Estimates of suspended-sediment flux and bedform activity on the inner portion of the Eel continental shelf

Energetic waves, strong bottom currents, and relatively high rates of sedim ent discharge from the Eel River combined to produce large amounts of suspe nded-sediment transport on the inner continental shelf near the Eel River d uring the winter of 1995-1996. Bottom-boundary-layer (BBL) measurements at a depth of similar to 50 m using the GEOPROBE tripod showed that the strong est near-bottom flows (combined wave and current speeds of over 1 m/s) and highest sediment concentrations (exceeding 2 g/l at similar to 1.2 m above the bed) occurred during two storms, one in December 1995 and the other in February 1996. Discharge from the Eel River during these storms was estimat ed at between 2 and 4 x 10(3) m(3)/s. Suspended-sediment flux (SSF) was mea sured 1.2 m above the bed and calculated throughout the BBL, by applying th e tripod data to a shelf sediment-transport model. These results showed ini tially northward along-shelf SSF during the storms, followed by abrupt and persistent southward reversals. Along-shelf flux was more pronounced during and after the December storm than in February. Across-shelf SSF over the e ntire measurement period was decidedly seaward. This seaward transport coul d be responsible for surficial deposits of recent sediment on the outer she lf and upper continental slope in this region. Sediment ripples and larger bedforms were observed in the very fine to fine sand at 50-m depth using a sector-scanning sonar mounted on the tripod. Ripple wavelengths estimated f rom the sonar images were about 9 cm, which compared favorably with photogr aphs of the bottom taken with a camera mounted on the tripod. The ripple pa tterns were stable during periods of low combined wave-current bottom stres ses, but changed significantly during high-stress events, such as the Febru ary storm. Two different sonic altimeters recorded changes in bed elevation of 10 to 20 cm during the periods of measurement. These changes an thought to have been caused principally by the migration of low-amplitude, long-wa velength sand waves into the measurement area. (C) 1999 Elsevier Science B. V. All rights reserved.