Surface Tidal Currents Research Articles

Characterization of the principal tidal current constituents on the Texas‐Louisiana shelf

We analyzed 81 current meter records of varying lengths (3 to 30 months) to describe the principal diurnal (O1, K1, P1, and Q1) and semidiurnal (S2, M2, K2, and N2) tidal current constituents on the Texas‐Louisiana continental shelf. The Louisiana‐Texas Shelf Physical Oceanography Program (LATEX) had 81 current meters at 31 sites and varying depths from April 1992 to December 1994. The local inertial period range across the shelf (24.4 hours to 26.2 hours) and thermal diurnal cycling during the summer season make it difficult to estimate the diurnal tidal constituents. Dominant tidal modes on the shelf are K1, O1, and M2. Absolute and relative energy contained in each tidal constituent varies with shelf location. The northeast corner of the shelf, near Atchafalaya Bay, has the largest tidal currents with maximum surface current amplitudes (at 3 m depth) of about 9 cm s−1, while typical maximum tidal surface currents near the shelf break are between 1 and 2 cm s−1 for each of the K1, O1, and M2 components. In general, the surface tidal currents decrease in magnitude as water depth increases toward the shelf break, although the semidiurnal components are amplified more at midshelf locations than the diurnal components. Examination of tidal ellipses at different depths suggests that the M2 tide has less vertical structure, while the diurnal tides exhibit more shear, particularly at the more shallow locations. Sea surface height constituents estimated at five locations along the Texas‐Louisiana coast are in agreement with historical values.

Bedform distribution and inferred sand transport on Georges Bank, United States Atlantic Continental Shelf

ABSTRACTFour bedform provinces have been identified on Georges Bank using sidescan‐sonar and echo‐sounding techniques: large sand waves superimposed on sand ridges, small sand waves, megaripples, and featureless seafloor. The large sand waves and sand ridges are found on the bank crest where the surface tidal currents are strongest. Areas of small sand waves and megaripples, formed where tidal currents are moderate in strength, border the area of large sand waves to the north and south. Featureless seafloor is found farthest from the bank crest where surface tidal currents are weakest.Sand‐wave asymmetry and surface‐sediment texture have been used to infer bedload transport paths on Georges Bank. In the large sand‐wave area, bedforms indicate a clockwise transport around each of the linear north‐west‐striking sand ridges with slight convergence of the sand waves on the ridge crests. This transport pattern implies erosion from the troughs and accumulation on the sand ridges. The asymmetry of the small sand waves along the south side of Georges Bank indicates that sand is also transported southward away from the linear sand ridges on top of the bank. Although the asymmetry of megaripples could not be determined, the occurrence of megaripples between the small sand‐wave province and areas of featureless seafloor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand dispersal pattern is further supported by the surface sediments which become progessively finer to the north and SW away from the crest of Georges Bank.

A mathematical model of an oil spill in the Upper Gulf of Thailand

Proper planning of oil pollution prevention, abatement as well as monitoring, for the Upper Gulf of Thailand requires a knowledge of the water mass transport. Magnitudes and directions of surface wind drift, fresh water and tidal currents have particular bearing on the movement and spreading processes of thin film layers of floating oil. Physical and chemical properties of oil govern the oil mass transfer. In response to the growing need for a knowledge of oil spill movement, a floating oil-spill movement model was constructed by the Division of Water Resources Engineering at the Asian Institute of Technology under a grant from the National Environment Board of Thailand. The results of the investigation leading to the construction of the model are presented in this report which contains the essential features involved in the analysis, and presents all significant findings of meteorologic and hydrodnamic conditions operating in the Upper Gulf. Techniques adopted in the quantitative assessments are also included. The model was satisfactorily verified using data from an actual spill. It is recommended that this oil spill movement model is used for further application in the Upper Gulf of Thailand.

Direction of drift of surface oil with wind and tide

Abstract The directions of surface oil slicks spreading from the wreck of t.e.v. Wahine during April 1968 are compared with predicted movements of surface water obtained by the vectorial addition of wind‐induced surface currents and tidal currents. Wind factors of 1%, 2%, 3%, 4%, and 5% of recorded surface wind speeds were used to calculate wind‐induced currents which were assumed to be in the same direction as the wind. The observed oil slick movements correspond best with the resultants obtained using a 3–5% wind factor, 60–64% of these falling within ± 20° of the observed directions of the oil slicks. Possible sources of error in the method are discussed and an example is given to illustrate how the method might be used to predict movements of surface oil resulting from an oil spillage.

Subsurface density inversions off Nantucket Island

Stations occupied south of Nantucket Island in the region of the discontinuity between the outflow waters of the Gulf of Maine and the denser offshore water show density inversions in the middle levels. The inversions are correlated with the tidal period, but not with the diurnal period. Rayleigh's instability criteria and other methods are used to investigate the conditions under which such inversions can exist. An equation is derived to obtain current velocities at different levels throughout the tidal period, from the surface tidal currents. Trajectories for each level indicate extensive overrunning of the lower layers by the upper layers during each tidal period. The magnitude of this overrunning is sufficient to reverse the slope of the discontinuity surface between the Gulf of Maine waters and the offshore waters, and to cause the inversions observed. A comparison between the conditions off Nantucket Island and cases of instability in other parts of the Atlantic Ocean shows a basic dissimilarity between them.