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 relat
ed to erosion rate. It was also proportional to sediment wet bulk dens
ity (rho(b)) in the form: tau(c)(0) = 7 x 10(-4) rho(b) 0.47. This tre
nd 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 positivel
y 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 fi
ndings 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 surf
ace 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 fores
lope, and occurred at current speeds in excess of 0.8 m s(-1). The ero
sion process began in all cases under turbulent transitional flows. Th
e change from Type I to Type II erosion corresponded to the onset of t
urbulent rough flows (60 < R-e < 70). This is supported by video obser
vations, which showed Type I erosion to be largely the result of entra
inment of small aggregates and flocs, whereas Type II erosion was the
result of enlargement of surface irregularities, and subsequent underc
utting and release of large aggregates. Mean erosion rate (E-m) was in
dependent of either azimuthal current speed or bed shear stress. It wa
s largely constant throughout each deployment and varied between 1.4 a
nd 7.4 x 10(-4) kg m(-2) s(-1). (C) 1997 Academic Press Limited.