in situ measurements of particle settling velocity on the northern california continental shelf

Pergamon Continental Shelf Research, Vol. 14, No. 10/11, pp. 1123-1137, 1994

Copyright © 1994 Elsevier Science Ltd 0278--4343(94)E0010-J Printed in Great Britain. All rights reserved

0278-4343/94 $7.00 + 0.00

In situ measurements of particle settling velocity on the northern California continental shelf

P. S. H I L L , * t C. R. SHERWOOD,*3~ R. W. STERNBERG* and A. R. M. NOWELL*

(Received 28 May 1993; in revised form 4 November 1993; accepted 9 November 1993)

Abstract--As part of the Sediment TRansport Events on Shelves and Slopes (STRESS) program, a remote optical settling box was deployed on the northern California continental shelf. The device operates by isolating a volume of sediment-laden fluid from the environment and then monitoring its sedimentation behavior with a transmissometer. Results show a bimodal distribution of suspended sediment during low-energy periods on the shelf that reflects the size distribution of bottom sediments. The coarse mode sinks at 0.026 cm s-1 (22 m day-l ) and the fine mode settles at 0.0025 cm s -I (2 m day-l) . Between one-quarter and two-thirds of the total mass resides in the coarse mode. Roughly one-quarter is in the fine mode. The remainder sinks too slowly (<0.0015 cm s-1 or < 1.3 m day -1) to be resolved during the 18-h measurement cycles. Greatest uncertainty in assigning mass to the various settling velocity classes comes from sensitivity to ill-constrained particle geometry of the conversion from light attenuation to mass. The device failed during higher energy periods, probably due to penetration of fluid into the box. Complete isolation of the fluid from the environment would improve the performance of settling boxes in energetic settings.

1. I N T R O D U C T I O N

SUSPENDED-Sediment flUX depends critically on the settling velocity of the sediment in suspension. Grains with relatively large settling velocities are more concentrated near the bed where fluid flow is retarded. Slower sinking grains are distributed more evenly through the bottom boundary layer, so that a substantial fraction of finer material moves with higher velocity fluid far from the bed.

Grain size, shape and density determine settling velocity. For sands and coarse silts these parameters may be estimated by direct examination of recovered bottom sediment. Finer silts and clays, however, tend to aggregate into flocs of variable size and density that sink at rates much greater than the component grains. Flocs lack resilience, so settling velocities of disturbed and disrupted flocs bear little resemblance to in situ settling velocities (ALLDREDGE and GOTSCHALK, 1988). The floc-dominated settling velocity of fine-grained sediment, therefore, must be measured in situ.

*School of Oceanography, WB-10, University of Washington, Seattle, WA 98195, U.S.A. tPresent address: Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada, B3H

4J1. ~Present address: Battelle Marine Sciences Laboratory, 1529 West Sequim Bay Road, Sequim, WA 98382,

U.S.A.

1123

1124 P.S. HILL et al.

To measure settling velocity of fine suspended sediment on the continental shelf of northern California, an in situ settling box was deployed as part of the Sediment TRansport Events on Shelves and Slopes (STRESS) program. The basic approach involves isolating an ambient sediment-fuid mixture in a settling box and relating changes in optical properties of the mixture to clearance rate of sediment due to settling in the column. This method has shown promise for determining settling velocities of natural flocculant sediment because it minimizes disturbance to aggregates, which alters settling velocities (ALLDREDGE and GOTSCHALK, 1988). Its use has been limited historically by uncertainty surrounding the relationship of observed optical properties to sediment properties. The introduction of monochromatic light sources, highly collimated beams and temperature-compensated sensors has reduced uncertainty (ZANEVELD et al., 1982), so the use of optics to interrogate sedimentation behavior of natural suspensions is enjoying a revival. For example, KINEKE et al. (1989) developed an in situ settling column instru- mented with five miniature nephelometers. Deployed in San Pablo Bay, California, the device measured in situ settling velocities much greater than predicted by examination of disaggregated bottom sediment. Measured settling velocities compared well with those inferred from vertical distributions of sediment in the boundary layer. MCCAVE and GROSS (1991) deployed the Remote Optical Settling Tube (ROST) on the Nova Scotian continental rise and similarly discovered a settling-velocity mode greater than that of the bottom sediment.

The remote optical settling box deployed on the California continental shelf in this study is the redesigned successor to ROST. Like ROST, it tracks settling in a rectangular box with a transmissometer. The goal of the study initially lay in monitoring changes in the settling velocity distribution of suspended sediment as a function of energy level on the shelf. This goal proved elusive because the device did not work as planned under energetic conditions. Focus then shifted to elucidation of the "fair weather" properties of the suspended load.

2. METHODS

The remote optical settling box was mounted 1.19 m off the bottom on the ABSS tripod, which is a modified version of the HEBBLE tripods (WILLIAMS et al., 1987). The ABSS tripod was deployed at the STRESS 90-m site, which is the CODE C3 site (BEARDSLEY and LENTZ, 1987) located at 38 ° 37.91' N, 123 ° 28.44' W on the northern California continental shelf (Fig. 1). The site was selected because of the wealth of physical and geological oceanographic data available and alongshelf uniformity in bathymetry and bottom sediments. Bottom sediment at the site is bimodal. A fine mode of clay and fine silt (<5 #m) and a coarse mode of medium silt (--~30 #m) are separated by a trough at ~10 ktm (WHEATCROFT, personal communication).

The rectangular Lexan settling box measures 100 cm high × 25 cm wide × 12.5 cm deep (Fig. 2). A Sea Tech 25-cm pathlength transmissometer monitors optical properties in the box at a position 5 cm above the bottom of the box. Transmissometer output ranges from 0-5 VDC and is digitized to I part in 1023, providing voltage resolution of 4.9 mV. The box is opened and closed with end plates that pivot into position. A stepper motor coupled to the endplates with a friction clutch moves the endplates from the fully opened to fully closed position in 32 s. A programmable computer (Onset Tattletale Model 4 with 384 Kbytes of memory) controls the device and logs output from the transmissometer.

The settling box was deployed twice. The first deployment took place on 18 November

Settling velocities on the continental shelf 1125

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38* 20'N

38* 10'N

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124" 0 0 ~ 123" 50"q¢

Fig. 1.

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:::::::::::::::::::::::: " k~ RUSSIAN RIVER

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123" IO'W 123" OIYW 122" 50'W

Location of the STRESS site on the continental shelf off northern California. The shaded area is the silt deposit, delineated by the 22-pm mean grain-size isoline.

1990 at 1600 UTC. Recovery was on 5 January 1991 between 1645 and 1845 UTC. The second deployment was on 7 January 1991 at 2026 UTC with final recovery on 9 March 1991 between 1800 and 2000 UTC.

The box was closed once per day for 18 h, during which time sedimentation was monitored• After each 18-h run, the box was opened for 6 h so that it was replenished with ambient fluid. While the box was open, voltage, which is linearly related to transmission, was measured at 1 Hz for 10 min each hour. For each 10-min period number of observations, sum of measured voltages, sum of squares of voltages, and minimum and maximum voltage were recorded. U p o n initiation of closure, voltage was measured at 1 Hz for 10 min, including the 32 s between initiation and complet ion of closure• Each of these observations was recorded. During the subsequent 18-h sedimentation period, 50 obser-

1126 P.S. HILL etal.

Fig. 2. Schematic of the optical settling box: a 100 x 25 x 12.5 cm Lexan box (a) is opened and closed by pivotable doors (b), driven by a stepper motor equipped with a friction clutch (not shown). A transmissometer mounted 95 cm below the top of the box (c) monitors optical

properties.

vations were made at exponentially increasing intervals. Each observation consisted of 10 samples taken at 1 Hz. Only statistics of these observations were recorded.

During each deployment the friction clutch coupling the motor to the box doors corroded and failed after several weeks. The times of failure were not recorded, but they were deduced from examination of the data. In each deployment the friction clutch functioned for 16 cycles.

During individual cycles measured voltage did not always increase monotonically, as expected (Fig. 3). In most runs high-frequency voltage fluctuations persisted for much of the 10-min initial sampling period. These fluctuations likely reflect fluid agitation with the box just after closure. More troublesome are runs that show large decreases in voltage, indicating increased sediment concentration, well after the box had closed (Fig. 3). Such decreases suggest either mixing within the box or penetration of sediment-laden fluid into the box. Comparison of the transmissometer readings in the box with the output of optical backscatter sensors mounted on the same tripod reveals correlation between mid-run decreases in voltage and increases in ambient sediment concentration. Using suspended- sediment load as a proxy for wave and current energy on the shelf, such correlation suggests that the likely cause of mid-run voltage decreases is fluid exchange between the closed box and the surrounding fluid. Pumping of fluid into and out of the box could churn the suspension in the box, yielding spurious data.

Using the correlation between optical backscatter sensor output and spurious decreases in transmissometer voltage as rationale, all runs that occurred during times that the optical backscatter sensor output showed substantial variability during the course of the 18-h closed period were eliminated from the ensuing analysis. Applying this standard left 13


"d

4.0

3.0

2.0 I I I I I I I 0.2 0.4 0.6 0.8

D a y s

Fig. 3. Voltage vs time for a good (solid line) and a bad run (dotted line). In the absence of stirring in the box voltage should increase monotonically as sediment settles and the clarity of the

water increases.

usable runs. The seven from the first deployment were begun on 20, 24, 25, 26, 27, 29 and 30 November 1990. The six from the second deployment were begun on 11, 16, 19, 20, 21 and 22 January 1991 (Fig. 4).

By focusing on runs free of substantial variability in background suspended sediments, the goal of delineating changes in the suspended-sediment settling-velocity distribution with changing energy levels on the shelf was surrendered. The data remained useful for exploring the background, "fair weather", suspended-sediment settling velocity distribution. In this new context the data were considered an ensemble of measurements of the sedimentation behavior of the low-energy suspended load.

Transmissometer voltage varies in direct proportion to percentage light transmission. In passage through a volume of water, light transmission is attenuated by the water itself and by suspended particulate and dissolved material. Attenuation is expressed with an attenuation coefficient, a, that is the sum of the attenuation coefficients for water and for particles. The particle attenuation coefficient, ap, is (cf. MCCAVE and GRoss, 1991)

ap = In(Vo/V)/L, (1)

where V is measured voltage, Vo is measured voltage in clear water and L is beam pathlength (25 cm). Clear-water voltages were determined from baseline voltages during periods of apparent high water clarity. Baseline voltages were 4.085 and 3.681 V for the first and second deployments respectively.

This method for setting Vo assigns the ambient washload and dissolved material invariably present in shelf waters to the water portion of the attenuation. In so doing, it focuses attention on the part of the suspended sediment load in excess of background levels. The method will allot less material to the fine fraction than if a true clear-water or

1 1 2 8 P . S . HILL et al.

320 340 360 380 400 420 440

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320 340 360 380 400 420 440

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320 340 360 380 400 420 440 Days 1991

Fig. 4. T ime ser ies of s ignif icant wave he ight at N D B C buoy 46013 (a); opt ica l backsca t t e r sensor

o u t p u t for sensor m o u n t e d at 0.19 m a b (b); t r a n s m i s s o m e t e r ou tpu t (c); and s t andard dev ia t ion of t r a n s m i s s o m e t e r ou tpu t (d) showing t ime of c lutch fai lure.


Fig. 5.

1 . 2

1.0

0.8

~/c~r 0.6

0.4

0.2

, I , , , ~ , , , , I

1 0 - 2 10-1

I I I I I I

10 0

Days

Ensemble normalized attenuation vs time: solid line, median; dashed line, third quartile; dotted line, first quartile.

air value for V o was used. The effect of different strategies for setting Vo will be explored in the discussion.

The attenuation coefficients for each time after closure were normalized by the mean attenuation coefficient of the 32 observations made during closure of the box. Normalized attenuations from the 13 usable runs were then pooled and succeeding analysis was on the behavior of the first, second and third quartiles of the data (Fig. 5).

The fractional amount by which the total particle attenuation coefficient was reduced over each interval between observations was calculated by subtracting a five-point running average of normalized attenuation centered on time after closure, t( i) , from the average normalized attenuation at time t(i - 1). Settling velocity, wsi , of material lost in the interval between t(i - 1) and t(i) was set as

h w s i - t( i) ' (2)

where h is distance of the transmissometer from the top of the box (95 cm). The attenuation due to particle size class i is (MCCAVE and GROSS, 1991)

zcdi2 N R ai = T i ki' (3)

where ai is the particle attenuation coefficient for size class i, di is the diameter of class i, Ni is the number of particles per unit of volume in size class i and Rki is a dimensionless scattering efficiency factor. The coefficient Rki is not a strong function of diameter for the sediment sizes on the shelf (ZANEVELD et al. , 1982; McCAvE and GROSS, 1991). Normalized attenuation ( a i ) / a r is given by


a i - "4 NiRki (4)

aT + ztd 2 ' N R 2. T i ki

i = 1

where a r is the mean attenuation coefficient during closure and n is the number of size classes. Proceeding under the assumption of constant Rki, equation (4) may be rewritten as

ai +Jrd~ N. ar / -~ 4 '

Ni = 1 ard/2 (5)

4

Mass per unit of volume in a size class, Mi, in its most general form is

(dild3 M i=-~mo ~do] Ni' (6)

where d3 is a three-dimensional ffactal dimension (ORBACH, 1986), mo is the mass of one of the particles of which the aggregates are made, and do is the diameter of these component particles. The use of a fractal dimension allows accommodation of observed decreases in particle bulk density with increased particle size of natural marine aggregates (e.g. LOGAN and WILKINSON, 1990). Larger suspended particles in the sea tend to be aggregates which become more porous as they grow. Fractional mass is

at [d .\d3 M i -6m°[~oo) Ni

- , ( 7 ) Mr (d¢3 -~mO~do] Ni

1

where M r is the initial mass in the settling box. With equation (5), equation (7) may be rewritten as

n

Mi 2 1 ddS-2 ai. (8)

m r 3 ~ jr d/d 3 Ctr g

1

The ratio of total projected particle area to total particle mass is constant for a given suspension, so equation (8) becomes

Mi _ Cdd3-2 a j_i (9) M T a T'

where C is a constant for a given suspension. To convert from normalized attenuation to fractional mass requires values for C and d3 and an expression for particle diameter, d~, as a function of settling velocity, wsi.


The ratio of aggregate diameter, di, to the diameter of the component particles within an aggregate, do, is (cf. HILL, 1992)

d i - - ( W s i l l l ( d 3 - 1 ) , (10)

do \Wsol

where Wso is the settling velocity of particles of diameter do. For particles of constant density, d3 = 3 and equation (10) states that for Stokesian particles diameter goes as the square root of settling velocity. Component-grain settling velocity may be related to diameter by Stokes' law as

Wso - (& - p)gd2, (11) 18/t

where Ps is sediment density, p is fluid density, g is gravitational acceleration and/~ is molecular dynamic viscosity. Inserting equation (11) into equation (10) yields

[ 1 8 / ~ ~ l / ( d 3 - 1 ) di = dd°3-3/d3-1 [(Ps - - P)g wsi) " (12)

Substitution of equation (12) into equation (9) yields

Mi ..-, ,A [ 18/t ~B ai - - (..a o i Wsi I - - ,

MT \(Ps -- P)g ] aT (13)

where

d3 2 - 5d3 + 6 A = d3 - 1 (14)

and

d3 - 2 B - d 3 ~ " (15)

For d3 = 3 (constant-density particles) equation (13) reduces to a statement that fractional mass is proportional to normalized attenuation times particle diameter.

The constant C in equation 13 may be determined by noting that

Mi (Ps ) aT = 1 = Cd ws i - - . 1 1

(16)

Solving for C yields

n Gt- - i

k(o,- o)g/ a,-j 1

(17)


M/M

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0.00

!1 7 I I I

I

i "1

.... ,-, ...... i - i - - ~ - : _ " " " : . - : :

, - - , . - - , , . .

I i i i i i i11 i i

10-2 i

10-1

w s (cm s'l)

Fig. 6. Histogram of median fraction of total initial mass vs settling-velocity class for different values of fractal dimension, d3, when d o is 2/tm: solid line, d3 = 2.4; dot-dashed line, d3 = 3.0;

dotted line, d3 = 2.0.

Inserting equation (17) back into equation (13) yields

w a/ Mi a T (18)

a T 1

Fractional mass can be estimated from equation (18) by assuming that the diameter of the component grains within the flocs is identical to the diameter of material too fine to be resolved during the 18-h sampling period. Then by inputting values for the smallest particle diameter in the suspension, do, and the three-dimensional fractal dimension of aggregates, d3, one can solve for fractional mass. The value of do determines the value of Wso needed in equation 18 through Stokes' law [equation (11)].

To gauge sensitivity of the results to assumed minimal particle size, do, and fractal dimension, d3, ranges of values were used. The fraction of the total attenuation due to particles smaller than could be measured during the 18-h runs was alternately assigned to particles of diameter 1, 2 and 4/~m. Fractal dimensions are variable and depend on a variety of factors including mechanism of formation and fluid agitation in the suspension (JIANG and LOCAN, 1991). Smaller aggregates are thought to have d3 values greater than those for larger aggregates (McCAvE, 1984). Values of 2.0, 2.4 and 3.0 were used here.

3. DISCUSSION

The "fair weather" suspended sediment on the continental shelf possesses a basic bimodal distribution (Figs 6-8). A coarse mode composing 0.25-0.66 of the total mass


0 .08

M / M T

0.07

0 .06

0.05

0 .04

0.03

0 .02

0.01

0 .00 i

10 -2 10-1

w (cm s 1)

Fig. 7. Histogram of median fraction of total initial mass vs settling-velocity class for a value of d3 of 2.4 and different assumed diameters of the smallest particles in the box, do : solid line, d o = 2.0;

dot~:tashed line, d o = 4.0; dotted line, d o = 1.0.

M / M T

0.08

0 .07

0 .06

0.05

0 .04

0.03

0 .02

0.01

0.00

f - 1

L

I

r t i 1 i . . i

i - i

! i ! i ~! ~ ~ L ~I~Ltl t , , , ,

10-2 1 I

10 -1

w s (cm s l )

Fig. 8. Histogram of fraction of total initial mass vs settling-velocity class for the first, second (median) and third quartiles. Values for d3 and d o are 2.4 and 2.0 Hm respectively: solid line,

median; dot~iashed line, third quartile; dotted line, first quartile.

1134 P . S . HILL et al.

Table 1. Distribution of mass among settling velocity classes for different assumed fractal dimension, d3, generated from median normalized

attenuation vs time data. Assumed diameter of smallest particles is 2 izm

d3 ws < 0.0015 0.0015 <- w~ 0.008 -< w s < 0.0792

2.0 0.49 0.26 0.25 2.4 0.25 0.26 0.49

3.0 0.12 0.22 0.66

Table 2. Distribution of mass among settling velocity classes for different assumed smallest particle diameter, do, generated from median normalized

attenuation vs time data. Assumed fractal dimension is 2.4

do (~m) ws < 0.0015 0.0015 - w s < 0.008 0.008 -< ws < 0.0792

1.0 0.18 0.29 0.53

2.0 0.25 0.26 0.49

4.0 0.33 0.24 0.43

Table 3. Distribution of mass among settling velocity classes for different quartiles. Assumed fractal dimension is 2.4, and assumed smallest particle

diameter is 2.0/zm

quartile w s < 0.0015 0.0015 -< w s < 0.008 0.008 -< ws < 0.0792

1 0.16 0.31 0.53

2 0.25 0.26 0.49

3 0.36 0.14 0.50

sinks at ~ 0.026 cm s -1 (Tables 1-3). A fine mode settles at ~ 0.0025 cm s -I , and its contribution to the total mass varies between 0.14 and 0.31 (Tables 1-3). The fraction of the original mass remaining in the box after 18 h ranges from 0.12 to 0.49 (Tables 1-3). This fine sediment sinks at speeds less than 0.0015 cm s -1.

The variability of estimated partitioning of mass among settling-velocity classes arises from three sources. The first is choice of fractal dimension d3. The value of d3 determines sensitivity to particle diameter of the attenuation-to-mass conversion. A value of d3 of 3 makes the attenuation-to-mass conversion scale linearly with particle diameter, whereas a value of 2 eliminates diameter dependence. The amount of mass assigned to the coarse mode therefore decreases with the value of d3. The second source of variability in estimates of mass partitioning is choice of the diameter d o of sediment too small to be resolved during the 18-h sampling intervals. When attenuation-to-mass conversion increases with diameter (i.e. d3 > 2), large do results in the assignment of more mass to these small particles. Finally, the third source of variability is measurement error.

Of the three sources of variability of estimated mass partitioning, the choice of fractal dimension is the greatest (Fig. 6, Table 1). Varying the diameter d o of sediment too small


to be resolved during the 18-h sampling has little influence on the distribution of mass among settling velocity classes (Fig. 7, Table 2). Variability due to measurement error is minimal in the coarse mode (Fig. 8, Table 3), but it is somewhat larger for the smaller settling velocity classes. Overall, however, uncertainty in the measurements is smaller than uncertainty associated with choice of fractal dimension (Tables 1, 3). Better constraint may be placed on estimates of mass distribution among settling-velocity classes by investing effort in better defining the fractal dimension of marine particles.

By setting Vo [equation (1)] as the measured voltage in the clearest water during a given deployment, the mass in coarser sizes is greater than if Vo were assigned a truly clear-water value. The former strategy focuses attention on that part of the suspended load above ambient washload. Differences in partitioning of mass among size classes for the two approaches were assessed by comparing fractions assigned to the finest material with a Vo of 3.681 V and one of the air calibration for the instrument, which is 4.67 V. Use of the higher Vo increases the fine fraction by -~ 10%. This change is much less than those caused by different values of d3, again emphasizing sensitivity of results to choice of fractal dimension.

More problematic is estimating the uncertainty associated with choice of the scattering efficiency factor, R~i [equation (3)]. Theory suggests that for mineral grains between 7 and 30/~m, R~i remains relatively constant (ZANEVELD et al., 1982). Assuming the particles in the column are spheres of density 2.65 g cm-3, sizes range for 5/~m for the slowest sinking particles (w~ = 0.0015 cm s-a) to 36/~m for the fastest sinking ones (Ws 0.0792 cm s-1). This size range falls, for the most part, in the predicted region of constancy for Rt, i. Particles are probably aggregates, however, pushing the particle size range monitored outside the range of constant Rki. Scattering efficiencies for aggregates are unknown. Because Rk~ may fall steeply for natural particles larger than 30/~m (BAKER and LAVELLE, 1984), more of the total suspended mass than estimated here may reside in coarser sizes. Estimating how much more is presently not possible.

A bimodal distribution in a fine-sediment suspension presumably indicates insufficient time has passed since the formation of the suspension for aggregation to blur modal peaks (MCCAVE, 1984). The bimodal suspended load on the shelf theoretically should reflect recent resuspension with the distribution matching that of the local bottom sediment. Grain size analysis of bottom sediment reveals a bimodal distribution with a fine (<5/~m) mode separated from a coarser (20-60/~m) mode by a trough at ~ 10/~m (Wheatcroft, personal communication). Mass is roughly equally divided between the clay-fine silt mode and the medium-to-coarse silt mode. Particles that survive grain-size analysis are likely to be tough, densely packed aggregates or primary grains. Assuming a fractal dimension of 3, therefore, yields settling velocities of 0.0065 cm s -x for 10/~m particles and 0.024 cm s-1 for 20-ktm particles. These settling velocities match closely the settling velocities of the trough and coarse mode in the settling velocity histogram generated from the settling column (Figs 6-8). Allocation of mass to various modes by the settling column also parallels bottom sediment distributions, with roughly half of the mass assigned to the coarse mode and half assigned to finer material (Tables 1-3). The settling velocity distribution of sediment in suspension during "fair weather" on the continental shelf resembles that of the local bottom sediment. This suggests that aggregation is not acting fast enough to substantially alter the size distribution of sediment in suspension during these times.

During low-energy sediment transport on the shelf the grain-size distribution of the bottom sediment is near that of the suspended load. Use of the bottom-sediment size


distribution would not produce substantial error in calculations of sediment transport. This may not be the case during higher energy periods. Aggregation rate varies with the square of the concentration of suspended sediment (eg. MCCAVE, 1984), SO aggregation can be expected to be a more active modifier of the suspended-sediment size distribution during stormy periods.

4. CONCLUSIONS

A remote optical settling box deployed on the northern continental shelf of California revealed a bimodal distribution of settling velocity. A coarse mode sinks at 0.026 cm s- (22 m day - t ) and a fine mode settles at 0.0025 cm s -1 (2 m day- l ) . Between one-quarter and two-thirds of the total mass resides in the coarse mode. Roughly one-quarter is in the fine mode. The remainder sinks too slowly to be resolved during the 18-h runs.

Uncertainty regarding the partitioning of mass among modes arises primarily from incertitude surrounding particle geometry. If particles are treated as uniform-density quartz spheres with constant scattering efficiency the conversion from attenuation to mass scales linearly with particle diameter. If particle bulk density is a decreasing function of particle size, the conversion scales as diameter to a power less than unity. The former assumption weights the coarse mode more than the latter. The sensitivity of the results to assumed geometry makes it imperative to better constrain the fractal geometry of marine aggregates.

The bimodal distribution in suspended sediment reflects the bimodality of bottom sediments at the STRESS site. The preservation of the distribution while in suspension suggests that aggregation is not acting fast enough to alter the size distribution of suspended sediment during fair weather.

Remote optical settling boxes should continue to be a useful tool for measuring in situ

settling velocities of fine sediments. Their simplicity and lack of disturbance to fragile aggregates make them attractive. Two problems must be overcome, however, to realize the full potential of such devices. First, an inviolable seal on the box is crucial for minimizing stirring within the box that produces spurious results. Second, uncertainty regarding the relationship of particle mass to particle diameter currently muddies the interpretation of results and must be overcome if data are to be better constrained.

Acknowledgements--This work is truly a group effort. Rex Johnson redesigned the settling box. Randy Fabbro built the box and associated electronics. Chris Sherwood programmed the logger/controller and supervised deployment, recovery and preliminary data reduction. Paul Hill performed the data analysis and writeup. Dick Sternberg and Arthur Nowell provided support in all aspects of the project. We thank Sandy Williams, John Bouthilette and Nick Lesnikowski, and the crew of the R.V. Wecoma for expert assistance in deployment and recovery. C. R. Sherwood acknowledges the support of Battelle Marine Sciences Laboratory, Sequim, WA, which is part of Pacific Northwest Laboratories operated by Battelle Memorial Institute for U.S. Dept. of Energy under contract DE-AC06-76RL0-1830. This work supported by ONR contracts N00014-91-j-1183, N00014-90-j- 1926, and N00014-89-j-1035. Contribution number 1992 from the School of Oceanography, University of Washington.

REFERENCES

ALLDREDGE A. L. and C. C. GOTSCHALK (1988) In situ settling behavior of marine snow. Limnology and Oceanography, 33,399-351.

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in situ measurements of particle settling velocity on the northern california continental shelf

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