Asymmetries in tidal flow over a Seto Inland Sea scour pit

Underway profiles of current velocity were combined with stationary profiles of temperature and salinity around a vertically mixed scour pit of the Seto Inland Sea throughout a semidiurnal tidal cycle. This was done with the purpose of determining (a) whether flood flow is asymmetric relative to ebb over a pit with weakly stratified conditions, and (b) whether there is a dynamic transition from frictionally dominated tidal flow to advectively dominated tidal flow over the pit. These questions arose from previous studies elsewhere under stratified water columns, in contrast to the unstratified conditions at the study site. Observations showed an acceleration of the flood tidal flow over the pit and a deceleration during ebb. The flow acceleration over the pit during flood and deceleration during ebb was attributed to asymmetric patterns of flow convergence/divergence. In turn, these divergence patterns were influenced by the direction and strength of the baroclinic pressure gradient force, which was 10 to 30% of the advective term, despite the relatively weak horizontal gradients of order 10−5 kg/m4. The non-negligible influence of the baroclinic pressure gradient was possible from the relatively large depths that exceeded 100 m at the deepest part of the pit, compared to the surrounding depths of 30 m. From depth-averaged dynamical terms derived for flood and ebb phases of the tidal cycle, it was found that the advective terms became more important than frictional terms over the deep parts of the pit. Advection became more prominent than friction where the bottom slope exceeded the value of the bottom drag coefficient (∼0.003). Otherwise, frictional effects dominated outside the pit

A Two-Dimensional Analytic Tidal Model for a Narrow Estuary of Arbitrary Lateral Depth Variation: The Intratidal Motion

An innovative method is introduced to solve a two-dimensional, depth-averaged analytic model for narrow estuaries or tidal channels with arbitrary lateral depth variations. The solution is valid if the lateral variation of the amplitude of tidal elevation (\Delta a\) is small, i.e., \Delta a\ much less than a, where a is the amplitude of the tidal elevation. This assumption is supported by a 60-day observation of elevation in the James River Estuary using pressure sensors at both sides of a cross section of the estuary. The error introduced by the solution is of the order of \Delta a\/a, which has a maximum of similar to 5% in the James River Estuary. The propagation of the tidal wave (elevation) is therefore essentially one- dimensional (along the estuary), regardless of the depth distribution, whereas tidal velocity has a strong transverse shear and is three- dimensional in general. Dozens of depth functions in six groups of various forms are used to calculate the solution. The tidal velocity is highly correlated with the bathymetry. The largest amplitude of the along-channel velocity is in the deepest water. The phase of the along-channel velocity in the shallow water leads that in deep water, causing a delay in time of flood or ebb in the deep water. The transverse velocity is generally small in the middle of a channel but reaches its maximum over the edges of bottom slopes. The depth function has a significant effect on the ellipticity and the sense of rotation of the tidal ellipses. By fitting the observed phase of semidiurnal tide in the James River Estuary to the phase of the momentum equation, we have obtained optimal values of the drag coefficient: 1.5 x 10(-3) and 1.8 x 10(-3) for the spring and neap tides, respectively. Then we apply these values of the drag coefficient and the model to the James River Estuary using the real bathymetry. Results show remarkable agreement between the observations and the model along the transects for both spring and neap tides. The cross-channel phase difference of the along-channel velocity between the channel and the shoal is found to be similar to 1 hour, a value consistent with that from the model. The model-estimated lateral variation of elevation is 2.5% of the tidal amplitude, which is slightly smaller than the observed value. Copyright 1999 by the American Geophysical Union

Reversing Circulation Patterns in a Tropical Estuary

A combination of current velocity and water density measurements was used to characterize the basic patterns of water exchange in the Gulf of Fonseca, a tropical estuary on the Pacific Ocean side of Central America. The measurements were obtained during spring and neap tides in March ( dry season) and June ( wet season) of 2001 and consisted of profiles of current velocity and density along four transects. From mid-March to mid-April a time series of hourly surface current velocity maps was also obtained with a high-frequency radar system of two antennas. The sampling transects and the radar coverage concentrated in the portion of the estuary that has open communication with the ocean. During the dry season, water exchange at the entrance to the gulf suggested an inverse estuarine circulation that was more robust, and its dynamics were closer to geostrophy during neap than during spring tides. It is likely that salinity increased toward the tributaries of the system and then decreased within those tributaries because of the persistent influence of fresh water. In contrast, during the wet season, salinity decreased into the estuary, and the circulation resembled that of a typical estuary. In this season the fortnightly modulation of exchange flows was masked by wind effects, which also played a relevant role in the dynamics. The net volume inflows measured in both seasons suggested that the residence time of the Gulf of Fonseca varies from 2 weeks to 1 month

Transverse Structure of Wind-Driven Flow at the Entrance to an Estuary: Nansemond River

Observations of current velocity profiles were combined with an analytical solution to study the transverse partition of the wind-driven flow in an estuary, the Nansemond River, which is a tributary of the James River in the Chesapeake Bay. Observations spanned two periods of nearly 3 months in autumn-winter of 2003-2004 and spring-summer 2004. The wind-driven circulation consisted of downwind flow over the shoal and upwind flow in the channel at the entrance to the estuary. This pattern developed mainly with landward winds and provided observational evidence that sustains analytical and numerical model results. The transverse structure of the flow showed synoptic temporal variability (3-7 days), which corresponded to the variability of winds and sea level. Synoptic variability seemed to be more influential in autumn-winter than in spring-summer. However, variability of 1-2 days was persistent in both periods of observation. Also, the transverse structure of the wind-driven flows was linked to a counterclockwise recirculation pattern previously observed with survey data. Part of the flow going into the tributary over the shoal might recirculate and form or enhance the outflow in the channel. As suggested by the temporal scale of the wind, the recirculation might weaken or even reverse direction every 3-7 days at the entrance to the estuary. Further detailed studies are needed to better define the extent of this recirculation

Observations of wind influence on exchange flows in a strait of the Chilean Inland Sea

A \u3e 100-day time series of velocity profiles, sea level and wind velocity at a strait in the Chilean Inland Sea was analyzed to examine the effects of wind forcing on the mean two-layer exchange. Measurements took place in the Meninea Constriction of the Moraleda Channel during a period dominated by northerly winds. The mean flow in the strait, and in general in the Moraleda Channel, showed net surface northward outflow and net bottom southward inflow that likely resulted from the dynamical balance between pressure gradient and friction. The influence of tidal mixing on mean exchange flows was further suggested by isopycnals intersecting the bottom. The same momentum balance between pressure gradient and friction, applied with temporally varying sea level slopes, satisfactorily described the subtidal modifications to the mean exchange flows produced by wind forcing. The use of sea level slopes to explain the subtidal variability of velocity profiles at the Meninea Constriction was justified by the strong correlation between sea level slopes and wind stresses (0.84). In fact, the vertically integrated dynamics was essentially explained by the balance between wind stress and barotropic pressure gradient for northerly winds. Addition of bottom stress improved the dynamical explanation during periods of weak or southerly winds. This dynamical response was confirmed by the first two empirical orthogonal functions of the record. Two-layer exchange flows were weakened by northerly winds as depicted by the first empirical function mode, which was unidirectional throughout the water column in response to depth-integrated dynamics. The second empirical function mode was related to the depth-dependent response that followed the wind-induced sea level set-up

Simulation and energy partition of the flow through Paso Galvarino, Chile

Paso Galvarino is a constriction in Seno Ventisquero, a tidally-energetic Chilean fjord. The pass is about 1500 m long and constricts in width by about 90 % near its sill, which has a depth of about 8 m. A laterally-averaged numerical model is compared to ADCP and backscatter observations of the hydraulic flow near the sill, during maximum flood, the slack tide after the flood, maximum ebb, and the slack tide after the ebb. The model is also used to examine how the energy flux into the fjord is partitioned in the region of the constriction. Energy is removed from the surface tide near the sill and is largely dissipated near the sill. The model predicts that the internal tide is unimportant and that energy transport by advection is much more important than that due to radiation. Advection is significant only near the sill, however, and a counteracting surface flux develops that suppresses the influence of the advection

On the relative importance of the remote and local wind effects on the subtidal exchange at the entrance to the Chesapeake Bay

Water velocity data from acoustic Doppler current profilers and electromagnetic current meters deployed at six separate locations across the entrance of the Chesapeake Bay from mid-April to early July of 1999 and from early September to mid-November of 1999 were used in conjunction with wind velocity and sea level records to describe the characteristics of the wind-induced subtidal volume exchange between the bay and the adjacent continental shelf. The current measurements were used to estimate volume fluxes associated with the local and remote wind-induced bay-shelf exchange over time scales of 2–3 days. The results show that at these relatively short subtidal time scales (1) the net flux integrated over the entrance to the estuary adequately describes the unidirectional (either inflow or outflow over the entire cross-section) barotropic volume flux associated with the coastally forced remote wind effect, (2) during the first deployment there is always a bi-directional exchange pattern (inflow and outflow existing simultaneously over different parts of the cross-section) superimposed on the sectionally integrated unidirectional exchange, (3) the magnitude of the bi-directional transport associated with the local wind effect may be a significant fraction of the unidirectional transport associated with the remote wind effect, and (4) the relative importance of the local wind effect in producing estuary-shelf exchange changes appreciably with season, depending on the characteristic frequency of the wind events and the degree of stratification in the estuary

Observations of Cross Channel Structure of Flow in an Energetic Tidal Channel

[1] Measurements of velocity and density profiles were made to describe the transverse structure of flow in Chacao Channel, Southern Chile (41.75 degreesS), where typical tidal velocities are similar to4 m/s. Current profiles were obtained with a 307.2 kHz Acoustic Doppler Current Profiler (ADCP) over 25 repetitions of a cross-channel transect during one semidiurnal tidal cycle. The 2.2 km long transect ran northeast/southwest across the channel. A northern channel (120 m deep) and a southern channel (85 m deep) were separated by Remolinos Rock, a pinnacle that rises to 20 m depth at similar to0.7 km from the southern side. Density measurements to depths of similar to50 m were obtained with a Conductivity, Temperature, and Depth (CTD) recorder at the north and south ends of each transect repetition. One CTD profile was also taken in the middle of the northern channel. The mean flow exhibited weak vertical structure because of strong vertical mixing. The predominant lateral structure consisted of mean outflow ( toward the ocean) in the channels and mean inflow ( toward Gulf of Ancud) over the pinnacle and the sides of the channel. This lateral structure pattern was consistent with the mean flow pattern expected from tidal rectification, as robust overtides were generated throughout the transect. The contributions to flow divergence and vorticity by the lateral variations of the lateral flow (partial derivativev/partial derivativey) and by the lateral shears of the along-channel flow (partial derivativeu/partial derivativey), respectively, were both of the order of 10(-3) s(-1). This caused advective and frictional forces ( both horizontal and vertical) to be dominant in the across-channel momentum balance, as they were more than twenty times the Coriolis acceleration. The present work then represents one of the few examples reported where lateral friction ( proportional to partial derivative(2)v/partial derivativey(2)) appears relevant to the transverse momentum balance

Observations of Intratidal Variability of Flows Over a Sill/Contraction Combination in a Chilean Fjord

Underway velocity measurements were carried out for the first time in a Chilean fjord using an acoustic Doppler velocimeter with the purpose of elucidating the intratidal variability of flows through a pass, Paso Galvarino. The pass included a sill, where the bottom sloped by roughly 30%, and a coastline contraction of -90%. The relatively small dimensions of the pass allowed for rapid sampling of the flow evolution throughout the tidal cycle. The backscattered sound signal from the velocimeter and from an echo sounder were used to describe the vertical excursions of the pycnocline throughout the domain and to identify regions of enhanced vertical mixing within the pass. The spatial variability of the flow in the pass was consistent with uniform two-layer flow. At the narrowest section of the contraction the pycnocline dropped sharply around both maximum flood and maximum ebb, while the flow accelerated downstream relative to the tidal flow. The slope of the pycnocline changed sign from flood to ebb, which was atypical of other fjord observations but could be explained by the transitions from subcritical to supercritical flow. These transitions switched location at either side of the narrowest section of the contraction. Leeward of this section, increased sound backscatter suggested intensified turbulence that extended over a greater area during ebb than during flood because the distance between the point of pycnocline drop and the end of the pass was longer during ebb. Enhanced vertical mixing within the pass was reflected in the tidally averaged fields by a three-layer flow that consisted of near-surface and near-bottom flow converging toward the pass and flow around the pycnocline diverging away from the pass

Observations of the wind-induced exchange at the entrance to Chesapeake Bay

Water density and velocity data from two ~75-day deployments across the entrance to the Chesapeake Bay were used in conjunction with wind velocity and sea level records to describe the transverse structure of wind-induced subtidal exchange. Acoustic Doppler current profilers, electromagnetic current meters, and conductivity-temperature-depth recorders were deployed at the entrance to the bay from mid-April to early July of 1999 and from early September to mid-November of 1999. Three main scenarios of wind-induced exchange were identified: (1) Northeasterly (NE) winds consistently drove water from the coast toward the lower Chesapeake Bay as well as water from the upper bay to the lower bay, which was indicated by the surface elevation slopes across the lower bay and along the bay. This resulted in water piling up against the southwestern corner of the bay. The subtidal flow over the southern portion of the bay entrance was directed to the left of the wind direction, likely the result of the influence of Coriolis and centripetal accelerations on the adjustment of the sea level gradients. Over the northern shallow half of the entrance, the subtidal flows were nearly depth-independent and in the same direction as the wind. (2) Southwesterly (SW) winds caused opposite sea level gradients (relative to NE winds), which translated into near-surface outflows throughout the entrance and near-bottom inflows restricted to the channels. This wind-induced circulation enhanced the two-way exchange between the estuary and the adjacent ocean. (3) Northwesterly winds produced the same exchange pattern as NE winds. Water piled up against the southwestern corner of the bay causing net outflow in the deep, southern area and downwind flow over the shallow areas. Northwesterly winds greater than 12 m/s caused the most efficient flushing of the bay, driving water out over the entire mouth of the estuary