Paul H. LeBlond

Estuary/ocean exchange controlled by spring-neap tidal mixing

Abstract This paper documents the role played by tidal mixing and wind forcing in modulating the export of freshwater from an estuarine system. Nineteen years of daily salinity data collected at lighthouses on the shores of the Strait of Georgia and Juan de Fuca Strait have been analysed and related to local wind, runoff and the strong tidal currents. Seaward freshwater export is found to be greatest at neap tides, at which time it may be considerably enhanced by favourable winds. It thus appears that the gravitational circulation of the estuary system is controlled by vertical mixing due to the tidal currents. The origin and impact of the baroclinic monthly and fortnightly ‘density tides” generated in the process are discussed. It is found that the combined influence of the wind and tidal currents may be characterized by a parameter resembling a Froude number, providing some insight into the dynamics of the mixing processes at work in the system.

Velocity and temperature fluctuations in a buoyancy‐driven current off Vancouver Island

An experiment to study buoyancy driving of the poleward flowing coastal current off the west coast of Vancouver Island was carried out from June to November 1984. The experiment consisted of two hydrographic surveys and a three-dimensional instrumented array that included 51 current/temperature sensors distributed on 18 moorings. Moorings were maintained across the mouth of the Strait of Juan de Fuca (to monitor the fluctuating buoyant source/sink) and along three cross-shelf transects roughly 40, 90, and 130 km north of the mouth of the strait. To detect possible incoming propagating coastal-trapped waves (CTWs), moorings were also located ∼10 and ∼30 km south of the mouth of the strait.The main thrust of the experiment was to attempt to identify signals specifically associated with buoyant outflow from the Strait of Juan de Fuca. Four such signals were identified. (1) A seasonally varying mean poleward flow over the mid and inner shelf. (2) Velocity and temperature fluctuations associated with buoyant plumes over the shelf. The plumes, which occur at roughly monthly and bimonthly intervals, are driven by anomalously strong freshwater pulses from the strait, and the associated fluctuations in the along-isobath component of velocity accounted for about 20% (10%) of the variance during summer (fall) in the region within about 50 km of the mouth of the strait. (3) Positive temperature anomalies (∼3°C) related to the cessation of strait outflow. These anomalies, which are strongest at and south of the mouth of the strait, occur during periods of regional southwesterly winds and have a south-to-north lag consistent with poleward advection of the pre-existing (usually cold) plume water at about 30 km d−1. (4) Phase modification of the wind-driven CTW and local wind-driven velocity signals across the mouth of the strait: a south to north phase difference of about 90° is observed across the mouth of the strait at low (<0.12 cpd) frequencies during both summer and fall. Statistical analysis as well as application of a first-mode wind-driven CTW model demonstrated that the dominant variance over the Vancouver Island shelf is related primarily to the first-mode wind-driven CTW (summer, ∼55%) or local wind forcing (fall, ∼80%). This being the case, and also because of the rugged shelf topography in the area, the analysis proved insufficient to identify definitively more subtle results of plume and strait processes such as the local generation of CTWs.

Three-dimensional modeling of tsunami generation due to a submarine mudslide

Abstract Submarine landslides are a common cause of tsunamis in coastal and estuarine areas. To study this phenomenon, a numerical model is developed to simulate tsunami generation due to a viscous mudslide on a gentle uniform slope. A formulation of the dynamics of the problem is presented, where the mudslide is treated as an incompressible three-dimensional viscous flow. Seawater is treated as an inviscid fluid, and the water motion is assumed irrotational. The long-wave approximation is adopted for both water waves and the mudslide. The resulting differential equations are solved by a finite-difference method. The focus of this paper is to examine the effects of the longitudinal spreading as well as the transversal spreading of the slide upon surface wave generation, and the spreading of water waves sideways. Three-dimensional pictures are presented for successive profiles of the mud surface, the horizontal velocities of the mudslide, the evolution of the surface elevations, and the velocities of the w...

Deep and intermediate water replacement in the Strait of Georgia

Abstract We have studied deep‐water replacement processes in the Strait of Georgia using data from two different observational programs. From the monthly hydrographic data of Crean and Ages (1971) we have recognized the propagation of cold, brackish and well oxygenated waters northwards from Boundary Passage at depths between 75 and 200 m. We found a significant correlation over the years 1967–78 between surface cooling and temperature drops at those depths some months later. Measurements at an array of moorings in the central Strait of Georgia (Stacey et al., 1987) revealed the presence during summer months of currents concentrated near the bottom and varying with fortnightly and monthly periods. We have interpreted this phenomenon in terms of gravity currents originating from Boundary Passage during periods of neap tides and introducing at depth salty waters from the Strait of Juan de Fuca. Our analysis confirms in part the validity of the deep‐water replacement mechanisms advanced by Waldichuk; however...

Forced fortnightly tides in shallow rivers

Abstract The presence and the basic features of the forced fortnightly wave observed in some shallow rivers are explained through scaling arguments which show that this wave is generated by the fortnightly modulation of the frictional forces due to the variation in tidal velocities. Comparison of the results with sea‐level records from a shallow reach of the St Lawrence River shows reasonable agreement between data and theory.

Analysis of deep‐drogued satellite‐tracked drifter measurements in the northeast pacific

Abstract We analyse the trajectories of 24 deep‐drogued, satellite‐tracked drifters launched between 50 and 52°N in the northeast Pacific during June and October 1987. Three aspects of the observed motions at the drogue depths of 100 to 120 m are studied: (i) the spatial structure of the mean and variance velocity fields; (ii) the dispersion and eddy diffusion characteristics of the fluctuating motions; and (iii) the properties of selected mesoscale eddies. The mean Lagrangian velocity field is consistent with the mean flow pattern derived from the historical dynamic height topography. Fluctuating motions within the region are dominated by mesoscale eddies and meanders. Several instances of persistent O(100 days) westward flowing countercurrents were also observed. Based on the Lagrangian integral time‐ and length scales, drifter motions become decorrelated within a period of 10 days and a separation of 100 km. The mean zonal and meridional integral time‐scales of 4.5 and 3.6 days, respectively, are nearl...

Satellite observations of labrador current undulations

Abstract Analysis of visual images of the offshore ice margin of the Labrador Coast, taken on four consecutive days from the NOAA‐5 satellite, reveals horizontal oscillations with a mean wavelength of 75 km and amplitude of 15 km. The oscillations travel downstream, with the Labrador Current, at a speed C ≈ 0.2 m s‐1. Oscillations of similar periods are seen in moored current meter records. An examination of available models of baro tropic and baro‐clinic instability shows that the latter mechanism could account for the generation of the observed oscillations from the shear in the Labrador Current.

ON THE FORMATION OF SPIRAL BEACHES

The data for the spectra of wind-generated waves measured in a laboratory tank and in a bay are analyzed using the similarity theory of Kitaigorodski, and the one-dimensional spectra of fetch-limited wind waves are determined from the data. The combined field and laboratory data cover such a wide range of dimensionless fetch F (= gF/u ) as F : 10 ~ 10 . The fetch relations for the growthes of spectral peak frequency u)m and of total energy E of the spectrum are derived from the proposed spectra, which are consistent with those derived directly from the measured spectra.A solution of finite amplitude long waves on constant sloping beaches is obtained by solving the equations of the shallow water theory of the lowest order. Non-linearity of this theory is taken into account, using the perturbation method. Bessel functions involved in the solution are approximated with trigonometric functions. The applicable range of this theory is determined from the two limit conditions caused by the hydrostatic pressure assumption and the trigonometric function approximation of Bessel functions. The shoaling of this finite amplitude long waves on constant sloping beaches is discussed. Especially, the effects of the beach slope on the wave height change and the asymmetric wave profile near the breaking point are examined, which can not be explained by the concept of constancy of wave energy flux based on the theory of progressive waves in uniform depth. These theoretical results are presented graphically, and compared with curves of wave shoaling based on finite amplitude wave theories. On the other hand, the experiments are conducted with respect to the transformation of waves progressing on beaches of three kinds of slopes ( 1/30, 1/2.0 and 1/10 ) . The experimental results are compared with the theoretical curves to confirm the validity of the theory.Measurements of drift were made in a wind and wave facility at different elevations below the mean water level. The drift profiles were obtained for reference wind speeds, Ur = 3.1, 5.7 and 9.6 m/sec. The measurement technique involved tracing the movement of small paper discs which were soaked in water to become neutrally buoyant at the elevation of release. A logarithmic drift profile is proposed. The water shear velocity, U*w, predicts a surface stress, TS = pw U*S, in agreement with that obtained from the wind shear velocity, s = Pa U*li where pa and pw refer to air and water densities, respectively.The paper describes a procedure for obtaining field data on the mean concentration of sediments in combination of waves and currents outside the breaker zone, as well as some results of such measurements. It is assumed that the current turbulence alone is responsible for the maintenance of the concentration profile above a thin layer close to the bottom, in which pick-up of sediments due to wave agitation takes place. This assumption gives a good agreement between field data and calculated concentration profiles.A section of beach on the south coast of England has been under surveillance for five years, from March 1966 until March 1971. During this period, two permeable groynes of the Makepeace Wood type were constructed. Beach cross sectional areas and rates of accretion were compared before and after groyne construction. The groynes caused a buildup in beach levels updrift.The results of model tests, carried out to evaluate the stability of submarine slopes under wave action are presented. A Bentonite clay was sedimented in a glass walled tank 6 feet long by 0.5 feet wide by 2.5 feet deep. The sedimentation and consolidation processes were studied and sediment densities were measured at various depths in the profile. Vane shear strength profiles were also measured afvarious average degrees of consolidation. Plastic markers were placed in the sediment adjacent to a glass wall so that the soil movements under both gravity and wave induced slides could be documented by photography. Dimensional similitude is discussed and the model test data are presented in a dimensionless form. All instabilities were observed to be of the infinite slope type. Analysis of the data shows that wave action is instrumental in initiating downslope mass movements in gently to steeply sloping off-shore sediments. General lack of agreement between the model test results and published theoretical analyses was found but there was close similarity in the depths and form of failure under wave action and under gravity stresses alone. The loss of stability under wave action is analyzed on the concept that failure is gravity controlled and the soil strength is reduced to a value commensurate with gravity sliding by the cyclic shearing stresses imposed by progressive waves. A method of evaluating the stability of prototype slopes using a model test correlation and field vane strength measurements is proposed. INTRODUCTION Instabilities in submarine slopes have been observed or have been inferred over a wide range of slope angles from less than half a degree up to about 30°. These subaqueous landslides are believed to have caused rupture of submarine cables and to have generated many of the geomorphological features on the ocean bottom. There are numerous records describing these landslides but very few publications discuss the application of the principles of soil mechanics to the analysis of the stability of submarine slopes. Associate Professor of Civil Engineering, Queens University at Kingston, Canada 2 Soils Engineer, Geocon Ltd., Toronto, Canada 3 Associate Professor of Civil Engineering, Cornell University, Ithaca, N.Y.Several mathematical models have been lately presented which describe the tidal wave propagation within an estuary. The existing models derived from the method for damped co-oscillating tides are based on sinusoidal wave profile. Meanwhile a tidal wave which moves upstream, generally exhibits a progressive deformation which tends to unbalance the length of time between flood and ebb tides. The actual profile is therefore no longer sinusoidal. Our investigation uses the potential method, and takes into account the wave amplitude which is usually neglected compared with the water depth. Finally, the velocity potential is obtained explicitely, using a double iterative method. Tidal elevation, particle velocities and trajectories are given by the same computer programmed algorithm. Our study shows that l) the phenomenon can be clearly visualized on the theoretical curves and 2) the magnitude of this deformation is inversely proportional to the water depth, becoming significant when the ratio f|/h reaches the critical value of 1/10. Damping and geometrical effects are also considered and the theory was applied to the St.Lawrence Estuary. A partial positive reflection of the incoming tidal wave is assumed at the narrow section near Quebec, whereas a complete negative reflection is assumed at the entrance to Lake St.Peter. The calculated and observed wave profiles, velocity distributions, and phase shifts are in good agreement.A numerical model is presented to describe the hydromechanics of lagoons connected to the ocean by relatively narrow inlets. Because special attention is given to the flushing, all second order terms in the hydrodynamic equations are retained. The study is restricted to lagoons with a onedimensional flow pattern and water of uniform density. In designing a numerical solution to the equations, the inlet equations are regarded as implicit boundary conditions to the equations describing the flow in the lagoon proper. The advantages of this approach are: (1) the size of the computational grid in the lagoon can be chosen independently of the relatively small dimensions of the inlets and (2) the flow at branching inlets (an inlet connecting a lagoon to the ocean such that branching of the inlet flow can occur) still can be described by a one-dimensional tidal model. The predictive capability of the numerical model is confirmed by favorable comparison between measured and computed particle paths and net transport for a series of laboratory experiments. In the experiments a canal of uniform width and depth is freely connected to a tidal basin at one end and at the other end is connected to the same basin by a submerged weir.

Observations of seamount‐attached eddies in the North Pacific

Satellite-tracked drifters were used to examine eddy activity in the vicinity of the Emperor Seamount Chain (ESC) in the North Pacific during 1991–1993. The trajectories of two drifters drogued at a depth of 120 m revealed a pair of counterrotating mesoscale eddies attached to the leeside of Ojin/Jingu Seamount in the summer of 1992. The eddies had diameters of 75–100 km and rotational speeds of 20–40 cm/s at 120 m. Sea surface height anomalies derived from blended TOPEX/ERS-1 satellite altimetry revealed that the eddies had a surface manifestation as well. One of the drifters made five loops within the cyclonic eddy over a period of 62 days, during which time the eddy translated westward, toward the seamount, at 2.9 cm/s. This is one of the first observations demonstrating an extended attachment of a topographically generated eddy to a seamount. Drifters drogued at a depth of 15 m which crossed the ESC in the summer of 1991 and winter of 1993 revealed no eddy activity, most likely because of a decoupling of the topographic influence to the 15 m flow at their crossing latitude over the Nintoku Seamount (summit depth at 1000 m). The implication is that eddy formation within the mixed layer near the ESC is confined to the region around the taller Ojin/Jingu and Kinmei Seamounts (summit depths at 800 m and 100 m, respectively). It is suggested that long-lived eddies attached to the leeside of the ESC can profoundly influence local biological production and water exchange between the western and eastern basins of the North Pacific.

Surface circulation in the Western Labrador Sea

Abstract The residual circulation in the western Labrador Sea is discussed in the light of recent current data combining information from moored instruments, satellite-tracked surface buoys, and geostrophic calculations. The characteristic features of the surface flow pattern observed near the mouth of Hudson Strait are explained qualitatively in terms of the response of the flow to the form of the coastline and to the nearshore bathymetry.