SEA CAROUSEL - A BENTHIC, ANNULAR FLUME

A benthic annular flume (Sea Carousel) has been developed and tested to measure in situ the erodibility of cohesive sediments. The flume is equipped with three optical backscatter sensors, a lid rotation switch, and an electromagnetic (EM) flow meter capable of detecting azimuthal and vertical components of flow. Data are logged at rates up to 10·66 Hz. Erodibility is inferred from the rate of change in suspended sediment concentration detected in the annulus. The energy-density/wave number spectrum of azimuthal flow showed peaks in the energy spectrum at paddle rotation wave numbers (k) of 14 and 7 m-1 (macroturbulent time scales) but were not significant. Friction velocity (U*), measured (1) at 1 Hz using a flush-mounted hot-film sensor, and (2) derived from measured velocity profiles in the inner part of the logarithmic layer gave comparable results for Ū* < 0·064 m s-1. At higher values of U*, method (2) underpredicted by up to 20%. Method (1) showed radial increases in Ū* in the annulus for Ūy > 0·32 m s-1. Radial velocity gradients were proportional to (Ūy - 0·32 m s-1). Maximum radial differences in U* were 10% for Ūy = 0·5 ms-1. Suspended sediment mass concentration (S) in the annulus resulted in a significant decrease (10·5%) in Ū* derived by method (1) over the range 0<S<208 mg l-1. These decreases were not evident in method (2). Method (1) may, therefore, be subject to changes in stress sensor calibration with changes in S. Subaerial deployments of Sea Carousel caused severe substrate disturbance, water losses, and aeration of the annulus. Submarine deployments produced stable results, though dispersion of turbid flume water took place. Results clearly demonstrated the existence of 'Type I' and 'Type II' erosion documented from laboratory studies. © 1992 Academic Press Limited.