Suspended sediment transport in the surf zone: Response to cross-shore infragravity motion

Abstract

A field experiment to investigate the magnitude and temporal and spatial scales of suspended sediment transport in the inner surf zone was conducted at San Marine, Oregon on October 7–13, 1984. Instrumentation consisted of five electromagnetic current meters, three strain gauge-type pressure transducers, and 25 optical backscatter sensors to measure the concentrations of suspended sediment. The instruments were deployed approximately 70 m ( x) seaward of the mean high water line (mean depth h ̄ = 1.1−1.3 m ) in a closely spaced cross-shaped pattern. Data were collected for 68 min periods spanning high tide during the 7 day experiment. During the experiment, waves varied in angle of approach from shore-normal to southwesterly. Offshore significant swell heights were 3–5 m with periods of 12–14 s. Local sea conditions with wind waves of approximately 8 s also occurred. A visual estimation of the width of the surf zone indicated it to be approximately 500 m and the breaker pattern suggested that there were two longshore bars at approximately 200 and 500 m offshore. The data resulting from the experiment encompass a broad range of wave and current conditions which included periods of extreme infragravity motion (30 < T < 300 s) and weak longshore currents, and periods of low infragravity motion and strong longshore currents. Periods of incident wave dominance also occurred. The field study section of this paper qualitatively describes the sediment response to strong (± 240 cm s −1), low-frequency (0.01 Hz), cross-shore fluid motions which dominated the inner surf zone during two of the data runs. The infragravity band accounted for 85% of the cross-shore velocity variance, the peak (0.01 Hz) of which was associated with shore-normal standing waves. Concentration gradients of suspended sediment appear to be associated with diffusive processes and develop and decay on time scales of the infragravity motions rather than those of incident waves. Suspension events during times of infragravity dominance often persisted for periods of 30–45 s and reached peak concentrations of 20–40 g l −1 at elevations in excess of 26 cm. Mean suspended load was 3–4 times larger than that associated with incident wave motions. The modeling section of the paper attempts to quantitatively describe the observed time-dependent, vertical suspended sediment distribution with a one-dimensional, turbulent diffusion model. Formulations of eddy diffusion and erosion rate coefficients associated with peak cross-shore flows are found to agree with or fall within the range of values from previous laboratory and field studies and are used in the model. The model is evaluated by its ability to reproduce the time-dependent vertical distribution of suspended sediment and to estimate the observed mean suspended load, and the correct direction and magnitude of the mean longshore and cross-shore flux (level above the bed z = 31 cm) of suspended sediment.