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Effects of sediment bulk properties on erosion rates
The Science of the Total Environment 266 Ž2001. 41᎐48
Effects of sediment bulk properties on erosion rates Wilbert LickU , Joe McNeil Department of Mechanical and En¨ ironmental Engineering, Uni¨ ersity of California, Santa Barbara, CA 93106, USA Received 17 September 1999; accepted 28 June 2000
Abstract Considerable work has been done recently on the effects of sediment bulk properties on erosion rates. From this it is known that erosion rates depend on at least the following parameters: bulk density, average particle size, particle size distribution, mineralogy, organic content, volume of gas in the sediment, salinity of the pore waters, and time after deposition. This work is reviewed and discussed here with the purpose of presenting a quantitative overview of the effects of each of these parameters. This information is then used to demonstrate a procedure for estimating erosion rates of sediments based on a knowledge of their bulk properties. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Sediments; Erosion rates; Bulk properties
1. Introduction In order to accurately predict the transport of sediments and associated contaminants in aquatic systems, it is necessary to quantitatively measure and predict sediment erosion rates. These rates depend not only on hydrodynamic conditions Žthe applied shear stress due to currents and waves. but also on the bulk properties of the sediments.
U
Corresponding author. Tel.: q1-805-893-4295; fax: q1805-893-8651. E-mail address: [email protected] ŽW. Lick..
In a sediment bed, these properties vary in both the horizontal and vertical directions, and their variations can cause changes in the erosion rates by several orders of magnitude. The purpose of much of our recent laboratory and field work is to measure, quantitatively understand, and be able to predict the effects of sediment bulk properties on erosion rates. For this purpose, laboratory experiments with sediments with well-defined properties are essential. A unique flume, called Sedflume, has been developed at the University of California at Santa Barbara which is especially useful for this type of laboratory experiment, as well as for the determi-
0048-9697r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 0 . 0 0 7 4 7 - 6
42
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
nation of erosion rates of relatively undisturbed sediments from field cores. In the laboratory, Sedflume has been used to investigate the effects of various bulk properties on erosion rates of reconstructed, but otherwise natural, sediments. The emphasis has been on fine-grained, cohesive sediments and sediments which contain a significant amount of fine-grained sediments. In the present paper, this work is reviewed and discussed. The purpose is to present a quantitative overview of our present knowledge of the effects of bulk properties on erosion rates, especially their relative importance. The ultimate goal is to be able to predict erosion rates based on a knowledge of bulk properties. The effects of individual bulk properties will be discussed in the following section, while a qualitative procedure for predicting erosion rates based on bulk properties is given in the third section. A summary and concluding remarks are given in the final section.
2. Effects of individual properties Laboratory tests to determine erosion rates as a function of bulk properties were done by means
of Sedflume ŽFig. 1.. Sedflume is essentially a straight flume which has a test section with an open bottom through which a coring tube containing sediment can be inserted. The coring tube has a rectangular cross-section, 10 cm by 15 cm, and is generally 20᎐100 cm in length. As the sediment at the sediment᎐water interface is eroded by the flow through the test section, the sediments in the coring tube are moved upwards by the operator by means of a piston and jack inside the coring tube; this is done at such a rate that the sediment᎐water interface is level with the bottom of the test and inlet sections at all times. The shear stress at the sediment᎐water interface is a known function of the flow rate and can be varied by changing the flow rate. In this manner, erosion rates can be measured as a function of depth in the sediments with shear stress as a parameter. By means of Sedflume, erosion rates can be measured in the laboratory and in the field at high shear stresses Žup to stresses on the order of 20 Nrm2 . and with depth in the sediments Ždown to 1 m or more.. It has been used to measure erosion rates of relatively undisturbed sediments in the field, e.g. the Detroit River in Michigan,
Fig. 1. Schematic diagram of Sedflume.
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
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Table 1 Properties of reconstructed sediments Sediment
Mean particle diameter Žm.
Organic content Ž%.
Mineralogy
Quartz Fox River Detroit River Santa Barbara Grand River Long Beach Žseawater. Kaolinite Bentonite
5᎐1350 20 12 35 48 70
0 2.3, 4.1, 6.7 3.3 1.8 4.8 0, 0.25
Quartz Some clay Mica, no clay No clay No clay Some clay
0 0
Kaolinite Bentonite
4.5 5
the Lower Fox River in Wisconsin, Lake Michigan, Long Beach Harbor in California, a dump site offshore of New York Harbor and the Grand River in Michigan ŽMcNeil et al., 1996; Jepsen et al., 1997a, 1998, 2000.. These tests have illustrated the large differences in erosion rates Žby as much as several orders of magnitude. at different sites, with depth in the sediments, and as a function of shear stress. From these data, limiting cases for erosion rates of a coarse, sandy sediment and a fine-grained, cohesive sediment are evident as well as highly stratified sediments. From these field tests as well as from laboratory experiments, it has been determined that erosion rates depend on at least the following parameters: bulk density, average particle size, particle size distribution, mineralogy, organic content, volume of gas contained in the sediment, salinity of pore waters and time after deposition. Existing work on the effects of each of these parameters is described below.
of depth and the erosion rate as a function of depth and shear stress were measured. Bulk densities for a 40 cm core of Detroit River sediments are shown in Fig. 2. It can be seen that the densities increase with time and generally increase with depth. This is to be expected as the pore waters are forced upwards and out of the bottom sediments due to the weight of the overlying sediments. The increase of density with time is most rapid initially and then decreases as time increases and consolidation slows. It can also be seen that there are deviations from a monotonic increase of density with depth. These deviations tend to appear between 5 and 21 days and are due to increases in water content at a particular depth as the waters above this layer are transported upwards slower than the waters below
2.1. Bulk density Experiments were first done to investigate the effects of bulk density on the erosion rates of reconstructed Žwell-mixed., but otherwise natural, sediments from three locations: the Fox River, the Detroit River, and the Santa Barbara Slough ŽJepsen et al., 1997b.. Properties of these sediments are given in Table 1. For each of these sediments and for consolidation times varying from 1 to 60 days, the bulk density as a function
Fig. 2. Bulk density as a function of depth for consolidation times of 1, 2, 5, 12, 21, 32 and 60 days. Cores are 40 cm long. Sediments are from the Detroit River.
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W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
on the bulk properties of average particle size, particle size distribution, mineralogy, organic content, volume of gas, and salinity, as well as time after deposition. Erosion rates for the Detroit, Fox, and Santa Barbara sediments as a function of bulk density are compared in Fig. 4. For convenience, only erosion rates for a shear stress of 1.6 Nrm2 are shown. Results for other shear stresses are similar. It can be seen that the range of bulk densities for each sediment are quite different. 2.2. Particle size Fig. 3. Erosion rates as a function of bulk density with shear stress ŽNrm2 . as a parameter. Sediments are from the Detroit River. Solid lines are approximations by means of Eq. Ž1..
this layer. This trapping effect disappeared by 60 days in all cores. The general character of the dependence of bulk density on depth and time for the Fox and Santa Barbara sediments was the same as that for the Detroit sediments. For each sediment, erosion rates for shear stresses of 0.2, 0.4, 0.8, 1.6, 3.2 and 6.4 Nrm2 and for different consolidation times were determined as a function of depth. For each shear stress, these erosion rates were then related uniquely to the local bulk density. For the Detroit River sediments, the results are shown in Fig. 3. The rapid decrease of the erosion rate as the bulk density increases can be clearly seen. The data can be approximated by an equation of the form Es A n m
Laboratory experiments have been done to understand and quantify the individual and combined effects of bulk density and particle size on the erosion of quartz particles ŽRoberts et al., 1998.. These experiments were done with sediments with different mean sizes ranging from approximately 5 to 1350 m. The size distribution for each sediment was fairly narrow. Bulk densities ranged from approximately 1.65᎐1.95 grcm3. Typical results of these experiments are shown in Fig. 5 where erosion rates are plotted as a function of density for quartz particles from 5 to 1350 m in diameter, all at a shear stress of 1.6 Nrm2 . It can be seen that erosion rates are a very strong decreasing function of density for the finer particles and are essentially independent of density for the coarser particles. In addition, the
Ž1.
where E is the erosion rate Žcmrs., is the shear stress ŽNrm2 ., is the bulk density Žgrcm3 . and A, n and m are constants. This equation has been shown to be a valid approximation to almost all of our existing data for sediments with a wide range of properties in both laboratory and field experiments. Exceptions and the reason for these exceptions are noted below. Eq. Ž1. shows the effects of hydrodynamics Ždependence on . and consolidation Ždependence on where s Ž t . and t is time after deposition.. The parameters A, n and m are different for each different sediment and depend
Fig. 4. Erosion rates as a function of bulk density at a shear stress of 1.6 ŽNrm2 . for sediments from the Fox River, the Detroit River and the Santa Barbara Slough.
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
45
Fig. 5. Erosion rates as a function of bulk density for different uniform size quartz sediments at a shear stress of 1.6 ŽNrm2 .. Particle diameters in micrometers.
experimental results generally demonstrated that: Ž1. for the larger particles, the sediments behaved in a non-cohesive manner, i.e. they consolidated rapidly and the surface eroded particle by particle; Ž2. for the smaller particles, the sediments behaved in a cohesive manner, i.e. they consolidated relatively slowly and the surface eroded in chunks as well as particles; Ž3. critical stresses for erosion were strongly dependent on particle size and, for the smaller particles, were also strongly dependent on bulk density. 2.3. Particle size distribution A preliminary investigation of the effects of particle size distribution was made by mixing two size classes of quartz particles together ŽJesse Roberts, personal communication.. In particular, 5- and 75-m quartz particles were mixed together as follows: Ža. 100% 5.7 m, Žb. 90% 5.7 m and 10% 75 m, Žc. 50% 5.7 m and 50% 75 m, Žd. 10% 5.7 m and 90% 75 m and Že. 100% 75 m. For these mixtures, erosion rates as a func-
tion of bulk density and shear stress were then measured. From these experiments, it was shown that: Ž1. erosion rates for the mixtures are in between the erosion rates for the single size class sediments making up the mixture; and Ž2. erosion rates for mixtures are not proportional to the average size of the mixtures but are proportional to the logarithm of the average size of the mixture. For example, the 50r50 mixture has an average size of 42 m. At a bulk density of 1.85 grcm3, this mixture has an erosion rate of approximately 2.2= 10y3 cmrs while uniform size 42 m particles have an erosion rate of approximately 1 = 10y2 cmrs Žsee Fig. 5., i.e. the two differ by a factor of four to five. These experiments demonstrate that particle size distribution has a significant effect on erosion rates; however, the effects seem to be modest Žfactors of two to five. compared with the effects of other properties, which can affect erosion rates by orders of magnitude. These experiments were meant to give a preliminary estimate of the effects of particle size distribution on erosion rates, and much more needs
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W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
to be done before effects of mixtures of different size particles are understood and quantified. 2.4. Mineralogy It is well known that mineralogy has a significant effect on soil and sediment properties. However, quantitative information on erosion rates as a function of mineralogy is sparse. For this reason, preliminary experiments were done with two different clays, kaolinite and bentonite, and are reported here. For kaolinite, the sediments behaved in a similar manner to other fine-grained sediments. In particular, their erosion rates could be described by Eq. Ž1. ŽJesse Roberts, Personal Communication.. For bentonite, the behavior of the sediments was quite different from other sediments. The bentonite densities were generally quite low Žapprox. 1.1 grcm3 . and almost constant with time. When stirred, bentonite behaved initially as a very viscous liquid. It also eroded relatively easily. As time progressed, the bentonite Žwithout changing density. tended to form a relatively firm gel which was then much more difficult to erode. This gelled bentonite often failed catastrophically by fracture rather than by erosion of particles or chunks of particles. These experiments indicate that erosion rates for bentonite Žand presumably other fine-grained, cohesive sediments. are a function of time after deposition Žwith all other bulk properties remaining constant. or, more generally, the strain rate history of the sediments, i.e., the sediments are thixotropic. This phenomenon is well-known in soil mechanics Že.g. see Mitchell, 1993. but its effect on sediment erosion rates has not been investigated carefully. Preliminary results for kaolinite and bentonite are compared below with results for other sediments. 2.5. Organic content Preliminary experiments to investigate the effects of organic matter Žmeasured as organic carbon, or o.c.. on consolidation and erosion rates were done by heating both Fox River Ž6.7% o.c.. and Long Beach Ž0.25% o.c.. sediments at 500⬚C for 24 h to remove the organic matter. The o.c. of
Fig. 6. Erosion rates as a function of bulk density for Fox River sediments with different organic carbon contents at a shear stress of 1.6 ŽNrm2 ..
the Fox River sediments was reduced to 2.3% while the o.c. of the Long Beach sediments was reduced to essentially zero. In addition, Fox River sediments with high and low organic carbon were mixed in equal amounts so that a Fox River sediment with 4.1% o.c. was obtained. Measurements of erosion rates of these sediments were then made. Results are shown in Fig. 6. It can be seen that organic content has a significant effect on the initial density of the sediments. For example, the initial density of the Fox River sediments varies from approximately 1.3 grcm3 for sediments with 6.7% o.c. to 1.4 grcm3 for 4.1% o.c. to 1.6 grcm3 for 2.3% o.c. In addition, the slope of E as a function of changes so that E becomes almost independent of as the organic content becomes small, as for coarse, almost non-cohesive sediments. The effect of organic carbon on Long Beach sediments also is significant despite the fact that these only had 0.25% o.c. before treatment. 2.6. Gas Effects of gas on the erosion of sediments have been investigated ŽJepsen et al., 2000.. Erosion rates and bulk properties of relatively undisturbed and reconstructed sediments Žboth of which often contained considerable amounts of
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
gas. from the Grand River in Michigan were measured. The organic carbon contents of these sediments ranged from 1 to 5%, while gas volumes ranged from 1 to 10%. In laboratory consolidation studies, the density was a function of depth, time and temperature; it generally increased due to pore waters moving up and out of the solid᎐water matrix and decreased due to gas generation and movement. As the temperature increased, the rate of gas generation and the gas volume increased, and the bulk density decreased. For reconstructed sediments at approximately 20⬚C, the effects of gas were to decrease the sediment densities by up to 10%, to increase the erosion rates by as much as a factor of 60, and to decrease the critical shear stress for erosion by as much as a factor of 20 compared with sediments with no gas present. 2.7. Salinity Experiments are being done to investigate the effects of salinity on the erosion of fine-grained particles. These are being done with quartz particles with average diameters of 5, 15 and 48 m and with water salinities ranging from almost zero Žde-ionized water. to sea water Ža salt concentration of approx. 35 000 mgrl.. The differences in erosion rates between sediments in sea water and tap water were relatively small Žapprox. a factor of two. but were much larger for sediments in sea water and very pure waters Žan order of magnitude.. As with the bentonite, effects of gellation were present, i.e. small amounts of stirring or mixing of the sediments with high salinities decreased their strength and erosion rates but did not change the values of the other bulk properties. Experiments are continuing.
47
cept for Long Beach which was for experiments in seawater. and without gas. In Fig. 7, the dashed lines are for quartz particles of different sizes Žas in Fig. 5.. These sediments contain no organic carbon, and the mineralogy Žpure quartz. is well-defined. As such, these data serve as a base against which data from other sediments can be compared. In this way, the effects of other bulk properties on erosion rates can be understood. For example, consider the reconstructed sediments from the Fox River Žmean size of 20 m.. The effect of eliminating most of the organic carbon from these sediments is to increase the initial bulk density from approximately 1.3 grcm3 Ž6.7% o.c.. to 1.6 grcm3 Ž2.3% o.c... Fox River sediments contain a small amount of clay ŽTable 1.. Eliminating this clay would also increase the bulk density. The result of this qualitative argument is that the erosion rates for the Fox River sediments, after taking into account the effects of organic carbon and clay content, are very similar to those for non-cohesive quartz particles. Sediments from the Detroit River have a mean size of 12 m and 3.3% o.c.; they contain no clay but do contain mica which is fine-grained and behaves similarly to a clay. By taking into account the effects of o.c. and mica, the erosion rates for
3. Erosion rates from bulk properties Existing data for erosion rates of reconstructed sediments tested by us are shown as a function of density at a shear stress of 1.6 Nrm2 in Fig. 7. Information on mean particle size, organic content and mineralogy are presented in Table 1. All these data are for sediments in fresh water Žex-
Fig. 7. Erosion rates as a function of bulk density for reconstructed sediments at a shear stress of 1.6 ŽNrm2 .. Dashed lines are for quartz particles of different sizes Ždiameters in micrometers..
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W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
Detroit sediments are similar to those for 12 m quartz. In a similar manner, the effects of bulk properties on the erosion rates of other sediments can be qualitatively considered. With additional data and understanding, it is hoped that erosion rates for all sediments can be collapsed into a family of curves such as that for quartz particles, i.e. EŽ, d ..
4. Summary and concluding remarks An overview of recent work on the erosion rates of a wide range of reconstructed, but otherwise natural, sediments has been given. Much of this work is preliminary but does lead to a reasonable procedure for estimating the effects of bulk properties on sediment erosion rates. Almost all sediments that have been tested by us can be described by Eq. Ž1., i.e. erosion rates are a relatively simple and unique function of bulk density and shear stress. The parameters A, n and m that appear in Eq. Ž1. are generally functions of the mean particle size, particle size distribution, mineralogy, organic content, gas volume and salinity. For bentonite, 5 m quartz, and probably other fine-grained, very cohesive sediments, the effects of gellation on erosion rates are significant and need to be considered. For these sediments, the parameters A, n and m are dependent on time after deposition and, more generally, on the strain rate history of the sediments. As with soils, this is a complex process and needs further investigation. Another difficult problem is the effects and determination of the mineralogy. Erosion rates can be readily determined for each of the clays commonly found in sediments, and also for mixtures of these clays. However, in applications to real sediments, a major difficulty is the accurate determination of the mineralogy of the sedi-
ments. This determination is complex and expensive, and may not be sufficiently accurate for the purpose of predicting erosion rates. The work reviewed above is a first step toward the prediction of erosion rates from bulk properties. Extensive investigations need to be done before this goal is accomplished. However, despite this, our knowledge at present is sufficient to qualitatively predict erosion rates from bulk properties and, more quantitatively, to interpolate erosion rates of a sediment from the erosion rates of two or more sediments where the bulk properties are known and where Sedflume has been used to measure erosion rates.
Acknowledgements This research was supported by the US Environmental Protection Agency and the US Army Corps of Engineers. References Jepsen R, Roberts J, Lick W. Long Beach Harbor Sediment Study, Report. Los Angeles District: US Army Corps of Engineers, 1997a. Jepsen R, Roberts J, Lick W. Effects of bulk density on sediment erosion rates. Water Air Soil Pollut 1997b;99:21᎐31. Jepsen R, Roberts J, Gotthard D, Lick W. New York Sediment Study, Report. University of California, Santa Barbara, CA: Department of Mechanical and Environmental Engineering, 1998. Jepsen R, McNeil J, Lick, W. Effects of gas generation on the density and erosion of sediments from the Grand River. J Great Lakes Res 2000; in press. McNeil J, Taylor C, Lick W. Measurements of erosion of undisturbed bottom sediments with depth. J Hydraul Eng 1996;122:316᎐324. Mitchell JK. Fundamentals of soil behavior. New York: Wiley, 1993. Roberts J, Jepsen R, Gotthard D, Lick W. Effects of particle size and bulk density on erosion of quartz particles. J Hydraul Eng 1998;124:1261᎐1267.
Effects of sediment bulk properties on erosion rates Wilbert LickU , Joe McNeil Department of Mechanical and En¨ ironmental Engineering, Uni¨ ersity of California, Santa Barbara, CA 93106, USA Received 17 September 1999; accepted 28 June 2000
Abstract Considerable work has been done recently on the effects of sediment bulk properties on erosion rates. From this it is known that erosion rates depend on at least the following parameters: bulk density, average particle size, particle size distribution, mineralogy, organic content, volume of gas in the sediment, salinity of the pore waters, and time after deposition. This work is reviewed and discussed here with the purpose of presenting a quantitative overview of the effects of each of these parameters. This information is then used to demonstrate a procedure for estimating erosion rates of sediments based on a knowledge of their bulk properties. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Sediments; Erosion rates; Bulk properties
1. Introduction In order to accurately predict the transport of sediments and associated contaminants in aquatic systems, it is necessary to quantitatively measure and predict sediment erosion rates. These rates depend not only on hydrodynamic conditions Žthe applied shear stress due to currents and waves. but also on the bulk properties of the sediments.
U
Corresponding author. Tel.: q1-805-893-4295; fax: q1805-893-8651. E-mail address: [email protected] ŽW. Lick..
In a sediment bed, these properties vary in both the horizontal and vertical directions, and their variations can cause changes in the erosion rates by several orders of magnitude. The purpose of much of our recent laboratory and field work is to measure, quantitatively understand, and be able to predict the effects of sediment bulk properties on erosion rates. For this purpose, laboratory experiments with sediments with well-defined properties are essential. A unique flume, called Sedflume, has been developed at the University of California at Santa Barbara which is especially useful for this type of laboratory experiment, as well as for the determi-
0048-9697r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 0 . 0 0 7 4 7 - 6
42
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
nation of erosion rates of relatively undisturbed sediments from field cores. In the laboratory, Sedflume has been used to investigate the effects of various bulk properties on erosion rates of reconstructed, but otherwise natural, sediments. The emphasis has been on fine-grained, cohesive sediments and sediments which contain a significant amount of fine-grained sediments. In the present paper, this work is reviewed and discussed. The purpose is to present a quantitative overview of our present knowledge of the effects of bulk properties on erosion rates, especially their relative importance. The ultimate goal is to be able to predict erosion rates based on a knowledge of bulk properties. The effects of individual bulk properties will be discussed in the following section, while a qualitative procedure for predicting erosion rates based on bulk properties is given in the third section. A summary and concluding remarks are given in the final section.
2. Effects of individual properties Laboratory tests to determine erosion rates as a function of bulk properties were done by means
of Sedflume ŽFig. 1.. Sedflume is essentially a straight flume which has a test section with an open bottom through which a coring tube containing sediment can be inserted. The coring tube has a rectangular cross-section, 10 cm by 15 cm, and is generally 20᎐100 cm in length. As the sediment at the sediment᎐water interface is eroded by the flow through the test section, the sediments in the coring tube are moved upwards by the operator by means of a piston and jack inside the coring tube; this is done at such a rate that the sediment᎐water interface is level with the bottom of the test and inlet sections at all times. The shear stress at the sediment᎐water interface is a known function of the flow rate and can be varied by changing the flow rate. In this manner, erosion rates can be measured as a function of depth in the sediments with shear stress as a parameter. By means of Sedflume, erosion rates can be measured in the laboratory and in the field at high shear stresses Žup to stresses on the order of 20 Nrm2 . and with depth in the sediments Ždown to 1 m or more.. It has been used to measure erosion rates of relatively undisturbed sediments in the field, e.g. the Detroit River in Michigan,
Fig. 1. Schematic diagram of Sedflume.
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
43
Table 1 Properties of reconstructed sediments Sediment
Mean particle diameter Žm.
Organic content Ž%.
Mineralogy
Quartz Fox River Detroit River Santa Barbara Grand River Long Beach Žseawater. Kaolinite Bentonite
5᎐1350 20 12 35 48 70
0 2.3, 4.1, 6.7 3.3 1.8 4.8 0, 0.25
Quartz Some clay Mica, no clay No clay No clay Some clay
0 0
Kaolinite Bentonite
4.5 5
the Lower Fox River in Wisconsin, Lake Michigan, Long Beach Harbor in California, a dump site offshore of New York Harbor and the Grand River in Michigan ŽMcNeil et al., 1996; Jepsen et al., 1997a, 1998, 2000.. These tests have illustrated the large differences in erosion rates Žby as much as several orders of magnitude. at different sites, with depth in the sediments, and as a function of shear stress. From these data, limiting cases for erosion rates of a coarse, sandy sediment and a fine-grained, cohesive sediment are evident as well as highly stratified sediments. From these field tests as well as from laboratory experiments, it has been determined that erosion rates depend on at least the following parameters: bulk density, average particle size, particle size distribution, mineralogy, organic content, volume of gas contained in the sediment, salinity of pore waters and time after deposition. Existing work on the effects of each of these parameters is described below.
of depth and the erosion rate as a function of depth and shear stress were measured. Bulk densities for a 40 cm core of Detroit River sediments are shown in Fig. 2. It can be seen that the densities increase with time and generally increase with depth. This is to be expected as the pore waters are forced upwards and out of the bottom sediments due to the weight of the overlying sediments. The increase of density with time is most rapid initially and then decreases as time increases and consolidation slows. It can also be seen that there are deviations from a monotonic increase of density with depth. These deviations tend to appear between 5 and 21 days and are due to increases in water content at a particular depth as the waters above this layer are transported upwards slower than the waters below
2.1. Bulk density Experiments were first done to investigate the effects of bulk density on the erosion rates of reconstructed Žwell-mixed., but otherwise natural, sediments from three locations: the Fox River, the Detroit River, and the Santa Barbara Slough ŽJepsen et al., 1997b.. Properties of these sediments are given in Table 1. For each of these sediments and for consolidation times varying from 1 to 60 days, the bulk density as a function
Fig. 2. Bulk density as a function of depth for consolidation times of 1, 2, 5, 12, 21, 32 and 60 days. Cores are 40 cm long. Sediments are from the Detroit River.
44
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
on the bulk properties of average particle size, particle size distribution, mineralogy, organic content, volume of gas, and salinity, as well as time after deposition. Erosion rates for the Detroit, Fox, and Santa Barbara sediments as a function of bulk density are compared in Fig. 4. For convenience, only erosion rates for a shear stress of 1.6 Nrm2 are shown. Results for other shear stresses are similar. It can be seen that the range of bulk densities for each sediment are quite different. 2.2. Particle size Fig. 3. Erosion rates as a function of bulk density with shear stress ŽNrm2 . as a parameter. Sediments are from the Detroit River. Solid lines are approximations by means of Eq. Ž1..
this layer. This trapping effect disappeared by 60 days in all cores. The general character of the dependence of bulk density on depth and time for the Fox and Santa Barbara sediments was the same as that for the Detroit sediments. For each sediment, erosion rates for shear stresses of 0.2, 0.4, 0.8, 1.6, 3.2 and 6.4 Nrm2 and for different consolidation times were determined as a function of depth. For each shear stress, these erosion rates were then related uniquely to the local bulk density. For the Detroit River sediments, the results are shown in Fig. 3. The rapid decrease of the erosion rate as the bulk density increases can be clearly seen. The data can be approximated by an equation of the form Es A n m
Laboratory experiments have been done to understand and quantify the individual and combined effects of bulk density and particle size on the erosion of quartz particles ŽRoberts et al., 1998.. These experiments were done with sediments with different mean sizes ranging from approximately 5 to 1350 m. The size distribution for each sediment was fairly narrow. Bulk densities ranged from approximately 1.65᎐1.95 grcm3. Typical results of these experiments are shown in Fig. 5 where erosion rates are plotted as a function of density for quartz particles from 5 to 1350 m in diameter, all at a shear stress of 1.6 Nrm2 . It can be seen that erosion rates are a very strong decreasing function of density for the finer particles and are essentially independent of density for the coarser particles. In addition, the
Ž1.
where E is the erosion rate Žcmrs., is the shear stress ŽNrm2 ., is the bulk density Žgrcm3 . and A, n and m are constants. This equation has been shown to be a valid approximation to almost all of our existing data for sediments with a wide range of properties in both laboratory and field experiments. Exceptions and the reason for these exceptions are noted below. Eq. Ž1. shows the effects of hydrodynamics Ždependence on . and consolidation Ždependence on where s Ž t . and t is time after deposition.. The parameters A, n and m are different for each different sediment and depend
Fig. 4. Erosion rates as a function of bulk density at a shear stress of 1.6 ŽNrm2 . for sediments from the Fox River, the Detroit River and the Santa Barbara Slough.
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
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Fig. 5. Erosion rates as a function of bulk density for different uniform size quartz sediments at a shear stress of 1.6 ŽNrm2 .. Particle diameters in micrometers.
experimental results generally demonstrated that: Ž1. for the larger particles, the sediments behaved in a non-cohesive manner, i.e. they consolidated rapidly and the surface eroded particle by particle; Ž2. for the smaller particles, the sediments behaved in a cohesive manner, i.e. they consolidated relatively slowly and the surface eroded in chunks as well as particles; Ž3. critical stresses for erosion were strongly dependent on particle size and, for the smaller particles, were also strongly dependent on bulk density. 2.3. Particle size distribution A preliminary investigation of the effects of particle size distribution was made by mixing two size classes of quartz particles together ŽJesse Roberts, personal communication.. In particular, 5- and 75-m quartz particles were mixed together as follows: Ža. 100% 5.7 m, Žb. 90% 5.7 m and 10% 75 m, Žc. 50% 5.7 m and 50% 75 m, Žd. 10% 5.7 m and 90% 75 m and Že. 100% 75 m. For these mixtures, erosion rates as a func-
tion of bulk density and shear stress were then measured. From these experiments, it was shown that: Ž1. erosion rates for the mixtures are in between the erosion rates for the single size class sediments making up the mixture; and Ž2. erosion rates for mixtures are not proportional to the average size of the mixtures but are proportional to the logarithm of the average size of the mixture. For example, the 50r50 mixture has an average size of 42 m. At a bulk density of 1.85 grcm3, this mixture has an erosion rate of approximately 2.2= 10y3 cmrs while uniform size 42 m particles have an erosion rate of approximately 1 = 10y2 cmrs Žsee Fig. 5., i.e. the two differ by a factor of four to five. These experiments demonstrate that particle size distribution has a significant effect on erosion rates; however, the effects seem to be modest Žfactors of two to five. compared with the effects of other properties, which can affect erosion rates by orders of magnitude. These experiments were meant to give a preliminary estimate of the effects of particle size distribution on erosion rates, and much more needs
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W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
to be done before effects of mixtures of different size particles are understood and quantified. 2.4. Mineralogy It is well known that mineralogy has a significant effect on soil and sediment properties. However, quantitative information on erosion rates as a function of mineralogy is sparse. For this reason, preliminary experiments were done with two different clays, kaolinite and bentonite, and are reported here. For kaolinite, the sediments behaved in a similar manner to other fine-grained sediments. In particular, their erosion rates could be described by Eq. Ž1. ŽJesse Roberts, Personal Communication.. For bentonite, the behavior of the sediments was quite different from other sediments. The bentonite densities were generally quite low Žapprox. 1.1 grcm3 . and almost constant with time. When stirred, bentonite behaved initially as a very viscous liquid. It also eroded relatively easily. As time progressed, the bentonite Žwithout changing density. tended to form a relatively firm gel which was then much more difficult to erode. This gelled bentonite often failed catastrophically by fracture rather than by erosion of particles or chunks of particles. These experiments indicate that erosion rates for bentonite Žand presumably other fine-grained, cohesive sediments. are a function of time after deposition Žwith all other bulk properties remaining constant. or, more generally, the strain rate history of the sediments, i.e., the sediments are thixotropic. This phenomenon is well-known in soil mechanics Že.g. see Mitchell, 1993. but its effect on sediment erosion rates has not been investigated carefully. Preliminary results for kaolinite and bentonite are compared below with results for other sediments. 2.5. Organic content Preliminary experiments to investigate the effects of organic matter Žmeasured as organic carbon, or o.c.. on consolidation and erosion rates were done by heating both Fox River Ž6.7% o.c.. and Long Beach Ž0.25% o.c.. sediments at 500⬚C for 24 h to remove the organic matter. The o.c. of
Fig. 6. Erosion rates as a function of bulk density for Fox River sediments with different organic carbon contents at a shear stress of 1.6 ŽNrm2 ..
the Fox River sediments was reduced to 2.3% while the o.c. of the Long Beach sediments was reduced to essentially zero. In addition, Fox River sediments with high and low organic carbon were mixed in equal amounts so that a Fox River sediment with 4.1% o.c. was obtained. Measurements of erosion rates of these sediments were then made. Results are shown in Fig. 6. It can be seen that organic content has a significant effect on the initial density of the sediments. For example, the initial density of the Fox River sediments varies from approximately 1.3 grcm3 for sediments with 6.7% o.c. to 1.4 grcm3 for 4.1% o.c. to 1.6 grcm3 for 2.3% o.c. In addition, the slope of E as a function of changes so that E becomes almost independent of as the organic content becomes small, as for coarse, almost non-cohesive sediments. The effect of organic carbon on Long Beach sediments also is significant despite the fact that these only had 0.25% o.c. before treatment. 2.6. Gas Effects of gas on the erosion of sediments have been investigated ŽJepsen et al., 2000.. Erosion rates and bulk properties of relatively undisturbed and reconstructed sediments Žboth of which often contained considerable amounts of
W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
gas. from the Grand River in Michigan were measured. The organic carbon contents of these sediments ranged from 1 to 5%, while gas volumes ranged from 1 to 10%. In laboratory consolidation studies, the density was a function of depth, time and temperature; it generally increased due to pore waters moving up and out of the solid᎐water matrix and decreased due to gas generation and movement. As the temperature increased, the rate of gas generation and the gas volume increased, and the bulk density decreased. For reconstructed sediments at approximately 20⬚C, the effects of gas were to decrease the sediment densities by up to 10%, to increase the erosion rates by as much as a factor of 60, and to decrease the critical shear stress for erosion by as much as a factor of 20 compared with sediments with no gas present. 2.7. Salinity Experiments are being done to investigate the effects of salinity on the erosion of fine-grained particles. These are being done with quartz particles with average diameters of 5, 15 and 48 m and with water salinities ranging from almost zero Žde-ionized water. to sea water Ža salt concentration of approx. 35 000 mgrl.. The differences in erosion rates between sediments in sea water and tap water were relatively small Žapprox. a factor of two. but were much larger for sediments in sea water and very pure waters Žan order of magnitude.. As with the bentonite, effects of gellation were present, i.e. small amounts of stirring or mixing of the sediments with high salinities decreased their strength and erosion rates but did not change the values of the other bulk properties. Experiments are continuing.
47
cept for Long Beach which was for experiments in seawater. and without gas. In Fig. 7, the dashed lines are for quartz particles of different sizes Žas in Fig. 5.. These sediments contain no organic carbon, and the mineralogy Žpure quartz. is well-defined. As such, these data serve as a base against which data from other sediments can be compared. In this way, the effects of other bulk properties on erosion rates can be understood. For example, consider the reconstructed sediments from the Fox River Žmean size of 20 m.. The effect of eliminating most of the organic carbon from these sediments is to increase the initial bulk density from approximately 1.3 grcm3 Ž6.7% o.c.. to 1.6 grcm3 Ž2.3% o.c... Fox River sediments contain a small amount of clay ŽTable 1.. Eliminating this clay would also increase the bulk density. The result of this qualitative argument is that the erosion rates for the Fox River sediments, after taking into account the effects of organic carbon and clay content, are very similar to those for non-cohesive quartz particles. Sediments from the Detroit River have a mean size of 12 m and 3.3% o.c.; they contain no clay but do contain mica which is fine-grained and behaves similarly to a clay. By taking into account the effects of o.c. and mica, the erosion rates for
3. Erosion rates from bulk properties Existing data for erosion rates of reconstructed sediments tested by us are shown as a function of density at a shear stress of 1.6 Nrm2 in Fig. 7. Information on mean particle size, organic content and mineralogy are presented in Table 1. All these data are for sediments in fresh water Žex-
Fig. 7. Erosion rates as a function of bulk density for reconstructed sediments at a shear stress of 1.6 ŽNrm2 .. Dashed lines are for quartz particles of different sizes Ždiameters in micrometers..
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W. Lick, J. McNeil r The Science of the Total En¨ ironment 266 (2001) 41᎐48
Detroit sediments are similar to those for 12 m quartz. In a similar manner, the effects of bulk properties on the erosion rates of other sediments can be qualitatively considered. With additional data and understanding, it is hoped that erosion rates for all sediments can be collapsed into a family of curves such as that for quartz particles, i.e. EŽ, d ..
4. Summary and concluding remarks An overview of recent work on the erosion rates of a wide range of reconstructed, but otherwise natural, sediments has been given. Much of this work is preliminary but does lead to a reasonable procedure for estimating the effects of bulk properties on sediment erosion rates. Almost all sediments that have been tested by us can be described by Eq. Ž1., i.e. erosion rates are a relatively simple and unique function of bulk density and shear stress. The parameters A, n and m that appear in Eq. Ž1. are generally functions of the mean particle size, particle size distribution, mineralogy, organic content, gas volume and salinity. For bentonite, 5 m quartz, and probably other fine-grained, very cohesive sediments, the effects of gellation on erosion rates are significant and need to be considered. For these sediments, the parameters A, n and m are dependent on time after deposition and, more generally, on the strain rate history of the sediments. As with soils, this is a complex process and needs further investigation. Another difficult problem is the effects and determination of the mineralogy. Erosion rates can be readily determined for each of the clays commonly found in sediments, and also for mixtures of these clays. However, in applications to real sediments, a major difficulty is the accurate determination of the mineralogy of the sedi-
ments. This determination is complex and expensive, and may not be sufficiently accurate for the purpose of predicting erosion rates. The work reviewed above is a first step toward the prediction of erosion rates from bulk properties. Extensive investigations need to be done before this goal is accomplished. However, despite this, our knowledge at present is sufficient to qualitatively predict erosion rates from bulk properties and, more quantitatively, to interpolate erosion rates of a sediment from the erosion rates of two or more sediments where the bulk properties are known and where Sedflume has been used to measure erosion rates.
Acknowledgements This research was supported by the US Environmental Protection Agency and the US Army Corps of Engineers. References Jepsen R, Roberts J, Lick W. Long Beach Harbor Sediment Study, Report. Los Angeles District: US Army Corps of Engineers, 1997a. Jepsen R, Roberts J, Lick W. Effects of bulk density on sediment erosion rates. Water Air Soil Pollut 1997b;99:21᎐31. Jepsen R, Roberts J, Gotthard D, Lick W. New York Sediment Study, Report. University of California, Santa Barbara, CA: Department of Mechanical and Environmental Engineering, 1998. Jepsen R, McNeil J, Lick, W. Effects of gas generation on the density and erosion of sediments from the Grand River. J Great Lakes Res 2000; in press. McNeil J, Taylor C, Lick W. Measurements of erosion of undisturbed bottom sediments with depth. J Hydraul Eng 1996;122:316᎐324. Mitchell JK. Fundamentals of soil behavior. New York: Wiley, 1993. Roberts J, Jepsen R, Gotthard D, Lick W. Effects of particle size and bulk density on erosion of quartz particles. J Hydraul Eng 1998;124:1261᎐1267.