Results

1
A multi-sensor approach to measuring
size-specific sediment transport in bottom
boundary layers
Philip Orton, Doug Wilson, David Jay OHSU /
Oregon Graduate Institute, Hydrodynamic Processes
and Ecosystems Lab http//www.ese.ogi.edu/ja
ylab
Challenge
Test Application
Results
Approach
We use an inverse analysis calibration approach
to take advantage of dynamical information
available from sediment concentration profile
shapes, and the differing particle-size
responses of optical and acoustical
instrumentation
The output of this analysis is high-resolution
concentration and transport fields, expressed in
terms of several discrete settling velocity
classes.
Issues in application of this method involve
Choosing a simple test site, where sediment
distributions are dominantly governed by
diffusion and settling Choosing
parameterizations for several variables Evaluati
ng the uncertainty in two realms a) Ws-class
definitions b) Concentration estimates
Conventional instrumentation for monitoring in
situ sediment settling velocity and concentration
often fails under extreme conditions.
Furthermore, such instrumentation generally
relies on settling tubes, which have been shown
to alter the ambient environment of the falling
particles. There has been little in situ
cross-verification using methods that lack this
problem. We seek a measurement technique
applicable in extreme environments, which
does not reduce the tubulence surrounding
particles is not limited by particle size
range is not limited by high suspended
sediment concentrations may be used in highly
turbulent conditions may be used in regions
with high ship traffic
From 0.3 km
8m depth
3m depth
TRAN2 C1
Measurements
TRAN2 C2
Verified with calibration data pumped water
samples in-situ laser particle-sizer Owen tube
Coulter counter
Acoustic Backscatter (ABS), 1200kHz Optical
Backscatter (OBS) Pumped Sediment
Concentration Sediment settling rate Sediment
size spectra
TRAN2 C3
TRAN1
TRAN2 C4
O OBS ABS
TRAN2
Calibration
Optics and Acoustics separately, have limitations
TRAN3
TRAN3 C1
Stage I
TRAN3 C2
Acoustic response to MASS concentration is
strongly dependent on particle size, r ?D3
Optical response to MASS concentration decreases
with increasing particle size, r ?D-1 Plotted
below is a real (Coulter-measured) size
distribution as multiplied by ABS and OBS
responses
Table 1 Uncertainty due to methods (as opposed
to data variance) simply offsets Ws-class
definitions, and has no effect on accuracy of
final concentrations
TRAN3 C3
TRAN3 C4
Stage II
Conclusions
This multi-instrument observational approach has
several advantages Useful for monitoring
sediment transport, residence times, erosion
and deposition rates, and pollutant transport
High resolution, able to resolve a wide range of
sediment sizes Could be used for inexpensive,
moored monitoring Is applicable to acoustic
instruments or joint optic/acoustic approach
Table 2 Definitions of Ws-classes, with a
preliminary analysis of sediment types and actual
settling velocities after consideration of
diffusivity parameter b (the ratio of diffusion
of mass to that for momentum)
Using Rose and Thorne (2001), equation B 3.1
exp( -0.17 U/Ws )