Multibeam Sonar on the Inner Shelf of the Eel Margin: Nearshore gravel-floored troughs

A high-frequency (300 kHz) multibeam sonar survey was conducted on the inner shelf of the Eel River Margin in August 2000 using a hull-mounted Simrad EM 3000D echosounder. The survey, which was conducted as part of the STRATAFORM project, was intended to provide high-resolution bathymetry and backscatter data on the inner shelf in water depths of 8 to 65 m. In addition to the multibeam data we collected nearly 100 grab samples with which to ground truth the backscatter imagery. One year prior to this survey, a test survey was conducted in a small portion of the survey area. This preliminary survey was done with a single hull-mounted EM 3000 transducer, and provided the opportunity to assess bathymetry and backscatter change on a time scale of 1 year. The largest changes between the two surveys were in the nearshore area, water depths of 8 to 15 m, a region of wave-induced sediment transport. During both surveys we noted the presence of nearshore troughs in less than 15-m water depth. The troughs are typically 0.2-1.0 m deeper than the adjacent seabed and are characterized by high backscatter. Gravel-sized sediment was recovered from the floors of the troughs in 2000. Although the shapes of individual troughs are irregular, the data collected in 2000 reveal that on a larger-scale, the scours create a cuspate pattern in the along-shore direction. The water depth and morphology of the features suggests that they may be the seaward extent of longshore bars. These troughs indicate active transport of large volumes of sediment and are likely related to nearshore processes. In one small region that was surveyed both years, we document both erosion and accretion with bathymetric change of up to 2-m in isolated locations. Similar to the pattern associated with the scours, bathymetric change reveals a cuspate pattern of erosion and accretion in the along-shore direction. In this subsampled area (1 km²) we observe significant bathymetric change and estimate a net removal of about 1 x 10⁵ m³ of sediment.