Tidal Excursion Research Articles

Cooling water discharges into a sea with a sloping bed

Analytical solutions for the rise in background water temperatures due to the discharge of cooling water from a power station are developed. Previous work has taken account of the cooling water flow rate and temperature increase through the station, the tidal excursion and residual drift, cross-tide dispersion and heat exchange between the water and atmosphere in a sea with constant depth. The treatment in the present paper incorporates the slope of the sea bed. The mathematical feature of the analysis is that a closed form for the background (or far-field) temperature distribution is obtained in terms of confluent hypergeometric functions. The parameters which define these functions involve the site hydrographic feature and cooling water system parameters. Results are presented which indicate the sensitivity of the temperature rise to bed slope and the position of the discharge. The temperature at the shore, which may be an important ecological indicator and station design parameter, decreases as the distance of the outlet from the shore increases; the effect of the bed slope on the rate of decrease is quantified.

On Lagrangian residual currents with applications in south San Francisco Bay, California

The Lagrangian residual circulation has often been introduced as the sum of the Eulerian residual circulation and the Stokes' drift. Unfortunately, this definition of the Lagrangian residual circulation is conceptually incorrect because both the Eulerian residual circulation and the Stokes' drift are Eulerian variables. In this paper a classification of various residual variables are reviewed and properly defined. The Lagrangian residual circulation is then studied by means of a two‐stage formulation of a computer model. The tidal circulation is first computed in a conventional Eulerian way, and then the Lagrangian residual circulation is determined by a method patterned after the method of markers and cells. To demonstrate properties of the Lagrangian residual circulation, application of this approach in South San Francisco Bay, California, is considered. With the aid of the model results, properties of the Eulerian and Lagrangian residual circulation are examined. It can be concluded that estimation of the Lagrangian residual circulation from Eulerian data may lead to unacceptable error, particularly in a tidal estuary where the tidal excursion is of the same order of magnitude as the length scale of the basin. A direction calculation of the Lagrangian residual circulation must be made and has been shown to be feasible.

Pulmonary injury in rats following continuous exposure to 60% O2 for 7 days.

Morphological, biochemical, and physiological studies were done on rats exposed to 60% O2 for 7 days. This exposure did not induce O2 tolerance but instead caused a significant decrease in survival time of animals subsequently exposed to pure O2. The activity of lung superoxide dismutases and glucose-6-phosphate dehydrogenase were unchanged after exposure to 60% O2. A decrease in lung compliance was suggested by changes in the total lung capacity and in the pressure-volume curves of excised lungs. Ventilation of these animals with large tidal excursion resulted in pulmonary edema. Morphometric analyses revealed a significant decrease in alveolar air volume and an increase in the number of alveolar macrophages. The most significant lesions involved the pulmonary vascular bed. The volume and thickness of the capillary endothelium was decreased. There were focal areas of pericapillary fluid accumulations, and a number of the smaller vessels had perivascular edema. These findings suggest that significant pulmonary injury occurs in rats exposed to 60% O2 and that the primary site of injury is the pulmonary capillary endothelium.

Longitudinal mixing in the upper reaches of the Bay of Fundy

A time-dependent, one-dimensional, advection-diffusion equation is used to predict the salinity distribution in the vertically well mixed upper regions of the Bay of Fundy. It is shown that the longitudinal diffusion coefficient must increase in a seaward direction from the head of the bay to give a good agreement between the model and observations, and a simple parameterization of the coefficient is sought in terms of the product of appropriate length and velocity scales. A coefficient equal to b1U0l0 where b1 is a constant, U0 the amplitude of the tidal current and l0 the tidal excursion, is tried but does not give this increase and produces a poor description of the salinity distribution. However, a diffusion coefficient of the form cu*h, where c is a constant, u* the r.m.s. friction velocity from the tidal current averaged over the M2 tide, h, is the water depth and the overbar denotes a cross-sectional average, does give the required increasing value and allows a good agreement between the model and observations for c=200. This form of parameterization is the same as that for shear dispersion except that the coefficient c is in order of magnitude too large. It is suggested, instead, that the diffusion process results from the interaction of tidal residual eddies with the oscillatory tidal flow, a mechanism described by Zimmerman (1976) and capable of producing large diffusion coefficients. Preliminary estimates show that the construction of a tidal power barrage across Minas Basin would considerably reduce the salinity in the headpond of the barrage but have little effect seaward. A sinusoidally varying freshwater input to the estuary is shown to have a significant effect on the salinity distribution only for periods of oscillation exceeding a month, while variations in the longitudinal diffusion coefficient due to an apogee-perigee tidal cycle have little effect on the salinity distribution.

Microwave radiometer measurement of tidally induced salinity changes off the Georgia coast

A quasi‐synoptic survey of tidally induced salinity changes off the Georgia coast was performed by using a L band microwave radiometer onboard a NASA aircraft. Salinity maps were obtained for ebb and flood conditions in order to define the salinity distributions near rivers and sounds and major changes that occur from ebb flow to flood flow. The Savannah River plume dominated the salinity regime and extended out from the Savannah River mouth about 12 km during ebb tidal conditions. The plume merged into a band of low salinity water extending along the Georgia‐South Carolina coast which was produced by the many river sources of freshwater entering the coastal waters. The changes in salinity observed offshore of the river plume area were consistent with estimates of the changes that would occur over a typical tidal excursion perpendicular to the observed gradient.

Effects of sulfuric acid aerosols on pulmonary function of guinea pigs.

Forty-seven Hartley guinea pigs were exposed for 1 h to approximately 1-micrometer (mass median aerodynamic diameter) sulfuric acid aerosols at concentrations that ranged from 1.2 to 48.3 mg/m3. Ten animals (controls) were exposed to filtered room air only. Eight H2SO4-exposed animals exhibited marked increases in total pulmonary resistance and marked decreases in dynamic compliance. Four of these eight "responsive" animals died during exposure. All other H2SO4-exposed animals exhibited no major difference from controls and were termed nonresponsive. The proportion of responsive to nonresponsive animals increased with exposure concentration, but the magnitude of pulmonary function change was similar for all responsive animals regardless of concentration. Compared to nonresponders, responsive animals had higher preexposure values of tidal transpulmonary pressure excursions and total pulmonary resistance and lower values of dynamic compliance. Preexposure transpulmonary pressure excursions were positively correlated with minute volume only for nonresponsive animals; transpulmonary pressure excursions were positively correlated with total pulmonary resistance in responsive animals. The results suggest that the Hartley guinea pig reacts to inhaled H2SO4 with an essentially all-or-none airway constrictive response and that an animal's sensitivity to this response may be related to its preexposure airway caliber.

The Application of a Segmented Tidal Mixing Model to the Great Bay Estuary, N. H.

Measurements show that in general salt is vertically well-mixed everywhere in the Great Bay Estuary, New Hampshire except near the river entrances at the head of the estuary. Dyer and Taylor’s (1973) modified version of Ketchum’s segmented tidal prism model has been applied to the Great Bay Estuarine System in order to predict high and low water salinity distribution for a specified river flow. The theory has been modified here to account for the mixing which occurs at the junction of two branches of an estuary. The mixing parameter, which in this model is related to the tidal excursion of water in the estuary, has been determined for different segments in the estuary on the basis of a comparison between predictions and a comprehensive data set obtained for a low river flow period. Using a mixing parameter distribution based on the low river flow calibration procedure the salinity distribution has been predicted for high river flow. The resulting salinity distribution compares favorably with observations for most of the estuary. The corresponding flushing times for water parcels entering at the head of the estuary during periods of low and high river flow is 54.5 and 45.9 tidal cycles respectively.

Topographic Rectification of Tidal Currents on the Sides of Georges Bank

The rectification of M2 tidal currents on the sloping sides of Georges Bank is predicted to make an important year-round contribution to its observed mean clockwise circulation. A rectification mechanism involving continuity and Coriolis effects, but regulated by bottom friction (Huthnance, 1973), is operative. Huthnance's (1973) depth-averaged theory for the along-isobath mean Eulerian current associated with this mechanism and with a second, purely frictional, mechanism is extended to include mean current-tidal current interaction, spatially-varying bottom friction and rotary tidal currents. The ratio of cross-isobath tidal excursion Le to topographic length scale L is found to be an important nondimensional parameter in determining the degree of nonlinearity of the Coriolis mechanism. A significant Stokes velocity is associated with both rectification processes, so that, for the Coriolis mechanism, the mean Lagrangian current is only about two-thirds of the mean Eulerian current. On the sides of Georges Bank, Le and L are of the same order, and the rectification is sufficiently non-linear that interaction of the mean current with the tidal current is important. The mean Eulerian and Lagrangian currents, and the cross-isobath mean sea surface slopes, are predicted for half-sinusoid representations of bottom topography on the northwestern, northern and open ocean sides of the Bank. The mean flow is clockwise and concentrated over the edge of the Bank, but smeared out onto the top of the Bank by the mean current-tidal current interaction. The predicted current speeds, which are greatest on the northwestern and northern sides, are of the same order as those observed.

Tidal Dynamics and Low-Frequency Exchanges in the Aransas Pass, Texas

Current, water level and wind data collected from a study site at the Aransas Pass, Texas, during a 45-day period in mid 1977 are used to describe tidal motions and low-frequency, meteorologically-forced exchanges between the inner shelf and a series of intracoastal bays. Analysis of individual tidal constituents indicates a mixed but principally diurnal tide. Tidal constituents move through the Pass as nearly progressive waves. Asymmetry in the co-oscillating tidal motions is explained as a result of unequal frictional forces acting on flood and ebb currents in the Pass. Tidal excursions computed from the tidal harmonic constants and from cumulative net displacements suggest that tidal-period exchanges are not effective in flushing the bays even under tropic tidal conditions. Nontidal water levels are related to the cross-shelf component of the coastal windstress at statistically significant levels. This suggests that low-frequency local meteorological forcing, as a set-up or set-down of coastal water levels, plays a valuable role in assisting tidally-driven exchanges.

Effects of body position change on thoracoabdominal motion.

With a linearized respiratory magnetometer, measurements of anteroposterior and lateral diameters of both the rib cage and the abdomen were made at functional residual capacity and continuously during tidal breathing. Twenty-five subjects with normal respiratory systems were studied in the sitting, supine, lateral decubitus, and prone body positions. When subjects changed from sitting to supine position anteroposterior diameters of both rib cage and abdomen decreased while their lateral diameters increased. Both anteroposterior and lateral tidal excursions of the rib cage decreased; those of the abdomen increased. When subjects turned from supine to lateral decubitus position both anteroposterior diameters increased and the lateral diameters decreased. This was associated with an increase in both lateral excursions and a decrease in the abdominal anteroposterior excursions. Diameters and tidal excursions in the prone position resembled those in the supine position. Diameter changes could be explained by gravitational effects. Differences in tidal excursions accompanying body position change were probably related to 1) differences in the distribution of respiratory muscle force, 2) differences in the activity or mechanical advantage of various inspiratory muscles, and 3) local compliance changes in parts of the rib cage and abdomen.

A SAND BYPASSING SYSTEM USING A JET PUMP

All harbors and tidal inlets that are located in coastal areas have one characteristic in common-the need to bypass littoral materials that collect nearby. If natural harbors and tidal inlets are left unattended, bypassing will often occur naturally, but in the process, the harbor or inlet is usually rendered unfit for commercial or navigation purposes. Quite often, the inattention results in the total closure of the inlet. Therefore, at almost all harbor entrances and controlled tidal inlets, the natural bypassing must be augmented by secondary, usually mechanical, means. The customary technique for bypassing sand and maintaining harbors and inlets is the use of floating dredge equipment. This equipment is rugged, reliable, has been proved over and over, and appears to be irreplacable for many applications and locations. However, there are many locations and situations for which this floating equipment is not suitable and may, in fact, be detrimental or prohibitively costly. Waves of even moderate height, moderateto- high tidal excursion and currents, draft limitations, limited maneuvering area, and interference with normal navigation operations are examples of conditions which decrease the desirability and application of floating dredge equipment. Small volumes of material to be bypassed are an economic liability for the floating plant since mobilization and demobilization costs contribute extraordinarily to the unit cost for bypassing work.

The turbulent boundary layer on the continental shelf

IN the late summer of 1973 we made measurements of the current structure in the thermal boundary layer on the continental shelf. The measurements were taken under conditions of complete neutrality, well away from the influence of local coastlines. Figure 1 shows the positions of two moorings, 001 (48° 10.5′N, 07° 54′W), and 002 (48° 16.5′N, 07° 47′W) separated by 15 km. This compares with a tidal excursion of about 10 km for the surface water. Both moorings were in 180 m of water and more than 200 km from the nearest mainland. The current meters were supported by subsurface buoyancy. The thermal structure of the water column, and the depths at which current meter observations were made in relation to the thermal boundary layer is shown in Fig. 2. The temperature of the bottom 100 m of the water column increases with depth. This marginal increase of temperature at the adiabatic rate (1.3 × 10−6° C cm−1) is expected of a turbulent boundary layer, because the production of turbulent energy is much greater than the stabilising effect of the buoyancy flux. The absolute accuracy is only 0.02° C, using a mercury-in-glass reversing thermometer as the calibration standard. The resolution is, however, considered quite genuine to ±0.0005° C(ref. 1).

Shrimps in relation to oxygen depletion and its ecological significance in a polluted estuary

Monitoring of the shrimp ( Crangon vulgaris L.) catch off West Thurrock in the Thames estuary shows that they occur from September to February–April, but that within this period they are subject to great fluctuations. There is a substantial body of evidence that the lack of shrimps at any one time is associated with the low oxygen tension of the water. Experiments have been devised to determine (a) the level of oxygen depletion at which shrimps are asphyxiated, (b) the behaviour of shrimps as a consequence of oxygen depletion and (c) the effects of varying oxygen tensions on shrimp behaviour. A major effect of reducing oxygen tensions is for shrimps to alter their swimming pattern from the normal ‘background’ type to one in which stimulated swimming takes over and in which greater numbers of individuals of the population participate. During stimulated swimming the shrimps move upwards more vertically through the water body and this frequently involves ‘escape’ and ‘multiple escape’ reactions and passive sinking to the substrate. The ecological implications in a polluted estuary are that the higher the vertical height attained by the shrimps the longer the shrimps take to sink passively downwards. The longer they take to sink downwards the greater will be the distance covered horizontally when picked up by the tidal current and the faster the current the greater will be this distance. Within the context of the tidal Thames with its 14·5 km tidal excursion, the results have been computer-simulated and a model of shrimp migration suggested. The validity of computer simulation of a polluted estuary was tested against that of an unpolluted estuary, which latter can also be analysed mathematically.

Use of radioactive microspheres to assess distribution of cardiac output in rabbits.

Use of radioactive microspheres to assess distribution of cardiac output in rabbits.

Role of the Coastal and Upper Estuarine Waters Contributing Phytoplankton to the Shoals of the Niantic Estuary

Niantic Bay and the estuarine basin of the Niantic River were studied as source areas of phytoplankton transported across the shoals which separate the upper estuarine basin from the coastal waters and which support dense populations of bay scallops and other benthic filter feeders. Surface and bottom stations in the bay and estuarine basin and two stations over the shoals, where tidal excursions alternately transport water from offshore and from the estuary, were sampled at 5—week intervals for 1 year. Dinoflagellates predominated in the basin of the estuary and, to a lesser extent, were significant seaward into the Bay. The diatoms were most abundant in the Bay where they followed population trends characteristic of Long Island Sound and coastal waters. Two species of chrysophyceans were abundant, chiefly restricted to these near—shore waters but showing no indications that either the Bay or the more confined estuarine waters were more favorable. When the Bay was the predominant phytoplankton source area, as judged by the counts and population trends, the plankton of the mixed waters over the shoals was similar to that of the Bay in kind and in numbers. When the basin was the predominant source area, the plankton over the shoals, though similar in kind, commonly dropped to one—tenth the abundance found up—estuary. This drop may relate to the vertical migration of the flagellates into deeper waters which do not flow freely across the shoals. The lower plankton counts over the shoals may also result from the mixing of the waters ebbing from the basin with the relatively large volume of water over the shoals following the high tide. In the basin of the estuary the phytoplankton volume fluctuated around a mean of 6.90 mm3/liter. The average for the Bay was 0.23 mm3/liter. Over the shoals the average was 0.63 toward the upper estuary and 0.45 toward the Bay. These averages and other data constitute evidence that the upper estuarine area contributes more phytoplankton to the shoals than do the offshore waters.

The Exchanges of Fresh and Salt Waters in the Bay of Fundy and in Passamaquoddy Bay

The results of 385 previously published hydrographic stations in the Bay of Fundy are summarized, and the average distribution of salinity at various depths is derived from these data.The total quantity of river water accumulated within the Bay of Fundy is equivalent to the quantity introduced by the rivers during a period of about 76 days.The exchange ratios for tidal excursion segments of the Bay of Fundy range from 0.17 in the neighborhood of Cape Chignecto to 0.056 in the segment immediately south of Grand Manan. In the upper tidal reaches of the Petitcodiac River and of Minas Basin the exchange ratios are about 0.95.The estuary of the St. Croix River has been studied and calculations indicate that about 15 tides, or 8 days are required on the average to replace one day's river flow. The exchange ration in this case range from 0.905 to 0.30.From a summary of existing data in Passamaquoddy Bay it is calculated that the total accumulation of river water is equal to the quantity introduced by the rivers in a period of about 16 days.

A mixing‐length theory of tidal flushing

The salinity distribution in an idealized estuary where mining is predominantly due to the tides is computed, using an eddy diffusivity based on the distribution of tidal currents and excursions. Comparison is made to the salinity distribution in real estuaries.

Practical views on the proposed improvement of the Ohio River

Recent studies suggest that tidal forcing can be as important as gravitational circulation in maintaining an estuarine turbidity maximum (ETM). It is further postulated that a long-term mass balance between the import and export of sediment in an estuary may require episodic large river discharges or ‘freshets’ to flush sediment out of the ETM towards the open sea. In this study, we use a 2-month data set from a mooring in a tidal river that drains into a large drowned-river-valley estuary (Kaipara Harbour, New Zealand) to investigate interactions between tidal-current asymmetry and gravitational circulation. During baseflow river discharge and on spring tides, suspended-sediment transport was directed up-channel (landwards), driven by tidal pumping due to tidal-current asymmetry. During neap tides, the suspended-sediment flux was approximately zero. The data suggest that the bed was not locally erodible and that bed sediments at the site were being supplied by an ETM. The ETM only migrated far enough down channel to be observed at the mooring site during spring tides when the tidal excursion was longer. Suspended sediments were effectively trapped and recycled within the ETM. During and after two freshets, high river discharge displaced saline water from the tidal river, water-column stratification strengthened and the surface and bed current speeds increased. As a result, the advective component of the down-channel directed suspended-sediment flux increased. This provided a transport pathway for sediment out of the otherwise tidally pumped, flood-dominant system. We conclude that largest export of sediments out of the tidal river would potentially occur when a large freshet coincides with an apogean spring tide.