• TABLE OF CONTENTS
HIDE
 Report documentation page
 Title Page
 Errata
 Table of Contents
 List of Figures
 List of Tables
 Abstract
 Part I. Kaolinite and Cedar Key...
 Part II. San Francisco Bay mud
 Appendix: Velocoity and shear stress...
 Literature cited






Group Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 86/007
Title: Experiments on the erosion of deposited and placed cohesive sediments in an annular flume and a rocking flume
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 Material Information
Title: Experiments on the erosion of deposited and placed cohesive sediments in an annular flume and a rocking flume
Series Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 86/007
Physical Description: Book
Creator: Villaret, Catherine
Paulic, Mary
Affiliation: Coastal and Oceanographic Program -- Department of Civil and Coastal Engineering
Publisher: Coastal and Oceanographic Engineering Department
Publication Date: 1986
 Subjects
Subject: Mud
Sediments   ( lcsh )
Shear flow
 Notes
Funding: This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.
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Bibliographic ID: UF00076169
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida

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Table of Contents
    Report documentation page
        Unnumbered ( 1 )
        Unnumbered ( 2 )
    Title Page
        Page 1
    Errata
        Page 2
    Table of Contents
        Page 3
        Page 4
    List of Figures
        Page 5
        Page 6
        Page 7
    List of Tables
        Page 8
    Abstract
        Page 9
        Page 10
    Part I. Kaolinite and Cedar Key mud
        Introduction
            Page 11
            Page 12
            Page 13
        Part II. Methods and materials
            Page 14
            Page 13
            Page 15
            Page 16
            Page 17
        Part III. Results
            Page 18
            Page 19
            Page 20
            Page 17
            Page 21
            Page 22
        Part IV. Concluding remarks
            Page 23
            Page 24
            Page 22
    Part II. San Francisco Bay mud
        Introduction
        Page 25
        Sediment and fluid properties
            Page 25
            Page 26
        Test results
            Page 27
            Page 28
            Page 29
            Page 30
        Concluding remarks
            Page 31
            Page 30
    Appendix: Velocoity and shear stress calculations
        Page 32
        Page 33
        Page 34
        Page 35
    Literature cited
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
Full Text



REPORT DOCUMENTATION PAGE
I. Report No. 2. 3. Recipient's Accession No.


4. Title and Subtitle 5. Report Date
Experiments on the Erosion of Deposited and Placed June, 1986
Cohesive Sediments in an Annular Flume and a 6.
Rocking Flume
7. Author(s) 8. Performing Organization Report No.
Catherine Villaret UFL/COEL-86/007
Mary Paulic
9. Performing Organization Name and Address 10. Project/Task/Work Unit No.
Coastal and Oceanographic Engineering Department
University of Florida 11. Contract or Grant No.
336 Well Hall DACW39-84-C-0013
Gainesville, FL 32611 13. Type of Report
12. Sponsoring Organization Name and Address
U.S. Army Engineer Waterways Experiment Station
P.O. Box 631
Vicksburg, MS 39180
14.
15. Supplementary Notes



16. Abstract
The effects of bed structure and flow regime on the erosional behavior of fine,
cohesive sediments were investigated in a series of laboratory experiments. Two types
of beds, placed and deposited, were used in both a rotating annular flume under a
constant shear stress, Tb, and in a rocking flume under an oscillatory shear stress.
The deposited bed represents the top sediment layers of an estuarine bed which is
frequently resuspended by the action of currents and waves. In the flumes they were
formed by allowing a dilute suspension of sediment to settle out of the water column
and consolidate into a bed. The placed bed represents those layers of the estuarine
bed which are not regularly perturbed; thus they have had time to consolidate. They
were prepared as a dense slurry and then placed into the apparatus. A commercial
kaolinite, and estuarine sediments collected from a tidal mud flat in Cedar Key,
Florida and from San Francisco Bay, were used to prepare the beds.

Kaolinite and Cedar Key Mud: Comparative analyses of the results from both types of
beds yielded distinct concentration-time profiles and different relationships for the
rate of erosion as a function of excess shear stress, Tb-Ts, above the bed shear
strength, Ts. The deposited beds yielded a concentration-time profile representing a
succession of steady states. The placed beds yielded a linear profile. The
difference in profiles is best explained by corresponding differences in the vertical
distribution of bed shear strength.
Continued -

17. Originator's Key Words 18. Availability Statement
Annular flume Erosion rate
Cedar Key mud Kaolinite
Cohesive sediment Rocking flume
Critical shear stress San Francisco Bay mud

19. U. S. Security Classif. of the Report 20. U. S. Security Classif. of This Page 21. No. of Pages 22. Price
Unclassified Unclassified 61









The relationship between the mass rate of erosion, e, and excess shear stress was
found to be approximated by log(E/Ef) = a[Tb-Ts(z)]1/2 for the deposited bed,
and e=M(Tb-Ts)/Ts for the placed bed, where z is depth below the bed surface. The
coefficients ef (floc erosion rate), a and M were, in general, not the same for the
two flumes; they were higher under oscillatory current in the rocking flume than under
uni-directional current in the annular flume. Sediment type, bed structure and
current regime are important factors in determining the erosional behavior of a
cohesive sediment.

San Francisco Bay Mud: An erosion rate expression was found relating the rate of
erosion to the bulk density of the bed and the current speed. Bed density was found
to be a strong influential parameter. Soft beds (1.2 g/cm3 density) generally showed
a significantly higher rate of erosion than dense beds (~ 1.6 g/cm3) at the same
current speed.










UFL/COEL-86/007


EXPERIMENTS ON THE EROSION OF DEPOSITED AND PLACED

COHESIVE SEDIMENTS IN AN ANNULAR FLUME

AND A ROCKING FLUME






by





Catherine Villaret

Mary Paulic








Coastal and Oceanographic Engineering Department

University of Florida

Gainesville, FL 32611


June, 1986










FOREWORD


The methodology for the reported erosion studies is based on laboratory
experimental procedures developed previously at the University of Florida.
For details, which have been omitted here, the reader should refer to Parchure
and Mehta (1985), who used the annular flume with flow deposited beds.
Results using placed beds in the annular flume are being reported here for the
first time. Likewise, the rocking flume was used for the first time in this
study. This flume was designed under a previous project supported by the
Florida Sea Grant College, NOAA, through Grant IR-84-25. Drs. C. Montague and
A. J. Mehta were the principal investigators. The present study was supported
by the U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS,
Contract No. DACW39-84-C-0013. Dr. A. J. Mehta was the principal
investigator. Support through both agencies is sincerely acknowledged.










TABLE OF CONTENTS


Page
FOREWORD ............ ......................... ............................ 2
LIST OF FIGURES...... .................................... .............. 5

LIST OF TABLES........................................................... 8
ABSTRACT............................. .................................... 9

Kaolinite and Cedar Key Mud............. ............................ 9
San Francisco Bay Mud ............................................... 9
PART 1. KAOLINITE AND CEDAR KEY MUD.................. ................... 11

I. INTRODUCTION ............................................... ..... 11
Bed Structure...................... ......... ............... 12

Concentration-Time Profiles.................................. 12
Erosion Rate Expressions................................... 13
II. METHODS AND MATERIALS............................................ 13

Apparatus.. .. .............. ............. ..... ...... ...... ........ 13
Annular Flume.............................................. 13

Rocking Flume............................................. 14

Flume Calibration.................... ................... 15

Bed Preparation................................................. 15
Placed Bed................................................ 15

Deposited Bed.............................................. 15
Test Procedure.. ................................................ 16
Annular Flume ........................ .. ....... ............. 16

Rocking Flume................................. ............. 16

Materials ......... ...... ......... ..... ....... .................. 16
Estuarine Sediment ........ ......... .......... ............ ... 16
Kaolinite........... ... ... ........ .................. ...... 17
Fluid................................ ...................... 17
Density Measurement.. .................. ...................... .. 17
III. RESULTS ....... ........ ................................. ...... .. 17

Density Measurement.. ............................................ 17
Deposited Bed....................................... ......... 17
Placed Bed.. ................ ............................. .. 17
Concentration-Time Profiles.................................................. 18
Deposited Bed............................................... 18










TABLE OF CONTENTS
(Continued)


Page
Placed Bed................................... ........... 18
Bed Shear Strength.............................................. 19
Deposited Bed.............................................. 19
Placed Bed.................................................. 19
Relationship of Shear Strength to Depth.......................... 20

Deposited Bed.............................................. 20
Placed Bed........................ ....... ............ .... 20
Erosion Rate................................................... 21

Deposited Bed..................................... ...... 21
Placed Bed................................................. 21
IV. CONCLUDING REMARKS .............................................. 22
PART 2. SAN FRANCISCO BAY MUD............................................ 25
I. INTRODUCTION ................................... ... ....... ...... 25
II. SEDIMENT AND FLUID PROPERTIES................................... 25
III. TEST RESULTS.................... ............................. 27
IV. CONCLUDING REMARKS.............................................. 30
APPENDIX VELOCITY AND SHEAR STRESS CALCULATIONS........................ 32
LITERATURE CITED........................o ............................... 36










LIST OF FIGURES


Figure Page


1. Variation of Bed Shear Strength with Depth for a Deposited Bed,
Type I Profile (after Parchure, 1984).............................. 37
2. Variation of Bed Shear Strength with Depth for a Placed Bed,
Type II Profile (after Parchure, 1984)............................. 37
3a. Concentration-Time Profile for a Deposited Bed (Type I) (after
Parchure, 1984).................................................... 37
3b. Concentration-Time Profile for a Placed Bed (Type II) (after
Parchure, 1984) .................. ... .............. ........ .......... 37
4. Schematic View of Annular Flume (after Mehta, 1973)................ 38
5a. Plan View of Rocking Flume........................................ 39
5b. Elevation View of Rocking Flume................................... 39
6a. Side View of Rocking Flume........................................ 40
6b. Top Constriction for Rocking Flume................................. 40
6c. Top Constriction Placed in the Rocking Flume....................... 40
6d. Removable Sheet Metal Bed for Rocking Flume..................... 40
7. Experimental Test Procedure (Shear Stress Variation)............... 41
8. Apparatus for Measurement of Density as a Function of Depth
(after Parchure, 1984) ............................ .................. 41
9. Density (Dry) Profile as a Function of Depth for Deposited
Kaolinite...... ................................................. 42
10. Density (Dry) Profile as a Function of Depth for Deposited Cedar
Key Mud..................... ................................... 42
11. Density (Dry) Profile as a Function of Depth for Placed
Kaolinite**....... ...................... ........... ................ 43
12. Density (Dry) Profile as a Function of Depth for Placed
Cedar Key Mud ........................ *........................ 43
13. Suspended Sediment Concentration versus Time for Deposited
Kaolinite, Annular Flume........................................... 44
14. Suspended Sediment Concentration versus Time, Deposited Kaolinite,
Rocking Flume..................................................... 44
15. Suspended Sediment Concentration versus Time for Deposited Cedar
Key Mud, Annular Flume.......................... .. ............... 45










LIST OF FIGURES
(Continued)


Figure Page
16. Suspended Sediment Concentration versus Time, Deposited Cedar Key
Mud, Rocking Flume .............................................. 45
17. Suspended Sediment Concentration versus Time, Placed Kaolinite,
Annular Flume............................... ..................... 46
18. Suspended Sediment Concentration versus Time, Placed Kaolinite,
Rocking Flume ....................... ............................ 46
19. Suspended Sediment Concentration versus Time, Deposited Cedar
Key Mud, Annular Flume............................................. 47
20. Suspended Sediment Concentration versus Time, Placed Cedar Key Mud,
Rocking Flume ............................. ............. .... ..... 47
21. Mass Per Unit Surface Area versus Shear Stress, Deposited
Kaolinite, Both Flumes ............................................. 47
22. Mass Per Unit Surface Area versus Shear Stress, Deposited Cedar
Key Mud, Both Flumes .............................................. 48
23. Erosion Rate versus Shear Stress, Placed Kaolinite, Both Flumes.... 48
24. Erosion Rate versus Shear Stress, Placed Cedar Key Mud, Both
Flumesa .................................... .......... ........ ..... 49
25. Variation of Bed Shear Strength with Depth for Deposited Kaolinite,
Both Flumes........ ..-*.**.* ................... .. 49
26. Variation of Bed Shear Strength with Depth for Deposited Cedar
Key Mud, Both Flumes............................................... 50
27. Variation of Bed Shear Strength with Depth for Placed Cedar Key
Mud, Annular Flume..... ....... ....... .... ................ ......... 50
28. Log e versus (Tb Ts)0*5 for Deposited Kaolinite, Both Flumes..... 51
29. Log e versus (Tb Ts)0*5 for Deposited Cedar Key Mud, Both
Flumes............................................................. 51
30. Bed Dry Density Profiles, Bay Mud.................................. 52
31. Time-Concentration Relationship, Bay Mud, Dense Bed, Annular
Flume (Test 1).................................. ................. 52
32. Time-Concentration Relationship, Bay Mud, Deposited Bed, 0.5 Day
Consolidation, Annular Flume (Test 2).............................. 53










LIST OF FIGURES
(Continued)


Figure Page
33. Time-Concentration Relationship, Bay Mud, Deposited Bed, 0.5 Day
Consolidation, Rocking Flume (Test 3).............................. 53
34. Time-Concentration Relationship, Bay Mud, Deposited Bed, 3.8 Day
Consolidation, Annular Flume (Test 4).............................. 54
35. Time-Concentration Relationship, Bay Mud, Deposited Bed, 3.8 Day
Consolidation, Rocking Flume (Test 5).............................. 54
36. Rate of Erosion versus Bed Shear Stress, Bay Mud, Dense Bed and
Results of Partheniades.............. .............................. 55
37. Rate of Erosion versus Bed Shear Stress, Bay Mud, Dense Bed and
Soft Beds.. ................ ...................................... 55
38. Rate of Erosion versus Bed Shear Stress, Bay Mud, Soft Beds, 0.5
Day Consolidation.................................................. 56
39. Rate of Erosion versus Bed Shear Stress, Bay Mud, Soft Beds, 3.8
Day Consolidation......................................... ......... 56
40. Final Concentration, C90, versus Bed Shear Stress, Tb, Bay Mud,
Soft Beds, Annular Flume .......................................... 57
41. Final Concentration, C90, versus Bed Shear Stress, Tb, Bay Mud,
Soft Beds, Rocking Flume......................................... 57
42. Critical Shear Stress, Tc, as a Function of Bed Bulk Density, pB'
Bay Mud............................................................ 58
43. Erosion Rate Constant, M, as a Function of Critical Shear Stress,
Tc, Bay Mud.. ..................................................... 58
44. Erosion Rate Dependence on Bed Bulk Density and Current Speed, Bay
Mud................................................................ 59
A.1. Oscillatory Motion of the Rocking Flume.......................... 60
A.2. Calibration Curve between Maximum Velocity, um, and Wave Period,
T, in the Rocking Flume........................................... 60
A.3. Calibration Curve between Bed Shear Stress, Tb, and Wave Period,
T, in the Rocking Flume............................................ 61










LIST OF TABLES


Table Page


1. Experimental Design................................................ 12
2. Bed Surface Shear Strength, Tso, and Characteristic Shear
Strength, Tsc, of Deposited Beds in the Annular Flume
and the Rocking Flume............................................. 19
3. Shear Strength of Placed Beds in the Annular Flume and the
Rocking Flume ................................................... 20
4. Values of a and ef for Deposited Beds.............................. 21
5. Values of M and Ts for Placed Beds................................. 21
6. Bay Mud Sample Locations.......................................... 26
7. Bay Mud Sample Properties......................................... 26
8. Bay Mud Test Conditions.............................. ........ 27
9. Bay Mud Erosion Rate Constants.......................... .... .. 30
A.1. Maximum Velocities Obtained by Measurement of Horizontal
Displacement, without and with Top Constriction.................... 33
A.2. Periods of Oscillation and Velocities Obtained from Current
Meter in the Rocking Flume.............................. ....... 34
A.3. Maximum Velocities Obtained by Considering Flow Continuity......... 34










ABSTRACT


The effects of bed structure and flow regime on the erosional behavior of
fine, cohesive sediments were investigated in a series of laboratory
experiments. Two types of beds, placed and deposited, were used in both a
rotating annular flume under a constant shear stress, Tb, and in a rocking
flume under an oscillatory shear stress. The deposited bed represents the top
sediment layers of an estuarine bed which is frequently resuspended by the
action of currents and waves. In the flumes they were formed by allowing a
dilute suspension of sediment to settle out of the water column and
consolidate into a bed. The placed bed represents those layers of the
estuarine bed which are not regularly perturbed; thus they have had time to
consolidate. They were prepared as a dense slurry and then placed into the
apparatus. A commercial kaolinite, and estuarine sediments collected from a
tidal mud flat in Cedar Key, Florida and from San Francisco Bay, were used to
prepare the beds.


Kaolinite and Cedar Key Mud: Comparative analyses of the results from both
types of beds yielded distinct concentration-time profiles and different
relationships for the rate of erosion as a function of excess shear stress,

Tb_ s, above the bed shear strength, Ts. The deposited beds yielded a
concentration-time profile representing a succession of steady states. The
placed beds yielded a linear profile. The difference in profiles is best
explained by corresponding differences in the vertical distribution of bed
shear strength.

The relationship between the mass rate of erosion, c, and excess shear
stress was found to be approximated by log(e/ef) = a[Tb-T (z)] 2 for the
deposited bed, and e=M(Tb-Ts)/s for the placed bed, where z is depth below
the bed surface. The coefficients ef (floc erosion rate), a and M were, in
general, not the same for the two flumes; they were higher under oscillatory
current in the rocking flume than under uni-directional current in the annular
flume. Sediment type, bed structure and current regime are important factors
in determining the erosional behavior of a cohesive sediment.


San Francisco Bay Mud: An erosion rate expression was found relating the rate
of erosion to the bulk density of the bed and the current speed. Bed density









was found to be a strong influential parameter. Soft beds (1.2 g/cm3 density)
generally showed a significantly higher rate of erosion than dense beds (~ 1.6
g/cm3) at the same current speed.










PART 1. KAOLINITE AND CEDAR KEY MUD


I. INTRODUCTION


The erosion of fine, cohesive sediments in estuaries is important to both
the engineer and the scientist. The resuspension and transport of fine
sediments can cause shoaling in ship channels resulting in increased time and
cost of dredging. From an environmental perspective, resuspension of sediment
increases turbidity, thus degrading water quality and possibly harming aquatic
organisms.

Under mild to moderate flow conditions in the estuary, erosion of the mud
surface typically occurs by the entrainment of aggregates rather than by mass
erosion. The erosional behavior of a mud bed depends on four principal
factors; physico-chemical properties of the mud, chemical properties of the
eroding fluid, flow characteristics, and bed structure (Parchure and Mehta,
1985). Bed structure can be classified as either placed or deposited, in
relation to the procedure for bed preparation. For the purposes of this
report, a placed bed is defined as one in which the bed has been prepared by
placing a thick slurry of mud into the laboratory apparatus. A deposited bed
is produced by allowing a dilute mud suspension to settle from the water
column and consolidate. The deposited bed represents the top sediment layers
of an estuarine sediment which are frequently resuspended by the action of
waves and currents. A placed bed is more representative of the lower sediment
layers which do not regularly receive perturbations from waves and currents.

The influence of the first three parameters on the erosion rate of
cohesive sediments has been extensively studied (Parchure and Mehta, 1985).
The majority of laboratory experiments performed have used only one bed
structure and flow regime without comparative studies of different bed
structures and flow regimes. The main purpose of this study was to show the
effect of bed structure on the rate of surface erosion under both steady and
oscillatory currents. Two different apparatuses, a rotating annular flume and
a rocking flume, were used to generate a steady current and an oscillatory
current, respectively. Both bed types, using both kaolinite and estuarine
mud, were tested in each apparatus. Table 1 is a list of the experiments
performed.










Table 1. Experimental Design


Sediment/Bed

Apparatus Kaolinite Estuarine Mud

Annular Flume Deposited bed Deposited bed
Placed bed Placed bed


Rocking Flume Deposited bed Deposited bed
Placed bed Placed bed




Bed Structure

The primary difference between placed and deposited beds is the
distribution of bed shear strength (and density) with depth. A deposited bed
shows an increase in shear strength with increasing depth into the bed (Figure
1). This is a Type I profile. Placed beds have a nearly constant shear
strength from top to bottom (Figure 2). Such a profile is referred to as Type
II (Parchure, 1984; Hunt and Mehta, 1985).

A profile of density with depth is critical to determining erosion
rates. Bed density increases in a deposited bed from top to bottom. On the
other hand, a placed bed has nearly uniform density from top to bottom.
Deposited beds undergo both primary and secondary consolidation as compared to
mainly secondary consolidation for placed beds (Parchure, 1984). Due to their
mode of preparation, deposited beds are generally weaker (lower density and
shear strength) than placed beds for a comparable period of consolidation.


Concentration-Time Profiles

For a deposited bed the rate of erosion, c (the time-rate of change of
suspended sediment mass per unit bed surface area), which is proportional to
the time-rate of change of suspension concentration, decreases as erosion
proceeds and eventually stops. Once this steady state condition has been
reached, the concentration of suspended mass remains constant, as in Figure
3a. Erosion is no longer occurring. At this stage, the bed shear strength at
the mud-fluid interface is equal to the applied shear stress, Tb.










Placed beds behave differently. The suspended sediment concentration
increases linearly with time for a given shear stress in excess of the shear
strength, as in Figure 3b. Thus, the rate of erosion of these beds is
constant for a given shear stress.


Erosion Rate Expressions

Erosion of a deposited bed can be empirically modeled as a logarithmic
relationship correlating the erosion rate to the excess shear stress above the
bed shear strength. This relationship is:


log -= a[Tb (z)]2 (1)


where E is the erosion rate, Tb is the time-mean bed shear stress, Ts(z) is
the bed shear strength as a function of depth, z, below the bed surface, a is
an empirical rate constant and ef is defined as the floc erosion rate
(Parchure, 1984; Parchure and Mehta, 1985).

The erosion rate of a placed bed can be related to the bed shear stress
by:

(b s)
M = M T (2)
s

where Ts is the constant (critical) bed shear strength and M is an empirical
coefficient (Parchure and Mehta, 1985).


II. METHODS AND MATERIALS


Apparatus

Two different flumes were used for these experiments; a rotating annular
flume and a rocking flume.

Annular Flume. The annular flume had a channel width of 20 cm, depth of
46 cm, and a mean radius of 76 cm. Inside the channel a 20 cm plexiglass
annular ring was suspended by means of four vertical supports attached by
horizontal supports to the central vertical shaft (Figure 4). The equipment
was calibrated to produce a bed shear stress up to 0.9 N/m2. Complete details
of flume calibration are contained in Mehta (1973). The total depth of










sediment and water in the flume could be up to 33 cm. For the described
experiments a bed of 7 cm depth and water column height of 23 cm were used.

When the ring was rotated, a shear stress was transmitted to the sediment
bed through the water column. To operate properly the ring was required to be
in complete surface contact with the water column. During operation the ring
and channel were rotated in opposite directions to minimize the effects of
secondary currents and to maintain a uniform flow in the channel.

Taps were located on the outside wall of the channel to allow sampling
from the water column. Samples were collected over a variable time regime.
Total suspended sediment was determined by filtering water samples with a 0.45
micron Millipore filter and filtering apparatus. Samples were then dried at
50C for at least two hours and then weighed on a Mettler balance (model H80)
with an accuracy of 0.1 mg.

Rocking Flume. The rocking flume was constructed of 1.25 cm thick
plexiglass. It was 2.4 meters in length and 36 cm high with an inner width of
15 cm. A false bottom was built into the flume at a height of 7 cm. The
actual depth of the flume channel was therefore 29 cm. Figures 5a, 5b and 6a
illustrate plan, elevation and side views of the flume. The entire assembly
was mounted on a table with dimensions of 2.75 meters in length, 91 cm in
width, and 91 cm in height. The flume was mounted on a pivot 16 cm above the
table allowing it freedom of rocking motion. Directly above the pivot the
channel had been deepened an additional 5 cm for a length of 54 cm to allow
for the placement of a sediment bed. The flume was operated by a hydraulic
transmisison attached to a 3/4 hp motor. A metal shaft (rocking arm) at one
end of the flume was attached by a circular hub to the flume and to the
hydraulic transmission by a hub attached to a rotating plate (Figures 5a,b and
6a). When the flume was in operation, the transmission turned a shaft which
turned the rotating plate. This caused the shaft to move up and down
resulting in the flume rocking back and forth. Different periods of rocking
could be obtained by increasing the speed of the motor and the attached
shaft. Amplitude of rocking motion could be varied by changing the
eccentricity of the rocking arm/rotating plate connection.

When the flume was operated a standing wave was produced which had its
node at the center of the flume, in the middle of the sediment bed. The waves
produced were of shallow water type so that the oscillatory velocities were










Placed beds behave differently. The suspended sediment concentration
increases linearly with time for a given shear stress in excess of the shear
strength, as in Figure 3b. Thus, the rate of erosion of these beds is
constant for a given shear stress.


Erosion Rate Expressions

Erosion of a deposited bed can be empirically modeled as a logarithmic
relationship correlating the erosion rate to the excess shear stress above the
bed shear strength. This relationship is:


log -= a[Tb (z)]2 (1)


where E is the erosion rate, Tb is the time-mean bed shear stress, Ts(z) is
the bed shear strength as a function of depth, z, below the bed surface, a is
an empirical rate constant and ef is defined as the floc erosion rate
(Parchure, 1984; Parchure and Mehta, 1985).

The erosion rate of a placed bed can be related to the bed shear stress
by:

(b s)
M = M T (2)
s

where Ts is the constant (critical) bed shear strength and M is an empirical
coefficient (Parchure and Mehta, 1985).


II. METHODS AND MATERIALS


Apparatus

Two different flumes were used for these experiments; a rotating annular
flume and a rocking flume.

Annular Flume. The annular flume had a channel width of 20 cm, depth of
46 cm, and a mean radius of 76 cm. Inside the channel a 20 cm plexiglass
annular ring was suspended by means of four vertical supports attached by
horizontal supports to the central vertical shaft (Figure 4). The equipment
was calibrated to produce a bed shear stress up to 0.9 N/m2. Complete details
of flume calibration are contained in Mehta (1973). The total depth of










nearly uniform over depth. Maximum horizontal displacement occurred at the
node where the velocity was predominantly in the horizontal direction, along
the bed surface. Wave period could be determined by timing the rotation of
the plate. Wave amplitude could be determined by measuring the vertical
displacement of water from still water level at the end of the flume.

A modification was made to the flume to increase the flow velocity at the
bed surface. A plexiglass top constriction of height 19 cm and 54 cm length
was placed in the water column above the sediment bed (Figure 6b,c). The ends
of it were sloped to reduce turbulence at the entrance to the bed. Its height
above the bed could be varied. With the top constriction in place, free
surface flow in the flume was thus replaced by flow in a "tunnel" in the
central portion of the flume. Over time the current generated at the sediment
surface had a sinusoidal velocity variation.

Flume Calibration. The flume was calibrated to produce a maximum shear
stress up to 0.8 N/m2. Maximum shear stress was calculated as 0.5 pfwum2,
where p is water density, fw is the coefficient of friction, and um is the
maximum horizontal water velocity. A number of different techniques were used
to determine velocity. These included direct measurement of the displacement
of the water level relative to the mean, mean surface particle displacement at
the node, and velocity of the water above the bed. For these experiments a
water depth of 10 cm was maintained above the bed and 17.5 cm at the ends.
Complete details, calculations, and calibration curves are contained in the
Appendix.


Bed Preparation

Placed Bed. A thick slurry of sediment and salt water (salinity 10 ppt)
was mixed for one hour in a mixer and then placed into the flume to uniform
depth. Water was then carefully added to the flume to the appropriate
depth. A separate bed was placed in a bucket for determination of bed
density.

Deposited Bed. An appropriate volume of sediment was added to the
annular flume and water added to a depth of 30 cm. The flume was then rotated
to generate a bed shear stress of 0.9 N/m2, in order to assure complete
mixing. After 24 hours, the flume was stopped and the sediment allowed to
settle under quiescent conditions. After mixing, but before significant










settling of the sediment, water containing suspended sediment was withdrawn
from the channel and deposited into removable beds (Figure 6d) that could be
placed directly into the rocking flume. The ends of these beds were
temporarily sealed with plexiglass to allow a water column to be poured over
the bed. A second sample was withdrawn from the annular flume and allowed to
deposit in a bucket. This was later used for bed density measurement.


Test Procedure

Annular Flume. For each experiment six different shear stresses were
selected. They were applied in a step-wise fashion starting at 0.1 N/m2 and
continuing until 0.6 N/m2 in increments (90 min duration) of 0.1 N/m2
Suspension samples were removed, in approximately 50 ml aliquots, at 2,5,10,
15,20,25,30,40,50,60,75, and 90 minutes with an initial sample taken at the
start of the test. Samples were taken from taps at the top and bottom of the
water column to give an average suspension concentration for the entire water
column. Salt water was periodically added to the flume to maintain a 23 cm
water depth.

Rocking Flume. Shear stresses selected in this flume were 0.1, 0.2, 0.3,
and, in some cases, 0.4 N/m2. Note that these are wave-averaged rather than
maximum values. Samples were collected over the same time regime as for the
annular flume, excluding the 2 minute sample. Samples were taken from the
center of the flume, at one-quarter reach and at one end, including the top
and bottom at each location. Salt water was added periodically to the flume to
replace the volume of water lost to samples.

The test procedure with regard to the applied shear stress is summarized
in Figure 7 for both flumes. Note that with respect to deposition and
consolidation, the duration of deposition was typically quite small compared
with that of consolidation. In what follows, the combined duration is
referred to as consolidation period.


Materials

Estuarine Sediment. The mud was collected from a tidal flat in Cedar
Key, Florida. Mineralogically it was composed of 73% montmorillonite, 21%
kaolinite and 6% quartz. Prior to being used, the mud was sieved through a 1
mm screen to remove shells and plant materials. The median (dispersed)

16










particle size was ~ 2 microns, as obtained by hydrometer (ASTM, 1981). The
cation exchange capacity was = 100 millequivalents per hundred grams. Total
organic matter corresponded to 11% loss on ignition, as obtained by standard
procedure (American Public Health Association, 1976).

Kaolinite. The kaolinite was obtained from a commercial source. It was
prepared by soaking 90 kg dry kaolinite in thirty gallons of salt water
(salinity 10 ppt) for one month. The kaolinite-water mixture was stirred
every few days to ensure equilibration of the sediment with the fluid. The
median (dispersed) size was ~ 1 pm. The cation exchange capacity was ~ 6
milliequivalents per hundred grams and loss on ignition was 12%.

Fluid. All experiments were performed with salt water at a salinity of
10 ppt. Salt water was prepared by mixing sodium chloride in tap water.
Salinity was checked by a refractometer. Fluid temperature during the tests
was in the range of 240-270C. The pH varied from 8.5 to 9.5.


Density Measurement

The method used for determining bed density followed the procedure of
Parchure (1984). The apparatus used consisted of a 2.0 cm diameter coring
tube and a 15 cm diameter plexiglass cylinder with a 2.5 cm diameter metal
tube in the middle (Figure 8). Cores were taken from the bed and then the
cylinder was placed over the coring tube. The inside of the cylinder was
filled with ethanol and dry ice to snap freeze the cores in situ. Once frozen
the cores were sliced into thin sections between 2 mm and 10 mm, dried at 40C
and weighed.


III. RESULTS


Density Measurement

Deposited Bed. The density of deposited bed typically increases with
depth. Such a trend was observed for both kaolinite and estuarine mud.
Density (dry) profiles are contained in Figure 9 for kaolinite and Figure 10
for mud.

Placed Bed. The density of a placed bed is fairly constant with depth.
Density (dry) profiles are contained in Figure 11 for kaolinite and Figure 12
for mud. The measured values indicate deviations from uniformity with depth.










Concentration-Time Profiles


Deposited Bed. Figures 13 through 16 are plots of suspension
concentration versus time. Where deemed important, comments on the observed
trends have been made within the figures, e.g. Figure 16. Most comments made
here and in subsequent figures are either self-explanatory, or are discussed
in the text. The total (instantaneous) suspension concentration is
represented as a depth-averaged value for each flume. In general, deposited
beds in both flumes exhibited a series of steady states (characterized by
constant final concentrations). Higher suspension concentrations were
obtained with kaolinite than with mud at the same applied shear stress. At
high shear stresses, particularly in the annular flume, plots appear to
indicate a nearly linear increase of concentration with time (Figure 15). In
these cases, either the samples were not collected for a sufficient time
period to reach steady state concentrations, or the bed shear stress had
exceeded the maximum bed shear strength (Parchure, 1984).

Placed Bed. Figures 17 through 20 are concentration-time profiles of
placed beds. Again, the suspension concentration is a depth-averaged
quantity. In general, the profiles are linear. The placed mud bed in the
annular flume, Figure 19, exhibits an initial pattern of steady states at low
shear stresses. This behavior occurred because it was difficult to add water
to the flume initially without disturbing the bed; thus the top sediment
layers behaved like deposited beds. Also observed in this figure is a sudden
drop in the concentration at the beginning of the last three steps. It should
be noted that the concentration plotted here is based on measurements at a
single elevation approximately half way between the suspension surface and the
bed. The concentration drop can be attributed to a change in the vertical
concentration profile, rather than deposition, as a consequence of a change in
the inter-particle collision frequency at the beginning of each step
(Parchure, 1984). In the rocking flume, little erosion of the placed beds
occurred before 0.3 N/m2. In particular, the placed mud bed in the rocking
flume Figure 20, did not start to erode until 0.4 N/m2. Note that erosion
occurred suddenly without any increase in applied shear stress. This type of
behavior may be attributed to a decrease in the bed shear strength (bed
softening) under the oscillatory velocity field in the rocking flume (Maa,
1986).










Bed Shear Strength

Deposited Bed. The final, steady state suspension concentration for each
shear stress was first converted to mass per unit bed area and then plotted
against the applied bed shear stress. Two linear plots of slopes M1 and M2
are obtained (see for example Fig. 21). By extrapolating the M1 line back to
the abscissa the bed surface shear strength Tso, corresponding to initiation
of erosion can be determined (Parchure and Mehta, 1985). Likewise the point
of intersection of lines M1 and M2 gives the characteristic shear strength,
Tsc above which the rate of erosion increases significantly. Bed surface
(z=0) shear strength, Tso, and characteristic shear strength, Tsc, values are
given in Table 2. Figures 21 and 22 are plots of suspended sediment mass per
unit bed surface area versus applied shear stress from which the values given
in Table 2 have been obtained. Both the rocking flume and the annular flume
data are on the same plot. For the kaolinite beds, Figure 21, the same curves
were obtained in both flumes. Values of Tso and Tsc in Table 2 suggest that
the mud generally had a somewhat higher resistance to erosion than kaolinite.


Table 2. Bed Surface Shear Strength, Tso, and Characteristic Shear Strength,
Tsc, of Deposited Beds in the Annular Flume and the Rocking Flume


Kaolinite Mud

Tso Tsc TSO TSC
Apparatus (N/m2) (N/m2) (N/m2) (N/m2)

Annular Flume 0.08 0.25 0.18 0.40
Rocking Flume 0.08 0.25 0.03 0.20



Placed Bed. Table 3 contains values of bed shear strength (uniform over
depth) for placed beds in each apparatus. Figures 23 and 24 are plots of
suspended sediment mass eroded per unit bed surface area per unit time (i.e.
rate of erosion) versus shear stress for placed kaolinite and mud beds,
respectively. These plots were used to obtain values given in Table 3. The
mud bed may be considered to have a somewhat higher shear strength than the
kaolinite bed. However, contrary to the bed softening trend expected in the
rocking flume, the shear strength was higher in this flume than in the annular
flume. A possible explanation is noted later.










Table 3 Shear Strength of Placed Beds in the Annular Flume and the Rocking
Flume


Ts
(N/m2)

Apparatus Kaolinite Mud


Annular Flume 0.25 0.22
Rocking Flume 0.28 0.40


Relationship of Shear Strength to Depth

The density profiles coupled with concentration-time profiles presented
earlier were used to produce profiles of the bed shear strength with depth.
Details of procedure are given by Parchure and Mehta (1985).

Deposited Bed. Figures 25 and 26 are plots of bed shear strength versus
depth. The same density profile for a given sediment was used for both
flumes. The bed shear strength is observed to increase with depth below the
bed surface. For the kaolinite bed, the profiles resulting from the two
flumes are nearly coincident. For the mud bed, the profiles from the two
flumes differ; the shear strengths from the rocking flume are lower. This
difference is believed to be due to bed softening.

At corresponding depths in the bed, the shear strength of the mud is
generally higher than that of kaolinite in the annular flume. In the rocking
flume, shear strengths of kaolinite and mud at corresponding depths are nearly
the same.

Placed Bed. The kaolinite bed yielded a constant depth versus shear
strength profile, with a shear strength of 0.25-0.28 N/m2, see Table 3, with
only a small difference between the values obtained in the two apparatuses.
Figure 27 is a plot of depth versus shear strength for the placed mud bed in
the annular flume. Unlike the kaolinite beds, the profile is not constant,
but has a lower shear strength in the top few millimeters, due to the
deposited bed-like behavior noted previously. The shear strength of the
placed mud bed in the rocking flume was 0.40 N/m2, as estimated from
Figure 24.










particle size was ~ 2 microns, as obtained by hydrometer (ASTM, 1981). The
cation exchange capacity was = 100 millequivalents per hundred grams. Total
organic matter corresponded to 11% loss on ignition, as obtained by standard
procedure (American Public Health Association, 1976).

Kaolinite. The kaolinite was obtained from a commercial source. It was
prepared by soaking 90 kg dry kaolinite in thirty gallons of salt water
(salinity 10 ppt) for one month. The kaolinite-water mixture was stirred
every few days to ensure equilibration of the sediment with the fluid. The
median (dispersed) size was ~ 1 pm. The cation exchange capacity was ~ 6
milliequivalents per hundred grams and loss on ignition was 12%.

Fluid. All experiments were performed with salt water at a salinity of
10 ppt. Salt water was prepared by mixing sodium chloride in tap water.
Salinity was checked by a refractometer. Fluid temperature during the tests
was in the range of 240-270C. The pH varied from 8.5 to 9.5.


Density Measurement

The method used for determining bed density followed the procedure of
Parchure (1984). The apparatus used consisted of a 2.0 cm diameter coring
tube and a 15 cm diameter plexiglass cylinder with a 2.5 cm diameter metal
tube in the middle (Figure 8). Cores were taken from the bed and then the
cylinder was placed over the coring tube. The inside of the cylinder was
filled with ethanol and dry ice to snap freeze the cores in situ. Once frozen
the cores were sliced into thin sections between 2 mm and 10 mm, dried at 40C
and weighed.


III. RESULTS


Density Measurement

Deposited Bed. The density of deposited bed typically increases with
depth. Such a trend was observed for both kaolinite and estuarine mud.
Density (dry) profiles are contained in Figure 9 for kaolinite and Figure 10
for mud.

Placed Bed. The density of a placed bed is fairly constant with depth.
Density (dry) profiles are contained in Figure 11 for kaolinite and Figure 12
for mud. The measured values indicate deviations from uniformity with depth.









Erosion Rate

Deposited Bed. For a deposited bed under a constant shear stress the
rate of erosion decreases with time. The relationship given by Eq. 1
describes the rate of erosion. The calculated rate coefficients a and f are
contained in Table 4 (Parchure and Mehta, 1985). Figures 28 and 29 are plots
of the log of the erosion rate versus the square root of the applied shear
stress minus the bed shear strength, i.e., square root of the excess shear
stress.


Table 4. Values of a and ef for Deposited Beds


Kaolinite Mud

Apparatus a cf a ef
(m/N 2 ) (mg/cm2-hr) (m/N 1/2) (mg/cm2-hr)


Annular Flume 5.1 2.1 x 10-3 7.9 3.2 x 10-3
Rocking Flume 5.1 2.1 x 10-3 7.9 2.0 x 10-3



Placed Bed. For a placed bed the rate of erosion is given by Eq. 2. The
values of M and Ts are given in Table 5. Figures 23 and 24 are plots of
erosion rate versus applied shear stress for kaolinite and mud, respectively.
The erosion coefficient, M, was the same in both flumes for the kaolinite beds
until the applied shear stress equalled 0.4 N/m2 at which point the erosion
rate increased rapidly in the rocking flume. However, there were insufficient
data points to evaluate the coefficient M. The same situation occurred with


Table 5. Values of M and Ts for Placed Beds


Kaolinite Mud

Apparatus M Ts M Ts
(mg/cm2-hr) (N/m2) (mg/cm2-hr) (N/m2)

Annular Flume 18.6 0.25 5.8 0.22
Rocking Flume 18.6 0.28 0.40










the mud bed in the rocking flume. It is noteworthy that in the rocking flume,
the erosion rate increased suddenly in both cases kaolinitee and mud) in spite
of the fact that the shear stress was constant at 0.4 N/m2 (see Figs. 18 and
20). It is believed that bed softening under oscillatory current was a
possible cause of this behavior.


IV. CONCLUDING REMARKS


A comparison of results obtained in both the annular flume and the
rocking flume indicates trend similarities as well as quantitative differences
in the erosional behavior of the two cohesive sediments.

Comparisons have been made of concentration-time profiles, shear strength
variation with depth as a function of bed structure, and erosion rate. The
concentration-time profiles for deposited beds were characterized by a series
of steady states in both flumes and for both sediments. At high shear
stresses (equal to or greater than 0.5 N/m2), the concentration typically
continued to increase linearly for the entire sampling period. The
explanation for this behavior is the nature of the vertical distribution of
shear strength. With increasing depth the shear strength increased, but at
smaller rates until it was nearly constant. A one and a half hour sampling
period was apparently insufficient to erode away the material to a depth at
which the applied shear stress equalled the shear strength. Alternatively,
the same type of behavior can be shown to result if the applied bed shear
stress exceeds the maximum bed shear strength (Parchure and Mehta, 1985).

Placed beds exhibited a linear increase in suspension concentration with
time. The initial period of testing may exhibit a pattern more like that of a
deposited bed, as in Figure 19. The reason for this trend is that upon
initial addition of water to the flume some disruption of the surface occurred
even though care was taken in the addition of water. In general, the values
obtained for suspension concentration from the placed beds were lower than for
the deposited beds under the same flow conditions. Placed beds are typically
more dense to begin with and are less erodible than deposited beds.

An important observation to note about placed beds in the annular flume
is that after the applied shear stress was increased, the concentration of
sediment in suspension actually decreased, in some instances. A similar










observation was made by Parchure (1984). There are two possible mechanisms
involved in an interpretation of this phenomenon. The first is simply a delay
in the response of the bed to an increase in the shear force being exerted on
it. Secondly, increasing the rate of turbulent shearing in the water column
increases the number of collisions between particles which enhances the rate
of aggregation. Larger aggregates would be able to deposit, thereby reducing
the suspension concentration.

The deposited mud bed had a lower bed shear strength (with respect to
erosion) when subjected to an oscillatory current (in the rocking flume) as
compared to a steady current (in the annular flume). The difference between
shear strengths obtained with the two types of currents also increases with
depth in the bed (see Figure 26). In general, the bed shear strength was
lower under oscillatory currents than under steady currents. This feature is
probably due to the bed softening under oscillatory currents, implying a
degradation of bed shear strength due to a breakdown of the structure of the
deposited aggregates. The coefficients a and ef of the erosion rate
expression were comparable, however.

Placed beds in the rocking flume showed a sudden increase in the erosion
rate without increasing the applied shear stress. The mud bed began to erode
after about one hour at a shear stress of 0.4 N/m2, while the erosion rate of
kaolinite approximately doubled after about 45 minutes at the same shear
stress (0.4 N/m2). These sudden increases in erosion rate imply that at the
time of occurrence of these changes, the bed shear strength decreases to a
level below the applied shear stress.

The bed shear strength of placed beds was nearly the same for kaolinite
in both flumes, but was higher for mud in the rocking flume than in the
annular flume. This trend is seemingly in contradiction to the bed softening
phenomenon noted. Maa (1986) however noted that under certain conditions
depending upon the initial bed structure and flow conditions, a breakdown of
aggregate structure within the bed is accompanied by an enhanced rate of
consolidation. If the influence of consolidation on bed erodibility exceeds
that due to structural breakdown, the bed would become more erosion resistant
under oscillatory flows in comparison with steady flows.

The coefficient M of the erosion rate expression for kaolinite was the
same under both types of currents. The results for mud could not be compared
because there were insufficient data for mud from the rocking flume.










As noted, differences in the results between the two flumes may be the
result of softening of the bed when subjected to an oscillatory current. The
degree of softening is also dependent on the bed properties. The kaolinite
bed was weaker than the mud bed, partly because it was less cohesive than the
mud which contained montmorillonite as the predominant constituent. In a
sense, kaolinite was already "softer" so it was not as readily affected by
softening as mud.

The type of sediment used for an experiment had measurable influence on
the results obtained. Kaolinite had a narrower distribution of (primary)
particle size making a more homogeneous bed. The mud contained a sand
fraction which does not erode by the same mechanism that fine particles do.
The sand fraction can move as bedload or as suspended load, rather than as
suspended load alone. Also, the mud contained an organic fraction which can
sometimes lead to increased flocculation of particles.

In conclusion, higher bed shear strengths were generally obtained for mud
than kaolinite, making the mud more resistant to erosion than kaolinite.
Likewise, the erosion coefficient M for placed beds was 3 to 4 times larger
for kaolinite as compared to mud. The type of current (steady or oscillatory)
eroding the sediment appears to be an important factor in determining the
erosion rate.










the mud bed in the rocking flume. It is noteworthy that in the rocking flume,
the erosion rate increased suddenly in both cases kaolinitee and mud) in spite
of the fact that the shear stress was constant at 0.4 N/m2 (see Figs. 18 and
20). It is believed that bed softening under oscillatory current was a
possible cause of this behavior.


IV. CONCLUDING REMARKS


A comparison of results obtained in both the annular flume and the
rocking flume indicates trend similarities as well as quantitative differences
in the erosional behavior of the two cohesive sediments.

Comparisons have been made of concentration-time profiles, shear strength
variation with depth as a function of bed structure, and erosion rate. The
concentration-time profiles for deposited beds were characterized by a series
of steady states in both flumes and for both sediments. At high shear
stresses (equal to or greater than 0.5 N/m2), the concentration typically
continued to increase linearly for the entire sampling period. The
explanation for this behavior is the nature of the vertical distribution of
shear strength. With increasing depth the shear strength increased, but at
smaller rates until it was nearly constant. A one and a half hour sampling
period was apparently insufficient to erode away the material to a depth at
which the applied shear stress equalled the shear strength. Alternatively,
the same type of behavior can be shown to result if the applied bed shear
stress exceeds the maximum bed shear strength (Parchure and Mehta, 1985).

Placed beds exhibited a linear increase in suspension concentration with
time. The initial period of testing may exhibit a pattern more like that of a
deposited bed, as in Figure 19. The reason for this trend is that upon
initial addition of water to the flume some disruption of the surface occurred
even though care was taken in the addition of water. In general, the values
obtained for suspension concentration from the placed beds were lower than for
the deposited beds under the same flow conditions. Placed beds are typically
more dense to begin with and are less erodible than deposited beds.

An important observation to note about placed beds in the annular flume
is that after the applied shear stress was increased, the concentration of
sediment in suspension actually decreased, in some instances. A similar










PART 2. SAN FRANCISCO BAY MUD


I. INTRODUCTION


Erosion tests conducted with mud from San Francisco Bay were for the
purpose of evaluating the erosion potential of the mud at various bed
densities. The test methodology, apparatus and procedure were the same as
those of kaolinite and Cedar Key mud. Here therefore emphasis is placed
predominantly on data analysis and interpretation.


II. SEDIMENT AND FLUID PROPERTIES


The predominant clay mineral constituent in the bay mud is
montmorillonite, followed by illite, kaolinite, halloysite and chlorite.
Among the non-clay minerals, quartz is predominant. There is also some iron
(both structural, replacing some of the aluminum in illite, and non-
structural, i.e., independent of the clay mineral) and organic matter. The
cation exchange capacity of the samples used was 61 milliequivalents per
hundred grams.

Suspended or recently deposited bay mud typically has a light brown
color, while sediment from a depth of a few centimeters below the surface has
a color ranging from light grey to black. When a sample of wet dredged
sediment is placed in a glass cylinder and thoroughly stirred in water, a
color change from dark grey to brown takes place. When allowed to stand, the
color slowly changes back to greenish grey, and finally back to dark grey.
These color changes occur due to the following reasons: in the dark grey
sediment iron is present as ferrous sulfide. When stirred, ferrous sulfide is
easily oxidized due to aeration to ferric hydroxide, which imparts a brownish
color to the sediment. If allowed to stand, bacterial reduction first changes
ferric iron to ferrous iron which is greenish, and then finally back to
ferrous sulfide.

Table 6 gives sediment sample numbers and corresponding locations within
the bay. In Table 7, sample properties median size, bulk density, pB'
sediment density, ps, and total organic matter are given. Sample 3A contained
a sizeable fraction of sand; hence its median size (75 um) was in the fine










PART 2. SAN FRANCISCO BAY MUD


I. INTRODUCTION


Erosion tests conducted with mud from San Francisco Bay were for the
purpose of evaluating the erosion potential of the mud at various bed
densities. The test methodology, apparatus and procedure were the same as
those of kaolinite and Cedar Key mud. Here therefore emphasis is placed
predominantly on data analysis and interpretation.


II. SEDIMENT AND FLUID PROPERTIES


The predominant clay mineral constituent in the bay mud is
montmorillonite, followed by illite, kaolinite, halloysite and chlorite.
Among the non-clay minerals, quartz is predominant. There is also some iron
(both structural, replacing some of the aluminum in illite, and non-
structural, i.e., independent of the clay mineral) and organic matter. The
cation exchange capacity of the samples used was 61 milliequivalents per
hundred grams.

Suspended or recently deposited bay mud typically has a light brown
color, while sediment from a depth of a few centimeters below the surface has
a color ranging from light grey to black. When a sample of wet dredged
sediment is placed in a glass cylinder and thoroughly stirred in water, a
color change from dark grey to brown takes place. When allowed to stand, the
color slowly changes back to greenish grey, and finally back to dark grey.
These color changes occur due to the following reasons: in the dark grey
sediment iron is present as ferrous sulfide. When stirred, ferrous sulfide is
easily oxidized due to aeration to ferric hydroxide, which imparts a brownish
color to the sediment. If allowed to stand, bacterial reduction first changes
ferric iron to ferrous iron which is greenish, and then finally back to
ferrous sulfide.

Table 6 gives sediment sample numbers and corresponding locations within
the bay. In Table 7, sample properties median size, bulk density, pB'
sediment density, ps, and total organic matter are given. Sample 3A contained
a sizeable fraction of sand; hence its median size (75 um) was in the fine










sand range. This sample was therefore discarded from further analysis. The
remaining samples were mixed in approximately equal proportions since they all
had similar properties. Thus, erosion tests reported here are for the
composite sample, a mixture of 1, 2A, 2B and 2C.


Table 6. Bay Mud Sample Locations

Sample Location
No.

1 Larkspur Channel
2A Richmond Longwharf Manuevering Area
2B Richmond Longwharf Manuevering Area
2C Richmond Longwharf Manuevering Area
3A Southampton Shoal Channel


Table 7. Bay Mud Sample Properties


Sample Median Bulk Sediment Total
No. size density, PB density, ps organic
(1m) (g/cm3) (g/cm3) (%)

1 3 1.52 2.76 10.0
2A 7 1.56 2.67 7.6
2B 30 1.69 2.76 3.4
2C 12 1.65 2.72 4.7
3A 75 1.90 3.11 2.2



The (eroding) fluid was tap water to which sodium chloride was added to
raise the salinity to 33 ppt. The pH was maintained at ~ 9. The mean fluid
temperature was 240C during the experiments.

In tests with deposited beds, the pore fluid composition may be
considered to have been the same as the eroding fluid composition given
above. In the single test with a placed bed at natural density, the pore
fluid composition was as follows: Na++ 9,700 ppm, Ca++ 940 ppm, Mg++ 1,150
ppm, K 770 ppm, Cl- 16,930 ppm and SO4 2,640 ppm. Solution conductivity was
33 mmhos/cm.









III. TEST RESULTS


Test conditions are summarized in Table 8. Test 1 was with a placed
(dense) bed in the annular flume at the natural bulk density of 1.63 g/cm3
(corresponding to a dry density of 0.96 g/cm3). Tests 2 through 5 were for
deposited (soft) beds with consolidation periods of 0.5 day and 3.8 days.


Table 8. Bay Mud Test Conditions

Test Apparatus Consolidation PD OB
No. (days) (g/cm3) (g/cm3)

1 Annular flume dense bed 0.96 1.63
2 Annular flume 0.5 0.22 1.17
3 Rocking flume 0.5 0.22 1.17
4 Annular flume 3.8 0.40 1.28
5 Rocking flume 3.8 0.40 1.28
PI Straight flumea 40 (placed) 0.61b 1.36
P2 Straight flumea 15 (placed) 0.57b 1.34

aTests of Partheniades (1965).
bSediment density was 2.24 g/cm3


Density profiles for the dense bed (test 1) and soft beds (tests 2,3,4,5)
are given in Fig. 30. These are dry densities, pD (not to be confused with
sediment density, ps). The dense bed density did not vary with depth. For
the soft beds, pD and pB values given in Table 8 are representative depth-mean
values corresponding to the top bed layers which eroded during the tests.
Thus they are not averages over the entire mud bed thickness shown in Fig. 30.

Tests PI and P2 corresponding to series I and II of Partheniades (1965)
were conducted on remolded, placed beds. Since Partheniades also used
sediment from the San Francisco Bay which is spatially well mixed (Krone,
1978), results from these tests are included in the subsequent analysis.

Time-concentration data for tests 1 through 5 are given in Figs. 31
through 35. Data from P1 and P2 appear elsewhere (Partheniades, 1962).

The erosion rate, e, against bed shear stress, Tb, relationship from
test 1 (annular flume) is compared with P1 (series I) and P2 (series II) in










Fig. 36. The annular flume data agree with series I up to Tb = 0.8 N/m2
Disparities for larger Tb are attributed to likely corresponding differences
in the bed structure due to differences in the method of bed preparation,
i.e., the manner in which the beds were remolded and placed. In series II,
iron oxide from rust in the return pipe of the flume used by Partheniades
enhanced bed resistance to erosion due to cementing of aggregates.
Characteristically however, incipient erosion is observed to have begun at the
same Tb = Tco ~ 0.1 N/m2, in all three cases.

In Fig. 37, erosion rate, e, is plotted against Tb for tests 1 through 5,
i.e. for dense as well as soft beds, for the mere purpose of demonstrating
similarities and differences. For the dense bed, time-concentration profiles
(Fig. 31) were characteristically linear, hence e was constant for a given

Tb. On the other hand, time-concentration response of the soft beds (Figs.
32, 33, 34, 35) was a series of steady state steps also characteristic of such
beds. For all tests, E was calculated for each Tb by substracting the initial
concentration from final concentration for each particular step and dividing
the difference by the step duration (90 minutes). Thus, the e value is a
representative mean for the entire step. The most significant feature of Fig.
37 is the considerably higher resistance to erosion offered by the dense bed
compared to the soft beds. In tests with soft beds, the bed softening role of
oscillatory flow in the rocking flume is also evident, particularly in the 0.5
day consolidation test, when compared with the corresponding results from the
annular flume.

The following analysis is directed towards determining the erosion rate
constants, M and Tc (= Ts) of Eq. 2, from all the tests. Tc is then
correlated empirically to the bulk density and, finally, M is likewise
correlated to Tc. Equation 2 is an acceptable approximation for the erosion
behavior of dense beds. For soft beds, Eq. 1 is applicable (Parchure and
Mehta, 1985). However, Eq. 2 is a reasonable approximation of the erosion
behavior of soft beds, provided the erosion rate is calculated as a
representative mean of each steady state step as noted (Fig. 37).

In Figs. 38 and 39, e-Tb relationships for soft beds have been replotted
for clarity. With reference to Fig. 39 as an example, To is the value of Tb
corresponding to incipient erosion, while Tc is the "operational" or "design"
value of the critical shear stress for erosion applicable to Eq. 2. M is










evaluated from the slope of the second line. In Fig. 38, erosion rates at

Tb = 0.1 N/m2 appear to be excessively high in comparison with the trends
implied by other data from both flumes. These values, corresponding to points
A and B, suggest mass erosion as opposed to surface erosion behavior (Parchure
and Mehta, 1985). Therefore, points A and B were disregarded.

For the dense bed as well as tests of Partheniades, linear approximations
(dashed lines) shown in Fig. 36 were used to evaluate Tc and M. For the soft
beds, Parchure (1984) used an alternative procedure for estimating Tc. This
involves plotting the final suspension concentration in a steady state step
against the corresponding Tb. This is done in Figs. 40 and 41 where C90 is
the (final) concentration at 90 minutes, the step duration.

Results are summarized in Table 9. Characteristically, T values are
close to each other with a mean of 0.12 N/m2. For the same sediment,
incipient erosion occurs at the same shear stress because the surface shear
strength (equal to applied shear stress) is unaffected by overburden. Hence
bed preparation procedure or density do not significantly influence Tco Tc
has been calculated by two methods A corresponding to Figs. 38, 39 and B
corresponding to Figs. 40 and 41; the latter method being applied to deposited
(soft) beds only, since for dense beds the two methods yield identical
results. Values obtained by B are generally slightly lower (except in test 4)
than A, but are of comparable magnitudes. M values are obtained from linear
slopes in Figs. 36, 38 and 39.
In Fig. 42, T (both methods) is plotted against pg. The following may
be considered as a representative relationship encompassing all data:

Tc = 1.04 (PB-l) (3)

In Fig. 43, M is plotted against Tc yielding the following relationship
(without consideration for the influences of bed structure or flow):

O- -2.33 'T
M = 1.06 x 10-3e c (4)

With respect to Eq. 3, the trend of increasing Tc with bed density is in
agreement with previous observations (Mehta et al., 1982). Likewise, others
have previously reported the trend of decreasing M with increasing Tc evident
in Fig. 43 and Eq. 4 (Ariathurai and Arulanandan, 1978; Hunt, 1981).










Table 9. Bay Mud Erosion Rate Constants


c

Test Tco A B M
No. (N/m2) (N/m2) (N/m2) (g/cm2-min)

1 0.12 0.65 _b 2.8 x 104
2 0.16 0.35 0.23 3.2 x 104
3 -a 0.12 0.05 5.0 x 104
4 0.10 0.28 0.30 7.4 x 10-4
5 0.10 0.28 0.20 7.4 x 104
P1 0.12 0.38 -b 2.1 x 10-5
P2 0.12 1.20 -b 7.8 x 10-5

alnsufficient data
bMethod A not applied


IV. CONCLUDING REMARKS


The relationships considered to be representative of the rate of erosion
of bay mud are as follows:


= M ( -1)


T = 1.04 (pB 1)


M = 0.00106 exp(- 2.33 Tc)


pgn
=- u
b 1/3
h


noting that in Eq. 2, Tc and Ts, used previously, have the same meaning.

In Eq. 5, n is Manning's bottom resistance coefficient, h is depth of
flow and u is current speed. An example is considered in Fig. 44 where the
rate of erosion, e, is plotted against current speed, u, (0-1.5 m/sec), for
different values of the bed bulk density, pB (1.2, 1.4 and 1.6 g/cm3). h =
10 m and n = 0.020 were selected arbitrarily as typical representative
estuarine values. The influence of pB (which also reflects bed "aging") on e









in this "design chart" is observed to be quite significant. Soft beds (1.2
g/cm3) generally have an order of magnitude (~ 10-2 g/cm2-min) greater rate of
erosion than do dense (1.6 g/cm3) beds (~ 10-3 g/cm2-min) at the same speed (~
1.3 m/sec).










Table 9. Bay Mud Erosion Rate Constants


c

Test Tco A B M
No. (N/m2) (N/m2) (N/m2) (g/cm2-min)

1 0.12 0.65 _b 2.8 x 104
2 0.16 0.35 0.23 3.2 x 104
3 -a 0.12 0.05 5.0 x 104
4 0.10 0.28 0.30 7.4 x 10-4
5 0.10 0.28 0.20 7.4 x 104
P1 0.12 0.38 -b 2.1 x 10-5
P2 0.12 1.20 -b 7.8 x 10-5

alnsufficient data
bMethod A not applied


IV. CONCLUDING REMARKS


The relationships considered to be representative of the rate of erosion
of bay mud are as follows:


= M ( -1)


T = 1.04 (pB 1)


M = 0.00106 exp(- 2.33 Tc)


pgn
=- u
b 1/3
h


noting that in Eq. 2, Tc and Ts, used previously, have the same meaning.

In Eq. 5, n is Manning's bottom resistance coefficient, h is depth of
flow and u is current speed. An example is considered in Fig. 44 where the
rate of erosion, e, is plotted against current speed, u, (0-1.5 m/sec), for
different values of the bed bulk density, pB (1.2, 1.4 and 1.6 g/cm3). h =
10 m and n = 0.020 were selected arbitrarily as typical representative
estuarine values. The influence of pB (which also reflects bed "aging") on e










APPENDIX


VELOCITY AND SHEAR STRESS CALCULATIONS


The original plan for the rocking flume designated a height of 22 cm.
Early experiments determined that this height would not allow for sufficient
water depth to generate a large enough shear stress on the bed surface. The
height of the flume was therefore increased to 36 cm. A second modification
was made with the addition of a plexiglass top constriction over the center of
the flume (Fig. 6b,c). This constriction increased the flow velocity over the
sediment bed. The top could be set at any selected depth over the bed.

Flume Calibration. To calculate the shear stress in the flume it was
necessary to know the velocity of water above the bed. Three different
techniques were used to measure velocity. In all cases maximum velocity was
measured. Following is a brief description of the methods employed and a
comparison of the results obtained. Calculations are made of the shear stress
with and without the top in place. Calibration marks were added to the speed
controller of the flume, so that wave period and velocity could be determined
at specific settings.

For a shallow water wave the velocity profile over the water depth is
fairly constant, at least within the detection limits employed. The simplest
method of measuring velocity is to determine the horizontal displacement of a
particle floating on the surface at the node. From the distance traveled the
maximum velocity can be calculated from the relationship um=rd/T where d is
displacement and T is wave period. This measurement could only be made
without the top constriction in the flume. Once the top was in place new
estimates of velocity were made by assuming that the only effect of the top
was to increase water velocity at the center of the flume. From the equation
of continuity, the same volume of water, 17.5 cm deep, had to pass the center,
but there were only 10 cm of depth for it below the top. New velocities were
calculated from a ratio of water depths at the ends and the middle of the
flume. A ratio of 1.75 (velocity with top divided by velocity without top)
was determined. Table A.1 contains velocities obtained by the method of
horizontal displacement.










Table A.1.


Maximum Velocities Obtained by Measurement of Horizontal
Displacement, without and with Top Constriction


Wave Horizontal Maximum Maximum
period, T displacement, d velocity, um velocity, um
without top with top
(sec) (cm) (cm/sec) (cm/sec)

13.0 25 6.0 10.6
8.0 27 10.6 18.6
6.6 28 13.3 23.3
6.1 30 15.5 27.0
5.7 33 18.2 31.8
5.4 36 20.8 36.4
5.2 38 23.0 40.2
5.1 40 24.6 43.1
5.0 50 31.7 55.5



The most direct method of measuring velocity was with a current meter.
The current meter was an electro-magnetic unit made by Marsh McBirney (model
523), with an accuracy of 3 cm/s. Measurements were taken at a height of 2 cm
above the bed at the center of the flume. Data were recorded on a Hewlett-
Packard strip chart recorder so that the mean maximum velocity could be
determined. Table A.2 contains velocity (and wave period) data obtained using
the meter.

The third method involved measuring the displacement of water above and
below still water level, at the ends of the flume. Velocity was calculated by
determining the total volume of water that moved through the flume without and
with the top in place over one-half a wave period (see Fig. A.1). The maximum
velocity, um, without the top is


rAd
m hd
um = hT


where Ad is the longitudinal (vertical) area of water displaced during one-
half period, and h is the still water depth. Ad is obtained from (for small
displacements):


(A.1)










Table A.2. Periods of Oscillation and Velocities Obtained
from Current Meter in the Rocking Flume

Period, T Max.velocity, um
(sec) (cm/sec)

33.6 3.1
35.0 3.7
21.5 4.6
13.0 10.5
11.0 13.7
9.1 16.3
7.8 19.1
7.4 20.1
7.2 20.8
6.6 23.4
6.1 27.0
5.7 32.3
5.5 36.4
5.2 43.0
5.1 47.0


Table A.3. Maximum Velocities Obtained by Considering Flow Continuity

Wave Maximum Maximum
period, T velocity, um velocity, um
without top with top
(sec) (cm/sec) (cm/sec)

13.0 4.8 8.1
8.0 9.2 15.4
6.6 12.0 20.7
6.1 14.3 26.0
5.7 15.9 32.1
5.5 19.5 36.2
5.2 20.7 45.0
5.1 23.8 52.0











Ad = L (A + ) (A.2)


where A/2 = vertical displacement amplitude at flume ends relative to still
water level, B/2 = vertical displacement amplitude of flume bottom and L =
flume length. In Eq. A.2, the water surface profile is assumed to vary
sinusoidally. With the top in place, Ad was appropriately modified. Results
are presented in Table A.3.

The maximum applied shear stress was calculated from the following
relationships established by Jonsson (1966); Tmax=0.5pfu 2, where p is water
density, fw is the coefficient of friction, and um is maximum velocity. The
coefficient of friction can be calculated as fw=0.09 Re-02, where Re is the
2
wave Reynolds number. The Reynolds number can be calculated as u /av, where
m
um is maximum velocity, a is wave angular frequency (2r/T), and v is kinematic
viscosity of water. For these experiments the kinematic viscosity was taken
to be 1 x 10 cm /sec and p as 1 g/cm These calculations are based on
fresh water.

Calculation of shear stress using Jonsson's formula yields the maximum
applied shear stress. This formula is valid for progressive waves generating
(smooth) turbulent flows. In dealing with a standing wave, the applied shear
stress is not constant, but oscillates as a square sine function. To adjust
for this difference the maximum velocity was used to calculate a maximum shear
stress, Tm. By integrating shear stress over one-half a wave period the mean
shear stress was determined. The result is that mean shear stress is one-half
the maximum shear stress. Justification for this manipulation was based on
the correlation of results of critical shear stresses obtained in the rocking
flume compared to those obtained in the annular flume.

A calibration curve between maximum velocity, um, and wave period, T, is
presented in Fig. A.2, based on data in Tables A.1, A.2 and A.3. The
corresponding relationship between the average bed shear stress, Tb, and wave
period, T, is given in Fig. A.3.










LITERATURE CITED


American Public Health Association, Standard Methods for the Examination of
Water and Wastewater, 14th ed., American Public Health Association, New
York, 1975.
Ariathurai, R., and Arulanandan, K., "Erosion of Cohesive Soils," Journal of
the Hydraulics Division, ASCE, Vol. 104, No. HY2, February, 1978, pp.
279-283.
ASTM, 1981 Annual Book of ASTM Standards, American Society for Testing and
Materials, Philadelphia, Pennsylvania, 1981.
Hunt, S.D. and Mehta, A.J., "An Evaluation of Laboratory Data on Erosion of
Fine Sediment Beds," Paper Presented at the Annual Meeting of the Fine
Particle Society, Miami, Florida, April, 1985.
Jonsson, I.G., "Wave Boundary Layers and Friction Factors," Proceedings of the
10th Coastal Engineering Conference, ASCE, Vol. 1, Tokyo, Japan, 1966, pp.
127-148.
Krone, R.B., "Sedimentation in San Francisco Bay System," In: San Francisco
Bay: The Urbanized Estuary, California Academy of Sciences, San Fancisco,
California, 1979, pp. 177-190.
Maa, P.Y., "Erosion of Soft Muds by Waves," Ph.D. Dissertation, University of
Florida, Gainesville, Florida, 1986.
Mehta, A.J., "Depositional Behavior of Cohesive Sediments," Ph.D.
Dissertation, University of Florida, Gainesville Florida, 1973.
Mehta, A.J., Parchure, T.M., Dixit, J.G., and Ariathurai, R., "Resuspension of
Deposited Cohesive Sediment Beds," In: Estuarine Comparisons, V.S. Kennedy
Editor, Academic Press, New York, 1982, pp. 591-609.
Parchure, T.M., "Erosional Behavior of Deposited Cohesive Sediments," Ph.D.
Dissertation, University of Florida, Gainesville, Florida, 1984.
Parchure, T.M. and Mehta, A.J., "Erosion of Soft Cohesive Sediment Deposits,"
Journal of Hydraulic Engineering, ASCE, Vol. 3, No. 10, October 1985, pp.
1308-1326.
Partheniades, E., "A Study of Erosion and Deposition of Cohesive Soils in Salt
Water," Ph.D. Dissertation, University of California, Berkeley,
California, 1962.
Partheniades, E., "Erosion and Deposition of Cohesive Soils, Journal of the
Hydraulics Division, ASCE, Vol. 91, No. HY1, January, 1965, pp. 105-139.













TYPE I


Fig. 1. Variation of Bed Shear
Strength with Depth for a
Deposited Bed, Type I Profile
(after Parchure, 1984).





z


c-t profile of Type I
z

Z
Lii



z decrec
2 dt
U)
z

T
Sa) T



0O
c-t profile of Type



z
O

C)



U) b) T


Fig. 2. Variation of Bed Shear
Strength with Depth for a
Placed Bed, Type II Profile
(after Parchure, 1984).


II


-!i= constant
dt


IME (t)


Fig. 3a. Concentration-Time Profile for
Parchure, 1984).
Fig. 3b. Concentration-Time Profile for
Parchure, 1984).


a Deposited Bed (Type I) (after

a Placed Bed (Type II) (after


TYPE 1


IME ( t )





















-Beoring fcr Inner Shoft

Slipring ond Brush Block Assembly
Stroin Goges
Ring Suspending Blode
Imner Shoflt -Refilling Funnel
-berglass Channel
Fiberglass Stiffener
igloss -Somple Top
ingle o

-Sample Bolltle


Driving Motors


Fig. 4. Schematic View of Annular Flume (after Mehta, 1973).
















































Fig. 5a. Plan View of Rocking Flume.
Fig. 5b. Elevation View of Rocking Flume.


tt-J n


H


II


II


-d -II


11


L J


| t__ c


-` I





I I I rii -rr ri


I| P oI


i i


















GI~II0






b)


I I


0 20cm


Side View of Rocking Flume.
Top Constriction for Rocking Flume.
Top Constriction Placed in the Rocking Flume.
Removable Sheet Metal Bed for Rocking Flume.


Fig.
Fig.
Fig.
Fig.









Mixing
14 -ok4


Deposition and
Consolidation


Erosion


0 I 2 3 4 5 1.5 3 4.5 6 75 9
Days Hours

Fig. 7. Experimental Test Procedure (Shear Stress Variation).



2-- cm dia plastic tube


S*- 15 cm dia. plexiglass cylinder
15cm 2.5 cm dia metal tube

_L
Annular space for mixture of alcohol
and dry ice


Porcelain
D ish


Piston with Screw Rod


Fig. 8. Apparatus for Measurement of Density as a Function of Depth
(after Parchure, 1984).


U^
en-


E03
cr



IL










DRY DENSITY, (g/cm3)


E


4-
I O 0 p(z) = 0.128+ 0.046z

2.7 p (z) Q029+0.00548z
6





8

Fig. 9. Density (Dry) Profile as a Function of Depth for Deposited Kaolinite.


DRY DENSITY,p(g/cm3)
)0.1 0.2 0.3


0 < z < 1.3
p=0.33 z +
1.35< z
p= 0.025z


5
.063

+0.073


Fig. 10. Density (Dry) Profile as a Function of Depth for Deposited Cedar Key
Mud.










O.r
0r


E
N

.Q.
Fig. Density (Dry)






Fig. 11. Density (Dry)


Profile


DRY DENSITYp(g/cm3)
3 0.4 O.5


as a Function of Depth for


Placed Kaolinite.


DRY DENSITY,p(g/cm3)


0.3
or-


Fig. 12. Density (Dry) Profile as a Function of Depth for Placed Cedar Key
Mud.


S I




Representative I .
Mean


I


*I
I






"
I1


1
I




Representative
Mean ,


I

I











TIME (mins)


TIME (mins)


Fig. 13. Suspended Sediment Concentration versus
Kaolinite, Annular Flume.


2



O.IN/m2 0.2 N/m2

io
0I



z
5-
U)l r
r ^ -- -


Time for Deposited


TIME (mins)


Fig. 14. Suspended Sediment Concentration versus Time, Deposited Kaolinite,
Rocking Flume.





























100 150 200 250 300
TIME (mins)


350 400 450 500 540


Fig. 15. Suspended Sediment Concentration versus Time for Deposited Cedar Key
Mud, Annular Flume.




1.5
O1 0.1 N/mz2 0.2 N/m2 0.3 N/m2








z

0 0

LU
Q0.5


0 Possibly Localized
an Erosion
0.1 I

0 50 100 150 200 250
TIME (mins)



Fig. 16. Suspended Sediment Concentration versus Time, Deposited Cedar Key
Mud, Rocking Flume.


S"I II I.l
0.I N/m2 0.2 N/m2 0.3 N/m2 0.4 N/mz 2 0.5 N/m r 0.6 N/m2
0- Lineor Trend




;- ___ /
8


6


4


2


0 .-" ._


50









TIME (mins)


3 300 400


5 04N/m 0.5N/m2
5-

4-

0 3-
z

0 2- -

a. I 2 0.2N/m2
mi 0.1 N/m .

0" 100
TIME(m

Fig. 17. Suspended Sediment Concentration versus
Annular Flume.


200
ins)


Time, Placed Kaolinite,


50 100


150
TIME (mins)


Fig. 18. Suspended Sediment Concentration versus Time,
Rocking Flume.


Placed Kaolinite,


04


0.2


00'
C


I I I II


0.2N/m2 0.3 N/m2 0.4 N/m2
2 0
(- indicates mean trend at 0.4 N/m 2
( and () suggest possible increase
in erosion rate at 0.4 N/m2 due to /
bed softening at 220 min.

// 2


Step-Increase in Concentration
Sdue to Moss ErosionV *

I I I I I I


200


250


'--- ---


1











30

0.1 N/m 0.2 N/m 0.3 N/m2 0.4N/m2 05 N/m2 0.6 N/m2
2 0 -. Placed Bed .>
u Suggests Deposited Beh
SBed Behovior o

z 10 /
o1 Drop due to Chonge
in Vertical Structure of
SConcentration Profile

S 0 100 200 300 400 500
TIME (mins)

Fig. 19. Suspended Sediment Concentration versus Time, Deposited Cedar Key
Mud, Annular Flume.


z

e(fl



z
8


)A

0. I N/m2 0.2 N/m2 0.3 N/m2 0 0.4 N/m2
).2
Bed Shear Strength
Possibly due to Exceeded;Bed Softening
Localized Erosion o Factor
*Oflf r^rsfl 6- A


- -


CUU
TIME (mins)


+40


lI-\


Fig. 20. Suspended Sediment Concentration versus
Rocking Flume.


0.4
Tb (N/m2)


Fig. 21. Mass Per Unit Surface Area versus Shear
Both Flumes.


Time, Placed Cedar Key Mud,


Stress, Deposited Kaolinite,


300































N
E
z

-o


(Jq- W3/6o6W) '31Vd NOISOI3


S(zW3/b6w) V38V 3VdlnCIS IINn 83d SSVAI 03(N3dSnS


co
1 43







U)

$4
4-i






p4
1-1
r

(U
fai

TO


4-i
0
o













0






U;
a
0
o
n

0)
0



4-i
CO T

CO CD





















E 8 Erosion due to
a Mud Softening
E
Ov I
w 60
-cr
z
0
0 4- /
gr



2
Point Believed
/to Represent
1/Sheor Strength

0 0.2 0.4 0.6
STb (N/m2)

Fig. 24. Erosion Rate versus Shear Stress, Placed Cedar Key Mud, Both Flumes.


BED SHEAR STRENGTH,


E
N
0









4


Fig. 25. Variation of Bed Shear Strength
Both Flumes.


Ts (N/m2)


with Depth for Deposited Kaolinite,











BED SHEAR STRENGTH, Ts (N/mZ)


Fig. 26. Variation of Bed Shear Strength with Depth for Deposited
Mud, Both Flumes.


Cedar Key


BED SHEAR STRENGTH, Ts (N/m2)


E

N

12
4-







4


Fig. 27. Variation of Bed Shear Strength with Depth for Placed Cedar Key
Mud, Annular Flume.


II II





nulor Flume
king Flume o

Iower Chenr Strennth in o


the Rocking Flume Suggests
Bed Softening under Oscillotory Currents


-


o An
* Ro











o Annular Flume
Rocking Flume




-1.0 0
_0
0





-2.0-





-3.0 I
*





0.1 0.2 03

(Tb- T)'2


Fig. 28. Log e versus (Tb TS)0.5 for Deposited Kaolinite, Both Flumes.





o Annular Flume
Rocking Flume
0 0







-- l. -

CP











-2.0-
S*


0 0.1 0.2 0.3
(Tb


Fig. 29. Log e versus (Tb Ts)0.5 for Deposited Cedar Key Mud, Both Flumes.


































DRY DENSITY (g/cm3)


Fig. 30. Bed Dry Density Profiles, Bay Mud.


TIME (mins)


Fig. 31. Time-Concentration Relationship,
(Test 1).


Bay Mud, Dense Bed, Annular Flume














-J
S14-
Z

/

z
08-

9 6- -- -- --
S0.4 N/m2 -
a. 4 0.5 N/m 0.6 N/m 0.3N/m
|) --I-- 0-3N/m'
S0.1 N/m 0.2 N/m2
2- Step-like Time-
.- .-.. .,*-,r. Concentration Profile
I I< I _________ 1^ ______ I I -- I--I
R) 50 100 150 200 250 300 3
TIME (mins)



Fig. 32. Time-Concentration Relationship, Bay Mud, Deposited Bed, 0.5 Day
Consolidation, Annular Flume (Test 2).


03 0 350 400 450


50 100 1U0 200 250
TIME (mins)


Fig. 33. Time-Concentration Relationship, Bay Mud, Deposited Bed, 0.5 Day
Consolidation, Rocking Flume (Test 3).


0.1 N/m2 -4- 0.2N/m2 -- 0.3 N/mZ --O.4 N/m2 -4- 0.5 N/m2---


,,
















c \ 'J. .'1 1"1 W ".. ii/ in -

z






S-,
U




22
:) O 2 |- -.- ,0.4N/m2-
--- 0.1 N/m2 --- 0.2 N/m2 03N/m2



50 100 150 200 250 300
TIME (mins)



Fig. 34. Time-Concentration Relationship, Bay Mud, Deposited Bed, 3.8 Day
Consolidation, Annular Flume (Test 4).


50 100


0 250
TIME (mins)


400 '44


Fig. 35. Time-Concentration Relationship, Bay Mud, Deposited Bed, 3.8 Day
Consolidation, Rocking Flume (Test 5).


3 I I I I I I I

--.IN/m2 I- 02N/m2 -1 N-- 03N/m2 -.4N/m2 -- 0.5N/m2 ---

2-





j^^^ tn t^vt


n


--











2x10 /




E
N

E Linear / Annular Flume
S. Approximation pg = 1.63 g/cm

S-4
0 II0 I
Sl IIO


o Partheniodes -
w I Series I
SP/ p=1.36g/cm3/ /
:' I pB Portheniodes
S/ Series 11
T/ pe = 1.34 q/cm3

I I -

BO 0.4 0.8 1.2 1.6 2.0 2.4 2.8
BED SHEAR STRESS, Tb (N/m2)

Fig. 36. Rate of Erosion versus Bed Shear Stress, Bay Mud, Dense Bed and
Results of Partheniades.


T (N/m2)
b


Fig. 37. Rate of Erosion versus Bed Shear Stress, Bay Mud, Dense Bed and Soft
Beds.










2.5 x 63
2.5 x 10


E
NC
I lO -3
o 10 x 10 -









I
u, Rocking Flume-
0










T-b (N/m2)

Fig. 38. Rate of Erosion versus Bed Shear Stress, Bay Mud, Soft Beds, 0.5 Day
Consolidation.



o Annulor Flume
SRocking Flume
B o
/




















5 x10
CnSecond sLine
o>
w


Tb ( N/m 2)


Fig. 39. Rate of Erosion versus
Consolidation.


Bed Shear Stress, Bay Mud, Soft Beds, 3.8 Day










* Q5 day
o 3.8 days


Annular Flume


/
9/


Fig. 40. Final Concentration, C90,
Beds, Annular Flume.


/


/ ,/



/ /


/


0.2 Tc 0.4 0.6
Tb (N/m2)
versus Bed Shear Stress, Tb, Bay Mud, Soft


* 0.5 day "
o 3.8 days


Rocking Flume


.7/


-/


.7
7
7
Y


/.


0 /
0 o


1~~


Tb (N/m2)


Fig. 41. Final Concentration, C90, versus Bed Shear Stress, Tb, Bay Mud, Soft
Beds, Rocking Flume.


r


c~.























N
E
z




0.2-


/**




0.0
1.0 I.


Fig. 42. Critical Shear Stress,
Bay Mud.









1-
o *




i *


E


pa (g/cm3)
Tc, as a Function of Bed Bulk Density, pB


Fig. 43. Erosion Rate Constant, M, as a Function of Critical Shear Stress,
Tc, Bay Mud.

58






















U hl/3


E



n=0.020
z h= 1Om
o
0 3
O5x



i d
I-1.
S -3





P,=1.6 cm3
L 1I I


O 0.4 Q8 1.2
CURRENT SPEED, u(m/sec)






Fig. 44. Erosion Rate Dependence on Bed Bulk Density and Current Speed,
Bay Mud.




















t=0











Fig. A.1.






56


U
50-



S40-

E
E
u
30-



0 20-
2


x


= T/2


Oscillatory Motion of the Rocking Flume.


PERIOD, T (sec)


Fig. A.2. Calibration Curve between Maximum Velocity, um, and Wave Period,
T, in the Rocking Flume.







60












































BED SHEAR STRESS, Tb(N/m2)


Fig. A.3. Calibration Curve between Bed Shear Stress, Tb, and Wave Period,
T, in the Rocking Flume.


















61




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