The stability of fine-grained sediments from the Fraser River delta

The stability and erodibility of benthic fine-grained sediments on the foreshore and upper foreslope of the Fraser River delta were measured using the in situ benthic flume-Sea Carousel. The erosion threshold [tau(c)(0)], which varied between 0.11 and 0.50 Pa, was inversely related to erosion rate. It was also proportional to sediment wet bulk density (rho(b)) in the form: tau(c)(0) = 7 x 10(-4) rho(b) 0.47. This trend agreed with those obtained from four other sites across Canada that encompassed a wide range in sediment densities: 1000 > rho(b) < 2000 kg m(-3). Erosion threshold Shields parameter (theta(c)) was positively related to grain Reynolds number over the silt-clay size range (0.01 < R-e < 5): theta(c) = 2.86 R-e(0.58). This was similar to earlier findings for water-saturated sediment. This trend is also in continuity with published threshold values measured in biostabilized sand (5 < R-e < 50). Three erosion types were observed: Type IA erosion was a surface phenomenon caused by the presence of a thin organic 'fluff' layer. It occurred at current speeds less than 0.3 m s(-1). Type 1B erosion (asymptotically decaying with time) characterized the mid-portions of each erosion time series, and occurred at current speeds of 0.3-0.9 m s(-1). Type I/II erosion (transitional) was largely found on the foreslope, and occurred at current speeds in excess of 0.8 m s(-1). The erosion process began in all cases under turbulent transitional flows. The change from Type I to Type II erosion corresponded to the onset of turbulent rough flows (60 < R-e < 70). This is supported by video observations, which showed Type I erosion to be largely the result of entrainment of small aggregates and flocs, whereas Type II erosion was the result of enlargement of surface irregularities, and subsequent undercutting and release of large aggregates. Mean erosion rate (E-m) was independent of either azimuthal current speed or bed shear stress. It was largely constant throughout each deployment and varied between 1.4 and 7.4 x 10(-4) kg m(-2) s(-1). (C) 1997 Academic Press Limited.