and flexible artificial vegetation. Results showed that the flow resistance significantly depended on the lateral distance between longitudinal vegetation zones and the reason could be explained by the difference of convective momentum transport through the interaction between longitudinal vegetation zones.

植生群落間の相互作用が大規模水平渦の発生に与える影響

Aquatic vegetation frequently determines the mean and turbulent flow sttucture in natural river and impacts the transport of sediment and nutrients. These vegetation communities are often observed to grow in stripe pattern or patches within main stream channel. In this paper, the influence that interaction of longitudinal vegetation zones gave the momentum transport and coherent vortex generation in an open-channel was examined experimentally. Laboratory experiments were conducted using a two-component electromagnetic current-meter and a particle-image-velocimetry (PIV). Results showed that horizontal large scale vortex generation significantly depended on the lateral distance between longitudinal vegetation zones and these vortices controls the momentum transporthrough the interaction.

Mass transfer limitation of photosynthesis of coral reef algal turfs

Algal turfs are the major primary producing component on many coral reefs and this production supports higher levels in the complex reef trophic web. Rates of metabolism of algal turfs are related positively to water motion, consistent with limitation by the diffusion of a substance through a boundary layer. Based on engineering mass transfer theory, we hypothesized that photosynthesis of algal turfs is controlled by rates of mass transfer and responses of photosynthesis to increasing flow speed should be predicted by engineering correlations. This hypothesis was tested in ten experiments where photosynthesis was estimated in a flume/respirometer from changes in dissolved oxygen at eight flow speeds between 0.08 and 0.52 m/s. Flow in the flume and over the reef at Kaneohe Bay, Oahu, Hawaii was estimated using hot-film thermistor and electromagnetic current meters. Rates of photosynthesis were related positively to flow in all experiments and plots of the log of the average Sherwood number (Shmeas) versus log Reynolds number (ReD) for each experiment are lower than predicted for mass transfer through a turbulent boundary layer. Algal turf-covered plates are characterized as hydrodynamically transitional to fully rough surfaces and the lower than predicted slopes suggest that roughness reduces rates of mass transfer. A negative correlation between algal turf biomass and slopes of the log Shmeas−log ReD plots suggests that mass transfer to algal turfs is affected significantly by the physical structure of the algal community. Patterns of photosynthesis based on changes in dissolved oxygen and dissolved inorganic carbon concentrations (DIC) indicate that the flow speed effect is not the result of increased flux of oxygen from the algal turfs, and combined with the short response time to flow speed, suggest that DIC may limit rates of photosynthesis. Although there are differences between flow in the flume and flow over algal turfs on the reef, these results suggest that photosynthesis is controlled, at least in part, by mass transfer. The chemical engineering approach provides a framework to pose further testable hypotheses about how algal canopy height, flow oscillation, turbulence, and substratum roughness may modulate rates of metabolism of coral reef algal turfs.

Controls on spatial patterns of sediment deposition across a macro-tidal salt marsh surface over single tidal cycles

Abstract A field study was conducted to determine the controls on spatial patterns of sediment deposition across a salt marsh surface in the Bay of Fundy. Approximately 670 surface-mounted sediment traps were deployed over 28 different tidal cycles in a variety of spatial configurations and environmental conditions. Spatial patterns of deposition were derived using spatial interpolation procedures in ArcView GIS 3.2. Suspended sediment concentration, flow characteristics and water depth were measured using co-located optical backscatterance sensors, electromagnetic current meters and a pressure transducer. Sediment deposition was a complex function of variables controlling the availability of sediment and the opportunity for this sediment to be deposited. The relative importance of inundation time, distance from source material, relative roughness and suspended sediment concentration varied across the marsh surface. Wave activity, however, exerted a significant influence on both the temporal and spatial patterns of sediment deposition, particularly through an increase in suspended sediment concentrations and through transport of suspended sediment further up into the mid and high marsh during spring tides. In addition, the region around the mean high water level appears to form a transition zone for processes of sediment transport and deposition, with most deposition taking place around this region. This study also emphasizes the importance of knowing precisely where one's sample stations are within the tidal frame. Modelling exercises must therefore consider spatial and temporal variability in sedimentary processes across a salt marsh surface. In addition, this study suggests sedimentation models developed for sheltered coastal marshes cannot necessarily be applied to open coastal marshes exposed to wave activity.