ERDC/CHL TR-20-13 

Dredging Operations and Environmental Research Program

Erosion Thresholds and Rates for Sand-Mud 

Mixtures 

Coastal and Hydraulics Laboratory 

David W. Perkey, S. Jarrell Smith, and Anthony M. Priestas July 2020

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Dredging Operations and Environmental 

Research Program 

ERDC/CHL TR-20-13 

July 2020 

Erosion Thresholds and Rates for Sand-Mud 

Mixtures

David W. Perkey, S. Jarrell Smith, and Anthony M. Priestas

Coastal and Hydraulics Laboratory

U.S. Army Engineer Research and Development Center

3909 Halls Ferry Road

Vicksburg, MS 39180-6199

Final report

Approved for public release; distribution is unlimited

Prepared for Dredging Operations and Environmental Research Program

Vicksburg, MS 39180

Under Project No. 485304, “Cost Effective Stabilization of Dredged Sediment for 

Capping and Nearshore Placement”
ERDC/CHL TR-20-13 ii

Abstract

Differences in erosion behavior of non-cohesive and cohesive sediments 

are widely recognized. In many natural environments, sand and mud are 

not completely separated and occur as mixtures. Significantly less 

research has been conducted on the erosion behavior of sand-mud 

mixtures compared to the separate treatment of sand and mud erosion. 

Sedflume erosion experiments were conducted on sand-mud mixtures 

with varying mud content to define the relationships between mud 

content, critical stress for erosion (τc), and erosion rate. Sand-mud 

mixtures were prepared with three mud sources: (1) non-swelling clay 

(kaolinite), (2) swelling clay (kaolinite/bentonite), and (3) a swelling, 

natural mud from the Mississippi River.

Test results showed that critical shear stresses of the mixed sediments 

departed from that of pure sand with mud fractions on the order of 2% to 

10%. Peak τc was observed between 30% to 40% mud content, with 

swelling muds achieving a ten-fold increase in τc while a five-fold increase 

in τc was measured for kaolinite. Additionally, this study demonstrated 

that the introduction of small amounts (≤5%) of mud to sand reduced 

erosion rates by a factor of 10 to 100. This observed abatement of erosion 

rate has implications for the use of dredged materials in civil and 

environmental engineering projects.

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be construed as an official Department of the Army position unless so designated by other authorized documents.

DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR.
ERDC/CHL TR-20-13 iii

Contents

Abstract.................................................................................................................................... ii

Figures and Tables..................................................................................................................iv

Preface..................................................................................................................................... x

1 Introduction ...................................................................................................................... 1

1.1 Background........................................................................................................1

1.2 Objective(s) ........................................................................................................1

1.3 Approach ............................................................................................................2

2 Methods ............................................................................................................................ 3

2.1 Sediment properties..........................................................................................3

2.2 Erosion testing ...................................................................................................7

2.2.1 Sediment Erosion with Depth flume (Sedflume)....................................................... 7

2.2.2 Slurry preparation ....................................................................................................... 9

2.2.3 Core preparation ......................................................................................................... 9

2.3 Physical properties ......................................................................................... 13

2.3.1 Grain size................................................................................................................... 13

2.3.2 Bulk density ............................................................................................................... 13

2.3.3 Atterberg limits..........................................................................................................14

3 Results.............................................................................................................................15

3.1 Physical properties of test materials ..............................................................15

3.1.1 Atterberg limits..........................................................................................................15

3.1.2 Bulk density ............................................................................................................... 17

3.1.3 Grain size................................................................................................................... 20

3.2 Erosion tests ................................................................................................... 22

3.2.1 Erosion parameterization .........................................................................................22

3.2.2 Sediment erosion rates ............................................................................................29

4 Summary and Conclusions ...........................................................................................32

5 Future Work ....................................................................................................................34

References.............................................................................................................................36

Appendix A: Grain Size Data................................................................................................39

Appendix B: X-Ray Attenuation Bulk Density Plots ..........................................................52

Appendix C: Erosion Data ....................................................................................................57

Acronyms and Abbreviations...............................................................................................82

Report Documentation Page
ERDC/CHL TR-20-13 iv

Figures and Tables

Figures

Figure 1. X-ray diffractograms of kaolin material. Strong peaks at 12.5 and 25 2Θ

indicate kaolinite as the predominant mineral in the material.............................................. 5

Figure 2. X-ray diffractograms of CSP Forestry sodium bentonite material. The 

shifting in position and intensity of peaks in the 3-7 2Θ range after exposure to 

magnesium (Mg) indicates the presence of smectite mineralogy......................................... 6

Figure 3. X-ray diffractograms of sediments collected from the MR. Quartz and 

illite appear to dominate the mineralogy with smaller amount of kaolinite. The 

collapse of the broad peak below 9° 2Θ in the heating treatments also suggests 

the presence of a smectite clay. ............................................................................................... 6

Figure 4. Images of the Sedflume (upper left), erosion surface (lower left), and 

operator (lower center), along with operational test range of the flume (right). .................. 7

Figure 5. Diagram of sediment core erosion process. The brown arrow indicates 

advancement of sediment into the flume with erosion. The blue arrow indicates 

flow direction of water. An example erosion sequence is provided in the table to 

the right of the sediment core................................................................................................... 8

Figure 6. Photos depicting the mixing of sand and mud are presented in A – C. 

Photos demonstrating the methods utilized to place material in core tubes are 

presented in D – G. ..................................................................................................................12

Figure 7. PI plotted against mud content for materials with kaolinite (K), 

kaolinite-bentonite (KB), and MR muds.................................................................................16

Figure 8. Bulk densities obtained through x-ray attenuation are shown for kaolinbentonite cores with mud contents ranging from 1% to 5% (KB001 – KB005). A 

surface transition zone was commonly observed at the water-sediment interface 

down to a depth of ~30 mm. Note that the “5R” in KB005R refers to an 

independent, duplicate measurement of a 5% kaolin-bentonite core................................ 17

Figure 9. Plots showing bulk density versus mud content for each core. Blue, red, 

and black lines indicate cores made with kaolinite (K), kaolinite-bentonite (KB), 

and MR muds, respectively. Curves with solid lines depict density values 

calculated by wet-dry analysis. Curves with dashed lines represent density values 

obtained from XRA....................................................................................................................19

Figure 10. Plot of mean mud content measured by laser diffraction against mud 

content by mass of each test core. Dashed blue line indicates least squares 

regression while the solid black line shows 1:1 correlation.................................................22

Figure 11. Plot of mean erosion rate against shear stress for cores composed of 

21% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04 cm/s..................................................................23

Figure 12. Critical shear stress (τc) plotted against mud content. Blue, red, and 

black lines indicate cores made with kaolinite (K), kaolinite-bentonite (KB), and 

MR muds, respectively. Dashed lines represent the 95% confidence interval of 

τc. The τc obtained from replicate cores are plotted with triangles with error bars 

indicating the 95% confidence. ..............................................................................................28
ERDC/CHL TR-20-13 v

Figure 13. Erosion rate plotted against kaolinite mud content. The colored solid 

lines depict the calculated erosion rates at various shear stresses, with dashed 

lines indicating 95% confidence. Mean erosion rate data are plotted with 

circles. Triangles represent replicate mean erosion rates with error bars 

indicating the 95% confidence. .............................................................................................30

Figure 14. Erosion rate plotted against kaolinite-bentonite mud content. The 

colored solid lines depict the calculated erosion rates at various shear 

stresses, with dashed lines indicating 95% confidence. Mean erosion rate 

data are plotted with circles. Triangles represent replicate mean erosion 

rates with error bars indicating the 95% confidence...................................................... 31

Figure 15. Erosion rate plotted against MR mud content. The colored solid lines 

depict the calculated erosion rates at various shear stresses, with dashed lines 

indicating 95% confidence. Mean erosion rate data are plotted with circles. 

Triangles represent replicate mean erosion rates with error bars indicating the 

95% confidence. ......................................................................................................................31

Figure A-1. Cumulative grain size distributions for sand. .....................................................39

Figure A-2. Grain size distributions for sand..........................................................................39

Figure A-3. Cumulative grain size distributions for kaolinite cores with mud 

contents ranging from 1% to 5%............................................................................................. 41

Figure A-4. Cumulative grain size distributions for kaolinite cores with mud 

contents ranging from 8% to 21%. ......................................................................................... 41

Figure A-5. Cumulative grain size distributions for kaolinite cores with mud 

contents ranging from 29% to 100%......................................................................................42

Figure A-6. Grain size distributions for kaolinite cores with mud contents ranging 

from 1% to 5%...........................................................................................................................42

Figure A-7. Grain size distributions for kaolinite cores with mud contents ranging 

from 8% to 21%. .......................................................................................................................43

Figure A-8. Grain size distributions for kaolinite cores with mud contents ranging 

from 29% to 100%. ..................................................................................................................43

Figure A-9. Cumulative grain size distributions for kaolinite-bentonite cores with 

mud contents ranging from 1% to 5%....................................................................................45

Figure A-10. Cumulative grain size distributions for kaolinite-bentonite cores with 

mud contents ranging from 8% to 21%..................................................................................45

Figure A-11. Cumulative grain size distributions for kaolinite-bentonite cores with 

mud contents ranging from 29% to 100%.............................................................................46

Figure A-12. Grain size distributions for kaolinite-bentonite cores with mud 

contents ranging from 1% to 5%.............................................................................................46

Figure A-13. Grain size distributions for kaolinite-bentonite cores with mud 

contents ranging from 8% to 21%. ......................................................................................... 47

Figure A-14. Grain size distributions for kaolinite-bentonite cores with mud 

contents ranging from 29% to 100%...................................................................................... 47

Figure A-15. Cumulative grain size distributions for MR cores with mud contents 

ranging from 1% to 5%.............................................................................................................49

Figure A-16. Cumulative grain size distributions for MR cores with mud contents 

ranging from 8% to 21%. .........................................................................................................49
ERDC/CHL TR-20-13 vi

Figure A-17. Cumulative grain size distributions for MR cores with mud contents 

ranging from 29% to 100%......................................................................................................50

Figure A-18. Grain size distributions for MR cores with mud contents ranging 

from 1% to 5%...........................................................................................................................50

Figure A-19. Grain size distributions for MR cores with mud contents ranging 

from 8% to 21%. ....................................................................................................................... 51

Figure A-20. Grain size distributions for MR cores with mud contents ranging 

from 29% to 100%. .................................................................................................................. 51

Figure B-1. Bulk densities obtained through x-ray attenuation are shown for sand 

cores. .........................................................................................................................................52

Figure B-2. Bulk densities obtained through x-ray attenuation are shown for 

kaolinite cores with mud contents ranging from 1% to 5%..................................................52

Figure B-3. Bulk densities obtained through x-ray attenuation are shown for 

kaolinite cores with mud contents ranging from 8% to 21%................................................53

Figure B-4. Bulk densities obtained through x-ray attenuation are shown for 

kaolinite cores with mud contents ranging from 29% to 100%...........................................53

Figure B-5. Bulk densities obtained through x-ray attenuation are shown for 

kaolinite-bentonite cores with mud contents ranging from 1% to 5%.................................54

Figure B-6. Bulk densities obtained through x-ray attenuation are shown for 

kaolinite-bentonite cores with mud contents ranging from 8% to 21%. .............................54

Figure B-7. Bulk densities obtained through x-ray attenuation are shown for 

kaolinite-bentonite cores with mud contents ranging from 29% to 100%. ........................55

Figure B-8. Bulk densities obtained through x-ray attenuation are shown for MR 

cores with mud contents ranging from 1% to 5%. ................................................................55

Figure B-9. Bulk densities obtained through x-ray attenuation are shown for MR 

cores with mud contents ranging from 8% to 21%...............................................................56

Figure B-10. Bulk densities obtained through x-ray attenuation are shown for MR 

cores with mud contents ranging from 29% to 100%. .........................................................56

Figure C-1. Plot of mean erosion rate data against shear stress for cores 

composed of 100% sand. Regression lines fit to the data were used to determine 

τc at an erosion rate of 1.0E-04..............................................................................................57

Figure C-2. Plot of mean erosion rate data against shear stress for cores 

composed of 1% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04...........................................................................59

Figure C-3. Plot of mean erosion rate data against shear stress for cores 

composed of 2% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04...........................................................................59

Figure C-4. Plot of mean erosion rate data against shear stress for cores 

composed of 3% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04...........................................................................60

Figure C-5. Plot of mean erosion rate data against shear stress for cores 

composed of 4% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04...........................................................................60

Figure C-6. Plot of mean erosion rate data against shear stress for cores 

composed of 5% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................61
ERDC/CHL TR-20-13 vii

Figure C-7. Plot of mean erosion rate data against shear stress for cores 

composed of 8% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................61

Figure C-8. Plot of mean erosion rate data against shear stress for cores 

composed of 11% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................62

Figure C-9. Plot of mean erosion rate data against shear stress for cores 

composed of 15% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................62

Figure C-10. Plot of mean erosion rate data against shear stress for cores 

composed of 21% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................63

Figure C-11. Plot of mean erosion rate data against shear stress for cores 

composed of 29% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................63

Figure C-12. Plot of mean erosion rate data against shear stress for cores 

composed of 40% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................64

Figure C-13. Plot of mean erosion rate data against shear stress for cores 

composed of 60% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................64

Figure C-14. Plot of mean erosion rate data against shear stress for cores 

composed of 80% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................65

Figure C-15. Plot of mean erosion rate data against shear stress for cores 

composed of 100% kaolinite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................65

Figure C-16. Plot of mean erosion rate data against shear stress for cores 

composed of 1% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................67

Figure C-17. Plot of mean erosion rate data against shear stress for cores 

composed of 2% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................67

Figure C-18. Plot of mean erosion rate data against shear stress for cores 

composed of 3% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................68

Figure C-19. Plot of mean erosion rate data against shear stress for cores 

composed of 4% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................68

Figure C-20. Plot of mean erosion rate data against shear stress for cores 

composed of 5% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................69

Figure C-21. Plot of mean erosion rate data against shear stress for cores 

composed of 8% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................69
ERDC/CHL TR-20-13 viii

Figure C-22. Plot of mean erosion rate data against shear stress for cores 

composed of 11% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................70

Figure C-23. Plot of mean erosion rate data against shear stress for cores 

composed of 15% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................70

Figure C-24. Plot of mean erosion rate data against shear stress for cores 

composed of 21% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04.............................................................. 71

Figure C-25. Plot of mean erosion rate data against shear stress for cores 

composed of 29% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04.............................................................. 71

Figure C-26. Plot of mean erosion rate data against shear stress for cores 

composed of 40% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................72

Figure C-27. Plot of mean erosion rate data against shear stress for cores 

composed of 60% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................72

Figure C-28. Plot of mean erosion rate data against shear stress for cores 

composed of 80% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................73

Figure C-29. Plot of mean erosion rate data against shear stress for cores 

composed of 100% kaolinite-bentonite mud. Regression lines fit to the data were 

used to determine τc at an erosion rate of 1.0E-04..............................................................73

Figure C-30. Plot of mean erosion rate data against shear stress for cores 

composed of 1% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................75

Figure C-31. Plot of mean erosion rate data against shear stress for cores 

composed of 2% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................75

Figure C-32. Plot of mean erosion rate data against shear stress for cores 

composed of 3% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................ 76

Figure 33. Plot of mean erosion rate data against shear stress for cores 

composed of 4% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................ 76

Figure C-34. Plot of mean erosion rate data against shear stress for cores 

composed of 5% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................77

Figure C-35. Plot of mean erosion rate data against shear stress for cores 

composed of 8% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................77

Figure C-36. Plot of mean erosion rate data against shear stress for cores 

composed of 11% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................78
ERDC/CHL TR-20-13 ix

Figure C-37. Plot of mean erosion rate data against shear stress for cores 

composed of 15% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................78

Figure C-38. Plot of mean erosion rate data against shear stress for cores 

composed of 21% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................79

Figure C-39. Plot of mean erosion rate data against shear stress for cores 

composed of 29% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................79

Figure C-40. Plot of mean erosion rate data against shear stress for cores 

composed of 40% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................80

Figure C-41. Plot of mean erosion rate data against shear stress for cores 

composed of 60% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................80

Figure C-42. Plot of mean erosion rate data against shear stress for cores 

composed of 80% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................81

Figure C-43. Plot of mean erosion rate data against shear stress for cores 

composed of 100% MR mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04............................................................................81

Tables

Table 1. Wentworth classification and median grain size (D50) of test materials. ............... 4

Table 2. Masses of sand, slurry and water utilized in core preparation. The “R” 

indicates replicate cores..........................................................................................................10

Table 3. Atterberg limit results for cores with mud content >20%. .....................................16

Table 4. Average bulk densities of cores obtained from both XRA and 

wet-dry methods.......................................................................................................................18

Table 5. Kaolinite-bentonite mud laser diffraction data.......................................................21

Table 6. Erosion parameters for sand cores.......................................................................... 24

Table 7. Erosion parameters for kaolinite mud cores. .......................................................... 24

Table 8. Erosion parameters for kaolinite-bentonite mud cores..........................................25

Table 9. Erosion parameters for MR mud cores....................................................................26

Table A-1. Sand laser diffraction data. ...................................................................................39

Table A-2. Kaolinite mud laser diffraction data.....................................................................40

Table A-3. Kaolinite-bentonite mud laser diffraction data....................................................44

Table A-4. MR mud laser diffraction data. .............................................................................48

Table C-1. Sand erosion parameters. ..................................................................................... 57

Table C-2. Kaolinite mud erosion parameters. ......................................................................58

Table C-3. Kaolinite-bentonite mud erosion parameters......................................................66

Table C-4. MR mud erosion parameters. ............................................................................... 74
ERDC/CHL TR-20-13 x

Preface

This study was conducted for the Dredging Operations and Environmental 

Research (DOER) Program under Project No. 485304, “Cost Effective 

Stabilization of Dredged Sediment for Capping and Nearshore Placement”

The Program Manager was Dr. Todd Bridges. 

The work was performed by the Field Data Collection and Analysis 

Branch of the Navigation Division, U.S. Army Engineer Research and 

Development Center, Coastal and Hydraulics Laboratory (ERDC-CHL). 

At the time of publication of this report, Mr. William Butler was Chief, 

Field Data Collection and Analysis Branch; Dr. Jacqueline Pettway was 

Chief, Navigation Division; and Mr. Charles E. Wiggins, was the 

Technical Director for Navigation. The Deputy Director of ERDC-CHL 

was Mr. Jeffrey R. Eckstein, and the Director was Dr. Ty V. Wamsley.

The Commander of ERDC was COL Teresa A. Schlosser, and the Director 

was Dr. David W. Pittman.
ERDC/CHL TR-20-13 1

1 Introduction

1.1 Background 

The differences in erosion behavior of non-cohesive (sand, gravel) and 

cohesive sediments (mud) are widely recognized. Erosion of non-cohesive 

sediments such as sand is dependent primarily upon the balance of weight, 

buoyancy, drag, and lift forces on the sediment, which is related to the 

size, density, and shape of the non-cohesive particles (Graf 1984; Garcia 

2008). Erosion of cohesive sediments is largely influenced by 

electrochemical bonding between sediment particles, which is influenced 

by clay mineralogy, pore water chemistry, organic content, biological 

cementation, and bed density (among others) (e.g., Mehta et al. 1989;

Mehta and McAnally 2008; Mehta 2013). In many natural environments, 

sand and mud are not completely separated but instead occur as mixtures. 

Much less research has been conducted on the erosion behavior of sandmud mixtures compared to the separate treatment of sand erosion and 

mud erosion. Erosion and transport processes of these heterogeneous 

sediments are more complex than that of pure sand as they vary with 

factors such as mud content, particle packing efficiency, and clay type 

(e.g., Mehta et al, 1989; Mitchner and Torfs 1996; van Ledden et al. 2004; 

Barry et al. 2006; Dickhudt et al. 2011; Jacobs et al. 2011; Wu et al. 2018). 

These previous studies have shown that as little as 3% to 15% mud added 

to sand can change erosion behavior from non-cohesive to cohesive; 

however, their focus was largely placed on the influence that mud fraction 

had on the critical shear stress for initiation of erosion. Less attention has 

been placed on the effect of mud fraction on erosion rates of mixed 

sediments. A notable exception is the work by Gailani et al. (2001), which 

investigated the effect of bentonite clay on the erosion rates of sand-mud 

mixtures. Their results showed that erosion rates decreased 2 orders of 

magnitude with 2% added bentonite, though they noted that other clay 

types should also be investigated; the present work was pursued to help fill 

this knowledge gap. 

1.2 Objective(s) 

The objective of this research is to conduct erosion experiments on sandmud mixtures to define the relationships between mud content, critical 

stress for erosion, and erosion rate for muds of differing mineralogy. The 
ERDC/CHL TR-20-13 2

primary aim of the work is to increase the predictive capability of erosion 

resistance and sediment transport by experimentally determining the 

relationship between sand-mud mixtures and erosion rates to improve the 

planning and design of civil and environmental engineering projects (e.g.,

estuarine sediment modeling, environmental capping design, and 

management of dredged materials).

1.3 Approach

A series of laboratory experiments were conducted to quantify changes in 

critical shear stress and erosion rate of sand-mud mixtures as a function of 

mud content (by mass percent) and clay mineralogy. An experimental set 

consisted of 15 artificially created cores prepared from a known mass of 

fine to medium sand mixed with a mud slurry of known mass and density. 

The mud content for each core varied between 0% to 100%. Erosion rates 

were measured as a function of flow rate (and thus applied shear stress) in 

the U.S. Army Corps of Engineers (USACE)-developed Sedflume 

(Sediment Erosion with Depth) erosion device. 

This report presents the methodologies utilized and results from 

laboratory erosion experiments conducted on several sand-mud mixtures. 

Impacts of mud content on both critical shear stress and erosion rate will 

be discussed. In addition, this report will also describe how variations in 

mineral composition and the physical properties of mud also impact the 

critical shear stress and erosion rate of the test beds.
ERDC/CHL TR-20-13 3

2 Methods

2.1 Sediment properties

Three groups of sand-mud mixtures were prepared for the erosion 

experiments. The sand source, a natural grain silica sand obtained through 

U.S. Silica, was constant for all three mixtures. Mud fractions among the 

three groups varied the following way: (1) a 100% Edgar Plastic Kaolin 

(EPK), hereafter referred to as kaolin, (2) a mud mixture composed of 

80% kaolin with 20% sodium bentonite, and (3) a natural mud collected 

from the lower Mississippi River (MR). Both the kaolin and sodium 

bentonite clays were purchased through commercial vendors, Edgar 

Minerals and CSP Forestry, respectively. The natural sediment was 

collected from the bottom of a perched boat basin off the MR near Grand 

Gulf, MS. MR sediment was obtained with a 14 cm* diameter stainless 

steel bottom drag sampler. Multiple sampling passes (~150 m in length) 

were made across the boat basin until the appropriate volume of sediment 

(~20 gallons†) had been collected. 

Grain size analysis was performed on all four components of the mixtures 

via laser diffraction methods (described later in this section). Table 1 

presents the Wentworth classification and D50 results of these analyses for 

the silica sand, kaolin, bentonite, and MR sediment. Size distribution and 

additional grain size data for these materials are presented in Tables A-1 

through A-4 and Figures A-1 through A-20 in Appendix A. Results showed 

that the U.S. Silica sediment was 100% sand (>63 μm) and was primarily 

composed of fine sand grains (125–250 μm). Both the kaolin and 

bentonite clays, along with the MR sediment, were classified primarily as 

silt (4 μm < x <63 μm). Grain size data provided by Edgar Minerals 

reported the kaolin to be >98% clay (<4 μm). These results were obtained 

through settling techniques utilizing a sedigraph (Goossens 2008) and not 

via laser diffraction. The bentonite obtained from CSP Forestry was in 

 

* For a full list of the spelled-out forms of the units of measure used in this document, please refer to US 

Government Publishing Office Style Manual, 31st ed. (Washington, DC: US Government Publishing 

Office 2016), 248-52, https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2016/pdf/GPOSTYLEMANUAL-2016.pdf.

† For a full list of the unit conversions used in this document, please refer to US Government Publishing 

Office Style Manual, 31st ed. (Washington, DC: US Government Publishing Office 2016), 345-7, 

https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2016/pdf/GPO-STYLEMANUAL-2016.pdf.
ERDC/CHL TR-20-13 4

pellet form (500–2000 μm), and therefore a report describing clay size 

content was not available.

Table 1. Wentworth classification and median grain size (D50) 

of test materials. 

Sample %Sand %Silt %Clay 

D50 

(μm) 

Silica Sand 100 0.0 0.0 159

Kaolin 5.8 72.8 21.4 10

Bentonite 5.1 65.4 29.5 7.1

MR 2.7 81.8 15.5 11

While grain size results from laser diffraction indicated predominantly 

silt-sized particles in the kaolin, bentonite, and MR sediment material, 

disparities in clay/silt content between laser diffraction and traditional 

settling techniques have been well documented in previous studies (e.g.,

Konert and Vandenberghe 1997; Buurman et al. 2001; Eshel et al. 2004). 

These studies have indicated that biases associated with both laser 

diffraction and settling methods commonly produce results that indicate 

lower clay content of material analyzed by laser diffraction when 

compared to settling methods. 

To further evaluate the properties of the clay sources utilized in this study, 

powder X-ray diffraction analysis was performed on the kaolin, bentonite, 

and MR clays. Standard methods that included exposure of the clays to 

different heating treatments, solutions of potassium and magnesium, and 

exposure to glycol were utilized to elucidate the clay mineralogies present 

in each material (Moore and Reynolds 1989; Jackson and Barak 2005) 

Figures 1, 2, and 3 display the diffractogram patterns that resulted from 

these analyses. 

Kaolinite peaks located at 12.5 and 25 2-theta (2θ) dominate the 

diffractograms of the kaolin material, indicating kaolinite to be the 

primary mineral in the sample (Figure 1). The shifting of position and 

intensity of the peaks located in the 3-7 2Θ range in the various treatments 

of the bentonite samples indicates a strong presence of smectite 

interstratified with mica (Figure 2). The term smectite refers to a group of 
ERDC/CHL TR-20-13 5

monoclinic minerals, of which bentonite is a member. The exact variety of 

smectite present could not be determined by the X-ray diffraction analysis, 

but the analysis did confirm a large presence of smectite minerals in the 

sodium bentonite material. 

Quartz and mica peaks were more pronounced in the MR diffractograms 

(Figure 3) when compared to those of kaolin and bentonite, suggesting a 

higher concentration of these minerals in the MR clays. The stability in 

location of the 10Å peak at 9° 2θ during the magnesium and glycol 

treatments suggests that the mica present in the MR clay is from the illite 

group (Gaudette et al. 1966). Illites are members of the phyllosilicate 

mineral group and are commonly formed from the weathering of 

argillaceous rocks and sediments. However, the broad features below the 

10Å peak in the potassium treatment do collapse down to 9° 2θ after 

heating, which still indicates the presence of some form of smectite 

mineralogy. Additionally, the kaolinite peaks of 12.5 and 25 2θ are both 

still present, but far less pronounced in comparison to the kaolin 

diffractograms. These findings suggest that the MR material is composed 

predominantly of illite group clays with a significant amount of fine 

grained quartz. Kaolinite and some form of smectite clays are also present 

but likely in lower abundance.

Figure 1. X-ray diffractograms of kaolin material. Strong peaks at 12.5 and 25 2Θ 

indicate kaolinite as the predominant mineral in the material. 
ERDC/CHL TR-20-13 6

Figure 2. X-ray diffractograms of CSP Forestry sodium bentonite material. The shifting 

in position and intensity of peaks in the 3-7 2Θ range after exposure to magnesium 

(Mg) indicates the presence of smectite mineralogy. 

Figure 3. X-ray diffractograms of sediments collected from the MR. Quartz and illite 

appear to dominate the mineralogy with smaller amount of kaolinite. The collapse of 

the broad peak below 9° 2Θ in the heating treatments also suggests the presence 

of a smectite clay. 
ERDC/CHL TR-20-13 7

2.2 Erosion testing

2.2.1 Sediment Erosion with Depth flume (Sedflume)

Sedflume is a field- or laboratory-deployable flume for quantifying cohesive 

sediment erosion. The USACE-developed Sedflume is a derivative of the 

flume developed by researchers at the University of California at Santa 

Barbara (McNeil et al. 1996). The flume includes an 80 cm long inlet 

section (Figure 4) with cross-sectional area of 2 × 10 cm for uniform, fully 

developed, smooth-turbulent flow. The inlet section is followed by a test 

section with a 10 cm diameter open bottom. Coring tubes and flume test 

section, inlet section, and exit sections are constructed of clear 

polycarbonate materials to permit observation of sediment-water 

interactions during the course of erosion experiments. The flume includes a 

port over the test section to provide access to the core surface for physical 

sampling. The flume accepts sediment cores up to 80 cm in length.

Figure 4. Images of the Sedflume (upper left), erosion surface (lower left), and 

operator (lower center), along with operational test range of the flume (right). 

Prior to conducting an erosion experiment, descriptions of the core are 

recorded, including length, condition of the core, and any visual evidence 

of layering. Cores are inserted into the testing section of Sedflume, and a 
ERDC/CHL TR-20-13 8

screw jack is used to advance the plunger such that the core surface 

becomes flush with the bottom wall of the flume. Flow is directed over the 

sample by diverting water from a 5.5 hp trash pump, through a 5 cm inner 

diameter hose, into the flume. The flow through the flume produces a 

shear stress on the surface of the core. Numerical, experimental, and 

analytical analyses have been performed to relate flow rate to bottom 

shear stress (Figure 4). As sediment is eroded from the core surface, the 

operator advances the screw jack to maintain the sediment surface flush 

with the bottom wall of the erosion flume. Erosion experiments were 

performed by repeating a sequence of five increasing shear stresses. Initial 

erosion testing is conducted in the surface region (upper 1 – 3 cm) of each 

core to identify shear stresses that produced erosion velocities within the 

bounds of typical operation (1.7 × 10-4 cm/s – 1.7 × 10-2 cm/s).

Approximately 1 – 5 mm of sediment is eroded at each specified shear 

stress; thus, the duration of each test is dependent on the rate of erosion. 

These shear stresses were utilized to create the erosion sequence, which 

was typically repeated five times for each core. An average erosion rate was 

then calculated for each shear stress. A diagram depicting this erosion test 

process along with an example erosion sequence is shown in Figure 5.

Figure 5. Diagram of sediment core erosion process. The brown arrow indicates 

advancement of sediment into the flume with erosion. The blue arrow indicates flow 

direction of water. An example erosion sequence is provided in the table to the right 

of the sediment core. 
ERDC/CHL TR-20-13 9

Physical samples for bulk sediment property measurements were taken at 

approximately 3 – 5 cm intervals during erosion experiments, generally at 

the end of a shear stress cycle. Physical samples were collected by 

draining the flume channel, opening the port over the test section, and 

extracting a sample from the sediment bed. These samples were then 

measured for bulk density and grain-size distribution. These properties 

can strongly influence erosion; therefore, documenting any variation with 

depth is important in interpreting the erosion data. Core material 

remaining after completion of the erosion test was retained for Atterberg 

limit testing.

2.2.2 Slurry preparation

The commercial clays were allowed a wetting period of at least 72 hours 

using fresh tap water, and each slurry was mixed in a rotary drum for 

several hours to disaggregate clumps and ensure uniform mixing. Slurry 

water contents were tested periodically to ensure a constant density of 

approximately 1.4 g/cm3, which offered sufficient viscosity to prevent sand 

particles from settling at high mud contents. A full description of the bulk 

density measurements is provided in section 2.3.2.

2.2.3 Core preparation

The general guidelines for preparing sediment mixtures follows that of 

Jacobs et al. (2011) whereby sand and mud fractions are sufficiently 

homogenized and the pore volume of the mixture is fully saturated with 

water. The goal is to minimize downcore variability of sediment properties 

that would otherwise affect erosion rates. 

First, an appropriate mass of mud slurry was added to a prescribed mass 

of sand to achieve the targeted mud mass fraction (Table 2; Figure 6A). 

For mud contents of 11% or less, a prescribed mass of water was also 

added to saturate the mixture assuming a sand porosity of 0.35 and slurry 

density of 1.4 g/cm3. The sample was then hand mixed for 3 – 10 min until 

the mud and sand were evenly distributed throughout the sample (Figure 

5B). For mud contents less than 21%, this involved folding and shearing 

the sample; shearing helps to minimize the formation of mud balls and 

distributes the mud as evenly as possible (Figure 6C). The sample was then 

prepared into 10 cm diameter cores by one of two methods. For fluid 

mixtures, the sample was extruded from a bag into the bottom of a core to 

minimize gas entrapment (Figure 6D). For highly viscous and granular 
ERDC/CHL TR-20-13 10

mixtures that could not easily be extruded, water was added to the sample 

to achieve saturation, and the samples were tamped into the core tube in 

approximately 0.5 to 1 cm lifts (Figures 6E-G). The prepared sediment 

cores were allowed to consolidate in a 4°C cooler for approximately 

30 days prior to erosion. 

Replicate cores were prepared utilizing the same methodology previously 

described. A total of 16 replicate cores were prepared. For each clay type, 

five mud contents were randomly selected for replication. Additionally, a 

replicate core of the 100% sand core was also prepared.

Table 2. Masses of sand, slurry and water utilized in core preparation. The “R” 

indicates replicate cores. 

Non-Swelling Clay 

(100% kaolin) 

Swelling Clay 

(80% kaolin, 

20% bentonite) 

Natural 

(MR) 

Mud 

(mass %) 

Sand 

Mass 

(g) 

Slurry 

Mass 

(g) 

Water 

Mass 

(g) 

Sand 

Mass 

(g) 

Slurry 

Mass 

(g) 

Water 

Mass 

(g) 

Sand 

Mass 

(g) 

Slurry 

Mass 

(g) 

Water 

Mass 

(g) 

0 3657 - 760 - - - - - -

0R 3657 - 760 - - - - - -

1 3657 81 702 3566 79 684 3688 79 710

1R 3693 81 709 - - - - - -

2 3657 163 644 3664 164 643 3664 157 650

3 3657 247 584 3631 247 578 3755 247 606

3R 3657 247 584 - - - 3636 247 578

4 3657 333 523 3637 333 517 3761 333 546

5 3657 420 461 3658 423 457 3782 423 487

5R 3657 420 461 3698 419 471 - - -

8 3657 694 265 3656 698 259 3781 698 292

8R - - - 3683 689 275 - - -
ERDC/CHL TR-20-13 11

Non-Swelling Clay 

(100% kaolin) 

Swelling Clay 

(80% kaolin, 

20% bentonite) 

Natural 

(MR) 

Mud 

(mass %) 

Sand 

Mass 

(g) 

Slurry 

Mass 

(g) 

Water 

Mass 

(g) 

Sand 

Mass 

(g) 

Slurry 

Mass 

(g) 

Water 

Mass 

(g) 

Sand 

Mass 

(g) 

Slurry 

Mass 

(g) 

Water 

Mass 

(g) 

11 3657 986 57 3644 989 48 3768 989 84

11R - - - 3661 973 69 3646 991 47

15 4199 1617 - 3524 1365 - 3644 1365 -

15R 4199 1617 - - - - - - -

21 3312 1921 - 2902 1694 - 3001 1694 -

21R 2901 1683 - 2902 1659 - 2921 1708 -

29 2274 2027 - 2902 2603 - 3001 2603 -

29R - - - - - - 2905 2609 -

40 1620 2356 - 1621 2373 - 1676 2373 -

60 845 2766 - 845 2783 - 874 2783 -

80 348 3034 - 347 3048 - 359 3048 -

80R - - - 347 2988 - - - -

100 - 3212 - - 3210 - - 3254 -

100R - - - - - - - 3208 -
ERDC/CHL TR-20-13 12

Figure 6. Photos depicting the mixing of sand and mud are presented in A – C. Photos 

demonstrating the methods utilized to place material in core tubes are presented 

in D – G. 
ERDC/CHL TR-20-13 13

2.3 Physical properties

Sub-samples of each sediment type were set aside for the purpose of 

Atterberg Limit testing before the preparation of sediment cores. 

Additional physical samples for bulk sediment property measurements 

were taken at periodic intervals during Sedflume testing, generally at the 

end of an erosion cycle (Figure 5). These samples were collected by 

draining the flume channel, opening the port over the test section, and 

extracting a sample from the sediment bed. Samples collected in this 

manner were used to characterize the bulk density and grain-size 

distribution of the test bed. The methods used to determine these 

properties are presented in this section.

2.3.1 Grain size

Grain size distributions and characterizations were obtained through 

Laser Diffraction Particle Size Analysis (LDPSA) with a Malvern 

Mastersizer 2000, which measures particle sizes in the range of 0.02 to 

2000 μm. Sediments were homogenized and disaggregated overnight in a 

solution of sodium metaphosphate (40 g/L). To remove any macro organic 

material, samples were passed through a 1000 μm sieve into the 

instrument’s reservoir and sonicated for 60 s prior to analysis. The 

Wentworth scale (1929) was used for the classification of sand (>63 μm), 

silt (63-4 μm) and clay (<4 μm) sized particles. 

2.3.2 Bulk density

Bulk sediment density of the mud slurries was determined by two 

methods: X-ray attenuation and a wet-dry weight analysis. 

2.3.2.1 X-Ray Attenuation (XRA)

Considerable variation in bed density is often found within the upper 

portion of the sediment bed, associated with expulsion of pore water 

from fine sediments through the process of self-weight consolidation 

(Winterwerp and van Kesteren 2004). Additionally, cohesive sediment 

erosion has been shown to typically decrease with increasing bed density, 

owing to the associated increased particle-particle contact and cohesion 

(Jepsen et al. 1997). Consequently, high-resolution, non-destructive 

methods to estimate bed density are desired in order to relate erosion to 

bed density. The U.S. Army Engineer Research and Development Center 

XRA device was applied to provide high-resolution, density profiles.
ERDC/CHL TR-20-13 14

The cores were scanned with varying vertical resolution to allow high 

resolution of the sediment-water interface and bulk density variation near 

the water surface. The typical scanning scheme was to sample for 10 cm 

with 1 mm resolution starting approximately 5 mm above the sedimentwater interface. The next 10 cm were then scanned in a 10 cm long 

segment at 2 mm resolution, followed by another segment to the bottom of 

the core (typically less than 10 cm) at 3 mm resolution. The reference 

material for these scans was water, and the reference material and all 

specimen scans were conducted with a 40 s dwell time.

2.3.2.2 Wet-dry weight analysis

Water content (w) of each sample was measured through wet-dry weight 

analysis following ASTM D2216-19 (2019) in which w is given by

ww = �

mmww−mmdd

mmdd

� (1)

where mw and md are the wet and dry weights, respectively. The total 

volume of sample was assumed to consist of both solid particles and 

water, with assumed densities of 2.65 g/cm3 and 1.0 g/cm3, respectively. 

The bulk density as a function of w and the densities of the sediment 

particles (ρs) and water (ρw) were calculated with Equation 2, derived 

from Jepsen et al. (2010):

ρρ = ρρss + wwρρss(ρρww−ρρss)

ρρww+wwρρss

(2)

2.3.3 Atterberg limits

After completion of erosion testing, the remaining material in each core 

was retained for the purpose of obtaining the Atterberg limits of the 

mixture. The Atterberg limits are composed of the liquid limit (LL), plastic 

limit (PL), and plasticity index (PI) of a material. Standard methods, 

described in ASTM D4318-5, were utilized to measure and calculate each 

parameter. To obtain the LL, the multipoint method was utilized with a 

motorized liquid limit device to maintain a constant drop rate. PL testing 

was conducted on a polished granite counter top using the hand method. 

The difference between LL and PL yielded the PI for each mixture.
ERDC/CHL TR-20-13 15

3 Results

3.1 Physical properties of test materials

The results of the physical parameters measured for each test material are 

presented in this section. 

3.1.1 Atterberg limits 

Upon the completion of erosion testing, sediment remaining within the core 

tube was retained and utilized to determine the Atterberg limits (LL, PL, 

and PI) of each mixture. It was found that mixtures with mud contents less 

than 20% by mass did not display plastic behavior for any of the mud types. 

Therefore, no results for those materials are reported. Plastic behavior was 

observed in the kaolin-bentonite cores starting at a mud content of 21% 

while both the kaolin and MR mud mixtures did not display plastic behavior 

until mud contents reached 29% by mass (Table 3). 

Results showed that the LL, PL, and PI of each material increased with mud 

content. Seed et al. (1964) documented similar trends in their study where 

they likewise mixed different amounts of kaolin and bentonite with fine 

sand and evaluated the Atterberg limits of those mixtures. Reported values 

of LL and PI for similar mixtures showed substantial variability between the 

two studies. Seed et al. (1964) reported 100% kaolin to have values of 45.0, 

34.8, and 10.2 for the LL, PL, and PI, respectively. By comparison, this 

study reported 100% kaolin to have values of 65.5, 33.9, and 31.6 for the LL, 

PL, and PI, respectively. While the PLs of the kaolin in the two studies are 

within close agreement, the LLs show a difference of nearly 20. This 

variability may largely be due to the variety of kaolin that was utilized. The 

LL of various types of kaolin have been reported to range from 34.98 to 72.0 

(White 1949; Lemos and Vaughan 2000; Sasanian et al. 2014). This study 

utilized EPK kaolin, which had a similarly reported LL of 61.0 by Sasanian 

and Newson (2014). Seed et al. (1964) did not identify the variety of kaolin 

that was utilized, but it is possible that they utilized a material that had 

similar properties to those reported by White (1949), 34.98 – 58.35. 

Regardless of these differences, both studies showed that the PI of the 

mixtures of materials increased with increasing clay content. 
ERDC/CHL TR-20-13 16

Table 3. Atterberg limit results for cores with mud content >20%. 

Non-Swelling Clay 

(100% kaolin) 

Swelling Clay 

(80% kaolin, 

20% bentonite) 

Natural 

(MR) 

Mud 

(mass %) LL PL PI LL PL PI LL PL PI 

21 - - - 18.4 14.7 3.8 - - -

29 18.3 14.9 3.4 22.1 13.5 8.6 24.3 16.0 8.3

40 25.3 14.6 10.7 28.4 14.2 14.2 32.3 15.0 17.3

60 37.7 21.8 15.9 41.9 18.7 23.2 44.7 20.1 24.6

80 51.0 27.7 23.3 54.7 24.4 30.4 60.6 23.7 36.9

100 65.5 33.9 31.6 68.9 30.1 38.8 76.0 27.4 48.6

The PI of the materials tested in this study are plotted against the mass 

percent mud content in Figure 7. Since PI is the difference between the LL 

and PL, it can be thought of as the range of water content over which a soil 

or sediment will behave plastically. Therefore, larger PI values indicate 

larger ranges in plasticity for the corresponding material. While the 

kaolin-bentonite mixtures displayed plasticity behavior at the lowest mud 

content, the MR cores tended to have higher PI values, indicating that they 

had a wider range of plasticity at a given mud content (Figure 7). 

Figure 7. PI plotted against mud content for materials with kaolinite (K), 

kaolinite-bentonite (KB), and MR muds. 
ERDC/CHL TR-20-13 17

3.1.2 Bulk density

3.1.2.1 XRA

After a 30-day consolidation period, the length of each sediment core was 

scanned with a collimated x-ray beam to determine the bulk sediment 

density of the core prior to erosion analysis. In general, the density scans 

showed a density transition zone between overlaying water and the 

sediment for the upper few centimeters of each core. As seen in Figure 8, 

below a depth of ~3 cm, densities typically remained constant. Density 

values below this transition zone were averaged to produce a mean density 

for each core (Table 4). Plots showing the density profiles of each core can 

be found in Figures B-1 through B-10 in Appendix B.

Figure 8. Bulk densities obtained through x-ray attenuation are shown for kaolinbentonite cores with mud contents ranging from 1% to 5% (KB001 – KB005). A 

surface transition zone was commonly observed at the water-sediment interface 

down to a depth of ~30 mm. Note that the “5R” in KB005R refers to an independent, 

duplicate measurement of a 5% kaolin-bentonite core. 

3.1.2.2 Wet-dry weight analysis

Through the process of wet-dry analysis, bulk density measurements were 

also performed on the physical samples collected during erosion testing. 

Bulk density measurements of the physical samples from each core were 

averaged to produce a mean bulk density. These values are reported in 

Table 3. Note that mean bulk density values obtained through wet-dry 

analysis were consistently higher than x-ray attenuation density values. 

The discrepancies between the two measurements were also found to be 
ERDC/CHL TR-20-13 18

greatest for cores in which mud contents ranged between 1% to 21% 

(Table 4; Figure 9). One possible explanation for this disparity in bulk

density would be the presence of air in the void space of the cores.

Table 4. Average bulk densities of cores obtained from both XRA and 

wet-dry methods. 

Wet-Dry Analysis 

(g/cm3) 

XRA 

(g/cm3) 

Mud 

(mass %) Kaolin 

Kaolin/ 

Bentonite MR Kaolin 

Kaolin/ 

Bentonite MR 

0 1.93 1.93 1.93 1.91 1.91 1.91

1 1.97 1.99 1.97 1.85 1.76 1.75

2 2.00 2.02 1.96 1.82 1.78 1.74

3 2.02 2.03 2.01 1.84 1.77 1.81

4 2.03 2.04 2.02 1.83 1.82 1.80

5 2.06 2.04 2.03 1.82 1.85 1.86

8 2.05 2.04 2.08 1.84 1.86 1.84

11 2.05 2.09 2.05 1.88 1.88 1.91

15 2.04 2.06 2.05 1.89 1.89 1.91

21 1.96 1.97 1.97 1.86 1.88 1.90

29 1.86 1.84 1.87 1.80 1.76 1.84

40 1.74 1.71 1.78 1.68 1.69 1.75

60 1.57 1.57 1.62 1.53 1.51 1.60

80 1.47 1.46 1.51 1.44 1.43 1.50

100 1.39 1.38 1.43 1.38 1.33 1.43
ERDC/CHL TR-20-13 19

Figure 9. Plots showing bulk density versus mud content for each core. Blue, red, and 

black lines indicate cores made with kaolinite (K), kaolinite-bentonite (KB), and MR 

muds, respectively. Curves with solid lines depict density values calculated by wetdry analysis. Curves with dashed lines represent density values obtained from XRA. 

As previously described, the mixing process for samples with mud 

contents ≤21% involved repeated folding and shearing of the sediment. 

These folding and shearing actions could have resulted in the addition of 

air into the mixture. The wet-dry method assumes a completely saturated 

sample and therefore does not account for any void space within the 

sediment occupied by gasses. In the event that significant amounts of gas 

were present, the total sample volume would be underestimated by the 

wet-dry method. Therefore, the mass measured from the water and 

sediment would be attributed to a partial volume of the total sample, and 

thus produce an elevated density. Cores which were exposed to limited 

folding and shearing mixing action (cores with >21% mud content and the 

100% sand cores) had similar density values for both wet-dry and XRA

methods. This observation supports the idea that the mixing process 

utilized with mud content ≤21% may have impacted the bulk density 

results by introducing air into the sediment matrix. 

Analysis of the mean density data revealed maximum densities occurred 

when mud contents were between 10% to 20% by mass. This pattern was 

consistent for all three mud varieties and both methods of bulk density 

calculation, as seen in Figure 9. The higher mean densities of the mixtures 

with a 10% – 20% mud content may reflect the filling of the sand pore 

spaces with mud.
ERDC/CHL TR-20-13 20

3.1.3 Grain size 

Grain size distribution analyses were performed on the samples collected 

during the erosion testing of each core. Results of each sample were 

averaged to produce a mean grain size distribution for each core. Table 5

presents the sand, silt, and clay content, along with the D10, D50, and D90 of 

each kaolinite-bentonite mixture. Additional tables and figures presenting 

these results along with mean grain size distributions of all the sediment 

mixtures are found in Appendix A.

Mean LDPSA measured mud (silt % + clay %) content of each test core was 

plotted against the mass percentage of mud used in compositing the core 

material to evaluate the correlation between the two mud contents 

(Figure 10). While the majority of data in Figure 10 plotted below the 1:1 

line, indicating that LDPSA commonly reported a lower mud content than 

the mass percentage, a strong correlation (r2=0.94) was observed between 

the two mud contents. However, note that 100% mud was assumed for the 

kaolin, bentonite, and MR muds during the compositing process. Laser 

diffraction analysis reported these materials to have lower mud contents of 

94%, 95%, and 97%, respectively (Table 1). This fact helps explain the 

lower mud content commonly observed in the laser diffraction results. 

Additionally, winnowing of the fines during the erosion process and the 

formation of aggregates could also have contributed to a bias towards 

coarser materials in the diffraction data. 

Notes taken during erosion testing periodically described “clouds” of 

sediment coming off the core surface when little bed erosion was observed. 

These clouds were likely composed of fine silts and clays and allowed for 

finer sediment to be removed from the bed while coarser sand material 

remained. Since grain size distribution samples were collected after 

completion of a series of erosion tests, it is possible that the sampled core 

surface had been partially winnowed of fine material. Additionally, 

aggregate particles were visually observed during erosion testing in cores 

with mud content as low as 2% – 3% by mass. These aggregates were 

observed to erode by being plucked from the sediment surface as opposed 

to the transport of individual sand grains. As mud content increased, 

aggregates of mud were more commonly observed and became the 

dominant form of particle erosion. Although sediment samples were 

immersed in deflocculant and sonified prior to grain size analysis, it is 

possible that some sediments were not completely disaggregated prior to 

testing. Therefore, any aggregated particles that may have passed to the 
ERDC/CHL TR-20-13 21

laser diffraction instrument could potentially skew the results towards 

coarser size distributions. 

Table 5. Kaolinite-bentonite mud laser diffraction data. 

Sample % Sand % Silt % Clay 

D10 

(μm) 

D50 

(μm) 

D90 

(μm) 

KB001 99.9 0.1 0 100.5 162.0 261.4

KB002 99.9 0.1 0 99.4 162.2 265.2

KB003 99.9 0.1 0 98.0 159.9 260.4

KB004 99.7 0.2 0.2 96.2 160.1 265.6

KB005 98.7 1.0 0.3 93.4 157.2 260.8

KB005R 94.8 3.6 1.6 91.3 157.1 250.2

KB008 94.8 3.6 1.6 91.2 157.1 250.6

KB008R 90.1 6.9 3.1 63.5 153.7 258.5

KB011 93.5 4.5 2.0 88.7 158.4 254.5

KB011R 92.1 5.4 2.5 81.3 155.9 252.7

KB015 90.7 6.3 3.0 72.9 153.6 249.0

KB021 92.9 4.8 2.3 86.2 160.0 260.1

KB021R 77.5 16.1 6.4 6.3 132.4 244.8

KB029 81.4 13.3 5.3 8.0 138.7 244.8

KB040 81.8 13.0 5.3 8.2 147.6 261.6

KB060 57.0 31.4 11.5 3.6 94.5 248.0

KB080 42.6 41.8 15.6 3.0 23.7 234.9

KB080R 19.8 58.7 21.5 2.5 10.8 144.0

KB100 4.6 70.9 24.5 2.4 8.3 37.8
ERDC/CHL TR-20-13 22

Figure 10. Plot of mean mud content measured by laser diffraction against mud 

content by mass of each test core. Dashed blue line indicates least squares 

regression while the solid black line shows 1:1 correlation. 

3.2 Erosion tests

3.2.1 Erosion parameterization

Cohesive sediment erosion is sensitive to changes in grain size, bed 

density, mineralogy, gas content, organic content, and a host of other 

factors. These factors can change significantly at relatively small scales. 

Consequently, measured cohesive sediment erosion rates are notoriously 

variable. Multiple steps were taken to reduce these sources of variability 

in this study. As previously described, sediments in this study were 

prepared and placed in core tubes in an effort to ensure as much 

homogeneity throughout the core as possible. Additionally, results of the 

x-ray scans were utilized to ensure that erosion experiments were 

conducted within regions of the core that had constant density. Last, the 

erosion experiments were conducted in a manner to produce a large 

sample from which to derive statistically representative relationships for 

various numerical erosion algorithms. To ensure high quality in the data 

analysis, data and associated experimental notes were evaluated to 

identify outliers in the dataset. Outliers were rejected based on 

comparisons between adjacent data points and experiment log notes.

Erosion velocities showed consistency with shear stresses and did not 

indicate the presence of layers or heterogeneity within the cores. An 

example of the data collected during erosion experiments is presented in 
ERDC/CHL TR-20-13 23

Figure 11, which shows erosion rate data plotted against shear stress for 

the 21% kaolinite-bentonite mud mixtures on log-log scale (plots for each 

core can be found in Tables C-1 through C-4 and Figures C-1 through C-43 

in Appendix C). The circles indicate the mean erosion rate for each unique 

shear stress that was evaluated during the erosion testing. The error bars 

on each circle indicate the 95% confidence interval around those means. A 

multivariate least square fit was applied to the data providing the 

algorithm for parameterization of the erosion process in the form: 

EE = 0; (ττ < ττcc) (3)

EE = AAττnn; (ττcc < ττ) (4)

where E represents erosion rate (cm/s) from the bed, τ is bed shear stress, 

τc is critical stress for erosion, A is an empirical constant, and n is an 

empirical exponent. For this study, an erosion rate of 1.0E-04 cm/s was 

utilized to solve for τc. Tables 6 – 9 provide the erosion parameters τc , A, 

and n for each core. Additionally, these tables provide a low and high 

range for τc that was obtained from the 95% confidence intervals of the 

least square fit.

Figure 11. Plot of mean erosion rate against shear stress for cores composed of 21% 

kaolinite-bentonite mud. Regression lines fit to the data were used to determine τc 

at an erosion rate of 1.0E-04 cm/s. 
ERDC/CHL TR-20-13 24

Table 6. Erosion parameters for sand cores. 

Mud 

(% mass) τc 

τc 

(low) 

τc 

(high) A n 

0 0.17 0.15 0.19 1.64E-01 4.14

0R 0.16 0.15 0.18 2.89E-02 3.11

Table 7. Erosion parameters for kaolinite mud cores. 

Mud 

(% mass) τc 

τc 

(low) 

τc 

(high) A n 

1 0.17 0.14 0.18 1.12E-01 3.93

1R 0.15 0.14 0.17 2.14E-02 2.84

2 0.20 0.19 0.22 4.03E-02 3.76

3 0.15 0.16 0.18 7.56E-03 2.31

3R 0.20 0.19 0.21 6.57E-03 2.58

4 0.19 0.19 0.20 8.90E-03 2.73

5 0.30 0.28 0.32 4.79E-03 3.20

5R 0.28 0.26 0.29 2.48E-03 2.50

8 0.29 0.27 0.32 1.37E-03 2.13

11 0.34 0.31 0.36 7.83E-04 1.92

15 0.47 0.44 0.51 5.05E-04 2.17

15R 0.90 0.82 0.97 1.30E-04 2.40

21 0.36 0.32 0.39 6.38E-04 1.83

21R 0.83 0.74 0.92 1.55E-04 2.28

29 0.88 0.72 1.07 1.40E-04 2.67

40 0.76 0.64 0.91 1.91E-04 2.36

60 0.50 0.37 0.54 4.28E-04 2.08

80 0.54 0.49 0.60 4.44E-04 2.44

100 0.33 0.31 0.39 1.37E-03 2.35
ERDC/CHL TR-20-13 25

Table 8. Erosion parameters for kaolinite-bentonite mud cores. 

Mud 

(% mass) τc 

τc 

(low) 

τc 

(high) A n 

1 0.17 0.18 0.21 1.81E-02 3.13

2 0.19 0.18 0.20 7.90E-03 2.62

3 0.29 0.27 0.31 5.49E-03 3.20

4 0.30 0.29 0.31 2.38E-03 2.64

5 0.49 0.47 0.52 1.21E-03 3.52

5R 0.56 0.53 0.61 5.63E-04 3.02

8 0.34 0.31 0.38 6.67E-04 1.78

8R 0.64 0.60 0.67 3.62E-04 2.83

11 0.78 0.60 0.94 1.52E-04 1.69

11R 0.90 0.82 0.97 1.30E-04 2.40

15 1.30 0.98 1.73 5.68E-05 2.16

21 1.63 1.29 2.07 2.70E-05 2.66

21R 1.51 1.32 1.71 3.34E-05 2.68

29 1.80 1.52 2.12 1.76E-05 2.96

40 1.56 1.20 2.01 2.34E-05 3.27

60 1.52 1.28 1.83 2.25E-05 3.54

80 1.17 0.98 1.39 5.76E-05 3.57

80R 1.13 1.00 1.27 6.13E-05 4.12

100 1.40 1.14 1.72 2.80E-05 3.75

 
ERDC/CHL TR-20-13 26

Table 9. Erosion parameters for MR mud cores. 

Mud 

(% mass) τc 

τc 

(low) 

τc 

(high) A n 

1 0.18 0.17 0.20 1.55E-02 3.00

2 0.22 0.21 0.24 8.58E-03 2.96

3 0.22 0.21 0.23 6.49E-03 2.78

3R 0.25 0.23 0.26 4.17E-03 2.67

4 0.26 0.25 0.28 5.75E-03 3.03

5 0.25 0.23 0.27 4.73E-03 2.80

8 0.54 0.49 0.58 6.52E-04 3.00

11 0.31 0.23 0.37 9.54E-04 1.91

11R 0.77 0.68 0.86 1.93E-04 2.56

15 0.55 0.51 0.58 4.27E-04 2.41

21 1.05 0.84 1.31 9.03E-05 2.14

21R 1.47 1.23 1.76 3.45E-05 2.75

29 0.99 0.85 1.14 1.01E-04 2.31

29R 1.43 1.05 1.93 3.73E-05 2.77

40 2.02 1.78 2.29 9.58E-06 3.34

60 1.98 1.73 2.26 1.05E-05 3.31

80 1.84 1.46 2.51 1.50E-05 3.12

100 1.94 1.60 2.36 5.44E-06 4.40

100R 1.36 1.18 1.56 3.55E-05 3.37
ERDC/CHL TR-20-13 27

Data from the erosion testing revealed that the τc of the sand bed could be 

expected to range between 0.15 to 0.19 Pa. Using the D50 (159 μm) of the 

test sand, the Shields curve (Shields 1936) estimates a τc of ~0.14 Pa, a 

value in close agreement with the ranges observed in this test. In general, 

it was observed that τc values tended to increase slightly as small amounts 

of mud were added to the sand bed. A two- to threefold increase in critical 

shear stress was observed by 8% mud content with τc values ranging from 

0.27 to 0.32 Pa, 0.3 to 0.67 Pa, and 0.49 to 0.58 Pa for the kaolin, kaolinbentonite, and MR mud mixtures, respectively. Some previous studies 

(Torfs et al. 2000; Barry et al. 2006) observed a reduction of critical shear 

stress associated with the addition of small amounts of mud to sand and 

attributed these reductions to a lubrication effect on the sand grains. Note

that such a trend was not observed in any of the mud mixtures tested in 

this study.

The results from the erosion experiments also demonstrated that τc values 

tended to approach a maximum when mud content reached 30% – 40% by 

mass (Figure 12). In both the kaolinite-bentonite and MR mud mixtures, 

this maximum value was approximately an order of magnitude greater 

than the τc of sand alone (1.8 Pa and 2.02 Pa, respectively) and remained 

fairly constant as mud content increased to 100%. By comparison, the 

kaolinite mud cores showed an approximate fivefold increase in τc (0.88 

Pa). However, unlike the kaolinite-bentonite and MR muds, the kaolinite 

cores showed a decrease in τc after the maximum value was reached at 

30% mud content (Figure 12). 

These differences may be largely due to the mineralogical composition and 

properties of the muds. Swelling clays were present in both the kaolinitebentonite and MR mud mixtures. These mixtures were found to have higher 

PI values than the kaolinite (a non-swelling clay) mixtures at equal mud 

contents. As indicated in other research (Jacobs et al. 2011), higher PI 

values may suggest a more cohesive and erosion resistant sediment matrix. 

The greater cohesion due to the presence of the swelling clays would not 

only offer an explanation for the larger observed τc values but could also 

offer insight into the drop in τc in the non-swelling kaolin mixtures after a 

maximum was reached. As pore space within the sand bed was filled with 

mud and bridged the gaps in intergranular contact, the mud-sand mixtures 

may have reached a maximum τc that relied on both intergranular 

interaction and the cohesiveness of the mud. However, as the mud content 

increased, the τc of the sediment matrix may have become less dependent 
ERDC/CHL TR-20-13 28

upon intergranular interaction and more dependent on the cohesiveness of 

the mud. The more cohesive swelling clays with higher PI values may have 

allowed for the sediment matrix to maintain their same level of critical 

shear stress. Conversely, the less cohesive kaolin mud may have depended 

on intergranular interaction to reach the maximum τc. Therefore, as mud 

content increased beyond 30% – 40% and reduced the intergranular 

contact the τc of the sediment matrix began to fall, as seen in Figure 12. 

Similar observations relating erosion resistance and τc of sand-mud 

mixtures to the fraction of the particle matrix filled with mud have been 

reported in numerous other studies (e.g., Mitchener and Torfs 1996; van 

Ledden et al. 2004; Jacobs et al. 2011; Dickhudt et al. 2011; Wu et al. 2018).

Figure 12. Critical shear stress (τc) plotted against mud content. Blue, red, and black 

lines indicate cores made with kaolinite (K), kaolinite-bentonite (KB), and MR muds, 

respectively. Dashed lines represent the 95% confidence interval of τc. The τc 

obtained from replicate cores are plotted with triangles with error bars indicating the 

95% confidence. 

Cohesion also appeared to impact the variability in the erosion data 

collected. As seen in Figure 12, the variability observed in τc was often 

lower in sediment mixtures with mud contents ≤5%. Observations made 

during erosion testing tended to describe discrete particle erosion, typical 

of sandy, non-cohesive sediment. However, as mud content and 

cohesiveness of the sediment increased, the variability in the calculated τc

often became larger. It was also observed that the dominant mechanism of 

erosion transitioned from discrete particles to aggregates of varying size. 

This resulted in increased variability in the bed roughness. Increased bed 
ERDC/CHL TR-20-13 29

roughness would often produce localized scour and thus impact the 

erosion rates being calculated. As multiple erosion cycles were made for 

each core, results from each cycle and shear stress would often be 

dependent upon the bed roughness at the start of each cycle. Non-cohesive 

beds that displayed uniform bed roughness and erosion in the form of 

discrete particles produced more consistent results for each cycle. The 

more cohesive beds that displayed changes in bed roughness produced 

results that were more variable. 

3.2.2 Sediment erosion rates 

While results showed slight increases in τc for mixtures with small 

amounts of mud (1% – 5%), a larger impact was observed in the erosion 

rates of these sediments. The plots in Figures 13 – 15 show the change in 

erosion rate at various shear stresses as mud content increased in the 

cores. Solid lines indicate the resulting erosion rate from utilizing the 

erosion parameters obtained from the best fit line through the data

presented in Appendix C. The dashed lines indicate the 95% confidence 

interval of those erosion rates. When available, mean erosion rate data 

were plotted (circles) for each shear stress. Replicate data were plotted as 

triangles with error bars indicating the 95% confidence interval of the 

mean erosion rate. For all three varieties of mud, erosion rates reduced 

rapidly with the addition of small amounts of mud. As mud concentrations 

approached 5%, erosion rates diminished on the scale of 1 to 2 orders of 

magnitude for all three mud types tested in this study (Figure 13-15).

Mirroring the trends in critical shear stress (Figure 12), erosion rates were 

also minimized when mud content approaches 30% – 40% for all mud 

types. This information may prove useful for engineering purposes.

Recent changes in policy (via the Federal Standard; 33 CFR 335) have 

sought to increase the beneficial use of dredged materials and reduce 

disposal in Confined Disposal Facilities or Offshore Dredge Material 

Disposal Sites (ODMDSs) (EPA 2007). Therefore, the level of erosion 

rate reduction potentially realized using limited amounts of mud mixed 

with sand could have meaningful implications for planning and design of 

civil and environmental engineering projects where sediment 

stabilization is desired. Some examples include environmental capping of 

contaminated sediments, wetland and habitat restoration, and shoreline 

construction. The use of locally sourced sediment with adequate sandmud content to curtail erosion may provide a cost-effective solution for 

these types of projects. Alternatively, dredged sediments may also be 
ERDC/CHL TR-20-13 30

engineered for specific projects using amendments such as clays or claypolymer composites (e.g., AquaBlokTM) to enhance erosion resistance. 

Therefore, the results and insights gained from quantifying and 

predicting erosion rates will improve forecasts of project performance 

and life-cycle analysis, in addition to better predicting the erosion, 

transport, and fate of sediment mixtures. 

Figure 13. Erosion rate plotted against kaolinite mud content. The colored solid lines 

depict the calculated erosion rates at various shear stresses, with dashed lines 

indicating 95% confidence. Mean erosion rate data are plotted with circles. Triangles 

represent replicate mean erosion rates with error bars indicating the 95% confidence. 
ERDC/CHL TR-20-13 31

Figure 14. Erosion rate plotted against kaolinite-bentonite mud content. The colored 

solid lines depict the calculated erosion rates at various shear stresses, with 

dashed lines indicating 95% confidence. Mean erosion rate data are plotted with 

circles. Triangles represent replicate mean erosion rates with error bars indicating 

the 95% confidence. 

Figure 15. Erosion rate plotted against MR mud content. The colored solid lines depict 

the calculated erosion rates at various shear stresses, with dashed lines indicating 

95% confidence. Mean erosion rate data are plotted with circles. Triangles represent 

replicate mean erosion rates with error bars indicating the 95% confidence. 
ERDC/CHL TR-20-13 32

4 Summary and Conclusions 

To gain insight into the relationship between mud content, critical shear 

stress, and erosion rate, erosion experiments were conducted on sandmud mixtures with varying mud content utilizing three different muds: (1) 

a non-swelling clay (kaolinite), (2) an 80/20 mixture of kaolinite with a

swelling clay (sodium bentonite), (3) a natural mud containing swelling 

and non-swelling clays from the MR. Mixed sediments were prepared in 

the laboratory by mixing varying fractions of mud with fine quartz sand 

and placing the material in 10 cm diameter cores. 

Erosion experiments were conducted with Sedflume (McNeil et al. 1996). 

Results of the testing performed on the non-cohesive (100% sand) cores 

yielded τc values that agreed closely with values predicted by the shields 

curve. As small amounts of mud were introduced to the sand, τc values 

increased. This trend was constant for all three mud varieties. Typically, a 

two- to threefold increase in critical shear stress was observed when mud 

content reached 8%. A maximum critical shear stress was obtained when 

mud content reached 30% – 40%. In instances where the mud contained 

swelling clays (bentonite-kaolinite and MR), the maximum τc was 

approximately an order of magnitude greater than that of fine sand. By 

comparison, maximum τc values in the kaolinite clay were approximately 

five times greater than τc of fine sand.

Atterberg limit tests showed that the kaolinite-bentonite and MR mud 

mixtures had higher PI values than kaolinite mixtures with equivalent 

mud content. The direct correlation observed between PI and τc supports 

previous research, indicating that PI values can be used as an indication of 

the cohesiveness of a material (Jacobs et al. 2011). Data in these tests also 

showed that as cohesion of the test material increased, variability in the 

erosion data also increased. This increase in variability was attributed to 

bed aggregate erosion that was observed in more cohesive sediments. This 

mechanism of erosion resulted in variable bed roughness throughout a test 

sequence. As bed roughness increased, local scour was often observed, 

which impacted erosion rates. 

While τc was shown to increase slightly as small amounts of mud (1% –

5%) were added to sand, a larger impact was observed in the erosion rates 

of these mixtures. Compared to the erosion rate of fine sand, a ten- to one 

hundredfold fold decrease in erosion rate was observed as mud content 
ERDC/CHL TR-20-13 33

approached 5%, and minimized as mud content approached 30% – 40%, 

for each of the mud varieties tested in this study. These data suggest that 

dredged sand-mud mixtures may be a viable, cost-effective solution for use 

to erosion abatement and capping projects in ports, harbors, and 

waterways across the country. 
ERDC/CHL TR-20-13 34

5 Future Work

While three varieties of clay materials were evaluated in this study, the 

sand matrix with which those clays were mixed was a constant, 

well-sorted, fine sand. Previous works evaluating the impact of clay 

content on the erodibility of sediment mixtures (e.g., Barry et al. 2006) 

utilized coarser grained sands. To allow for a more direct comparison of 

results with previous work such as Barry et al. (2006) and to assess 

sediment mixtures that may be more representative of materials 

commonly observed in natural systems, future experiments should utilize 

a more poorly sorted (well-graded) sand that would include coarser 

grained material. Additional research should focus on the swelling 

properties of various clay types at the sediment-water interface and its 

effect on erosion resistance. 

Additional research is being conducted in which the results of the present 

work could be relevant, which include the production and durability of 

mud aggregates*, the erosion resistance of the AquaBlok commercial 

product†, and the separation and fate of fine sediment during the dredging 

and placement process (Smith et al. 2019). 

First, observations made during erosion testing suggest that as clay 

content increased, the occurrence of aggregate erosion became more 

prevalent. These eroded aggregates were noted to range in size and 

frequency throughout the course of erosion experiments. Ongoing 

research is being performed that will evaluate the size, density, and 

settling velocity of the aggregates that were eroded during testing. This 

research hopes to provide insight into the clay content at which these 

aggregates form and if their size and concentration change with varying 

clay content and shear stress. This information can then be utilized in 

conjunction with the erosion rate data to help characterize how the 

mechanical process of erosion may vary with clay content. 

 

* Perkey, D. W., K. A. Fall, and S. J. Smith. In review. Physical Factors That Influence Muddy Bed 

Aggregate Production, Size, and Durability. ERDC Technical Report. Vicksburg, MS: U.S. Army Engineer 

Research and Development Center. 

† Priestas, A. M., C. F. Mackenzie, J. H. Hull, R. Ellis, and J. Z. Gailani. In review. Laboratory Evaluation of 

Aquablok® Erosion Resistance: Implications for Geotechnical Applications. ERDC Special Report. 

Vicksburg, MS: U.S. Army Engineer Research and Development Center.
ERDC/CHL TR-20-13 35

Erosion testing of AquaBlok is being performed to assess its performance 

under non-compacted and compacted states. These results of these tests can 

inform how AquaBlok may perform as an amendment to dredged material.

Finally, sediments dredged using hopper dredges with induced overflow 

tend to coarsen their load. This research attempts to predict the 

subsequent change in the fine sediment content of the hopper load. The 

results of the present study can be used to inform sediment transport 

models to determine the ultimate fate of these materials eroded from 

ODMDSs or other placement sites. 
ERDC/CHL TR-20-13 36

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ERDC/CHL TR-20-13 39

Appendix A: Grain Size Data

Table A-1. Sand laser diffraction data. 

Sample % Sand % Silt % Clay 

D10 

(μm) 

D50 

(μm) 

D90 

(μm) 

Sand 100.0 0.0 0 100.3 160.3 256.8

SandR 99.9 0.1 0 96.6 157.5 258.2

Figure A-1. Cumulative grain size distributions for sand. 

Figure A-2. Grain size distributions for sand. 
ERDC/CHL TR-20-13 40

Table A-2. Kaolinite mud laser diffraction data. 

Sample % Sand % Silt % Clay 

D10 

(μm) 

D50 

(μm) 

D90 

(μm) 

K001 99.9 0.1 0 98.5 160.0 259.4

K001R 99.9 0.1 0 99.8 162.5 264.3

K002 99.6 0.2 0.2 95.5 160.5 268.8

K003 98.1 1.1 0.8 95.3 158.3 258.5

K003R 98.3 0.8 0.9 100.3 165.5 269.4

K004 96.2 2.8 1.1 93.6 157.1 250.0

K005 94.6 3.8 1.6 90.4 156.4 249.9

K005R 94.8 3.5 1.7 92.2 161.4 261.5

K008 92.3 5.3 2.5 82.0 154.2 248.6

K011 92.2 5.3 2.5 80.8 153.0 248.0

K015 89.5 7.2 3.3 23.1 150.0 253.0

K015R 81.5 12.8 5.7 7.6 138.9 248.0

K021 82.3 12.2 5.4 8.2 138.0 242.5

K021R 79.0 14.4 6.5 6.3 134.5 241.8

K029 79.7 14.6 5.8 7.1 138.9 248.0

K040 65.7 24.6 9.7 4.1 111.4 242.0

K060 53.0 33.8 13.2 3.3 78.2 229.0

K080 40.1 43.2 16.7 2.9 20.0 232.1

K100 5.6 72.7 21.7 2.5 10.2 46.1
ERDC/CHL TR-20-13 41

Figure A-3. Cumulative grain size distributions for kaolinite cores with mud contents 

ranging from 1% to 5%. 

Figure A-4. Cumulative grain size distributions for kaolinite cores with mud contents 

ranging from 8% to 21%. 
ERDC/CHL TR-20-13 42

Figure A-5. Cumulative grain size distributions for kaolinite cores with mud contents 

ranging from 29% to 100%. 

Figure A-6. Grain size distributions for kaolinite cores with mud contents ranging 

from 1% to 5%. 
ERDC/CHL TR-20-13 43

Figure A-7. Grain size distributions for kaolinite cores with mud contents ranging from 

8% to 21%. 

Figure A-8. Grain size distributions for kaolinite cores with mud contents ranging from 

29% to 100%. 
ERDC/CHL TR-20-13 44

Table A-3. Kaolinite-bentonite mud laser diffraction data. 

Sample % Sand % Silt % Clay 

D10 

(μm) 

D50 

(μm) 

D90 

(μm) 

KB001 99.9 0.1 0 100.5 162.0 261.4

KB002 99.9 0.1 0 99.4 162.2 265.2

KB003 99.9 0.1 0 98.0 159.9 260.4

KB004 99.7 0.2 0.2 96.2 160.1 265.6

KB005 98.7 1.0 0.3 93.4 157.2 260.8

KB005R 94.8 3.6 1.6 91.3 157.1 250.2

KB008 94.8 3.6 1.6 91.2 157.1 250.6

KB008R 90.1 6.9 3.1 63.5 153.7 258.5

KB011 93.5 4.5 2.0 88.7 158.4 254.5

KB011R 92.1 5.4 2.5 81.3 155.9 252.7

KB015 90.7 6.3 3.0 72.9 153.6 249.0

KB021 92.9 4.8 2.3 86.2 160.0 260.1

KB021R 77.5 16.1 6.4 6.3 132.4 244.8

KB029 81.4 13.3 5.3 8.0 138.7 244.8

KB040 81.8 13.0 5.3 8.2 147.6 261.6

KB060 57.0 31.4 11.5 3.6 94.5 248.0

KB080 42.6 41.8 15.6 3.0 23.7 234.9

KB080R 19.8 58.7 21.5 2.5 10.8 144.0

KB100 4.6 70.9 24.5 2.4 8.3 37.8
ERDC/CHL TR-20-13 45

Figure A-9. Cumulative grain size distributions for kaolinite-bentonite cores with mud 

contents ranging from 1% to 5%. 

Figure A-10. Cumulative grain size distributions for kaolinite-bentonite cores with mud 

contents ranging from 8% to 21%. 
ERDC/CHL TR-20-13 46

Figure A-11. Cumulative grain size distributions for kaolinite-bentonite cores with mud 

contents ranging from 29% to 100%. 

Figure A-12. Grain size distributions for kaolinite-bentonite cores with mud contents 

ranging from 1% to 5%. 
ERDC/CHL TR-20-13 47

Figure A-13. Grain size distributions for kaolinite-bentonite cores with mud contents 

ranging from 8% to 21%. 

Figure A-14. Grain size distributions for kaolinite-bentonite cores with mud contents 

ranging from 29% to 100%. 
ERDC/CHL TR-20-13 48

Table A-4. MR mud laser diffraction data. 

Sample % Sand % Silt % Clay 

D10 

(μm) 

D50 

(μm) 

D90 

(μm) 

MR001 99.9 0.1 0 98.9 158.9 254.5

MR002 99.9 0.1 0 99.3 161.4 262.3

MR003 99.9 0.1 0 100.4 164.5 269.4

MR003R 98.0 1.6 0.4 95.3 159.3 259.2

MR004 95.9 3.1 0.9 96.4 163.7 262.2

MR005 96.3 2.8 0.9 95.9 162.5 261.0

MR008 93.6 4.7 1.7 92.8 164.8 262.8

MR011 88.9 8.6 2.5 20.9 150.3 246.5

MR011R 90.0 8.0 2.0 62.0 153.1 251.5

MR015 87.2 10.1 2.7 16.8 155.1 260.6

MR021 80.5 15.9 3.6 10.2 140.7 248.0

MR021R 77.3 18.5 4.2 8.8 137.6 248.4

MR029 66.3 28.0 5.7 6.1 118.5 246.1

MR029R 70.2 24.5 5.2 6.6 127.8 253.7

MR040 60.9 32.7 6.3 5.6 104.9 247.7

MR060 22.1 65.4 12.4 3.5 14.7 172.9

MR080 14.9 70.6 14.5 3.2 12.9 117.9

MR100 3.0 80.9 16.1 3.1 10.9 35.5

MR100R 2.6 82.6 14.8 3.2 11.2 34.5
ERDC/CHL TR-20-13 49

Figure A-15. Cumulative grain size distributions for MR cores with mud contents 

ranging from 1% to 5%. 

Figure A-16. Cumulative grain size distributions for MR cores with mud contents 

ranging from 8% to 21%. 
ERDC/CHL TR-20-13 50

Figure A-17. Cumulative grain size distributions for MR cores with mud contents 

ranging from 29% to 100%. 

Figure A-18. Grain size distributions for MR cores with mud contents ranging from 

1% to 5%. 
ERDC/CHL TR-20-13 51

Figure A-19. Grain size distributions for MR cores with mud contents ranging from 

8% to 21%. 

 

Figure A-20. Grain size distributions for MR cores with mud contents ranging from 

29% to 100%. 
ERDC/CHL TR-20-13 52

Appendix B: X-Ray Attenuation Bulk Density 

Plots

Figure B-1. Bulk densities obtained through x-ray attenuation are shown 

for sand cores. 

Figure B-2. Bulk densities obtained through x-ray attenuation are shown for kaolinite 

cores with mud contents ranging from 1% to 5%. 
ERDC/CHL TR-20-13 53

Figure B-3. Bulk densities obtained through x-ray attenuation are shown for kaolinite 

cores with mud contents ranging from 8% to 21%. 

Figure B-4. Bulk densities obtained through x-ray attenuation are shown for kaolinite 

cores with mud contents ranging from 29% to 100%. 
ERDC/CHL TR-20-13 54

Figure B-5. Bulk densities obtained through x-ray attenuation are shown for kaolinitebentonite cores with mud contents ranging from 1% to 5%. 

Figure B-6. Bulk densities obtained through x-ray attenuation are shown for kaolinitebentonite cores with mud contents ranging from 8% to 21%. 
ERDC/CHL TR-20-13 55

Figure B-7. Bulk densities obtained through x-ray attenuation are shown for kaolinitebentonite cores with mud contents ranging from 29% to 100%. 

Figure B-8. Bulk densities obtained through x-ray attenuation are shown for MR cores 

with mud contents ranging from 1% to 5%. 
ERDC/CHL TR-20-13 56

Figure B-9. Bulk densities obtained through x-ray attenuation are shown for MR cores 

with mud contents ranging from 8% to 21%. 

Figure B-10. Bulk densities obtained through x-ray attenuation are shown for MR 

cores with mud contents ranging from 29% to 100%. 
ERDC/CHL TR-20-13 57

Appendix C: Erosion Data

Table C-1. Sand erosion parameters. 

Mud 

(% mass) τc 

τc 

(low) 

τc 

(high) A n 

0 0.17 0.15 0.19 1.64E-01 4.14

0R 0.16 0.15 0.18 2.89E-02 3.11

Figure C-1. Plot of mean erosion rate data against shear stress for cores composed of 

100% sand. Regression lines fit to the data were used to determine τc at an erosion 

rate of 1.0E-04. 
ERDC/CHL TR-20-13 58

Table C-2. Kaolinite mud erosion parameters. 

Mud 

(% mass) τc 

τc 

(low) 

τc 

(high) A n 

1 0.17 0.14 0.18 1.12E-01 3.93

1R 0.15 0.14 0.17 2.14E-02 2.84

2 0.20 0.19 0.22 4.03E-02 3.76

3 0.15 0.16 0.18 7.56E-03 2.31

3R 0.20 0.19 0.21 6.57E-03 2.58

4 0.19 0.19 0.20 8.90E-03 2.73

5 0.30 0.28 0.32 4.79E-03 3.20

5R 0.28 0.26 0.29 2.48E-03 2.50

8 0.29 0.27 0.32 1.37E-03 2.13

11 0.34 0.31 0.36 7.83E-04 1.92

15 0.47 0.44 0.51 5.05E-04 2.17

15R 0.90 0.82 0.97 1.30E-04 2.40

21 0.36 0.32 0.39 6.38E-04 1.83

21R 0.83 0.74 0.92 1.55E-04 2.28

29 0.88 0.72 1.07 1.40E-04 2.67

40 0.76 0.64 0.91 1.91E-04 2.36

60 0.50 0.37 0.54 4.28E-04 2.08

80 0.54 0.49 0.60 4.44E-04 2.44

100 0.33 0.31 0.39 1.37E-03 2.35
ERDC/CHL TR-20-13 59

Figure C-2. Plot of mean erosion rate data against shear stress for cores composed of 

1% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-3. Plot of mean erosion rate data against shear stress for cores composed of 

2% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 60

Figure C-4. Plot of mean erosion rate data against shear stress for cores composed of 

3% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-5. Plot of mean erosion rate data against shear stress for cores composed of 

4% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 61

Figure C-6. Plot of mean erosion rate data against shear stress for cores composed of 

5% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-7. Plot of mean erosion rate data against shear stress for cores composed of 

8% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 62

Figure C-8. Plot of mean erosion rate data against shear stress for cores composed of 

11% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-9. Plot of mean erosion rate data against shear stress for cores composed of 

15% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 63

Figure C-10. Plot of mean erosion rate data against shear stress for cores composed 

of 21% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-11. Plot of mean erosion rate data against shear stress for cores composed 

of 29% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 64

Figure C-12. Plot of mean erosion rate data against shear stress for cores composed 

of 40% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-13. Plot of mean erosion rate data against shear stress for cores composed 

of 60% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 65

Figure C-14. Plot of mean erosion rate data against shear stress for cores composed 

of 80% kaolinite mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-15. Plot of mean erosion rate data against shear stress for cores composed 

of 100% kaolinite mud. Regression lines fit to the data were used to determine τc at 

an erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 66

Table C-3. Kaolinite-bentonite mud erosion parameters. 

Mud 

(% mass) τc 

τc 

(low) 

τc 

(high) A n 

1 0.17 0.18 0.21 1.81E-02 3.13

2 0.19 0.18 0.20 7.90E-03 2.62

3 0.29 0.27 0.31 5.49E-03 3.20

4 0.30 0.29 0.31 2.38E-03 2.64

5 0.49 0.47 0.52 1.21E-03 3.52

5R 0.56 0.53 0.61 5.63E-04 3.02

8 0.34 0.31 0.38 6.67E-04 1.78

8R 0.64 0.60 0.67 3.62E-04 2.83

11 0.78 0.60 0.94 1.52E-04 1.69

11R 0.90 0.82 0.97 1.30E-04 2.40

15 1.30 0.98 1.73 5.68E-05 2.16

21 1.63 1.29 2.07 2.70E-05 2.66

21R 1.51 1.32 1.71 3.34E-05 2.68

29 1.80 1.52 2.12 1.76E-05 2.96

40 1.56 1.20 2.01 2.34E-05 3.27

60 1.52 1.28 1.83 2.25E-05 3.54

80 1.17 0.98 1.39 5.76E-05 3.57

80R 1.13 1.00 1.27 6.13E-05 4.12

100 1.40 1.14 1.72 2.80E-05 3.75
ERDC/CHL TR-20-13 67

Figure C-16. Plot of mean erosion rate data against shear stress for cores composed 

of 1% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 

Figure C-17. Plot of mean erosion rate data against shear stress for cores composed 

of 2% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 68

Figure C-18. Plot of mean erosion rate data against shear stress for cores composed 

of 3% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 

Figure C-19. Plot of mean erosion rate data against shear stress for cores composed 

of 4% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 69

Figure C-20. Plot of mean erosion rate data against shear stress for cores composed 

of 5% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 

Figure C-21. Plot of mean erosion rate data against shear stress for cores composed 

of 8% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 70

Figure C-22. Plot of mean erosion rate data against shear stress for cores composed 

of 11% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 

Figure C-23. Plot of mean erosion rate data against shear stress for cores composed 

of 15% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 71

Figure C-24. Plot of mean erosion rate data against shear stress for cores composed 

of 21% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 

Figure C-25. Plot of mean erosion rate data against shear stress for cores composed 

of 29% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 72

Figure C-26. Plot of mean erosion rate data against shear stress for cores composed 

of 40% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 

Figure C-27. Plot of mean erosion rate data against shear stress for cores composed 

of 60% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 73

Figure C-28. Plot of mean erosion rate data against shear stress for cores composed 

of 80% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 

Figure C-29. Plot of mean erosion rate data against shear stress for cores composed 

of 100% kaolinite-bentonite mud. Regression lines fit to the data were used to 

determine τc at an erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 74

Table C-4. MR mud erosion parameters. 

Mud 

(% mass) τc 

τc 

(low) 

τc 

(high) A n 

1 0.18 0.17 0.20 1.55E-02 3.00

2 0.22 0.21 0.24 8.58E-03 2.96

3 0.22 0.21 0.23 6.49E-03 2.78

3R 0.25 0.23 0.26 4.17E-03 2.67

4 0.26 0.25 0.28 5.75E-03 3.03

5 0.25 0.23 0.27 4.73E-03 2.80

8 0.54 0.49 0.58 6.52E-04 3.00

11 0.31 0.23 0.37 9.54E-04 1.91

11R 0.77 0.68 0.86 1.93E-04 2.56

15 0.55 0.51 0.58 4.27E-04 2.41

21 1.05 0.84 1.31 9.03E-05 2.14

21R 1.47 1.23 1.76 3.45E-05 2.75

29 0.99 0.85 1.14 1.01E-04 2.31

29R 1.43 1.05 1.93 3.73E-05 2.77

40 2.02 1.78 2.29 9.58E-06 3.34

60 1.98 1.73 2.26 1.05E-05 3.31

80 1.84 1.46 2.51 1.50E-05 3.12

100 1.94 1.60 2.36 5.44E-06 4.40

100R 1.36 1.18 1.56 3.55E-05 3.37
ERDC/CHL TR-20-13 75

Figure C-30. Plot of mean erosion rate data against shear stress for cores composed 

of 1% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-31. Plot of mean erosion rate data against shear stress for cores composed 

of 2% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 76

Figure C-32. Plot of mean erosion rate data against shear stress for cores composed 

of 3% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure 33. Plot of mean erosion rate data against shear stress for cores composed of 

4% MR mud. Regression lines fit to the data were used to determine τc at an erosion 

rate of 1.0E-04. 
ERDC/CHL TR-20-13 77

Figure C-34. Plot of mean erosion rate data against shear stress for cores composed 

of 5% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-35. Plot of mean erosion rate data against shear stress for cores composed 

of 8% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 78

Figure C-36. Plot of mean erosion rate data against shear stress for cores composed 

of 11% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-37. Plot of mean erosion rate data against shear stress for cores composed 

of 15% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 79

Figure C-38. Plot of mean erosion rate data against shear stress for cores composed 

of 21% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-39. Plot of mean erosion rate data against shear stress for cores composed 

of 29% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 80

Figure C-40. Plot of mean erosion rate data against shear stress for cores composed 

of 40% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-41. Plot of mean erosion rate data against shear stress for cores composed 

of 60% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 81

Figure C-42. Plot of mean erosion rate data against shear stress for cores composed 

of 80% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 

Figure C-43. Plot of mean erosion rate data against shear stress for cores composed 

of 100% MR mud. Regression lines fit to the data were used to determine τc at an 

erosion rate of 1.0E-04. 
ERDC/CHL TR-20-13 82

Acronyms and Abbreviations

EPK Edgar Plastic Kaolin 

LDPSA Laser Diffraction Particle Size Analysis 

LL liquid limit 

MR Mississippi River 

ODMDS Offshore Dredge Material Disposal Site

PI plasticity index 

PL plastic limit

SED Sediment Erosion with Depth

USACE U.S. Army Corps of Engineers

XRA X-Ray Attenuation 
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1. REPORT DATE 

July 2020

2. REPORT TYPE

Final Report

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Erosion Thresholds and Rates for Sand-Mud Mixtures

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

David W. Perkey, S. Jarrell Smith, and Anthony M. Priestas

5d. PROJECT NUMBER

485304

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) (see reverse)

Coastal and Hydraulics Laboratory

U.S. Army Engineer Research and Development Center

3909 Halls Ferry Road

Vicksburg, MS 39180-6199

8. PERFORMING

ORGANIZATION REPORT

NUMBER

ERDC/CHL TR-20-13

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

Dredging Operations and Environmental Research Program

Vicksburg, MS 39180

10. SPONSOR/MONITOR'S ACRONYM(S)

DOER

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REPORT NUMBER(S)

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Approved for public release; distribution is unlimited.

13. SUPPLEMENTARY NOTES

14. ABSTRACT

Differences in erosion behavior of non-cohesive and cohesive sediments are widely recognized. In many natural environments, sand and 

mud are not completely separated and occur as mixtures. Significantly less research has been conducted on the erosion behavior of sandmud mixtures compared to the separate treatment of sand and mud erosion. Sedflume erosion experiments were conducted on sand-mud 

mixtures with varying mud content to define the relationships between mud content, critical stress for erosion (c), and erosion rate. 

Sand-mud mixtures were prepared with three mud sources: (1) non-swelling clay (kaolinite), (2) swelling clay (kaolinite/bentonite), and 

(3) a swelling, natural mud from the Mississippi River.

Test results showed that critical shear stresses of the mixed sediments departed from that of pure sand with mud fractions on the order of 

2% to 10%. Peak τc was observed between 30% to 40% mud content, with swelling muds achieving a ten-fold increase in τc while a fivefold increase in τc was measured for kaolinite. Additionally, this study demonstrated that the introduction of small amounts (≤5%) of 

mud to sand reduced erosion rates by a factor of 10 to 100. This observed abatement of erosion rate has implications for the use of 

dredged materials in civil and environmental engineering projects.

15. SUBJECT TERMS 

Dredging, Dredging spoil, Erosion, Mud, Sand

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 

ABSTRACT

SAR

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95

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a. REPORT David Perkey

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