Spring-neap Variations Research Articles

Tidal resuspension of microphytobenthic chlorophyll a in a Nanaura mudflat, Saga, Ariake Sea, Japan: flood–ebb and spring–neap variations

The resuspension of microphytobenthic chlorophyll a (chl a) and suspended particulate matter (SPM) from the upper intertidal flat was studied at Nanaura, Ariake Sea, Japan, and a spatial comparison of chl a and SPM in the northern Ariake Sea is presented here. A 15 d time series of chl a and SPM records (measured at 15 cm above the bottom) revealed that resuspension was clearly asso- ciated with flood-ebb and spring-neap tidal characteristics, where significant resuspension was found during early flood, particularly in the high-energy period (viz. spring tide). The peaks of resus- pended chl a and SPM during 28 tidal periods were observed to consistently occur shortly after the current velocity reached a maximum and when the threshold current velocity was found to be ca. 15 cm s -1 . Much more highly fluctuating, irregular peaks of chl a and SPM were occasionally observed, these were attributable to high wind speeds, ≥3 m s -1 . Computation of chl a and SPM fluxes showed clear spring-neap variation, and time-integrated fluxes denoted predominant offshore residual transport during the observation period. Benthic chl a measured in the surficial sediment (top 0.5 cm) during 13 exposure periods showed a decreasing trend from spring to neap tide, similar to spring-neap variations of chl a in the water column during the semi-lunar tidal period. The daily mean percentage of resuspended chl a in the water column relative to the amount of benthic chl a was estimated to be ca. 10 to 70% (mean = 33%), implying that a certain portion of the microphyto- benthos is resuspended during high tide and partly contributes to the total biomass in the water column of the upper intertidal flat.

Boundary Layer Forcing of a Semidiurnal, Cross-Channel Seiche

Abstract The outer Strait of Juan de Fuca is a stratified, tidal channel about 100 km long, 20 km wide, and 200 m deep. Tidal currents of O(0.5 m s−1) occur at both diurnal and semidiurnal periods and there is a pronounced spring–neap variation in the stratification due to changes in tidal mixing. Vertical isotherm excursions of O(50 m) have previously been observed along the northern side of the channel that appear to be phase locked to the tidal currents. Analyses of recent ADCP and thermistor chain data confirm the isotherm excursions and find that they are accompanied by distinctive baroclinic structures in the horizontal currents that persist across spring–neap cycles, and from year to year. The authors find that the phase of the semidiurnal signal does not vary along the channel, as would be expected of an along-channel internal tide. Instead, comparison of the semidiurnal measurements with two-dimensional analytical and numerical models indicate that much of the baroclinic structure can be explained as a cross-channel, internal seiche that is locally driven by reversing Ekman forcing in the bottom boundary layer. Near the bottom, the vertical motions are kinematic, resulting primarily from cross-channel flow on the side slopes. In the middepths, the seiche appears to explain part but not all of the vertical motions. The seiche appears to persist and to maintain a similar mode shape across large, O(2), changes in stratification. In Juan de Fuca, it is expected that these motions affect the overall salt balance of the system by enhancing mixing between the upper and lower layers of the estuary.

In situ measurements of spring–neap variations to unsteady island wake development in the Firth of Forth, Scotland

Field data and a numerical model have been used to investigate the von Kármán vortex street generated by a 50 m wide island in the Firth of Forth for which the characteristics differ between flood and ebb. During the flood, water depth in the lee of the island is shallow, hence little vertical structure develops in the velocity due to bottom-induced mixing. In contrast, the ebbing water depth is deeper in the lee of the island, allowing vertical structure to develop in the velocity field. However, the ebb wake is complicated by the presence of a lateral flow due to a bathymetric restriction. The effect of this lateral flow is to inhibit development of the ebb wake during spring tide conditions; whereas the effect is minimal during neap (lower freestream velocity) conditions. This change in wake development was clearly visible in the in situ horizontal and vertical velocity fields. While the observed flood wake varied considerably between neap and spring, the characteristics of the ebb wake were similar for neap and spring because of this overbank flow. The overbank flow has an important role in determining the varying magnitude of the vorticity of eddies generated either side of the island.

Ocean distribution of dungeness crab megalopae and recruitment patterns to estuaries in Southern Washington State

We investigated the distribution of meroplankton and water properties off southern Washington and simultaneously measured time series of larval abundance and water properties in two adjacent estuaries, Grays Harbor and Willapa Bay. The cruise period, in late May 1999, coincided with large variation in the alongshore wind stress that caused dynamic change in the position of the Columbia River plume, coastal upelling and downwelling, and offshore phytoplankton production. In the coastal ocean, meroplankton groups responded differently to this wind event and the associated advection of water masses. Dungeness crab (Cancer magister) megalopae were largely indifferent to the wide salinity variation, and were found throughout the surveyed area in both plume and recently upwelled waters. Megalopae of kelp crab (Pugettia producta) and hermit crab (Pagurus spp). were more abundant in upwelled water and low numbers were caught in the plume water. Barnacle cyprids appeared to track the advective transport suggesting that they may be more passively dispersed. Within the estuaries, hydrography responded rapidly and synchronously to variation in wind stress. Intrusions of both plume and newly upwelled waters were detected at estuarine sites, depending on the type of water present at the coast, indicating a tight link between the estuaries and the coastal ocean in this region. A 90-d record ofC. magister megalopae abundance was made at 3 estuarine sites using light traps. The bulk of theC. magister recruitment was limited to a relatively brief period in late May through June. Within this window, megalopae occurred in distinct pulses of 3–5 d interspaced with periods of low or zero abundance.C. magister megalopae recruited to the estuaries over a wide range of wind forcing, and were transported into the estuary within varied water types. There were no periodic patterns indicative of spring-neap tidal variations in the abundance time series. Abundance was only weakly cross-correlated between the adjacent Grays Harbor and Willapa Bay estuaries, which contrasts with the more synchronous estuarine-coastal linkages measured for water properties. These results suggest the interaction of larval aggregation size in the ocean with estuary-ocean exchange processes likely controls patterns of estuarine recruitment.

Tidal and seasonal nutrient dynamics and budget of the Chupa Estuary, White Sea (Russia)

This study is the third in a series of papers on the Chupa Estuary, White Sea, Russia, and investigates tidal and seasonal fate and fluxes of materials from a small fjord-like Arctic estuary to the coastal zone. Respective companion papers have detailed the hydrography (Estuar. Coast. Shelf Sci. 48 (1999) 1) and metals in sediments and mussels (Estuar. Coast. Shelf Sci. 48 (1999) 13). Two seasonal cruises were undertaken in summer (July 1994) and autumn (September, 1995). Nutrients (nitrate, nitrite, phosphate and silicate) and dissolved oxygen were recorded along the estuarine longitudinal axis and at diurnal anchor stations at key points in the estuary. The fluvial nutrient input was negligible throughout. Nutrient concentrations showed considerable inter-tidal and spring–neap variability. Evidence from isopycnals showed that this variability is related to water exchange processes in the deep basins where renewal takes place within the space of a tidal cycle. Deep-water renewal and cascading is more pronounced on neap tides due to reduced turbulent mixing. The downwelling of seawater ensures that the deep basin waters are well ventilated, and provides a means of coupling between bottom and surface water nutrient pools whereby remineralized nutrients are dispersed and recirculated. Nutrient fluxes to the coastal zone were quantified in autumn as a net import of dissolved inorganic silicate (DSi) and dissolved inorganic phosphate (DIP) at 2.10 and 0.51 mol s −1, respectively, and a dissolved inorganic nitrate + nitrite (DIN) export of 1.72 mol s −1. In July DIN, DIP and DSi were exported at 3.57, 0.41 and 7.04 mol s −1, respectively. An assessment of hydrodynamical parameters shows that the circulation is complex throughout the estuary, and so the resolved fluxes are semi-quantitative. Fjord-like circulation results in a large export of inorganic nutrients in the lower nutrient-rich layers compared to the incoming intermediate layers, which drives the system towards N-limitation in both summer and autumn, thus lending support to the concept of N-limiting tendencies observed in other sub-Arctic areas.

Effects of spatial and temporal variability of turbidity on phytoplankton blooms

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 254:111-128 (2003) - doi:10.3354/meps254111 Effects of spatial and temporal variability of turbidity on phytoplankton blooms Christine L. May1,*, Jeffrey R. Koseff1, Lisa V. Lucas2, James E. Cloern2, David H. Schoellhamer3 1Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, USA 2US Geological Service, 345 Middlefield Road, Menlo Park, California 94025, USA 3US Geological Service, Placer Hall, 6000 J Street, Sacramento, California 95819-6129, USA *Email: kmay@stanford.edu ABSTRACT: A central challenge of coastal ecology is sorting out the interacting spatial and temporal components of environmental variability that combine to drive changes in phytoplankton biomass. For 2 decades, we have combined sustained observation and experimentation in South San Francisco Bay (SSFB) with numerical modeling analyses to search for general principles that define phytoplankton population responses to physical dynamics characteristic of shallow, nutrient-rich coastal waters having complex bathymetry and influenced by tides, wind and river flow. This study is the latest contribution where we investigate light-limited phytoplankton growth using a numerical model, by modeling turbidity as a function of suspended sediment concentrations (SSC). The goal was to explore the sensitivity of estuarine phytoplankton dynamics to spatial and temporal variations in turbidity, and to synthesize outcomes of simulation experiments into a new conceptual framework for defining the combinations of physical-biological forcings that promote or preclude development of phytoplankton blooms in coastal ecosystems. The 3 main conclusions of this study are: (1) The timing of the wind with semidiurnal tides and the spring-neap cycle can significantly enhance spring-neap variability in turbidity and phytoplankton biomass; (2) Fetch is a significant factor potentially affecting phytoplankton dynamics by enhancing and/or creating spatial variability in turbidity; and (3) It is possible to parameterize the combined effect of the processes influencing turbidity‹and thus affecting potential phytoplankton bloom development‹with 2 indices for vertical and horizontal clearing of the water column. Our conceptual framework is built around these 2 indices, providing a means to determine under what conditions a phytoplankton bloom can occur, and whether a potential bloom is only locally supported or system-wide in scale. This conceptual framework provides a tool for exploring the inherent light climate attributes of shallow estuarine ecosystems and helps determine susceptibility to the harmful effects of nutrient enrichment. KEY WORDS: Estuaries · Coastal ecosystems · Light limitation · Turbidity · Phytoplankton · Numerical modeling · Coastal eutrophication · Nutrient enrichment Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 254. Online publication date: June 04, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Processes governing phytoplankton blooms in estuaries. I:The local production-loss balance

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 187:1-15 (1999) - doi:10.3354/meps187001 Processes governing phytoplankton blooms in estuaries. I: The local production-loss balance Lisa V. Lucas1,*, Jeffrey R. Koseff2, James E. Cloern1, Stephen G. Monismith2, Janet K. Thompson1,2 1United States Geological Survey, 345 Middlefield Road, MS #496, Menlo Park, California 94025, USA 2Environmental Fluid Mechanics Laboratory, Dept. of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, USA *E-mail: llucas@usgs.gov ABSTRACT: The formation and spatial distribution of phytoplankton blooms in estuaries are controlled by (1) local mechanisms, which determine the production-loss balance for a water column at a particular spatial location (i.e. control if a bloom is possible), and (2) transport-related mechanisms, which govern biomass distribution (i.e. control if and where a bloom actually occurs). In this study, the first of a 2-paper series, we use a depth-averaged numerical model as a theoretical tool to describe how interacting local conditions (water column height, light availability, benthic grazing) influence the local balance between phytoplankton sources and sinks. We also explore trends in the spatial variability of the production-loss balance across the topographic gradients between deep channels and lateral shoals which are characteristic of shallow estuaries. For example, under conditions of high turbidity and slow benthic grazing the highest rates of phytoplankton population growth are found in the shallowest regions. On the other hand, with low turbidity and rapid benthic grazing the highest growth rates occur in the deeper areas. We also explore the effects of semidiurnal tidal variation in water column height, as well as spring-neap variability. Local population growth in the shallowest regions is very sensitive to tidal-scale shallowing and deepening of the water column, especially in the presence of benthic grazing. A spring-neap signal in population growth rate is also prominent in the shallow areas. Population growth in deeper regions is less sensitive to temporal variations in tidal elevation. These results show that both shallow and deep regions of estuaries can act as sources or sinks for phytoplankton biomass, depending on the local conditions of mean water column height, tidal amplitude, light-limited growth rate, and consumption by grazers. KEY WORDS: Phytoplankton · Estuaries · Model · Benthic grazing · Bathymetry · Light Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 187. Publication date: October 14, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

The long-term salinity field in San Francisco Bay

Data are presented on long-term salinity behaviour in San Francisco Bay, California. A two-level, width averaged model of the tidally averaged salinity and circulation has been written in order to interpret the long-term (days to decades) salinity variability. The model has been used to simulate daily averaged salinity in the upper and lower levels of a 51 segment discretization of the Bay over the 22-yr period 1967–1988. Monthly averaged surface salinity from observations and monthly-averaged simulated salinity are in reasonable agreement. Good agreement is obtained from comparison with daily averaged salinity measured in the upper reaches of North Bay. The salinity variability is driven primarily by freshwater inflow with relatively minor oceanic influence. All stations exhibit a marked seasonal cycle in accordance with the Mediterranean climate, as well as a rich spectrum of variability due to extreme inflow events and extended periods of drought. Monthly averaged salinity intrusion positions have a pronounced seasonal variability and show an approximately linear response to the logarithm of monthly averaged Delta inflow. Although few observed data are available for studies of long-term salinity stratification, modelled stratification is found to be strongly dependent on freshwater inflow; the nature of that dependence varies throughout the Bay. Near the Golden Gate, stratification tends to increase up to very high inflows. In the central reaches of North Bay, modelled stratification maximizes as a function of inflow and further inflow reduces stratification. Near the head of North Bay, lowest summer inflows are associated with the greatest modelled stratification. Observations from the central reaches of North Bay show marked spring-neap variations in stratification and gravitational circulation, both being stronger at neap tides. This spring-neap variation is simulated by the model. A feature of the modelled stratification is a hysteresis in which, for a given spring-neap tidal range and fairly steady inflows, the stratification is higher progressing from neaps to springs than from springs to neaps. The simulated responses of the Bay to perturbations in coastal sea salinity and Delta inflow have been used to further delineate the time-scales of salinity variability. Simulations have been performed about low inflow, steady-state conditions for both salinity and Delta inflow perturbations. For salinity perturbations a small, sinusoidal salinity signal with a period of 1 yr has been applied at the coastal boundary as well as a pulse of salinity with a duration of one day. For Delta inflow perturbations a small, sinusoidally varying inflow signal with a period of 1 yr has been superimposed on an otherwise constant Delta inflow, as well as a pulse of inflow with a duration of one day. Perturbations in coastal salinity dissipate as they move through the Bay. Seasonal perturbations require about 40–45 days to propagate from the coastal ocean to the Delta and to the head of South Bay. The response times of the model to perturbations in freshwater inflow are faster than this in North Bay and comparable in South Bay. In North Bay, time-scales are consistent with advection due to lower level, up-estuary transport of coastal salinity perturbations; for inflow perturbations, faster response times arise from both upper level, down-estuary advection and much faster, down-estuary migration of isohalines in response to inflow volume continuity. In South Bay, the dominant time-scales are governed by tidal dispersion.

Biogenic sedimentary structures on a Korean mud flat: Spring-neap variations

Biogenic sedimentary structures created by dominant benthic animals of a mud flat were investigated in a small embayment at Panweol on the west coast of Korea. Burrows were replicated using the resin casting method on a sufficiently large scale to evaluate their variations statistically. The size and shape of burrows and other traces varied with locations and tidal phases. Burrows of the upper-intertidal crabs Helice tridens sheni and Ilyoplax dentimerosa were T- or I-shaped, vertical and deep whereas those of the mid-intertidal crab Macrophthalmus japonicus were U-shaped, oblique and shallow. The upper-intertidal polychaete Perineris aibuhitensis, a subsurface feeder, had sinuous and highly-branched burrows while the mid-internal one Periserrula leucophryna, a surface feeder, had vertical burrows without subsurface branches. Variations of crab burrows with tidal phases were distinct: The burrows of both H. tridens sheni and M. japonicus were significantly deeper and wider during neap tides than during spring tides. Sediment mounds were common in the upper intertidal zone, while crawling and feeding traces such as trails, trackways and cheliped scrapings were more prominent in the mid-intertidal zone. These traces were also observed in the upper intertidal zone during spring tides when surface sediments became watery. The size of the sediment mounds varied, being largest during neap tides. These biogenic sedimentary structures observed at Panweol were distinctly different from those found near Inchon only 50 km away, probably due to differences in hydrology and sedimentology.

Morphodynamics of intertidal dunes: a year-long study at Lifeboat Station Bank, Wells-next-the-Sea, Eastern England

Sand is driven clockwise by tidal currents round a diamond-shaped sand shoal in the entrance to the harbour at Wells-next-the-Sea. A population of dune bedforms occupying the ebb-dominated channel on the western side of the shoal was monitored for a year (October 1975 to October 1976) in terms of shape (height, wavelength, superimposed immature dunes), movement and bedform composition. Individual dunes and the population of dunes as a whole changed in response to sediment transport on astronomical tidal scales varying from the semi-diurnal to the equinoctial and in response to seasonal meteorological forcing. Most change occurred during the autumn and winter, when strong winds and gales created surges which, in some cases very significantly, enhanced the sediment transport ue purely to the astronomical tide. Dune height was on average greatest during the winter, when the sea temperature was low, and least during the period of summer warmth. Dune height also varied substantially on a spring-neap tidal scale, an increase in height with the onset of many of the springs being followed by a gradual lowering. Dune wavelength showed little response to the springneap variation of sediment transport, but decreased significantly between winter and summer. Varying degrees of time-lag accompanied all changes in bedform characteristics in response to hydraulic change. Although individual dunes had surprisingly large lifespans and ebb-directed excursions, some change was noted m the composition and statistical attributes of the bedform population as the result of appearances and disappearances. The more vigorous episodes of sediment transport created immature (nascent) dunes, some of which grew large enough to become incorporated as new members into the population of mature forms Internally, the dunes were dominated by ebb-oriented cross-bedding, complicated by a variety of intra-set discontinuities commonly associated with mud drapes Some recorded the smoothing of crests during tidal reversal and others the immobility of the bedform over a number of tidal cycles (occasionally many). Other discontinuities expressed the ‘capture’ of immature dunes by the main bedform within the duration of a single ebb-tide. Because of the frequent perturbation of the (astronomical) tidal sediment transport by meterological events, little order to the horizontal arrangement of discontinuities within the sets was detected in contrast to other reported cases.

Fourier and Autocorrelation Analysis of Estuarine Tidal Rhythmites, Lower Breathitt Formation (Pennsylvanian), Eastern Kentucky, USA

ABSTRACT Outcrops of the Pennsylvanian Breathitt Formation in eastern Kentucky reveal a rhythmic pattern of siliciclastic sedimentation in a marginal marine coastal setting. A 15-23 m thick stratigraphic interval of thinly interbedded, fine sandstone and shale displays tidally generated features such as flaser and wavy current ripple bedding, bipolar paleocurrents, and cyclic thickening and thinning of mud-draped sandstone layers. Bioturbation is common and increases toward the top of the section. Ichnogenera include Asterosoma, Arenicolites, Olivellites, Conostichus, Scalarituba, Planolites, Zoophycos, Teichichnus, Curvolithus, Chondrites, Neonereites, and Rosselia. A statistical analysis of sand layer thickness was carried out using shale partings as bounding surfaces for the individual sand units. Fourier and autocorrelation analyses were performed on two vertical sequences containing a total of over 2100 layers. The results reveal the presence of four cycles of thickness variation. First-order cycles consist of alternating thick-thin sand layers. These daily couplets may reflect unequal flood and ebb currents during a single tidal cycle or dominant and subordinate tidal deposits in an ebb or flood dominated semidiurnal or mixed system. Second-order cycles typically consist of 11-14 sand layers and reflect spring-neap variations in tidal range and current velocities. Third-order cycles are usually composed of 24-35 layers and are formed in resp nse to monthly variations in tidal range resulting from the ellipticity of the moon's orbit. Fourth-order cycles generally contain about 150 layers (range, 100-166) and were caused by seasonal maxima in tidal range associated with the solstice (winter, summer) and seasonal minima associated with the equinox (spring, fall).

Observations and analysis of a stratification-destratification event in a tropical estuary

A data set comprising 31 continuous tidal cycles was collected in the Sungai Merbok Estuary, Malaysia, in June 1987 as part of an ecological study of nutrient fluxes from a tropical mangrove estuary. Currents, salinity and salinity stratification at a deep-channel (15 m) station near the mouth of the Merbok Estuary showed a pronounced spring-neap variability. The slow currents and weak vertical mixing at neap tides favoured the formation of a stratified water column and generated a neap-spring cycle of water column stabilization and destabilization. A strong stratification event occurred during the period of observations. This was partly driven by a modest freshwater spate which coincided with neap tides. An eddy viscosity-diffusivity model of the stratification, which assumed a constant, longitudinal salinity gradient, demonstrated a pronounced stratification-destratification cycle due to neap-spring variations in vertical mixing. Larger and more realistic stratification was modelled when the estimated, time-varying longitudinal salinity gradient was incorporated. This gradient maximized in response to the peak in freshwater runoff. The measured and modelled density-driven circulations showed qualitative similarities and were of the order of 10 cm s −1 at neap tides. The circulation was weaker during spring tides. The tidally-filtered salt transport due to vertical shear was directed up-estuary and was an order of magnitude smaller during spring tides. The results are discussed in terms of their relevance to mangrove system oceanography.

Computed and Observed Currents, Elevations, and Salinity in a Branching Estuary

A one-dimensional, hydrodynamical model of the Tamar Estuary shows good agreement with measured tidal elevations and currents. Computed currents are used to drive a one-dimensional moving-element model of the salt balance. The moving-element model overcomes the numerical difficulties associated with strong tidal advection. Axial distributions of salinity at high water, computed using the moving-element model, compare well with measurements. The modelled and observed high water salinity distributions in this macrotidal estuary show little dependence on tidal range. The major variability in salinity is due to runoff. This strong and rapid dependence on runoff is a consequence of short residence (or flushing) times. Typically, residence times are less than one day throughout the year in the upper 10 km of estuary. The residence times maximize in summer, reaching 14 d for the whole estuary. During high runoff winter periods residence times are less than 5 d. Mixing coefficients for the moving-element salinity model are deduced from salinity measurements. Dispersion coefficients at fixed locations along the estuary are deduced from solutions of the salinity model. The spatially-averaged coefficients at mean spring and neap tides are 180 and 240 m2 s−1, respectively, for average runoff. Therefore, spring-neap variations in dispersion are fairly small and show a negative correlation with tidal range. The spatially-averaged dispersion coefficients at mean tides vary from 150 to 300 m2 s−1 for typical summer and winter runoff, respectively. The increase in dispersion with runoff and the decrease with tidal range implies that buoyancy-driven currents generate an important component of the shear dispersion in this estuary.

Transient salt-wedges in a tidal gulf: A criterion for their formation

The excess of evaporation over precipitation at the surface of a gulf or an embayment produces a horizontal gradient of salinity, with salinity increasing towards the head. Although there is a tendency to form salt wedges in such a situation, the water mass may be vertically well-mixed by tidal turbulence. In Gulf St Vincent of South Australia, during its neap or ‘dodge’ tides, which are characterized by relatively weak tidal streams, the intensity of tidally induced turbulence falls to such levels that sufficient vertical mixing cannot be sustained, allowing the formation of salt-wedges only to be dissipated again by the strengthening of tidal streams towards the next spring tide. The duration of stratification can extend up to a few days depending on tidal and wind conditions. By applying potential energy balance, it will be shown that for a given, uniform, horizontal density-gradient, there exists a critical value of the ratio h U 0 , where h is the local depth and U 0 is the depth-averaged amplitude of the tidal current, at which salt wedging is initiated as U 0 decreases towards the neap tide. This is achieved here by parameterizing the spring-neap variation of tidal turbulence by a depth-constant coefficient of vertical eddy viscosity proportional to (a) h U 0 ; (b) U 0 2 . The theory gives, for the initiation of transient salt-wedges in Upper Gulf St Vincent, estimates of the critical values of U 0 which are consistent with observations.

The extent of lateral water movement in the sediments of a New England Salt Marsh

Models of depth‐averaged hydraulic head are used with data from a water balance study in a New England salt marsh to describe horizontal pore water water fluxes near a creek bank. Three hydrologically distinct regions are identified in the sediment. In the marsh studied, semidiurnal tides may drive an oscillating horizontal flux in the narrow region within about 2.5 m of the creek bank. Farther than 15 m from the creek there is essentially no horizontal water movement. In the region between 2.5 and 15 m, drainage to the creek is driven by alternating periods of surface flooding and nonflooding due to the spring neap variation in tidal amplitudes. There is little or no input of fresh groundwater to the marsh sediment at this site. The spatial extent of the drained region depends on the duration of the nonflooded period, the morphology of the sediment surface, and the ratio of hydraulic conductivity to specific storage of the sediment.

Estimation and Verification of Tidally Induced Residual Currents

Using a multiple regression technique that includes both tidal and wind forcings, the tidally-induced residual current can be estimated from current meter records. However, the estimated mean (long-term averaged) tidally induced residual current is found to be very sensitive to an uncertain coefficient, ε, associated with the tidal forcing uaε, where ua is the semi-major axis of the tidal ellipse. Thus, the estimated mean tidally induced residual current cannot be used directly to verify model results. It is suggested that the verification be carded out on û2 = (û2/ûa)ūa where ûa and û2 are respectively the spring-neap oscillation of the semi-major axis and the tidally induced residual current, and ūa is the mean value of the semi-major axis. The suggestion is derived from the findings that 1) the value of ũ2 estimated from the current meter data is insensitive to the coefficient ε, and 2) the computed ũ2 from the numerical tidal model is insensitive to the specified spring-neap oscillation of the tidal forcing (surface elevation) at the open boundaries. By using Tee's three-dimensional tidal model, the mean and spring–neap variation of the tidally induced residual currents in the Cape Sable area, southwest of Nova Scotia were simulated. The computed values of ε are found to vary significantly in both the horizontal and vertical directions. The numerical model reproduces all the residual currents at the shallowest station where the estimation of the observed tidally induced residual current is most statistically reliable. At the other stations, the numerical model reproduces most of those currents that have high signal-to-noise ratios. To reduce the effect of uncertainty in ε on the estimation of the mean tidally induced residual current from current meter data, the estimation can be carried out by using the values of ε computed from a reliable tidal model. An example of the estimation using this method is shown for a Cape Sable station where the computed ũ2 has been verified by observation. In the absence of tidal modeling or in the case where accurate values of ε cannot be computed, it is suggested that ε = 2 be used for the estimation. This suggestion is derived mainly from the finding that the estimation using ε = 2 is reliable over a wider range of true ε values in the Cape Sable area than that using ε = 1 or 3.