Jane McKee Smith

SUPERTANK LABORATORY DATA COLLECTION PROJECT

The first part of this article gives an overview of the development works that have been carried out to date along the French coast, covering types, operating principles and impacts. A philosophy is identified with regard to the schemes implemented so far and those recommended for the future: rather than static structures that resist the action of the sea, it would often be preferable to substitute dynamic structures and schemes that work in harmony with it. Much research is being carried out on designs of this type. An analysis and summary of this work is given in the second part of this article. Most such work is currently at the experimental stage and no miracle solution has been found so far. Local conditions must be examined carefully before deciding to use any of the new alternatives being proposed.

Longshore current on a barred beach: Field measurements and calculation

Measurements of the longshore current on a barred beach made during the 1990 Duck Experiment on Low-Frequency and Incident-Band Longshore and Across-Shore Hydrodynamics (DELILAH) field data collection project conducted at Duck, North Carolina, revealed an unexpected and persistent broad peak in the current velocity in the trough between the nearshore bar and the shore. This paper introduces longshore current and associated wave measurements from DELILAH together with a numerical model capable of describing the field observations. An existing numerical model of the mean current is modified to include a general transport equation for the mean turbulent kinetic energy created by wave breaking, and Reynolds stress components needed to calculate the longshore current and mean water level are reexpressed to include the turbulent momentum transport. In comparison to predictions from the original model, the modified model produces much improved agreement with the measured current velocity on the barred profiles in the field measurements and with the velocity profile and mean water level generated on a uniformly sloping laboratory beach by monochromatic waves. Three forms of a bottom friction coefficient are examined, and the open-channel-flow Manning friction coefficient is selected because of best agreement and consistency. Values of the Manning friction coefficient required to calibrate the model agree with values normally assigned to the related bottom roughness in open-channel flow.

Equilibrium ranges in surf zone wave spectra

[1]xa0Laboratory measurements of two wave trains breaking on a plane slope demonstrate dramatic changes in spectral shape in the surf zone. The higher-frequency spectral peak is completely eliminated in the surf zone, and the resulting spectral shape is similar regardless of the peak frequency and relative energy content of the higher-frequency peak. Examination of both laboratory and field data show that surf zone wave number (k) spectra evolve to contain two equilibrium ranges. The higher-frequency range is similar to that proposed by Toba in deeper water with the form k−5/2 and is valid for approximately k > 1/depth. The second range falls between the peak wave number and k = 1/depth and has a wave number dependence of k−4/3, similar to that proposed by Zakharov on theoretical grounds. The equilibrium range coefficients, which are a function of wind speed in deep and finite water depths, are a function of the water depth in the surf zone. The laboratory and field data sets used in these analyses covered a broad range of conditions (unidirectional and multidirectional waves, plane and barred beaches, two orders of magnitude variation in wave height, and one order of magnitude variation in wave period), yet the equilibrium ranges identified were consistent and provide a robust parameterization of surf zone wave spectra.