ERDC/CHL CETN II-43
June 2000
in the summation that provides the vertically integrated flux rate. This usually includes the upper
bin, and often several bins below this.
Bins below the lowest EMCM must also be treated separately. As previously stated, there are
inherent difficulties in placing instruments near the bottom during storms. Inserting EMCM and
OBS into the wave boundary layer may not be feasible. Therefore, methods must be developed
for interpolating current between the lowest EMCM and the sediment bed/water interface to
provide a velocity value for each bin below the lowest EMCM. Three options for this estimate
are provided in Figure 5 and represent
a. constant velocity between lowest EMCM and sediment bed/water interface,
b. constant velocity from the lowest EMCM to the wave boundary layer and linear decrease in
velocity from top of the wave boundary layer to zero velocity at the interface,
c. quadratic decrease in velocity from the lowest EMCM to zero at the interface.
Option a will overestimate velocity in the wave boundary layer because velocity is, by definition
for viscous flow, zero at the interface. However, this overestimate of velocity may be offset by
the underprediction of concentration in the bottom bin. For estimating transport from the
STORM data, method b was employed.
After velocity/concentration time series are developed for each bin, time-averaged unit longshore
transport is estimated for each bin:
N
1
∑
qbin =
(1)
Ci ui hbin
N i=1
where N is the number of measurements in the deployment, C is the concentration, hbin is the
height of the bin, u is the longshore current, and qbin is the time-averaged longshore transport
through the bin in kg/m/hr. The transport rates for each bin are then summed to provide an
estimate of the longshore transport per meter width over the entire water column
m
∑ qbin
qi =
(2)
bin=1
where m is the number of bins being used to calculate the vertically integrated transport rate and
q is the vertically integrated longshore transport rate in g/m/s at offshore location i.
collection stations were performed in a manner such that water level and longshore currents
remained nearly constant across the surf zone. By collecting data in this manner, it is possible to
estimate the integrated longshore transport of sediment across the surf zone for a particular
period of time. The integrated longshore sand transport rate is estimated by first assuming that
the transport measured at a particular station is representative of transport within certain limits of
space band time. More precisely, the integrated estimate assumes that longshore transport
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