On the importance of nearbed sediment flux measurements for estimating sediment transport in the surf zone

Previous sediment studies in the surf zone typically have computed suspended sediment flux profiles by pairing a single current meter measurement at an elevation of 20 cm or more with measurements of suspended sediment concentration from sensor arrays located at 4–50 cm elevation. This note reports the results of a field experiment in which small impellor current meters were paired with OBS sensors at common elevations of 4, 9 and 19 cm from the bed and across the inner surf zone to obtain concurrent velocity and concentration profiles. The objectives of this experiment were to measure suspended sediment flux profiles within 20 cm of the seabed to determine the magnitude of flux very close to the seabed in comparison to measurements at higher elevation; and to evaluate the errors associated with estimating sediment flux using current meter measurements at only one elevation compared to flux estimates based on velocity profile measurements. Results show that the total sediment flux at z = 4 cm was greater than the flux higher in the water column (z = 9 and 19 cm) by a factor of at least 2. The flux profiles computed using a single impellor current meter at z = 19 cm and OBS sensors at z = 4, 9 and 19 cm typically were between 0.5 and 2.0 times the flux profiles computed using paired instruments. In one case the estimate of flux direction from the single current meter data predicted transport in the wrong cross shore direction. These results highlight the importance of nearbed measurements of concentration and velocity in estimating sediment transport in the surf zone.