Sediment-transport events on the northern California continental shelf

Abstract

A long-term monitoring tripod has been maintained in 60-m water depth at the northern end of the STRATAFORM study site on the northern California continental shelf. As part of this ongoing study, tripod data for 1 year beginning 24 September 1995 are used to provide a sediment-transport analysis on an event-by-event basis. The objective of this paper is to highlight the energetic nature of this shelf region in terms of the frequency, duration, and magnitude of sediment-suspension events and the associated particle flux. Analyses are based on measurements from two current meters and two optical backscatterance sensors located at 30 and 100 cm above the bed. Data from these instruments and a pressure sensor were averaged over 7.5 min every hour. During the year of record, 41 distinct sediment-suspension events occurred (i.e., sustained suspended-sediment concentrations greater than 20 mg/l above background level at 100 cm above bed). Suspended-sediment events were associated with significant wave activity, tidal currents, and river discharge. The average duration of a sediment-suspension event was 3.1 days and varied from 0.7 to 8 days. During events, mean suspended-sediment concentrations of 110 mg/l were observed with peak hourly observations exceeding 1000 mg/l. Concentrations between events averaged 35 mg/l. The overall sediment flux for the period of record was directed seaward and southward. The distribution and magnitude of sediment-suspension events and particle flux showed a strong seasonality. For example, of the sediment transport during events, 98% of the net along-shelf, and 73% of the net across-shelf sediment transport occurred in the winter. Three major winter storms could account for 72% of the total along-shelf transport but only 10% of the across-shelf transport. The across-shelf flux was more evenly distributed over the year and occurred as a result of mean currents and very low-frequency seaward flows associated with mesoscale circulation patterns.