Ck. Harris et Pl. Wiberg, APPROACHES TO QUANTIFYING LONG-TERM CONTINENTAL-SHELF SEDIMENT TRANSPORT WITH AN EXAMPLE FROM THE NORTHERN CALIFORNIA STRESS MID-SHELF SITE, Continental shelf research, 17(11), 1997, pp. 1389-1418
Modeling shelf sediment transport rates and bed reworking depths is pr
oblematic when the wave and current forcing conditions are not precise
ly known, as is usually the: case when long-term sedimentation pattern
s are of interest. Two approaches to modeling sediment transport under
such circumstances are considered. The first relies on measured or si
mulated time series of flow conditions to drive model calculations. Th
e second approach uses as model input probability distribution functio
ns of bottom boundary layer flow conditions developed from wave and cu
rrent measurements. Sediment transport rates, frequency of bed resuspe
nsion by waves and currents, and bed reworking calculated using the tw
o methods are compared at the mid-shelf STRESS (Sediment TRansport on
Shelves and Slopes) site on the northern California continental shelf.
Current, wave and resuspension measurements at the site are used to g
enerate model inputs and test model results. An Ii-year record of bott
om wave orbital velocity, calculated from surface wave spectra measure
d by the National Data Buoy Center (NDBC) Buoy 46013 and verified agai
nst bottom tripod measurements, is used to characterize the frequency
and duration of wave-driven transport events and to estimate the joint
probability distribution of wave orbital velocity and period. A 109-d
ay record of hourly current measurements 10 m above bottom is used to
estimate the probability distribution of bottom boundary layer current
velocity at this site and to develop an auto-regressive model to simu
late current velocities for times when direct measurements of currents
are not available. Frequency of transport, the maximum volume of susp
ended sediment, and average flux calculated using measured wave and si
mulated current time series agree well with values calculated using me
asured time series. A probabilistic approach is more amenable to calcu
lations over time scales longer than existing wave records, but it ten
ds to underestimate net transport because it does not capture the epis
odic nature of transport events. Both methods enable estimates to be m
ade of the uncertainty in transport quantities that arise from an inco
mplete knowledge of the specific timing of wave and current conditions
. (C) 1997 Elsevier Science Ltd.