|UFDC Home||myUFDC Home | Help|
This item has the following downloads:
EFFECT OF SUSPENDED
FINE SEDIMENT ON EQUILIBRIUM LOCAL
ELIZABETH ANNE SMYRE
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
Elizabeth Anne Smyre
To my family and friends
I would like to extend my gratitude to Dr. Max Sheppard, my advisor and
supervisory committee chair, for his guidance and support on this project. I would also
like to thank Dr. Robert Thieke and Dr. Ashish Mehta for serving on my supervisory
Several people provided invaluable assistance in the collection of the data used in
this report. I would like to thank Tom Glasser for his work on the Massachusetts scour
tests. I am grateful to Ken Kerr for his guidance on the operation of the RETA; his
assistance was instrumental in designing the tests conducted on the device. I would also
like to acknowledge Vernon Sparkman of the Coastal & Oceanographic Engineering
Laboratory for his assistance in modifying the RETA. Additionally, my sincere
appreciation and thanks go to Dougal Clunie at the University of Auckland for
conducting additional scour tests; the data and assistance he provided were invaluable in
the preparation of this report.
Finally, I would like to thank my parents, family, and friends for their support and
guidance throughout my graduate experience. Their ability to listen, advise, and even to
make me laugh, helped me to realize my goals; I only hope that I can be as helpful to
them in life as they have been to me.
TABLE OF CONTENTS
A C K N O W L E D G M E N T S ................................................................................................. iv
LIST OF TA BLE S .......................... .... ............. .............. .. vii
L IST O F F IG U R E S .......................................................................... ..... viii
LIST OF SYMBOLS ......................................................... xi
1 IN TR OD U CTION .................. ............................. ....... ...... .............. .
D definition of Scour ............ ............................................... .. ...... .. ........... 1
T h e S cou r P process ................ ................................................... ............... 2
Current Local Scour Prediction Equations ........................... ................... .............. 3
P ro b lem D e scrip tio n ..................................................................................................... 10
2 B A C K G R O U N D ................................................................................. ................ .. 13
University of Florida Local Scour Tests: USGS-BRD Laboratory ........................... 13
Analysis of Suspended Fine Sediment Water Samples .............................................. 20
V id eo D ata ...................................... ............................ ......... .............. 2 2
C alculation of B ed Shear Stress.......................................................... .... ................ 23
Discussion of U SG S-BRD D ata ........................................................ .............. 24
U university of A uckland Scour Tests...................................... ........................ ......... 25
3 LITER A TU RE REV IEW ........................................... .................. ............... 27
4 TEST APPARATUS AND PROCEDURE ....................................... ............... 33
B ack g rou n d ...................... ............................................................. 3 3
U university of Florida Testing A apparatus ..................................................................... 34
C u rrent T estin g ..................... ............................... ....... .............. 3 5
M modifications to RETA ....................................... .............. 35
Procedure ............ .............................. ............... 36
5 TEST RESULTS AND OVERALL CONCLUSIONS ............. ............... 42
Results........................... ...... .......... 42
D iscu ssio n ..................................................... 4 5
Conclusions...................... ............... ...... .............. 46
6 FU TU R E R E SE A R C H ............................................................................ .............. 50
A ROTATING EROSION TEST APPARATUS (RETA) TEST DATA........................54
B UNIVERSITY OF AUCKLAND SCOUR TESTING ...............................................79
Introduction .......................................... 79
Facility ..................................... ............. .............. 79
Test Parameters ............... ...... ...................... .................. 79
P ro c e d u re .............................................................................. 8 0
Results................................. .............. 82
D isc u ssio n .............. ..... ............ ................. ........................... .............. 8 3
LIST OF REFERENCES .......................... ..................89
B IO G R A PH IC A L SK E T C H ........................................................................................ 92
LIST OF TABLES
2-1. Summary of USGS-BRD scour tests parameters. ................... ............................. 16
2-2. Test Param eters: Experim ents A B ....................................... ........................ 16
2-3. Suspended Fine Sediment Concentration Test Data................................................21
2-4. Shear stress calculation parameters. .......................................................................... 23
2-5. Typical flat bed shear stress values for the local scour tests performed at the USGS-
B R D L laboratory ........ ...................................................................... .... .... .... 24
LIST OF FIGURES
2-1. Aerial photo of USGS-BRD Laboratory, Turners Falls, Massachusetts ..................14
2-2. Schem atic of U SG S-BRD flum e. ............ ......................................... ................... 14
2-3. Plot illustrating the effect of suspended fine sediment on scour depth ...................17
2-4. Post-experiment photos of Experiment A and Experiment B .............................18
2-5. Contour plots of Experiment A and Experiment B ..................................................19
2-6. Concentration of suspended fine sediment measured during one of the USGS-BRD
sc o u r te sts ................................................................................................ ..... 2 1
4-1. University of Florida Rotating Erosion Test Apparatus (RETA)..............................35
4-2. Photograph of cylinders used in RETA tests.................. ...............37
5-1. Average shear stress lines generated from tests conducted on 0.15 mm sediment
coated acrylic cylinder. ...................... ......... .. .. ..... ...............43
5-2. Average shear stress lines generated from tests conducted on 0.85 mm sediment
coated acrylic cylinder. ...................... ......... .. .. ..... ...............44
5-3. Proposed relationship between shear stress and scour depth. ...................................49
A-1. RETA Tests 5-8, 0.15 mm Sediment Coated Acrylic Cylinder. ............................54
A-2. RETA Tests 13-16, 0.15 mm Sediment Coated Acrylic Cylinder. .........................55
A-3. RETA Tests 25-28, 0.15 mm Sediment Coated Acrylic Cylinder. ...........................56
A-4. RETA Tests 29-32, 0.15 mm Sediment Coated Acrylic Cylinder. .........................57
A-5. RETA Tests 37-40, 0.15 mm Sediment Coated Acrylic Cylinder. .........................58
A-6. All 0.0 g/1 concentration tests conducted on 0.15 mm Sediment Coated Acrylic
C y lin der........................................................................................ .. . ..... 59
A-7. All 0.05 g/1 concentration tests conducted on 0.15 mm Sediment Coated Acrylic
C ylinder....................................................... ................... ... .... .. .... 60
A-8. All 0.5 g/1 concentration tests conducted on 0.15 mm Sediment Coated Acrylic
C ylinder....................................................... ................... ... ... .. .... 6 1
A-9. All 1.0 g/1 concentration tests conducted on 0.15 mm Sediment Coated Acrylic
C y lin der........................................................................................ .. . ..... 62
A-10. RETA Tests 1-4, 0.85 mm Sediment Coated Acrylic Cylinder. ..........................63
A-11. RETA Tests 9-12, 0.85 mm Sediment Coated Acrylic Cylinder. .........................64
A-12. RETA Tests 17-20, 0.85 mm Sediment Coated Acrylic Cylinder. .......................65
A-13. RETA Tests 21-24, 0.85 mm Sediment Coated Acrylic Cylinder. .......................66
A-14. RETA Tests 33-36, 0.85 mm Sediment Coated Acrylic Cylinder. .......................67
A-15. All 0.0 g/1 concentration tests conducted on 0.85 mm Sediment Coated Acrylic
C ylinder....................................................... ................... ... .... .. .... 68
A-16. All 0.05 g/1 concentration tests conducted on 0.85 mm Sediment Coated Acrylic
C ylinder....................................................... ................... ... .... .. .... 69
A-17. All 0.5 g/1 concentration tests conducted on 0.85 mm Sediment Coated Acrylic
C ylinder....................................................... ................... ... ... .. .... 70
A-18. All 1.0 g/1 concentration tests conducted on 0.85 mm Sediment Coated Acrylic
C ylinder....................................................... ................... ... ... .. .... 7 1
A-19. RETA Tests 45-48, Smooth Aluminum Cylinder. .............................................72
A-20. RETA Tests 49-52, Smooth Aluminum Cylinder. .............................................73
A-21. RETA Tests 41-44, Rough Aluminum Cylinder...............................................74
A-22. RETA Tests 53-56, Rough Aluminum Cylinder ..................................................75
A-23. RETA Tests 57-60, saltwater tests conducted on 0.15 mm Sediment Coated
A cry lic C y lin d er .................................................................................. 7 6
A-24. RETA Tests 61-64, saltwater tests conducted on 0.85 mm Sediment Coated
A cry lic C y lin d er .................................................................................. 7 7
B-1. University of Auckland Scour Tests, 0.95 Uc. .......................... ...............85
B-2. University of Auckland Scour Tests, 1.1 U ........................... ............ ............... 86
B-3. University of Auckland Scour Tests, 1.5 Uc. ..........................................87
B-4. University of Auckland Scour Tests, 2.0 Uc. ..........................................88
LIST OF SYMBOLS
b pier diameter
C suspended fine sediment concentration
D50 median bed sediment diameter
ds measured scour depth
dse equilibrium scour depth
Fr Froude number
g coefficient of gravity
ks Nikuradse roughness length
L length of RETA test cylinder
R radius of RETA test cylinder
U flow velocity
Uc critical velocity
yo flow depth
[t dynamic viscosity
v kinematic viscosity
p fluid density
ps sediment density
T shear stress
To critical shear stress
Co initial pier shear stress
Tu shear stress upstream of pier
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
EFFECT OF SUSPENDED FINE SEDIMENT ON EQUILIBRIUM
LOCAL SCOUR DEPTHS
Elizabeth Anne Smyre
Chair: D. Max Sheppard
Department: Civil and Coastal Engineering
Recent clearwater local scour experiments with cohesionless sediments have shown
that the presence of suspended fine sediments can impact equilibrium local scour depths.
Researchers have known for some time that suspended fine sediment can affect shear
stress at the flow boundaries and have observed large drag reductions due to suspended
fine sediment in the flow. Reduced bed shear stress is one possible explanation for the
observed reduction in local scour depths.
This thesis 1) presents the results of an attempt to quantify the effects of suspended
fine sediment on bed shear stress through laboratory experiments, 2) presents local scour
data (obtained by other researchers) that illustrate the reduction in scour depth due to
suspended sediment, and 3) discusses the possible causes for the reduced scour depths.
An understanding of these effects is not only important in scour depth prediction but also
in the proper interpretation of laboratory local scour results. This may also help explain
some of the scatter in reported laboratory and field local scour data.
Definition of Scour
The removal of sediment near a structure located in a flowing water body is
referred to as sediment scour, or simply as scour. Sediment scour can be due to several
mechanisms, and thus it is usually divided into categories or types. Lateral migration of
the channel or the channel thalweg can reduce the bed level at the structure; this is known
as general scour. General degradation of the channel bed can result in a reduced bed
level at the structure. The structure can cause a reduction in flow cross-section and a
corresponding general reduction in bed elevation at the structure (contraction or
constriction scour). Finally, the presence of the structure alters the flow field in the
vicinity of the structure causing an increase in bed stress and a corresponding removal of
sediment near the structure (local scour). The total scour is the sum of these components.
Scour at bridge piers is a major problem. According to Richardson and Davis
(1995), scour around bridge piles and foundations as a result of flooding is the most
common cause of bridge failure. The problems resulting from bridge scour is a
widespread problem and has the potential for tragic results. The potential cost, including
human toll and monetary cost, of bridge failures due to scour damage has highlighted the
need for better scour prediction methods and equations. Under-prediction of scour depth
can lead to costly bridge failure, while over-prediction can result in millions of dollars in
unnecessary construction costs.
Thus much of scour research has been directed at improving scour prediction
equations and methods. Current research is devoted primarily to predicting scour at
specific types of structures, predicting the rate at which scour occurs, predicting scour in
soils other than sand, and the development of 3-D computational models for predicting
scour. Despite continuing research into scour mechanisms, some aspects of the scour
process are still not understood.
The research reported in this thesis is directed at understanding the reasons why the
presence of suspended fine sediment (SFS) reduces equilibrium local scour depths.
Experiments that 1) discovered this effect and 2) attempt to quantify the dependence of
equilibrium scour depth on SFS concentrations and flow velocity are summarized first.
This is followed by experiments conducted as part of this work to measure the effects of
SFS concentrations on the wall (bed) shear stress.
The Scour Process
The depth of scour at a bridge pier or abutment depends on a range of flow,
sediment, and structure parameters. The results of empirical studies assist in reducing the
number of parameters to only those having a primary effect on the local scour processes
(Sheppard et al., 1998). Most research in bridge scour has been devoted to the accurate
prediction of the actual maximum scour depth for a particular bridge structure
arrangement; however, recent studies have also examined the time history of scour.
One of the primary flow mechanisms responsible for local scour is the horizontal
vortex at the base of the leading edge of the structure. This vortex is known as the
"horseshoe vortex" due to its horseshoe like shape when viewed from above. This vortex
increases in size and intensity as the scour hole develops up to a maximum value. The
dissertation of Melville (1975) details the process of scour hole formation. Scour begins
as the velocity around the circumference of the cylinder increases and reaches a
maximum value at approximately +1000 from the upstream edge. At these locations,
indentations begin to form as the shear stress exerted on the bed increases; these
indentations slowly progress to the front of the pier where they meet. Scoured material is
transported downstream by the flow. Though the initial horseshoe vortex is relatively
weak, its intensity increases as more material is eroded, thus causing the vortex to
descend into the hole. The horseshoe vortex derives its energy from the main flow. The
bed shear stress is extremely high near the structure at the base of the scour hole and
decreases radially outward. As sediment is removed from near the structure, the
surrounding sediment avalanches into the hole. The slope of the scour hole is
approximately the submerged angle of repose of the sediment (Melville, 1975). In the
clearwater scour range, scour continues until the shear stress causing the scour is
balanced by the gravitational forces. For the live bed scour regime, scour continues until
the sediment entering the scour hole equals that being transported out.
Current Local Scour Prediction Equations
Scour prediction equations vary greatly in format and in content due to the wide
variety of parameters that affect equilibrium scour depth. The depth of scour depends on
several flow, fluid, sediment, and structure parameters, as shown in the following
functionally dependent equation:
dse =f[p, g, Dso, q, ps, y,, U, Uc, b, h(pier), C] (Equation 1-1)
de = equilibrium scour depth,
p, p, = water and sediment densities, respectively,
u = dynamic viscosity of the water (temperature dependent),
g = gravity,
Dso = median sediment diameter,
a= gradation of the sediment,
yo = depth of flow upstream of the structure,
U= depth-average velocity,
Uc= sediment critical depth-average velocity,
b = pier diameter/width normal to the flow,
h(pier) = function that describes the shape and alignment of pier in
relation to the flow, and
C = concentration of suspended fine sediment in the water column.
A dimensional analysis using the variables in the above equation results in a
number of independent (7r) groups. Even though the equilibrium scour depth depends to
some degree on all of these groups, researchers at the University of Florida have found
the following groups to dominate:
d,, f y U b ,h(pier),C (Equation 1-2)
b b 'Uc'Do-
The aspect ratio 0- relates the depth upstream of the pier to the pier diameter
normal to the flow. According to Ettema (1980), the depth of the flow impacts the
formation of the vortices on the upstream side of the pier. Decreasing flow depth
decreases the effect of a surface roller that rotates in the opposite direction to that of the
horseshoe vortex at the bottom of the pier. If the two vortices interfere with each other,
the effect of the surface vortex decreases. Additionally, low values of the aspect ratio
decrease the percentage of the flow that moves through the scour hole (Ettema, 1980).
Laboratory data indicates that if the parameters and D0 are held constant, de
increases rapidly with increasing .o until the ratio reaches 2.5 to 3; de then remains
constant (Pristivelis, 1999).
velocity range. If the velocity ratio is greater than some value (that depends on the
structure shape) but less than 1, the scour is said to be clearwater scour. If the velocity
ratio is greater than or equal to 1, then the flow velocity is such that sediment motion is
initiated on a flat bed away from the structure; thus live-bed scour conditions exist. The
critical velocity is based on Shields' equations for the critical shear stress and shear
velocity. The Prandtl-Von Karman formula for a fully developed velocity profile can be
used to calculate the depth averaged velocity in terms of shear velocity and bed
roughness (Sleath, 1984). The critical velocity Uc is then calculated from these values.
The dependence of equilibrium scour depth on the ratio for larger values of
was not realized until recently. Ettema (1980) concluded that the effect of this ratio
on scour depth was only significant for low ratios. Sheppard and Ontowirjo (1994),
Sheppard (1997), and Sheppard et al. (2002) show that the scour depth dependence
extends to much larger values of Since there is a lower limit on the size of the
sediment before it becomes cohesive (-0.1 mm), the equilibrium scour depth dependence
on for extremely large values of is due entirely to the dependence on pier width,
Sediment gradation, or, is believed to affect scour depth due to the formation of an
armor layer around the structure; this armor reduces the depth of the scour hole. The
effect is more pronounced in the clearwater scour range; the data of Ettema (1980)
showed a reduction in scour depth as the standard deviation of the particle size
The physical pier properties, denoted by h(pier), are usually accounted for by a
multiplicative coefficient in the scour equation that is determined empirically.
As stated above, there are many predictive equations for local scour depth in the
literature. These equations differ significantly in their form and in the magnitude of their
predictions. Three of these equations are presented below. The FHWA recommends an
equation developed at Colorado State University in its Hydraulic Engineering Circular
No. 18 (HEC-18) report:
d, = 2.0yoKK b Fr043 (Equation 1-3)
dse = equilibrium scour depth,
yo = flow depth upstream of structure,
K1 = pier nose shape correction factor,
K2 = angle of attack flow correction factor,
b = pier width, and
Fr = Froude number,
This equation can be used for both clearwater and live-bed scour conditions and has
limiting values: ,e
2.4 for Fr < 0.8 andd = 3.0 forFr > 0.8 for a circular pile
(Richardson and Davis, 1995).
The following equation was developed by Melville (1997) and is based on the
results from laboratory experiments:
K1 = flow intensity factor
Kd= sediment-size factor
KyD = flow depth-pier width factor
for 0.7< -< 5,
Ka = pier-alignment factor, and
K, = pier-shape factor.
The values for Ka and K, are given in Melville and Sutherland (1988).
Sheppard and Ontowirjo (1994) published the original version of the following
equation in 1994. This equation has undergone minor modifications over the years as
more and improved data became available.
In the clearwater scour range 0.47 < < 1.0 :
"Se = Kcf(L-) f2 3 rj2, (Equation 1-5)
Sbb Uc D5
=( tanh[( c y C2
f U I n(UlUc)
2UC In(U/U) '
D50 c3 exp (x xpk)] 4 +cexp[-c3 X x
x =log D xpk = 0g pk)
D50o D 50 pk
c = 1.0,
c2 = 0.4,
c4 = 0.45,
c5 = 2.5,
= 44, and
For live bed scour conditions, the equations become
For 1.0 < U< U, U,
For > U
S=e Kc6 tanh c 2 (Equation 1-7)
c6 = 2.2, and
Up = velocity at which the peak scour depth occurs in the live bed
scour range (live bed peak velocity).
Note that SFS concentration, C, is not included in these, or to the author's
knowledge, in any of the published scour prediction equations. The dependence of
equilibrium scour depth of SFS concentration was discovered during tests preformed by
Sheppard et al. (2002) at the Conte USGS-BRD Laboratory. The flume used for these
tests was a "flow-through" type flume with the water being supplied by a power plant
reservoir adjacent to the Connecticut River. There was no control on the constituents in
the water, and for some of the tests the SFS concentrations were elevated due to rain
water runoff and/or snow melt. The USGS-BRD flume is similar to other flow-through
flumes used in scour research; thus, research into the effect of SFS has implications for
future scour studies conducted in similar flumes.
Most studies designed to measure scour around bridge pier configurations are
conducted in laboratory situations in which the experiment simulates a proposed bridge
pier design. These situations offer control over most of the parameters affecting local
scour depth. Tests can be designed for a specified bridge pier configuration, bed
sediment size distribution, and flow condition. However, the facilities available to
researchers provide their own limitations. Scour tests conducted in a recirculating flume,
for example, offer greater control of the water properties than those tests conducted in
flow-through type flumes. In the case of flow-through flumes, water properties, such as
suspended fine sediment concentration, cannot be controlled. Depending upon the
structure and flow velocity, scour experiments can last up to several days or weeks.
When a flow-through flume is used, the amount of sediment in the water source can vary
drastically depending upon weather conditions (specifically, the amount of runoff due to
rainfall events or snowmelt) during the duration of the test.
In a previous research project, University of Florida researchers (Sheppard et al.,
2002) conducted a series of clearwater scour tests at the Conte USGS-BRD Laboratory in
Turners Falls, Massachusetts. The facility consisted of a gravity-driven, flow-through
type flume that used an adjacent power plant reservoir as its water source. During several
of the scour tests, the water in the reservoir became turbid due to rainwater runoff and/or
snowmelt upstream of the laboratory. This reduced the equilibrium scour depths, forcing
researchers to repeat the tests. Since this was not expected, there were only few
measurements documenting the conditions during tests with heightened suspended
The purpose of research reported in this thesis was to confirm that the presence of
SFS does affect the measured local scour depth and to delineate possible reasons for the
reduced scour due to SFS and to examine one of these possible causes. One possible
cause is that the presence of SFS causes a reduction in bed shear stress. This would have
a similar effect to reducing the flow velocity. A second possibility for the reduction in
scour depth is that there may be deposition of fine sediment in the scour hole out from the
structure. This could retard the avalanching of sediment into the high bed stress region
near the structure and/or when avalanches occur, they would carry with them the fine,
cohesive, sediment. The mixture of sand and cohesive sediment has, in general, a higher
critical shear stress. Thus the equilibrium scour depth will be less.
This research concentrates on the first possible cause for scour reduction listed
above, namely the reduction in bed shear stress due to the presence of SFS. The
investigation of the SFS problem begins with a review of the clearwater scour data
obtained by Sheppard et al. (2002) in their tests performed at the USGS-BRD Conte
Laboratory in Massachusetts. The data provides insight into the flow, bed, and pier
conditions under which SFS impacts scour depths. Water samples collected during one
of the USGS-BRD scour tests were analyzed as part of this work. The results provide
information about the concentrations of SFS encountered in the USGS-BRD tests. In
addition, the results of local scour tests conducted at the University of Auckland with
various SFS concentrations and flow velocities are reviewed and discussed.
Because the effect of SFS on equilibrium scour depth has not been previously
investigated, the literature review focuses on the research that has been conducted on
drag reduction due to SFS. Laboratory tests were conducted in a Rotating Erosion
Testing Apparatus (RETA) in an attempt to quantify the reduction in bed shear stress as a
function of SFS concentration.
The objective of this research is to identify and discuss the most likely mechanisms
responsible for the reduction in scour and to investigate in some detail one of these
possible causes. Once the effects are understood and quantified, they can be incorporated
into scour prediction equations. The task of modifying scour prediction equations is,
however, not part of this research.
University of Florida Local Scour Tests: USGS-BRD Laboratory
Between August 1998 and June 2001, scour tests were conducted at the Conte
USGS-BRD Laboratory located in Turners Falls, Massachusetts adjacent to the
Connecticut River (Figure 2-1). The primary purpose of the tests was to extend the
structure-induced local sediment scour database to include data from larger structures
(larger values of ). Three different pile diameters [b= 0.915 m, 0.305 m, and 0.114
m] and three different bed sediment sizes (Dso= 0.22 mm, 0.80 mm, and 2.9 mm) were
used in the investigation. All of these tests were conducted within the clearwater scour
range of velocities (i.e., < 1).
The flume used for these tests measures 6.1 m wide, 6.4 m deep, and 38.4 m long.
The test section within the flume was 6.1 m wide, 9.8 m long, and began 24.4 m
downstream of the flume's entrance. The bed sediment height within the test section was
1.83 m deep. A hydroelectric power plant reservoir (connected to the Connecticut River
upstream of control structures that lower the river's elevation) supplied water for the
flume; water flowed from the reservoir through the flume and was then discharged into
the Connecticut River downstream of the river control structures. The flume itself is
gravity controlled with the elevation of the flume bottom approximately 10 m lower than
that of the reservoir. A sharp-crested weir at the downstream end of the flume controls
water depth and flow discharge within the flume. The advantage of this type of flume is
that pumps are not needed to produce the flow.
Figure 2-1. Aerial photo of USGS-BRD Laboratory, Turners Falls, Massachusetts
(reprinted from Sheppard et al., 2002)
NOT TO SCALE
All dimensions in meters
I----- 6. ----- t etion l 1 _
WWT ur 64 L2 x 12
H_ Sice gat Plan View
Test Sediment I 83 Flo, Dishare
--- ~38.4 |
I -L---- 0
Filter Matrial Base Scdiment Tcst Scdimcel Base Scdimcnl
Figure 2-2. Schematic of USGS-BRD flume (reprinted from Sheppard et al., 2002).
Flow-through flumes do, however, have disadvantages. There is little or no control
of the incoming water; thus, properties such as water temperature will be that of the water
source (in this case, the power plant reservoir). For these tests, the water temperature of
the Connecticut River varied from slightly above freezing to approximately 260C during
the summer months. In addition, the concentration of SFS within the water supply could
not be controlled. The level of rainwater and snowmelt runoff upstream of the reservoir
governed the concentration of SFS in the water supply. While the temperature of the
water can be accounted for in scour prediction, the effect of SFS had not yet been
identified as a quantity affecting scour depth.
During each of the fourteen scour tests, equilibrium scour depth measurements
were made using both acoustic transponders and video cameras located inside of the
piles. A real time scour depth plot was maintained during each test in order to know
when the scour depth had reached an equilibrium value. Flow velocity, water depth, and
water temperature were measured throughout the tests. A summary of scour test
parameters and results is presented in Table 2-1. Those tests affected by the suspended
fine sediment are not included in the summary, as most of the tests were halted once the
presence of SFS was detected and thus not included in the final data presentation.
In some of the longer duration tests, researchers noted a rather sudden increase in
SFS in the water column and a corresponding change in the rate of scour. In each case,
the rate of scour initially proceeded normally; at a point in the test, the rate of scour
suddenly decreased to zero, and the scour depth remained constant. Figure 2-3 is a
comparison time history plot that illustrates the change in the rate of scour. Experiment
A is clearly affected by the increase in SFS concentration approximately 10 hours into the
test, causing the scour hole depth to level out immediately. Experiment B, which was
with the same structure but at a slightly higher velocity and deeper water depth, was
conducted under ambient SFS conditions. The curve labeled Experiment B is the actual
Experiment B data adjusted to the water depth and flow velocity conditions of
Experiment A using Sheppard et al. (2002) scour prediction equations. Table 2-2
summarizes the parameters for the two experiments.
Table 2-1. Summary of USGS-BRD scour tests parameters (Sheppard et al., 2002).
Test Dso yo b (m) U Uc Duration y/b b/Dso dse
No. (mm) (m) (m/s) (m/s) (hrs) (m)
1 0.22 1.19 0.114 0.28 0.32 87 10.4 518 0.133
2 0.22 1.20 0.305 0.29 0.32 163 3.9 1386 0.257
3 0.80 1.27 0.915 0.43 0.47 362 1.4 1144 1.112
4 0.80 0.87 0.915 0.38 0.46 143 1.0 1144 0.638
5 0.80 1.27 0.305 0.37 0.47 87 4.2 381 0.416
6 0.80 1.27 0.114 0.38 0.47 42 11.1 143 0.185
7 2.90 1.22 0.915 0.68 0.84 188 1.3 316 1.270
8 2.90 0.56 0.915 0.60 0.76 330 0.6 316 1.058
9 2.90 0.29 0.915 0.56 0.69 448 0.3 316 0.896
10 2.90 0.17 0.915 0.48 0.65 616 0.2 316 0.659
11 2.90 1.90 0.915 0.60 0.93 350 2.1 316 1.004
12 0.22 1.22 0.305 0.31 0.33 256 4.0 1386 0.377
13 0.22 0.18 0.305 0.27 0.27 215 0.6 1386 0.296
14 0.22 1.81 0.915 0.21 0.32 579 2.0 4159 0.787
Table 2-2. Test Parameters:
in Figure 2-3
(Sheppard et al.,
Experiment b (m) D50 (mm) yo (m) U/Uc
A 0.915 0.22 1.22 0.92
B 0.915 0.22 1.8 0.97
The photographs in Figure 2-4 show the differences in the final scour holes for the
two tests. In Experiment A, fine sediment deposition was not apparent adjacent to the
structure; however, there was deposition in the scour hole away from the pier once the
experiment was halted (Sheppard et al., 2002). Contour plots of the scour hole for each
experiment indicate that the scour hole in Experiment A was slightly steeper than the hole
formed in Experiment B, but that the scour hole in Experiment B had a more uniform
shape. In Experiment A, scour only occurred on the upstream side of the pier (Figure 2-
Experiment B (low turbiditv)
** Experiment B Adjusted
D 0.3 :adusred to flow conditions of Experiment A
0 1 ExperimentA ,,,,gH :LirbdliT )
0 20 40 60 80 100 120 140
Figure 2-3. Plot illustrating the effect of suspended fine sediment on scour depth
(reprinted from Sheppard et al., 2002).
At the time of the experiments, the researchers were unaware of the sensitivity of
local scour rates and equilibrium values on SFS concentration; thus, water samples were
only taken during one of the tests that had higher than normal SFS concentrations. For
that test, water samples were taken at 11 different levels in the water column.
Figure 2-4. Post-experiment photos of Experiment A (top) and Experiment B (bottom)
(reprinted from Sheppard et al., 2002).
I' ~,, n
s .O ."'- _,. 2_
Figure 2-5. Contour plots of Experiment A (top) and Experiment B (bottom) (reprinted
from Sheppard et al., 2002). Flow direction is indicated.
Analysis of Suspended Fine Sediment Water Samples
In early 2001, 11 water samples were collected during a scour test with higher than
normal SFS concentrations. The water samples were collected at the following heights
above the sediment bed: 7.6 cm, 15.2 cm, 22.9 cm, 30.5 cm, 38.1 cm, 45.7 cm, 53.3 cm,
61.0 cm, 91.4 cm, 152.4 cm, and 182.9 cm. These samples were gathered in order to
determine the overall SFS concentration and the concentration profile during an actual
The procedure used to analyze the samples was to take a pre-measured volume of
the sample and filter it through a vacuum-pump filtration system. The filter paper was
oven-dried and the mass measured prior to the filtration. The water sample was then
filtered. The filter paper with the sediment was then dried overnight and the mass
measured again; the difference in the two measurements was recorded as the mass of the
sediment in the volume. The sediment mass was divided by the volume of the water
sample used in the test to get the concentration in g/1. Table 2-3 shows the results of the
suspended fine sediment concentration tests; the concentration profile is shown in Figure
There were errors involved in the concentration measurement process. There was
approximately a ten-month lapse between the time when the water samples were taken
and when they were analyzed. Some of the sediment that settled out during this time
adhered to the walls of the containers, thus causing the measured values to be lower than
the actual value. The pretest drying time for the filters was also not sufficient, again
causing a lower measured concentration.
Measured concentrations ranged from 0 to 0.08 g/l, with the highest concentration
occurring at 45.7 cm above the bed. The concentration profile varied significantly over
the water column. While these measurements were not very precise, they do provide
some insight as to the magnitude of the SFS concentrations in one of the experiments that
Table 2-3. Suspended Fine Sediment Concentration Test Data
Sample Depth Sample Blank Filter w/ Mass Concentration
Number Above Volume Filter Sediment Difference (g/l)
Bed (ft) (mL) Mass (g) (g)
1 6 85 1.1378 1.1395 0.0017 0.02
2 5 100 1.1231 1.1171 0 0.00
4 3 100 1.1246 1.1206 0 0.00
5 2 275 1.1246 1.1250 0.0004 0.00
6 1.75 225 1.1306 1.1310 0.0004 0.00
7 1.5 279 1.121 1.1433 0.0223 0.08
8 1.25 200 1.133 1.1320 0 0.00
9 1 100 1.1226 1.1174 0 0.00
10 0.75 100 1.1354 1.1385 0.0031 0.03
11 0.5 100 1.1272 1.1226 0 0.00
12 0.25 209 1.1313 1.1400 0.0087 0.04
Concentration Profile for Massachusetts Scour Test
000 001 002 003 004 005
Figure 2-6. Concentration of suspended fine sedime
BRD scour tests.
006 007 008 009
nt measured during one of the USGS-
Two small video cameras were mounted on a vertical traversing mechanism inside
of the test pier to monitor the water-sediment interface during the test. The cameras were
situated facing the upstream side of the pier and the scour hole. The camera platform was
made to traverse vertically with an electric motor that was controlled manually. Length
scales were mounted inside of the pier within the visual range of the cameras. Thus the
scour depth could be monitored from inside the pier via the video data. A programmable
control system was used to set the duration of the individual recordings as well as the
intervals between recordings. The control system also switched between the two cameras
at regular intervals (Sheppard et al., 2002).
The video recordings from two of the scour tests were reviewed as part of the
research in order to examine the differences in the scour hole formation between a test
without and one with SFS. In the test with little or no SFS present, the water was very
clear. The horseshoe vortex could be observed by the suspended bed sediment and small
debris particles in the water column. At the end of the test, movement of bed sediment
out of the scour hole was countered with sediment sliding back into the hole; this balance
of sediment transport indicated that the scour hole had reached equilibrium.
There were no major differences in the video from the experiment with SFS
(Experiment A) with the exception of a reduction in sediment movement in the bed.
Also, there was no indication of a sharp change in SFS (turbidity) at or near the time the
scour ceased. It should be pointed out, however, that in the tests with the large pile, the
pile was flooded with water and the cameras housed in waterproof containers. The view
of the external flow and scour hole was somewhat attenuated by the water in the pile. T
he only information obtained from the examination of the video was the reduced
sediment movement that could have been caused by any of the mechanisms outlined
earlier in the thesis.
Calculation of Bed Shear Stress
The equations for the hydraulically rough region of flow (see e.g. Sleath, 1984)
were used to estimate the shear stress values on the flat bed in front of the test pier. Table
2-4 summarizes the information used in the shear stress calculations.
Table 2-4. Shear stress calculation parameters.
Sediment density, p, 2650 kg/m3
Water density, p 1000 kg/m3
Gravity, g 9.81 m/s2
Water viscosity, v 1.12x10-6 m2/s
Depth-averaged velocity, U Used U given for each test (m/s).
Flow depth, yo Given for each test (m).
Roughness, k, 5D50 for the 0.22,0.8 mm sand; 2.5D5o for the
2.9 mm sand (m).
The equation for uq
in the hydraulically rough flow range is:
Solving for co results in:
The critical shear stress is calculated by finding the non-dimensional critical shear stress
ratio from the Shields' curve. Thus the critical shear stress is:
Tc = dimensionlesss) (p, p) g D (Equation 2-3)
Finally, the value of the ratio of u" was checked to ensure that the requirements
for hydraulically rough flow were met. This condition was met for the 2.9 mm and 0.8
mm sediment, but was not met for the 0.22 mm sediment. Similar shear stress values
were calculated for the 0.22 mm sediment using the equations for the transitional flow
region. Table 2-5 lists the average shear stress values calculated for each sediment size.
Table 2-5. Typical flat bed shear stress values for the local scour tests performed at the
Sediment Size (mm) Shear Stress (kg/m*s2)
Discussion of USGS-BRD Data
A review of the tests conducted at the Conte USGS-BRD Laboratory provides
insight as to the conditions needed for suspended fine sediment to cause a reduction in
local equilibrium scour depth. The majority of the tests affected by the suspended fine
sediment were those with a bed sediment diameter of 0.22 mm. Only one test with a bed
sediment diameter of 0.8 mm was affected by suspended fine sediment.
The tests affected by the suspended sediment were conducted over a range of flow
depths and pier sizes. The time at which the scour rate dropped to near zero varied for
each test, ranging from 20 hours to 200 hours. The point at which the scour rate
decreases sharply is more likely a function of the percentage of the equilibrium scour
depth without SFS.
The video data recorded during a test affected by suspended fine sediment did not
indicate a sharp increase in water turbidity at the time that the scour rate decreased. The
turbidity appeared to be the same throughout the test. This observation suggests that the
SFS has its greatest effect after the scour hole has reached a certain depth where the
effective shear stresses are diminished. In the early stages of scour hole development, the
effective shear stresses near the structure are large, and it is known that the effects of SFS
decrease with increased velocity/turbulence.
There was undoubtedly an increase in turbidity during the tests since otherwise the
tests would not have been started. The increase was, however, more gradual than
originally suspected. The analysis of the video data showed that the change in SFS
concentration was not rapid as believed, but it was not helpful in determining which, if
any, of the proposed scour reduction mechanisms were responsible for the observed
The measured SFS concentrations indicate that only small concentrations are
needed to cause a dramatic effect on the equilibrium scour depth. The measured
concentrations were less than 0.1 g/1. The dependence of scour depth on SFS
concentration, flow velocity, and possible other parameters is needed before accurate
predictions of scour attenuation can be made.
University of Auckland Scour Tests
In order to confirm that the effect of SFS was not confined to the USGS-BRD tests,
the data collected during a series of scour tests conducted at the University of Auckland
was examined. The tests involved four freshwater/bentonite suspension flows conducted
at four velocities. A reduction in d, occurred in the tests in the clearwater range;
however, the reduction in scour was less than those measured in the USGS-BRD tests.
The tests conducted in the live bed velocity range offered conflicting results. The
clearwater tests do confirm that the final measured scour depth is reduced by the presence
of SFS. A complete write-up of the test procedure and a presentation of the final results
are included in Appendix B.
The major thrust of this thesis was to investigate the effects of the suspended fine
sediment (SFS) on the shear stress exerted on the bed by the flow. Previous research
examined the effect of suspended clays on the drag forces exerted on a sediment bed in
an attempt to quantify the drag reduction due to SFS concentrations. Drag reduction is
defined as "the decrease in shear stress in the viscous sublayer with respect to the
apparent shear stress of the logarithmic layer in the upper water column" (Li and Gust,
2000, p.77). Research concerning the effect of drag reduction in open channel flows
began after similar studies, such as that of Toms (1949), measured drag reduction in
turbulent pipe flows with dilute polymer solutions. While several studies have shown
that the addition of suspended sediment within the flow causes a reduction in drag, the
exact mechanisms of drag reduction are not well understood. Several studies have tried
to quantify the effects of SFS concentrations on drag reduction (with concentrations as
high as 9% by volume).
Gust (1976) measured the mean streamwise velocity profiles for three smooth flow
systems: a Lucite and tap water fluid over a smooth bottom, a freshwater flow over a fine
quartz sand bed, and a seawater/clay mineral suspension over a mud bottom. Three
different fluid flows were used in order to determine the drag reduction for the clay
suspension flow at both non-eroding and eroding velocities. The research was designed
to examine whether the universal law of the wall,
U+ = K Ilny + C, (Equation 3-1)
U = dimensionless velocity-,
u = local mean velocity,
u*= shear velocity,
y = dimensionless wall distance y and
C = integration constant,
that describes the dimensionless velocity profile for hydrodynamically smooth flows, was
applicable in the case of a cohesive suspended sediment flow over a mud bottom. In
addition, the research was to verify the existence of the drag reduction observed in earlier
Both the Lucite/tap water flow and the freshwater flow considered in the study
fulfilled Newtonian flow expectations by yielding measured velocity profiles that
followed the law of the wall equation. However, all of the seawater/clay suspension
flows indicated deviations from the universal law of the wall. As the seawater flows
were increased to the turbulent flow velocity range, the measured viscous sublayer
thickened to 5 mm above the mud bottom, as compared to a 1 mm thickness for the
Lucite/tap water and freshwater flows. No substantial increase in clay concentration was
measured within the viscous sublayer, thus there was not an increase in the kinematic
viscosity that would indicate compatibility with the law of the wall.
The author presented his results with plots of the dimensionless velocity versus
dimensionless height above the bed. All of the velocity profiles for the Lucite/tap water
and the freshwater systems overlapped, indicating no change in the velocity profile with
increasing Reynolds number. However, the dimensionless velocity profiles of the
seawater/clay suspension flows indicated the presence of a thickened viscous sublayer;
the thickness of the sublayer generally increased with increasing Reynolds number. A
specific relationship between the Reynolds number and the thickness of the viscous
sublayer was not apparent. Thus a correlation between the clay concentrations and the
dimensionless velocity profile could not be obtained. The author assumed that the clay
concentration, the type of clay material, and the shear rate of the flow influenced the final
Best and Leeder (1993) noted that in previous work, drag reduction occurs when
the near wall turbulence structure is modified, thus linking drag reduction to the
mechanisms that produce turbulence. The authors conducted a series of experiments with
seawater/clay suspension (maximum concentration, 2.2 g/l) flows over a mud bed and
plotted the final velocity profiles for each seawater/clay mixture. The data showed a
decrease in the near bed velocities as the clay concentration increased; a plot of the
dimensionless velocity versus the dimensionless height revealed a thickening of the wall
layer with increasing clay concentration. The authors speculated that a decreased rate in
turbulent bursts within the turbulent boundary layers that leads to a reduced momentum
exchange within the boundary layer is a possible explanation for the reduction in drag.
In an additional series of tests on 0.5 mm and 0.8 mm sand, both plain seawater and
seawater/clay suspensions were allowed to flow over the bed sediment to investigate the
effects of the clay on bed formations. The flow velocity was allowed to increase to the
point of bed form movement initiation. Once the bed forms had reached equilibrium
position, the test was stopped and the bed forms were measured. For the 0.5 mm
sediment bed, the water mixed with clay was found to cause bed forms with much
smaller amplitudes and wavelengths than those generated in clear water even though the
test was run much longer than that of the test with plain seawater. It was believed that
the influence of drag reduction had not allowed the bed to reach a true movement
threshold. However, the tests conducted over the 0.8 mm sediment bed indicated that the
increased concentrations of clay had little to no effect on the bed forms created by the
flow. The sequence of bed forms generated in the seawater/clay suspension flow was
similar to that formed in plain freshwater flows. It was concluded that the thickened wall
layer formed by the clay concentrations had no effect on the course sediment.
Li and Gust (2000) examined the effect that clay suspensions have on the boundary
wall layer. The purpose of the research was to examine drag reduction at various clay
concentrations and flow velocities. In a series of experiments conducted in a
recirculating flume, velocity profiles and shear velocities within the viscous sublayer
were measured. The experiments involved suspended sediment kaolinitee) concentrations
ranging from 0.1 to 8 g/1 in both freshwater and seawater. The authors wanted to
examine the effect of higher clay concentrations on drag reduction, as previous studies
had mainly encompassed low clay concentrations (less than 2.2 g/1). Logarithmic
velocity profiles were plotted for each of the flows.
The results of the study indicated that drag reduction increased with an increase in
the suspended clay concentration. The data indicated that the measured shear velocity
decreased as the concentration of the clay suspension increased. Thus the use of the
shear velocity calculated from the velocity profile will result in the overestimation of the
actual bed shear stress. A corresponding thickening of the viscous sublayer was
presumed to be the cause of the drag reduction. The thickening of the wall layer led to a
reduction in bed shear stress. The effects of the suspended sediment were most
prominent at low flow velocities; the effect was weaker as the flow velocity increased.
While the effect of suspended sediment on drag reduction was clearly shown, a
direct relationship between clay concentrations and drag reduction has not been
determined. However, the authors formulated the following empirical relationship:
u, -72.73 +0.107u, og +0.451 (r2=0.760), (Equation 3-2)
us -114.93-L +0.653ulog +0.0410 (r2=0.903), (Equation 3-3)
u*, = measured shear velocity,
u*.og = profile-derived shear velocity,
c= clay concentration, and
ps = density of clay suspension sediment.
These equations show that if the clay concentrations and the velocity profiles are known,
the actual shear velocity, and the corresponding shear stress, can be quickly estimated.
Thus the magnitude of the drag reduction can be determined.
The authors concluded that, based on measurements from flume experiments that
utilized high kaolinite flows, the shear velocity at the top of the viscous sublayer could be
reduced by as much as 70%. The exact amount of the reduction depended on both the
clay concentration and the flow velocity; in general, the drag reduction increases with
increasing clay concentration at a given flow velocity. Also, the drag reduction is higher
at low flow velocity and decreases as the velocity increases for a given suspended clay
concentration. Finally, this thickening of the viscous layer as well as the turbulence
dampening was believed to be the cause of the reduced bed shear stress (Li and Gust,
In each of these studies, suspended fine sediment within the flow was shown to
increase the depth of the viscous sublayer, thus increasing the drag reduction. While the
effect of the suspended fine sediment itself has been proven, it has not yet been quantified
for varying flows. Further research is needed to quantify the effect of the suspended fine
sediment on the velocity profile and on the shear stress at the bed.
In each of the studies, the concentrations of the suspended fine sediment remained
fairly low, usually lower than 9 g/1. At higher concentrations, the suspended sediment
begins to flocculate; the effect of the flocculated particles is also not known.
TEST APPARATUS AND PROCEDURE
Rotating cylinder devices have been utilized in previous research in order to
measure the erosion rate of selected materials. The operation of the rotating cylinder
device is based on the theory of a rotational viscometer. A rotating viscometer is used to
measure the viscosity of liquids. A cylinder is rotated within a fixed outer cylinder the
annulus of which is filled with the fluid being tested. The viscosity of the fluid is
calculated from the measurements of the torque required to maintain the angular velocity
(Munson et al., 1994).
A rotating cylinder device was developed by Moore and Masch (1962) for the
purpose of measuring the scour resistance of cohesive soils. A cylindrical cohesive soil
sample, supported by a hollow tube, is centered and suspended within a slightly larger
cylinder. The annular gap between the outer and inner cylinder is filled with water. The
outer cylinder is rotated while the inner cylindrical sample remains fixed. A shear stress
is applied to the surface of the sample by the fluid. The torque exerted on the sample is
measured and the average shear stress on the sample surface computed.
Additional research has been conducted with rotating cylinder devices by Rektorik
(1964), Arulanandan et al. (1975), Sargunam et al. (1973), Alizadeh (1974), and Chapius
and Gatien (1986) (Kerr, 2001). Each set of research slightly modified and improved
upon the original device.
University of Florida Testing Apparatus
A Rotating Erosion Test Apparatus (RETA) was developed at the University of
Florida for the purpose of measuring erosion rates of certain types of rock (such as lime
rock, sand stone, coquina, etc.). Even though the RETA was designed to measure erosion
rates as a function of shear stress for erodible rock, it can be used with any sediment that
can support its own weight, such as a stiff clay.
The RETA is very similar to the device developed by Moore and Masch (1962) but
has improved instrumentation for measuring torque. As stated above, the computed shear
stress is the average value over the sample surface; thus the stress can be higher at certain
locations. There may also be variations in surface roughness that cannot be accounted for
using this approach. This problem is not unique to the current RETA. Rohan and
Lefebvre (1991) showed that the shear stress measurements generated by rotating
cylinder devices could be underestimated. The low estimates could be caused by the
centrifugal forces that cause the fluid to move toward the outer cylinder, creating a
helicodial, secondary flow. Fluctuations in the velocity radial components can also lead
to an underestimation of the shear stress when the fluid is in the fully turbulent regime
Figure 4-1 is a photograph of the University of Florida RETA. While the RETA is
based on the designs of earlier rotating cylinder devices, several practical modifications
have been incorporated into the new unit. A back slide plate has been added to allow the
test cylinder to be easily lowered into the outer cylinder as well as to allow the sample to
remain attached between tests. A torque cell is used to measure the torque exerted on the
inner cylinder. A variable speed motor is used to rotate the outer cylinder. The
tachometer, which is connected to the motor shaft, is programmed to show the corrected
RPM value based on the gear ratio of the gearing system (Kerr, 2001).
Figure 4-1. University of Florida Rotating Erosion Test Apparatus (RETA).
Modifications to RETA
As stated previously, the RETA was designed to measure the erosion rate as a
function of shear stress for various rock samples. Because rock specimens are highly
resistive to the erosive forces of water, these tests can last for days. Thus the gearing
system was originally designed for long tests at high angular velocities (revolutions per
minute). Even though these angular speeds were suitable for the rock samples (rock tests
required RPM values of approximately 1000 or greater), they were too large for the shear
stresses needed for this work. Therefore, the 3:1 step-up gear system used in the
apparatus was replaced by a 1:1 gear/belt system, allowing the outer cylinder to rotate at
speeds as low as 200 RPM. Since the original tachometer could not be reprogrammed to
show the actual RPM of the cylinder, the tachometer output was divided by three in the
analysis to give the actual RPM value.
In order to eliminate as much noise as possible from the digital readout of the
torque values, an electronic filter was used in accordance to the torque cell
manufacturer's guidelines. The filter eliminates noise as well as allows the torque
readout to stabilize faster once a particular torque value is achieved.
In previous tests using the RETA, the test cylinders were core samples taken
directly from rock beds. However, because these tests involved sand beds, a cylinder
could not be created in the same method. For these tests, two acrylic cylinders were
coated with similar sized sediment as was used in the USGS-BRD scour testing: one
cylinder was coated with 0.15 mm sediment and another with 0.85 mm sediment. A two-
part epoxy was used to adhere the sediment onto the cylinder. The cylinders are
approximately 10.2 cm in height and 6 cm in diameter and are closed at both ends. An
attempt was made to coat an additional cylinder with 2.0 mm sediment to simulate the 2.9
mm bed sediment size; however, due to the angularity and the size of the sediment it was
difficult to create a cylinder with an even roughness.
In addition to the sediment-coated acrylic cylinders, two additional test cylinders
were made from aluminum. The cylinders were knurled to simulate two different
roughnesses equivalent to those achieved with the 0.15 mm and 0.85 mm sediment
coated acrylic cylinders. The surface roughness was more uniform for the aluminum
cylinders than for those coated with sand. Figure 4-2 is a photograph of all four cylinders
used in the RETA tests.
Figure 4-2. Photograph of cylinders used in RETA tests. The left two are the aluminum
cylinders; the two cylinders on the right are acrylic cylinders coated with sand.
Tests were performed with each of the cylinders with four different (SFS)
(bentonite/water) concentrations: 0.0 g/l, 0.05 g/l, 0.5 g/l, and 1.0 g/l. The 0.0 g/l water
tests were performed first to establish the proper RPM values. These RPM values were
then replicated in the subsequent three sets of tests.
Once the test cylinder was placed in the apparatus and the outer cylinder filled with
distilled water, the cylinder was allowed to spin at the lowest possible speed in order to
generate a base torque (and shear stress) value. This shear stress value is compared to the
base shear stress originally calculated for the USGS-BRD tests. After the base torque is
generated, the torque values originally calculated from the USGS-BRD tests representing
increasing magnitudes of the base value were simulated. The RPM values generated
from these torque values are recorded. Following the tests, the clear water was discarded;
however, the cylinder remains on the torque cell for subsequent testing. The initial and
final water temperatures, as well as the final resting torque value, were noted for each test
The first bentonite/water mixture was then poured into the annulus between the
cylinders, and the initial set of tests was repeated. The difference in these tests was that
the RPM values, not the torque values, are the control values. The new torque values,
generated from the same RPM's measured in the 0.0 concentration test, were recorded.
The tests are to indicate if, at the same rotational speed, the shear stress changes with the
addition of bentonite in the fluid. The tests were designed to simulate velocities
equivalent of ratios between 1 and 4.
The torque values to be simulated in the RETA were calculated from the critical
shear stress values developed in Chapter 2. The torque is calculated from the critical
shear stress using the following equation:
T= 27rR2L-c (Equation 4-1)
R = radius of sample cylinder,
L = length of sample cylinder,
T= torque, and
c = critical shear stress.
For each cylinder, a minimum of five readings, representing increasing magnitudes of the
calculated torque value, were recorded.
The test procedure is outlined below:
0.0 g/l Test:
1. Attach the test cylinder onto threaded rod and secure in place with end plate. Place
acrylic lid over the sample and screw top end of threaded rod into torque meter.
2. Fill the plastic insert 1/3 full (approximately 200 mL) with distilled water. Record
the initial water temperature. Place the plastic insert into the acrylic annulus.
3. Lower the test cylinder into annulus, ensuring that the test cylinder is centered and
that water rises just above the top of the cylinder.
4. Add water if necessary.
5. Secure the cylinder lid. Make sure that the torque cell is locked into place on the
6. With the machine on and the cylinder still at rest, tare the torque meter so that it
reads 0.000 N-mm.
7. Make sure that the RPM control dial is set to the lowest speed.
8. With the directional switch pointing to the forward direction, turn on the motor.
9. Once the outer cylinder begins spinning, check that the water level is even; if not,
add more water.
10. Take an initial (base) reading at the lowest RPM.
11. The control values in the 0.0 concentration test are the torque values. Increase the
speed of the outer cylinder slowly until the first torque value is shown on the digital
readout. Record both the actual torque value achieved as well as the RPM.
12. Continue taking torque and RPM measurements at the values for 2*Initial Torque,
3*Initial Torque, etc.
13. Once finished, gradually reduce speed to the lowest possible RPM, then turn off
14. Record the resting torque value once the cylinder has stopped spinning.
15. Gently slide the test sample out of the acrylic annulus (test sample can remain
attached to the torque cell between tests).
16. Remove the plastic insert and record the final water temperature.
17. Discard water.
18. Carefully dry the test sample.
0.05 g/l 1.0 g/l Tests:
19. Measure 250 mL of distilled water.
20. Measure the appropriate amount of bentonite (record exact amount used), and mix
bentonite with the water.
21. Record the initial water temperature.
22. Fill the plastic insert 1/3 full (approximately 200 mL) with the bentonite/water
mixture. Retain remaining mixture in case additional is needed.
23. Place the plastic insert into the acrylic annulus.
24. Lower test sample into the outer cylinder, securing it in place.
25. Once the lid is tightened, tare the torque readout.
26. Repeat the test procedure from the 0.0 concentration test, this time simulating the
RPM values generated in that test. Record the new torque values achieved at each
27. Once complete, gradually slow the outer cylinder to the lowest speed, then turn off
28. Record the final resting torque value once the cylinder has stopped spinning.
29. Remove the test sample and the plastic insert from the acrylic annulus. Record the
final water temperature.
30. Dispose of the bentonite/water mixture and completely dry the plastic insert prior
to the next test. Carefully dry the test sample.
31. Repeat steps 19-30 for each remaining bentonite/water concentration.
Four sets of RPM versus torque data sets were obtained. The average shear stress
was computed from the measured torque values using Equation 4-1. The results were
plotted as shear stress versus RPM for different SFS concentrations.
As mentioned previously, the studies of Gust (1976), Best and Leeder (1993), and
Li and Gust (2000) incorporated mixtures of seawater and SFS in their research on drag
reduction. In order to see the effect of salinity on the SFS induced shear stress reduction,
one set of tests was conducting using seawater. The seawater was collected from the
Intracoastal Waterway on the east coast of Florida and had a measured salinity of 29.4
parts per thousand. The bentonite/seawater mixture was allowed to stand for ten days
prior to the tests in order to facilitate the reactions between the bentonite and salt
particles. The tests then followed the same procedure as was used in the freshwater tests.
The saltwater tests were limited to one on each sediment-coated acrylic cylinder in order
to reduce potential corrosion damage to the RETA.
TEST RESULTS AND OVERALL CONCLUSIONS
Five sets of tests were conducted on each of the two sediment-coated acrylic
cylinders, and two test sets were conducted on each of the two aluminum cylinders. A set
consisted of testing a range of RPM values at each of the four SFS concentrations. The
torques generated in these tests were near the lower limits that can be measured with the
torque cell on the RETA. This resulted in errors in the readings beyond those discussed
earlier. In order to minimize these errors (that are thought to be random in nature) the
tests were repeated for each set of conditions and the results averaged. Plots of the
individual tests are not conclusive, but there is a definite pattern of shear stress reduction
in the plots of the average values.
The tests with the acrylic cylinders (with bonded sand) show a greater reduction in
shear stress than the aluminum cylinders for some reason. There is also a greater shear
stress reduction with SFS concentration for the acrylic cylinder with the 0.15 mm sand
than for the one with the 0.85 mm sand. This is most likely due to the increased
turbulence generated by the larger sand particles and the corresponding reduction in the
viscous sublayer. The results of the average shear stress curves for the two acrylic
cylinders are shown in Figures 5-1 and 5-2. The results of all of the individual cylinder
tests are presented in Appendix A.
0.15mm Acrylic Cylinder- Average Concentration Lines
(Tests with excessive values removed)
0.4 -- 0.0g/1
i- 0.5 g/l
0.2 -X- 1.0 g/1 -
200 250 300 350 400 450 500
Figure 5-1. Average shear stress lines generated from tests conducted on 0.15 mm sediment coated acrylic cylinder.
0.85mm Acrylic Cylinder-Average Concentration Lines
(All freshwater tests)
2.5 ---- 0 0 0 g /1
0 05 g/1
A- 0 5 g/1
-X- -1 0 g/1
200 250 300 350 400 450 500
Figure 5-2. Average shear stress lines generated from tests conducted on 0.85 mm sediment coated acrylic cylinder.
The shear stress plot for the tests conducted with the 0.15 mm sediment coated
acrylic cylinder shows about a ten percent reduction in shear stress with the addition of
SFS. There is an increase in shear stress reduction with increasing RPM, but the data is
not accurate enough to discern an accurate dependence on SFS concentration. The results
of the 0.85 mm sediment coated acrylic cylinder were less conclusive; the plot of the
average concentration curves only shows a 7% shear stress reduction at a suspended
sediment concentration of 0.5 g/1. The curves for 0.05 g/1 and 1.0 g/1 concentrations
overlap the curve for the clear water test.
The results of the tests with the two aluminum cylinders were mixed. For both
cylinders, the first test set indicated a reduction in shear stress, while the second test set
showed an increase in shear stress with increasing SFS concentration. Because of the
conflicting results, an analysis of the average concentration lines was not conducted for
the aluminum cylinders. Why the test results with these cylinders were different from
those with the bonded sand is not clear.
The saltwater tests conducted on the 0.15 mm sediment-coated acrylic cylinder
yielded up to a 7% reduction in shear stress with the addition of SFS. This reduction was
slightly less than what was observed in the freshwater tests. The results of the 0.85 mm
sediment coated cylinder showed only a slight shear stress reduction, less than 5%. This
result was consistent with the fresh water results for the same cylinder. Results from the
saltwater tests are also provided in Appendix A.
Based on the results of the RETA tests, the SFS appeared to have a greater effect
on the shear stress of the cylinder coated with the smaller sediment. A reduction in shear
stress up to 10% was noted for the 0.15 mm sediment-coated cylinder. The reason for the
smaller shear stress reduction with the 0.85 mm sediment-coated cylinder is most likely
due to increased turbulence and a reduced viscous sublayer thickness. A similar result
would be expected in a field situation, i.e., less shear stress reduction would be expected
for a bed with large sediment than for one with fine sediment.
The RETA itself could be a source of some error in the tests. Though the gearing
system was modified to operate at lower RPM values, the angular velocity varied with
time at the lowest RPMs.
In spite of these problems, the tests with the RETA did conclusively show that a
reduction in shear stress does occur with the presence of SFS in the water column. It can
therefore be concluded that shear stress reduction is at least one of the possible causes for
the observed reductions in equilibrium local scour depths due to the presence of SFS in
the water column.
The results of the USGS-BRD and University of Auckland scour tests as well as
the shear stress measurements generated in the RETA provide valuable insight as to the
possible causes of the reduction in scour depth due to an increase in SFS. Video data
recorded during the tests at the USGS-BRD laboratory indicated that the turbidity of the
water did not increase suddenly; thus the SFS affected both the upstream flow conditions
as well as the formation of the scour hole. In Gust (1976) and others, an increase in
suspended sediment caused an increase both in drag reduction and in the height of the
viscous sublayer. If this were the case in the scour tests, the velocity upstream of the pier
would be reduced. Thus the velocity and subsequent shear stress within the scour hole is
lower than would be found in a clear water situation. A reduction in shear stress in the
scour hole reduces the sediment removed around the pier.
An additional aspect of the scour depth reduction is the pattern of the initial rate of
scour in comparison with the test not affected by the suspended sediment. The test
affected by the SFS initially follows the same scour rate as the non-affected test; at a
certain time into the test, the scour depth levels off and stops increasing. If the shear
stress was reduced upstream of the pier, the rate of scour would seemingly not be the
same as that of a test not affected by the suspended sediment. Since there is no sudden
increase in turbidity hours into the scour test, the effect of the suspended sediment must
have a gradual influence on the scour hole. The theory of this gradual influence is that
the suspended fine sediment is slowly deposited on the upstream slope of the scour hole.
Initially, this fine sediment acts as a lubricant, allowing bed particles to fall down the
slope of the scour hole slightly faster than normal. The scour hole development initially
proceeds as normal. Once the amount of deposition reaches a certain point, the cohesive
bonds between the fine sediment cause the bed particles to bond together, preventing both
sediment removal and the avalanching of sediment into the scour hole. Thus the scour
depth does not increase.
The results of the RETA tests indicate that the reduction in shear stress with
increasing suspended fine sediment concentration is greater with a smaller bed sediment
size. The shear stress reduction for the 0.15 mm sediment-coated cylinder was higher
than that found for the 0.85 mm sediment-coated cylinder. The tests affected by the
suspended fine sediment were all ones with 0.22 mm bed sediment. Thus one possible
condition of the scour depth reduction is the bed sediment size.
University of Florida researchers are developing a relationship between effective
shear stress in the scour hole and scour depth. This relationship is useful in explaining
the possible effect of the suspended sediment on the scour depth. The shear stress is
normalized by the upstream bed shear stress and the scour depth is normalized by the
equilibrium scour depth (see Figure 5-3). For clearwater scour tests, the predicted
curve pattern shows an initial increase in effective shear stress as the scour process
begins. The non-dimensional shear stress reaches a peak and then begins to decrease. At
some point, possibly when the horseshoe vortex is located entirely within the scour hole,
the slope of the shear stress curve changes sharply and the rate of scour remains small
until the critical shear stress is reached and the scour stops. The location of the "break
point" in this curve varies for flow, sediment, and pier conditions.
It is known that the level of bed shear stress reduction due to the presence of SFS
decreases rapidly with increased velocity and bed roughness. It seems reasonable that the
effects of SFS will be small during the initial stages of local scour, but will increase as
the scour hole develops and the turbulence level decreases. According to the normalized
shear stress versus normalized scour depth plot shown in Figure 5-3, a small reduction in
the shear stress in the latter stages of the scour hole development can reduce the
equilibrium depth significantly.
Another possible mechanism for equilibrium scour depth reduction is as follows.
Some of the SFS is deposited in the scour hole out from the structure due to the reduced
flow velocities in this region. As the surface material in this area avalanches into the
scour region near the structure, the fine sediment acts to increase the critical shear of the
material. As can be seen in Figure 5-3, an increase in Tc will decrease the equilibrium
While it is not possible with the limited amount of data to positively identify the
cause of scour depth reduction, the evidence points to the conclusion that the reduction in
shear stress due to SFS is at least one of the causes. A variety of experiments will be
required to determine the relative importance of the various proposed mechanisms.
Figure 5-3. Proposed relationship between shear stress and scour depth.
The scour experiments conducted at both the USGS-BRD Laboratory in
Massachusetts and at the University of Auckland have confirmed that an increase in
suspended fine sediment concentration within the flow affects the local equilibrium scour
depth. Because the exact effects of the suspended fine sediment on the local scour depth
appeared to differ with varying flow velocities and sediment diameters, continued
research is needed in order to quantify the change in scour depth for the purposes of scour
Clearwater versus live-bed scour: The tests conducted at the University of
Auckland were directed at establishing the relationship between equilibrium scour depth
and SFS concentration and flow velocity. There were, however, problems with the test
procedure that most likely affected the results. Due to the number of tests required and
the time available, the flume with SFS concentrations was allowed to stand overnight
(and even longer periods of time) between tests. It is suspected that this resulted in
deposition of fine sediment throughout the flume, which accumulated as the tests
progressed and impacted the critical shear stress. The initial test results were consistent
with the findings at the USGS-BRD Laboratory but started to deviate as the tests
progressed. In particular, two of the live bed tests actually indicated an increase in scour
depth with the presence of SFS. Additional tests should be conducted to further examine
the effect of SFS in both the clearwater and live bed velocity ranges; however, care
should be take to ensure that the test procedure itself does not cause unintended effects on
the test results.
Bed sediment diameter: Both the tests conducted at the USGS-BRD Laboratory
and at the University of Auckland utilized a similar median grain size sediment. Most of
the research summarized in Chapter 3 indicated an increase in the height of the viscous
sublayer over mud beds. Best and Leeder (1993) examined drag reduction on sand beds
and found that SFS had a greater impact on the bed forms in the finer sands. Thus the
Dso of the bed likely contributes to the effect of the SFS on the local scour depth. Further
testing is required to determine if the effect of the SFS changes with a change in the bed
Suspended fine sediment concentration: During one of the original USGS-BRD
scour tests, a depth-averaged suspended fine sediment concentration of 0.029 g/1 was
measured. Because this concentration was not measured during a test in which the
highest concentrations of suspended fine sediment were recorded, the tests in this paper
incorporated concentrations up to 1.0 g/1. Additional research is needed to determine if
there is a direct correlation between SFS concentration and a change in the measured
scour depth. Scour depth changes may be limited to flows with low suspended fine
Alteration of scour prediction equations: When the effect of suspended fine
sediment on equilibrium is quantified, the current equations used to predict equilibrium
scour depth will need to be modified. The causes of the scour depth reduction point to
possible ways in which this effect can be incorporated into scour prediction equations.
For example, the University of Florida currently uses the following formula for
clearwater scour prediction:
d= KcJ Uf2L fb (Equation 6-1)
b b Uc D o5
This equation can be modified in one of two ways: first, a constant that quantifies scour
depth reduction can be directly included in the equation. Second, the function of
- can be modified to include the effect, since the reduction in bed shear stress is
similar to a reduction in velocity.
These research topics are necessary in order to quantify the effect of SFS on local
scour depth and to facilitate the alteration of current scour prediction equations to
incorporate the effect of the suspended sediment.
ROTATING EROSION TEST APPARATUS (RETA) TEST DATA
0.15mm Acrylic Cylinder Tests 5-8
g s '----^ -----
200 250 300 350
Figure A-1. RETA Tests 5-8, 0.15 mm Sediment Coated Acrylic Cylinder.
0.15mm Acrylic Cylinder Tests 13-16
0--- 0 g/I
-m___-0 05 g/I
- -A- 05g/1I
-X- '1 0 g/I
--X- 1 0 g/l
Figure A-2. RETA Tests 13-16, 0.15 mm Sediment Coated Acrylic Cylinder.
0.15mm Acrylic Cylinder Tests 25-28
1 -- -------
-- 0 0 g/I
0. ----o 05 g/
0.2- A_- 0 5 g/
-X- '1 0 g/I
200 250 300 350 400 450 500
Figure A-3. RETA Tests 25-28, 0.15 mm Sediment Coated Acrylic Cylinder.
0.15mm Acrylic Cylinder Tests 29-32
Figure A-4. RETA Tests 29-32, 0.15 mm Sediment Coated Acrylic Cylinder.
250 300 350 400 450
0.15mm Acrylic Cylinder Tests 37-40
'' 0 0 g/l
-m-0-O 05 g/I
0 5 g/I
-X- -1 0 g/I
X 1 /
Figure A-5. RETA Tests 37-40, 0.15 mm Sediment Coated Acrylic Cylinder.
0.15mm Acrylic Cylinder All 0.0 g/I Tests
0' --Test 5
1 0.8 _____________
-A- Test 25
.2 K-Test 37
-- Test 57 (salt)
0 ---------------------------------------- ------------
200 250 300 350 400 450 500
Figure A-6. All 0.0 g/l concentration tests conducted on 0.15 mm Sediment Coated Acrylic Cylinder.
0.15mm Acrylic Cylinder All 0.05 g/I Tests
S-- Test 6
0.2 _Test 30
.2 --Test 38
--* --Test 58 (salt)
200 250 300 350 400 450 500
Figure A-7. All 0.05 g/l concentration tests conducted on 0.15 mm Sediment Coated Acrylic Cylinder.
0.15mm Acrylic Cylinder All 0.5 gll Tests
0.2 ------*- Test 7
--- Test 39
- Test 59 (salt)
200 250 300 350 400 450 500
Figure A-8. All 0.5 g/l concentration tests conducted on 0.15 mm Sediment Coated Acrylic Cylinder.
0.15mm Acrylic CylinderAII 1.0 g/l Tests
0.4 --^ *- Test 81
--- Test 28
0.2 x Test 32
--w- Test 40
--*--Test 60 (salt)
200 250 300 350 400 450 500
Figure A-9. All 1.0 g/l concentration tests conducted on 0.15 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder Tests 1-4
0-- 0.0 g/I
A 0.5 g/1I
-X- '1.0 g/I
Figure A-10. RETA Tests 1-4, 0.85 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder Tests 9-12
----*0 0 g/I
-- -0 05 g/I
A- 05 g/l
-X- '1 0 g/I
Figure A-11. RETA Tests 9-12, 0.85 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder Tests 17-20
0.5 ---*--0 0 g/I
-A -0 05 g/I
-A- 0 5 g/
-X- '1 0 g/I
200 250 300 350 400 450 500
Figure A-12. RETA Tests 17-20, 0.85 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder Tests 21-24
0.5 0 0 g/
S--o-0 05 g/I
-A 0 5 g/1I
-X- *1 0 g/I
200 250 300 350 400 450 500
Figure A-13. RETA Tests 21-24, 0.85 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder Tests 33-36
0.5 ----0 0 g/I
0 5 g/1
0.5 --4 -0 0 gI
-X- '1 0 g/1
200 250 300 350 400 450 500
Figure A-14. RETA Tests 33-36, 0.85 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder All 0.0 g/I Tests
-H- Test 9
0.5 ---Test 17
-x-- Test 21
-Test 61 (salt)
200 250 300 350 400 450 500
Figure A-15. All 0.0 g/1 concentration tests conducted on 0.85 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic CylinderAll 0.05 g/l Tests
-* Test 2
0.5 --*-Test 18
-"0 -Test 62 (salt)
200 250 300 350 400 450 500
Figure A-16. All 0.05 g/l concentration tests conducted on 0.85 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder All 0.5 g/l Tests
0 5 -_ -_- T e s t 1 1__ __-------------------------------
-- Test 35
-- -Test 63 (salt)
200 250 300 350 400 450 500
Figure A-17. All 0.5 g/1 concentration tests conducted on 0.85 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder All 1.0 g/l Tests
Figure A-1. All 1.0 gl concentrationTest 4
0.5 Test 12
0.5 -A- Test 20
-x- Test 24
--Test 64 (salt)
200 250 300 350 400 450 500
Figure A-18. All 1.0 g/l concentration tests conducted on 0.85 mm Sediment Coated Acrylic Cylinder.
Smooth Aluminum Cylinder Tests 45-48
250 300 350 400 450 500 550
Figure A-19. RETA Tests 45-48, Smooth Aluminum Cylinder.
Smooth Aluminum Cylinder Tests 49-52
250 300 350 400 450 500
Figure A-20. RETA Tests 49-52, Smooth Aluminum Cylinder.
Rough Aluminum Cylinder Tests 41-44
250 300 350 400 450 500 550 600
Figure A-21. RETA Tests 41-44, Rough Aluminum Cylinder.
Rough Aluminum Cylinder Tests 53-56
. 1 .
0.5 _________ ________
200 250 300 350 400 450 500 550 600 650
Figure A-22. RETA Tests 53-56, Rough Aluminum Cylinder.
0.15mm Acrylic Cylinder Tests 57-60
200 250 300 350 400 450 500
Figure A-23. RETA Tests 57-60, saltwater tests conducted on 0.15 mm Sediment Coated Acrylic Cylinder.
0.85mm Acrylic Cylinder Tests 61-64
-0 0.0 g/1
0.5 0.5 g
-X- -1.0 g/1
200 250 300 350 400 450 500
Figure A-24. RETA Tests 61-64, saltwater tests conducted on 0.85 mm Sediment Coated Acrylic Cylinder.
UNIVERSITY OF AUCKLAND SCOUR TESTING
In order to quantify the effect of suspended fine sediment (SFS) concentrations on
local scour in cohesionless sediments, a series of tests were initiated by University of
Florida researchers in a flume in the Hydraulics Laboratory at the University of
Auckland. An undergraduate student completed these tests as part of an Honors Project.
The purpose of these tests was not to provide information regarding the cause of the
scour reduction, simply the magnitude of scour reduction as a function of the suspended
sediment concentration and the normalized flow velocity.
The results are presented here with the permission of Mr. Thomas Macdougal
Clunie, the student who performed the tests. These results are designed to give further
evidence of the effect of SFS on local equilibrium scour.
The University of Auckland (UA) flume used for these tests is a glass-sided flume
that is approximately 0.440 m wide, 0.44 m deep, and 19 m long. Unlike the USGS-BRD
flume in Massachusetts, the UA flume is a closed flume; all of the water and sediment are
recycled. A 0.15 m diameter supply line controls the discharge, while a tailgate at the
downstream end controls the water level within the flume. Supported by a central pivot,
the slope of the flume can be adjusted using manual screw-jacks located at both ends of
the flume (Ettema, 1980).
Local scour tests were conducted for four SFS concentrations (0.0 g/l, 0.1 g/l, 0.5
g/l, and 1.0 g/l) and 4 normalized velocities (0.95 U, 1.1Uc, 1.5 U, and 2.0U,). The first
velocity ratio is in the clearwater scour range, while the remaining three are in the live
bed scour range. Note that the USGS-BRD Experiments A and B discussed in Chapter 2
were conducted at velocity ratios of 0.92 and 0.97, respectively; thus the clearwater tests
fall within the range of USGS-BRD tests where scour reduction due to SFS was
observed. The sediment concentrations used were designed to approximate the
magnitude of the SFS concentrations measured in the USGS-BRD scour tests. The test
structure was a 50 mm diameter Plexiglas cylinder. The bed sediment had a Dso of 0.24
mm. The water depth was maintained at 170 mm for all of the tests.
Even though the size and type of flume used in the UA tests was very different
from that at the USGS-BRD flume facility, the parameters -0- and in the UA tests
were similar to those in the USGS-BRD tests. The bed sediment diameters for the two
tests were also similar (0.22 mm for the USGS-BRD tests and 0.24 mm for the UA tests).
However, the UA pier diameter (and therefore the ratio) was only approximately 5%
for that of the USGS-BRD tests. As discussed in Chapter 1, local scour depth has been
y U b
found to depend primarily on the non-dimensional quantities -0, -, and The UA
b U, D ,
tests were conducted at about the same value of -0- as the USGS tests, but the value of
was much smaller. The UA tests covered a range ofU values including those in
D 50 U,
the USGS tests.
A procedure, similar to that used in the USGS-BRD tests, was followed in the UA
tests. Scour depth was measured using a single acoustic transponder that measured depth
approximately 15 mm in front of the circular pier. In order to ensure that the transponder
would not be affected by the addition of SFS, the transponder was tested for a known
depth over the range of suspended sediment concentrations. The transponder recorded
the correct depth each time.
The pre, during, and post-experiment procedure used for these tests is outlined
1. Compact and level the bed in the flume.
2. Thoroughly mix desired concentration of bentonite powder in water. To ensure that
bentonite flocs do not form, pass the mixture through a 60tlm sieve.
3. Pour concentrated solution into flume.
4. Start acoustic transponder.
5. Begin experiment, quickly increasing the velocity of the flow to predetermined
6. Measure the scour depth versus time using acoustic transponder.
7. Record water velocity, flow depth, and temperature, as well as observations of the
8. Gather samples of the water during the experiment to determine actual suspended
fine sediment concentrations.
9. Once the equilibrium scour depth has been reached, stop experiment.
10. Note the condition of the bed following the test.
11. Conduct data reduction and analysis; plot time-history of scour depth
The tests were conducted in the order of increased SFS concentrations, i.e., all of
the zero concentration tests at the four velocity ratios were conducted first. This was
followed by tests with the smallest SFS concentration at all four velocities, and so on
until the 1.0 g/1 concentration test was completed. Due to time constraints, the flume was
not emptied between scour tests. This appears to have influenced the test results.
The time-history scour plots are compiled into Figures B-1 through B-4. Each plot
represents a comparison of the entire scour tests conducted at a single velocity but at
varying SFS concentrations.
Initial results shown in the 0.95Uc tests indicate that the presence of suspended fine
sediment did cause up to a 15% reduction in equilibrium scour depth near the end of the
tests. The lowest equilibrium scour depths were recorded at the 0.84 g/1 and the 0.1 g/1
concentrations. The test at 0.5 g/1 suspended fine sediment yielded a 10% reduction in
equilibrium scour depth.
Three sets of tests were conducted in the live bed range of velocities. The first set,
with a velocity of 1.1 Uc, indicated that as suspended fine sediment was added to the flow,
the measured equilibrium scour depth actually increased; at the end of the allotted time,
almost a 10% increase in the scour depth was shown between the tests with 0.1 g/1 and
0.5 g/1 and the test with no suspended fine sediment.
The tests conducted at 1.5 U~ show the movement of bed forms through the scour
hole that is characteristic of tests conducted in the live bed velocity range. A reduction in
scour depth, which could approach 8% depending upon the position of the bed forms,
was indicated at the end of the allotted test time. However, the average difference in
scour depth between the suspended sediment tests and the tests without the sediment was
only around 5%.
The final test series, conducted at 2.0Uc, shows dramatic bed forms moving through
the scour hole during the test. While the final plot shows that the measured scour depths
overlap for the tests with the suspended fine sediment, it does indicate that the presence
of the suspended fine sediment causes an increase in the scour depth. This is in contrast
with the thinking that the presence of suspended sediment only decreases the equilibrium
scour depth. However, the fact that the flume was not drained between tests may have
affected these results. Additional sediment settling onto the bed between the tests may
actually change the velocity ratio of the 2.0Uc tests by increasing the critical velocity.
The four sets of scour tests conducted at UA provide some useful information about
the effect of SFS concentrations on equilibrium local scour depths. In the test conducted
in the clearwater velocity range, reductions in scour depth from 10-15% were measured.
The results from the live bed scour tests were somewhat conflicting, but they do indicate
that the effects of SFS on equilibrium scour depths is reduced significantly with increased
flow velocity. The tests at 1.5 U indicated a slight reduction (approximately 5%) in scour
depth, while the tests at 1.1 and 2.0U, actually yielded an increase in the measured scour
The magnitudes of the scour reduction in the UA tests were lower than those
experienced in the USGS tests (up to 40% in the USGS-BRD tests and only 15-20% in
the UA tests). The suspended sediment in the USGS-BRD tests represented that
occurring naturally from rainwater and snowmelt runoff. The suspended sediment in the
UA tests was bentonite. One explanation for the contradictory finding in the UA tests
that the scour depth increased with the presence of SFS is that fine sediment was
deposited on the bed during the intervals between tests. Small amounts of fine sediment
mixed with cohesionless sediments (sand) have been found to reduce the critical shear
stress compared to that for the sand alone. The thin coating of fine sediment on the sand
grains acts as a lubricant and reduces the stresses required to initiate sediment motion. A
reduced critical shear stress would mean an increased and thus an increased scour
0.95 Uc Local Sour Tests
I I "II II : 1111 III III
0.0 gl ----0. g0l- Og2gA -- -- 0.5 g/l 0.84 g/l ......1. og 4
Figure B-1. University of Auckland Scour Tests, 0.95 Uc.
1.1 Uc Local Scour Tests
0 203C 13C0 63C0 83CO 'X00 'C00
o- Oog ----. 0-1 g/I --O--- 0 ........ 1-OgAI
Figure B-2. University of Auckland Scour Tests, 1.1 Uc.
1.5 Uc Local Scour Tests
:C03 3CO .1030
.Io g/ ---- 0a.1gA 0-.5g/A -...--- 1.0 g/I
Figure B-3. University of Auckland Scour Tests, 1.5 Uc.