Application à la zone au large du Finistère d'une méthode d'estimation statistique des courants de houle au voisinage du fond

Application to an area off the Finistere coast of a statistical method of estimating wave currents near the sea bed

C.N.E.X.O.

Abstract

The spectral energy distribution of sea waves is supplied four times a day by the French Meteorological Office's DSAs model using the pressure distribution over the Atlantic as input. The surface-motion spectrum furnished by the model may then be used to estimate the local current spectrum near the sea bed. The present study is based on four years (1970-1974) of DSAs data for the so-called Brittany point (47°30 N, 5° W). The data correlates reasonably well with observations made from ships in the region lying between 40°-50° N and 0°-10° W (Figure 1). Plotted on Weibull paper in Figure 2 are the cumulative probability values of U_1/3 as calculated from the DSAs data (U_1/3 is the mean of the upper third of maximum current values at a depth of 100 m). The experimental U_1/3 statistics appear to obey Weibull's law quite closely. They also show that 0.7 m/s currents occur on average once a year. The principal objective of the study was to provide order-of-magnitude estimates of currents. Accordingly, factors such as reflection, diffraction, bed friction, and loss or gain of amplitude due to reduced depth were neglected. The extent to which this is justified might be assessed by carrying out model calibration tests in an area of particular interest in the course of one winter.