Near‐bottom sediment concentration and fluid velocity measurements on the inner continental shelf, New York

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Abstract
Prototype instrumentation designed to measure light scattering and transmission and the horizontal components of fluid velocity at a point 100 cm off the sea floor was deployed on the Long Island inner shelf for a 27‐day period in October and November of 1976. Depth at the point of deployment was 10.5 m. Data were taken from sensors in hourly bursts consisting of 468 consecutive 1‐s samples. Measurements taken with the electromagnetic current sensor were converted to absolute units by using a laboratory calibration, while absolute calibration of the turbidimeter in terms of suspended matter concentration was accomplished by using a combination of laboratory and field measurements. The results from the deployment show the effects of water motions at both wave and subtidal frequencies on sediment concentrations. Although burst mean fluid motions during the observation interval were primarily of tidal frequency, a single wind‐forced event caused burst mean currents in excess of 38 cm/s. During that same event, suspended particulate concentrations rose to a burst mean of 79 mg/l from a prestorm level of 5 mg/l, an order of magnitude increase occurring over a period of approximately 12 hours. During the storm event, individual burst records show that fluid motions at surface wave frequencies and with amplitudes of over 100 cm/s occurred at the experimental site. The near‐bottom suspended matter field at that time was found to have appreciable energy at surface wave frequencies as well. Changes in suspended matter concentration of the order of 130 mg/l were observed to occur in a period of 3–5 s. Increased burst mean concentration was seen to accompany increased wave activity, although increased mean flow with declining wave activity also led to increased suspended particulate concentration. When storm conditions (wave activity and mean flow much above deployment interval ambients) abated, concentrations returned to ambient levels in approximately 12 hours. Coherence between suspended matter concentration and horizontal fluid velocity was found to be statistically significant during four intervals of the deployment. In most cases the significance was centered around a broad frequency band near 0.1 Hz. Vertical near‐bottom wave velocities were inferred from the current measurements. The analysis of coherence and phase between the vertical velocity and concentration supports the hypothesis that part of the measured concentration signal was due to the vertical oscillatory advection of the mean concentration field past the turbidimeter during the passage of surface waves.