Sedimentology (2003) 50, 149–168
The stability of a remediated bed in Hamilton Harbour,
Lake Ontario, Canada
CARL L. AMOS*, IAN G. DROPPO, EDUARD O A. GOMEZ and TOM P. MURPHY
*School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton,
Hampshire SO14 3ZH, UK (E-mail: cla8@soc.soton.ac.uk)
National Water Research Institute, PO Box 5050, Burlington, Ontario, Canada
IADO, Bahia Blanca, Argentina
ABSTRACT
In situ measurements of lakebed sediment erodibility were made on three sites
in Hamilton Harbour, Lake Ontario, using the benthic flume Sea Carousel. Three
methods of estimating the surface erosion threshold (sc(0)) from a Carousel time
series were evaluated: the first method fits measures of bed strength to eroded
depth (the failure envelope) and evaluates threshold as the surface intercept;
the second method regresses mean erosion rate (Em) with bed shear stress and
solves for the floc erosion rate (Ef) to derive the threshold for Em ¼ Ef ¼ 1 · 10)5
kg m)2 s)1; the third method extrapolates a regression of suspended sediment
concentration (S) and fluid transmitted bed shear stress (s0) to ambient
concentrations. The first field site was undisturbed (C) and acted as a control;
the second (W) was disturbed through ploughing and water injection as part of
lakebed treatment, whereas the third site (OIP) was disturbed and injected with
an oxidant used for remediation of contaminated sediment. The main objectives
of this study were: (1) to evaluate the three different methods of deriving erosion
threshold; (2) to compare the physical behaviour of lacustrine sediments with
their marine estuarine counterparts; and (3) to examine the effects of ploughing
and chemical treatment of contaminated sediment on bed stability. Five
deployments of Sea Carousel were carried out at the control site. Mean
erosion thresholds for the three methods were: sc(0) ¼ 0Æ5 (±0Æ06), 0Æ27 (±0Æ01)
and 0Æ34 (±0Æ03) Pa respectively. Method 1 overpredicted bed strength as it was
insensitive to effects in the surface 1–2 mm, and the fit of the failure envelope
was also highly subjective. Method 2 exhibited a wide scatter in the data (low
correlation coefficients), and definition of the baseline erosion rate (Ef) is largely
arbitrary in the literature. Method 3 yielded stable (high correlation
coefficients), reproducible and objective results and is thus recommended for
evaluation of the erosion threshold. The results of this method correlated well
with sediment bulk density and followed the same trend as marine counterparts
from widely varying sites. Mass settling rates, expressed as a decay constant, k,
of S(t), were strongly related to the maximum turbidity at the onset of settling
(Smax) and were also in continuity with marine counterparts. Thus, it appears
that differences in salinity had little effect on mass settling rates in the examples
presented, and that biological activity dominated any effects normally
attributable to changes in salinity. Bedload transport of eroded aggregates
(2–4 mm in diameter) took place by rolling below a mean tangential flow
velocity (Uy) of 0Æ32 ms)1 and by saltation at higher velocities. Mass transport as
bedload was a maximum at Uy ¼ 0Æ4 ms)1, although bedload never exceeded
1% of the suspended load. The proportion of material moving as bedload was
greatest at the onset of erosion but decreased as flow competence increased.
2003 International Association of Sedimentologists
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C. L. Amos et al.
Given the low bulk density and strength of the lakebed sediment, the presence of
a bedload component is notable. Bedload transport over eroding cohesive
substrates should be greater in estuaries, where both sediment density and
strength are usually higher. Significant differences between the ploughed and
control sites were apparent in both the erosion rate and the friction coefficient
(/), and suggest that bed recovery after disruption is rapid (< 24 h). sc(0)
increased linearly with time after ploughing and recovered to the control mean
value within 3 days. The friction coefficient was reduced to zero by ploughing
(diagnostic of fluidization), but increased linearly with time, regaining control
values within 6 days. No long-term reduction in bed strength due to remediation
was apparent.
Keywords Erosion threshold, lacustrine sediments, benthic flume, mass
settling rates, deposition threshold.
INTRODUCTION
Erosion threshold is often used as the index of
bed stability in the study of cohesive/adhesive
muddy sediments, and a variety of methods have
been used to define this threshold. An evaluation
of some of these methods is given by Sutherland
et al. (1998a), who found large differences in
results depending on the method used. In the
present study, this work is expanded, and the
differences in results are examined in three
extrapolation methods used to define erosion
threshold. The purpose of the present investigation is to define a robust and objective means of
defining sediment erosion threshold from in situ
measures of suspended sediment concentration
(S) in order to evaluate changes in bed stability.
Absolute accuracy is not claimed in this study
because of the lack of a ‘traceable standard’ in the
estimation of bed shear stress over naturally
roughened surfaces (Gust & Muller, 1997) and
uncertainties in the definition of ‘erosion threshold’ (Lavelle & Mofjeld, 1985; Sutherland et al.,
1998a; Houwing, 1999).
Bioremediation of contaminated sediments has
been undertaken in Hamilton Harbour, Lake
Ontario (Murphy et al., 1995), and involves the
physical disruption of the topmost 0Æ3 m of
lakebed by a towed rake and the injection of
fluidized oxidant (calcium nitrate) into the sediment. Although the effects of this treatment on
the stability and mobility of the lakebed have not
been evaluated specifically, previous treatments
carried out in St Mary’s River have been shown
not to enhance mobility of the sediments
(Murphy et al., 1996) on the basis of sediment
traps placed both upstream and downstream
of the treatment site.
Several in situ measurements of bed stability
have been made in the marine environment, but
few measurements have been made in lakes.
Within the Great Lakes, analogous trends to
marine counterparts have been found in flocculation rate and floc diameter (Lick et al., 1992),
resuspension and transport mechanics (Lick
et al., 1994), resuspension rates and thresholds
(Ziegler & Nisbet, 1994) and sediment mass
balances incorporating settling rates (Cardenas
et al., 1995; Ziegler & Nisbet, 1995). These
observations have been incorporated within the
sophisticated sedimentation models of Gularte
et al., 1980), Lee et al. (1981) and Ziegler & Lick
(1997). The present study builds on this work to:
(1) define the stability of natural lakebed sediments and how these differ from estuarine
counterparts; (2) define the behaviour of sediments through controlled erosion events; and (3)
determine the impact of bioremediation on bed
stability. The study was undertaken using the
benthic, annular flume Sea Carousel (Amos et al.,
1992) on a treatment test site in Hamilton
Harbour, Lake Ontario, Canada.
BACKGROUND
Hamilton Harbour is an industrial port for the
steel manufacturing industry located at the western end of Lake Ontario (Fig. 1), which is a
freshwater setting. The harbour has a mean and
maximum depth of 13 and 24 m, respectively,
and an area of 2150 ha. The harbour is the
receiving water body for a watershed of
49 400 ha that drains the surrounding limestone/dolomite Niagara escarpment and is sheltered from strong currents and large waves, so the
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
151
Fig. 1. Location diagram of Hamilton Harbour, situated at the western
end of Lake Ontario, Canada.
site is depositional. The bed of the harbour is
composed of 20 000 000 m3 of gassy, organic-rich
(7% dry weight), contaminated silt with a water
content of 330% (Rukavina & Versteeg, 1996).
Most of the contamination of the harbour (heavy
metals, PCBs, PAHs, oil and grease) is the result
of long-term storm-water runoff, sewer overflow,
sewage treatment plant input, atmospheric input,
direct overland flow and industrial input (Irvine
et al., 1998). The study site is situated 100 m off
a steel plant in 5–7 m of water on a gently
undulating, featureless lakebed (Fig. 1).
METHODS
Three adjacent test sites were marked out with
Grimsby floats within a 50 · 50 m flat region of
Hamilton Harbour. The first site was designated
as control (C), the second (W) was ploughed and
injected with lake water to a depth of 0Æ3 m, and
the third (OIP) was ploughed and injected with
calcium nitrate. The bed was ploughed by towing
an 8 m wide injection rake over the lakebed in a
manner described by Murphy et al. (1995).
A benthic flume (Sea Carousel) was used to
carry out replicate in situ measurements of bed
stability within the three sites. Sea Carousel is a
benthic annular flume designed for field use in
subaqueous settings and is 1Æ0 m in radius with
an annulus 0Æ15 m wide and 0Æ30 m high. It
weighs 40 kg in water, is made of aluminium
(Fig. 2) and, by rotating a moveable lid that is
driven by a 0Æ75 hp surface-powered digital stepping motor, it induces flow in the annulus. Eight
small paddles, spaced equidistantly beneath the
lid, induce a flow of water in the annulus. The
Carousel is equipped with three optical backscatter sensors (OBSs; Downing, 1983) with two of
these being located non-intrusively in the inner
wall of the annulus at heights of 0Æ03 and 0Æ18 m
above the base. The third OBS detects ambient
suspended sediment concentration (S) outside
the annulus. A water-sampling port is situated in
the outer wall of the annulus at a height of 0Æ2 m
and is used to calibrate the OBS sensors and to
collect samples for physical, biological and
chemical analysis.
Mean flow in the Carousel was determined from
a relationship between azimuthal speed and lid
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
152
C. L. Amos et al.
Fig. 2. Sea Carousel with the major equipment annotated.
rotation (Amos et al., 1992) and was later verified
in laboratory measurements using a variety of
measurements (Thompson et al., 2002). The
mean tangential lid rotational speed was detected
by a shaft encoder resting on the lid (Fig. 2), and
the tangential (U) and vertical (W) mean current
speeds were measured by a Marsh-McBirney
electromagnetic current meter (EMCM model
513–2Æ54 cm head diameter) situated 0Æ18 m
above the bed. The current meter was calibrated
in a tow tank at Bedford Institute of Oceanography and was accurate to ± 20 mm s)1. Electrical
offsets in the EMCM depend on local conditions
and so were determined in situ in still water at the
start of each experiment. Output voltages from all
sensors were digitized and transformed to absolute units in real time on a Campbell Scientific
CR10 data logger and stored on an SM192 storage
module and a PC hard drive.
The sampling rate of all channels was 1 Hz, and
flow speed was increased in steps through a series
of script commands issued to the digital motor.
The erosion process was recorded through a
window in the side of the flume using a Sony
Handycam 8 mm video recorder held in an
underwater housing. The bed was illuminated
through a Perspex window by two 100-W lights
powered from the surface. Pitch and roll sensors
that were accurate to <1 monitored the flume
attitude, while a Parascientific pressure sensor
measured flow depth from within the electronic
housing to a resolution of ± 10 mm.
DEFINITIONS
Sea Carousel
The underlying assumption in the evaluation of
bed response is that the detection sensors account
for the total suspended mass (S) eroded from the
bed in Sea Carousel; that is, continuity of this
mass is maintained. Sediment continuity in three
dimensions is given by:
@S @S @S
@S ¼ 0
þU
þV
þW
@t
@x
@y
@z
ð1Þ
where the first term is the change in S with
time, which is measured by the inside OBS
sensors, and the last three terms represent tangential, radial and vertical advection by mean
currents U, V and W in the x, y and z directions
respectively.
Equation 1 is simplified within the closed
environment of the Sea Carousel as the second
and third terms are eliminated. The last term is
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
also simplified to a benthic flux resulting from the
small depth within the flume (usually 0Æ3 m, but
this varied depending on penetration of the flume
into the bed) and assumes well-mixed conditions:
@S
s S ¼ 0
W
@t
ð2Þ
where Ws is the vertical mass velocity (in this
notation, positive is downwards). Note that vertical diffusion through the water column, which
is proportional to ¶2S/¶z, is zero under the wellmixed conditions assumed here and so is ignored.
Leakage from the flume took place continuously
across the moving lid. During periods of bed
erosion, S in the flume was considerably greater
than the ambient S, causing sediment losses.
These losses were quantified using the diffusion
equation of Sverdrup (1952) with a diffusivity
constant (e) of 1Æ43 · 10)3 m2 s)1; dS/dx expresses
the gradient in concentration across the flume
wall ([(OBS1 + OBS3)/2)–OBS2]/Dy), where Dy
was evaluated from independent experiments on
dispersion (Amos et al., 1992):
h
@S
s S þ e @S
¼ W
@t
@Dy
ð3Þ
where h is the flume flow depth. The depth of
erosion (z) beneath the flume was determined
using Exner’s formula:
qb ð1 PÞ
@z
@S
s S þ e @S
¼h
¼ W
@t
@t
@Dy
ð4Þ
The depth of erosion (from McDowell & O’Connor, 1977) was derived from the sum of the
changes in S (measured), leakage (estimated) and
the benthic flux (derived) where P is the sediment
porosity, and qs is sediment density. In a wellmixed case, the buoyant (upward) flux is equated
with the benthic flux, where the benthic flux is
the difference between the deposition (D) and
erosion (E) rates:
s S ¼ D E
W
ð5Þ
It is assumed that, when E > 0, then D ¼ 0 and,
when D > 0, then E ¼ 0: Partheniades et al. (1968)
and Lau & Krishnappan (1994) have verified this
assumption. Assumptions made in estimating
eroded depth (z) are: (1) the sediment bulk
density profiles derived from CT scanner analysis
of cores are representative of the Sea Carousel site
as they were adjacent to each other; (2) the
density profile is constant over the footprint of
153
the Sea Carousel; and (3) erosion is constant
within the flume.
The time series of the erosion process may be
transformed into a depth profile of sediment yield
strength (sb(z)) to create a synthetic core. This is
based on the assumption that, at the point at
which erosion ceases, within each increment of
flow s0 ¼ sc(z) ¼ sb(z). Thus, by plotting the applied bed shear stress vs. eroded depth and fitting
lines to the data trends, structure within the
lakebed sediments begins to emerge. The trends
in strength with depth can be quantified in terms
of the friction coefficient, which is defined below.
The effect of S on the suppression of bed shear
stress is complex because of: (1) turbulence
dampening resulting from changes in the
Monin–Obhukhov length scale (the ratio of turbulent kinetic energy production by shear to
energy consumption by sediment mixing): Bagnold (1966), Best & Leeder (1993), Crapper & Ali
(1997), Li & Gust (2000) and Baas & Best (2002)
have examined this effect; (2) changes in viscosity
of the fluid with consequent effects on the
structure of the viscous sublayer (Gust, 1976);
and (3) fluid momentum transfer to accelerating
saltating aggregates brought into suspension during the erosion process (Bagnold, 1936; Amos
et al., 2000). Notwithstanding the above, experiments on stress reduction are inconclusive
because of the lack of a traceable standard of
bed shear stress over naturally roughened beds.
Nevertheless, the following stress reduction
algorithm has been applied to the present data
on the basis of results in Amos et al. (1992) and
Li & Gust (2000):
U s ¼ U f0 2267½log10 ðSÞ :ðU =6 35Þg cm s 1 ð6Þ
where S is evaluated in mg L)1 and clear-water
friction velocity U* ¼ (s0/q) and U*
s is the
turbidity-reduced friction velocity. s0 was evaluated using the quadratic stress law, where
s0 ¼ CdqU2 and Cd ¼ 4Æ0 · 10)2 (Amos et al.,
1992). The mean azimuthal flow in the flume
(U) was determined from mean lid rotational
speed (R) using a laboratory-derived equation:
U ¼ 0Æ574(R); r2 ¼ 0Æ92; lid rotation was checked
routinely in situ by tracking marks on the rotating
lid with video and was found to be constant. The
mean mass settling rate is calculated from:
W s ¼ ðdM =dtÞ=SðtÞ
ð7Þ
at 10 s intervals of t throughout the settling
period, and Ws is transformed according to Gibbs
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154
C. L. Amos et al.
et al. (1971) into an equivalent sedimentation
radius (d/2, in cm) by:
d=2 ¼ P1 þ
p
P2 þ ½P5ðP3 þ P4Þ =P5
ð8Þ
where
P1 ¼ 0Æ055804(q2Ws ),
P2 ¼ 0Æ003114
2
4
(q Ws ), P3 ¼ 4Æ5lWs, P4 ¼ 8Æ704 · 10)3 (q2Ws )
and P5 ¼ 981(qb–q), in which l is the absolute
viscosity of fresh water at 20 C (0Æ0131 poises),
qb is the aggregate bulk density equated with
the sediment bulk density at the equivalent
eroded depth (all evaluated in cgs units). Stokes
Law is not applicable to the fast settling sediments of this study (Middleton & Southard,
1984). The fluid density was adjusted for S as
follows:
q ¼ ½qo ð1 V s Þ þ qs V s
ð9Þ
where qo is the clear-water density (1000 kg m)3),
Vs is the suspended sediment volume, and qs is
the sediment density (2650 kg m)3).
Bed stability is defined in terms of four measurable characteristics:
(1) The erosion threshold (cohesion) (sc(0)) is
interpreted as the point at which the surface of
the bed begins to erode and is defined in three
ways in this study:
(a) Method 1: the surface intercept of the failure
envelope on plots of applied shear stress vs.
eroded depth. Such plots define changes in
sediment strength throughout the erosion process
and assume that, at an applied bed shear stress
(s0), mean bed erosion (Em) will stop at the time
(t) when the bed has eroded to a depth (z) at
which the bed strength (sb) equals the applied
stress: sb(z) ¼ sc(z) ¼ s0(t) and Em ¼ Ef (Mehta &
Partheniades, 1982).
(b) Method 2: the value at which the erosion
rate approaches a nominal erosion rate at zero
shear (Ef ¼ 1 · 10)5 kg m)2 s)1) in a regression of
erosion rate vs. applied bed shear stress.
(c) Method 3: the value at which sediment
concentration, S, approaches ambient conditions
(So) on a correlation plot of (S) vs. applied bed
shear stress.
(2) The friction coefficient (/) is adapted from
Terzaghi & Peck (1967): / ¼ tan)1 (sc(z)/r¢) and
uses results from the synthetic cores. Depth is
transformed to an effective stress (r¢) or buoyant
geostatic load from a knowledge of sediment (wet)
bulk density (qb): r¢ ¼ (qb–q)gz + P¢, where g is
the gravitational constant and P¢ is the ambient
pore pressure (unknown and so assumed to be
zero). In this study, / is defined as the rate of
change in bed strength with eroded depth. The
locus that describes this relationship is called ‘the
failure envelope’. The value of qb was taken as the
mean of the topmost 10 mm of the sediment
column: qb(C) ¼ 1150 kg m)2; qb(W) ¼ 1250 kg m)2;
qb(OIP) ¼ 1250 kg m)2.
(3) The mean erosion rate, Em, is a function of
applied bed shear stress and eroded depth; it is
defined as the difference between the starting and
final S within any velocity increment: Em ¼ dM/
dt ¼ (S(t)–S(t–Dt))Vsc/Dta, where M is the eroded
dry mass, Vsc is the Sea Carousel volume
(0Æ218 m3), a is the flume bed area (0Æ87 m2) and
Dt is the duration of the applied eroding bed shear
stress (after Krone, 1962).
(4) The type of erosion is either asymptotically diminishing with time (type I) or constant
(type II; after Villaret & Paulic, 1986). Type I
erosion is characterized by the release of flocs
and small pellets (surface erosion), and the
mode of transport is largely in suspension;
type II erosion occurs through the release of
rip-up clasts and large (up to 8 mm) aggregates
(mass or bulk erosion) often seen moving as
bedload.
Bottom sediment sampling
Bottom sediment samples were collected using an
NWRI box corer. The corer was 0Æ50 · 0Æ50 m in
plan and 0Æ80 m in length. Two syringe cores
(60 cm3), a push core 10Æ2 cm in diameter and a
bulk sample were collected from the undisturbed
central parts of the corer. The syringe cores were
frozen immediately by immersion in liquid nitrogen and kept frozen for the analyses of bulk
density and microfabric using a GE Hilite
scanner (Amos et al., 1996a). The push core was
frozen slowly and stored in a frozen state. Bulk
samples were collected and kept at ambient
temperatures for analyses of water content,
organic carbon content, chlorophyll a content
and grain size.
Water sampling
Water samples were collected from a sampling
port (mid-depth) about 60 s into each flow increment. Known, well-mixed volumes (Vl) of the
samples were filtered through a Swinnex system onto preweighed, Gelmann glass-fibre filters to remove the suspended solids. S was
determined from the gravimetric weight of the
suspended load (Wg): S ¼ Wg/Vl. Duplicate samples were collected for particulate organic carbon,
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
major ion and chlorophyll a analysis as well as
for conventional optical microscopy for observations of particle structure (Liss et al., 1996). These
samples were also analysed for suspended particle (floc/aggregate) size using an Optimax V
system. Settling velocity was measured as well,
and aggregate density was determined using a
Northern Exposure image analysis system
(Droppo et al., 1997). Size spectra of eroded
particles were measured using a Malvern particle size analyser (series 2600C) on water samples
pumped from the Sea Carousel. Analyses of
chlorophyll a and organic carbon were conducted
according to the methods described by Parsons
et al. (1984).
155
fraction was composed of illite, Fe-rich chlorite,
an abundance of quartz, small amounts of smectite, vermiculite and traces of kaolinite (Murdoch
& Zeman, 1975). The sediments were poorly
sorted, reflecting a variety of sources. The sand
fraction was largely made up of anthropogenic fly
ash and other airborne industrial emissions.
Analysis of major elements (EDAX) with time in
the Sea Carousel showed that only SiO2 varied
throughout the deployment, systematically
increasing in proportion to S. All other major
ions (K, Cl, Na, Mg and SO4) remained steady,
reflecting an association with porewater fluids
rather than siliceous sediment (Amos & Droppo,
1996; Droppo & Amos, 2001).
Sea Carousel
RESULTS
Bottom sediment analysis
The surface sediment texture was consistent,
comprising between 10% and 15% sand, 43%
silt with the remainder being clay. The disaggregated median diameter was 4–6 lm. The clay
A summary of the results obtained from all
deployments and tentative interpretations are
given in Table 1. Five successful deployments of
the Sea Carousel were completed at the control
site (C2 was abandoned as the flume dropped into
the sediments too rapidly causing unacceptable
disturbance) and four in each of the disturbed
Table 1. Sea Carousel station summary including data on surface erosion thresholds (correlation coefficients given
in brackets), computed friction coefficients and the range of wet weight sediment bulk densities determined from CT
scanner analysis of syringe cores.
Erosion threshold (Pa)
Deployment
site
Method 1
Eroded depth*
Method 2
Mean E
Method 3
Mean S
Friction
coeff. (/) (degrees)
Bulk density
(kg m)3)
C1
C3
C4
C5
C6
Mean
0Æ6
0Æ5
0Æ4
0Æ5
0Æ5
0Æ5 ± 0Æ06 Pa
–
0Æ29 (0Æ73)
0Æ28 (0Æ61)
0Æ25 (0Æ52)
0Æ28 (0Æ47)
0Æ27 (0Æ60)
0Æ27 ± 0Æ01 Pa
r2 ¼ 0Æ59
0Æ32 (0Æ93)
0Æ32 (0Æ95)
0Æ31 (0Æ93)
0Æ36 (0Æ95)
0Æ38 (0Æ95)
0Æ34 ± 0Æ03 Pa
r2 ¼ 0Æ94
9
12
11
18
11
12Æ2 ± 3Æ1
–
920–1380
–
–
–
–
W1
W2
W3
W4
Mean
0Æ5
0Æ5
0Æ4
0Æ6
0Æ5 ± 0Æ07 Pa
–
0Æ13 (0Æ78)
0Æ36 (0Æ61)
0Æ19 (0Æ63)
0Æ26 (0Æ36)
0Æ23 ± 0Æ08 Pa
r2 ¼ 0Æ59
0Æ17 (0Æ96)
0Æ40 (0Æ96)
0Æ28 (0Æ97)
0Æ48 (0Æ95)
0Æ33 ± 0Æ12 Pa
r2 ¼ 0Æ96
2
3
5
8
4Æ5 ± 2Æ3
–
–
–
1100–1430
–
OIP1
OIP2
OIP3
OIP4
Mean
0Æ3
0Æ4
0Æ4
0Æ5
0Æ4 ± 0Æ07 Pa
–
0Æ12
0Æ22
0Æ21
0Æ23
0Æ19
r2 ¼
0Æ17
0Æ29
0Æ30
0Æ38
0Æ28
r2 ¼
2
5
6
8
5Æ2 ± 2Æ2
–
–
–
1100–1530
–
(0.74)
(0.71)
(0.57)
(0.59)
± 0Æ04 Pa
0Æ65
(0.96)
(0.96)
(0.98)
(0.97)
± 0Æ07 Pa
0Æ97
* Calculated from synthetic core plots as surface intercept of bed strength.
Calculated with corrected (dispersion) erosion rate.
Calculated from corrected S.
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
156
C. L. Amos et al.
sites. A typical time series from station C4 from
the control site is shown in Fig. 3. Lid speed was
increased in a series of 11 equal steps to 1Æ7 ms)1
(Uy ¼ 0Æ97 ms)1). Fully turbulent conditions, evident as azimuthal flow fluctuations, occurred
above 0Æ3 ms)1 (Uy ¼ 0Æ17 ms)1); above these
velocities, the assumption of fully mixed conditions is appropriate. The EMCM flow sensor
showed very large and erratic fluctuations, probably caused by the highly magnetized nature of
the anthropogenic material from stack emissions
and surface runoff, which would influence the
EMCM sensor. Consequently, lid speed was used
to derive bed shear stress. Increases in S with
time denoted erosion, which followed a type I
pattern (asymptotically decaying). The erosion
rate was generally greatest at the onset of each
velocity increment and declined thereafter. Peak
erosion rate increased with increasing applied
flow velocity. Results of the corrected uppermost
OBS showed synchronized trends of increasing S
with current speed in Fig. 3A (the lowermost
OBS was usually buried and so was not used).
The plot of S, uncorrected for dispersion (leakage), agrees reasonably well with the pumped
samples (solid dots, Fig. 3B). Figure 3C illustrates
the erosion rate time series determined from the
changes in corrected S with time. The erosion
process began with movement of the organic-rich
detritus (type Ia), which then gave way to erosion
of bed material (type Ib) that moved partly as
surface creep and partly in suspension. The
erosion threshold was based on the onset of type
Ib erosion. Surface creep (bedload) infilled
depressions in the bed to create a smoothed
surface and took place before the onset of mass
Fig. 3. Time series plot for control
site C4. (A) Lid rotation and azimuthal current velocity. (B) Suspended sediment concentration (S)
derived from gravimetric analysis
and OBS sensors inside and outside
the flume. (C) Erosion rates calculated from dispersion-corrected
suspended sediment concentrations.
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
erosion of the bed. The mean size of aggregates
moving as bedload was 2–3 mm and increased
with mean velocity reaching maximum sizes of
10 mm. Bedload peaked at 0Æ3–0Æ4 ms)1 and
dropped thereafter as aggregates moved into
suspension. Above 0Æ3 ms)1, saltation of aggregates began, which dominated the bedload component above 0Æ4 ms)1. The finer fraction moved
into suspension immediately, but suspension
often lagged behind the onset of traction. Saltation
of rip-up clasts was evident in all deployments.
157
The mean frequency of aggregates through the
field of view varied up to 3 s)1.
Undisturbed site erosion thresholds
and erosion rates
Examples of the three methods used to determine
erosion threshold (sc(0)) are shown in Fig. 4; all
are taken from station C4. The erosion threshold
(sc(0)) in Fig. 4A is complicated by a change in /
at depths of 2 and 4Æ5 mm. The position of the
Fig. 4. Plots used in the determination of erosion threshold at control
site C4. (A) Method 1: sc(0) is
derived from the surface intercept of
the failure envelope in the synthetic
core plot. (B) Method 2: sc(0) is
derived from regression of erosion
rate on applied bed shear stress,
solving shear stress for
Em ¼ 1 · 10)5 kg m)2 s)1. (C)
Method 3: sc(0) is derived from
regression of S on applied bed shear
stress, solving for S at the ambient
concentration.
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
158
C. L. Amos et al.
failure envelop (joining points where erosion
ceases) is subjective, thus reducing the reliability
of the estimate of sc(0). The plot is, however,
valuable in detailing the macrofabric of the
sediment through changes in /. This fabric is
evident as three distinct layers of / ¼ 8, 20 and
15 with increasing depth. Method 2 is objective
and reproducible (using regression analysis to
derive sc(0)). However, the low mean correlation
coefficient (r2 ¼ 0Æ52) reflects the uncertainty in
the estimation of sc(0). Furthermore, extrapolation of the power function to an erosion rate of
zero is not possible and so a ‘background’ rate (Ef)
has been assigned a value of 1 · 10)5 kg m)2 s)1
(Parchure & Mehta, 1985). Method 3 is shown in
Fig. 4C, where the correlation of data is stronger
(r2 ¼ 0Æ93), and the intercept (at ambient
So ¼ 85 mg L)1) is unambiguous. A summary of
sc(0) values is given in Table 1. The mean erosion
thresholds for the control site for the three
methods were: (1) 0Æ5 (±0Æ06) Pa; (2) 0Æ27 (±0Æ01)
Pa, r2 ¼ 0Æ59; and (3) 0Æ34 (±0Æ03) Pa, r2 ¼ 0Æ94.
The numbers in brackets define spatial heterogeneity and errors, which account for about 10%
of the signal. Method 1 gave the highest scatter in
results, and method 2 yielded the lowest. Methods 2 and 3 predicted substantially lower thresholds than method 1. Method 3 was reproducible
and objective (yielding the highest r2 values) and
is considered to be the most reliable in this study.
For purposes of comparison, this method was
chosen as the standard.
Undisturbed site settling rates and size spectra
The mean still water mass deposition rate (dM/dt)
has been derived from the rate of change in S
within the Sea Carousel (after Krone, 1962;
Einstein & Krone, 1967). The mean settling curves
for the three sites (Fig. 5A) show an exponential
Fig. 5. Results of the still water
settling for the three sites, illustrating (A) exponential trend in S with
time, (B) fining in mean sedimentation diameter from fine sand to
coarse silt with time, and (C)
cumulative frequency distributions
of the mean sedimentation diameters (ds) for the suspended material
determined from still water time
series undertaken at the end of each
erosion experiment.
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
decay in S with time. The computed mean values
of mass settling rate (Ws) and equivalent sedimentation diameter (ds) are presented in Table 2,
and the latter are shown in Fig. 5B. Cumulative
frequency distributions of the suspended material
were derived from the settling time series. Siteaveraged cumulative curves are shown in Fig. 5C.
The control and water-injected sites show similar,
narrow size spectra with 80% between 40 and
80 lm. The mean control ds ¼ 42 (±8) lm.
159
The still water-settling trend is expressed by
the equation: S(t) ¼ –klog10(t) + b.Ws varied by a
factor of three, whereas ds varied from coarse silt
(0Æ04 mm) to medium sand (0Æ37 mm). The mean
decay constant in the settling equation varied as a
function of the starting Smax and had the linear
form: k ¼ 0Æ35 Smax)14 s)1 (Fig. 6). Both Ws and
ds varied throughout the settling period. The
settling trend showed a decreasing settling
rate with time. The equivalent sedimentation
Table 2 . Calculated values of mass settling rate in terms of the decay constant (k) and mean particle settling rate (Ws);
also shown is the mean sedimentation diameter (ds) determined from Ws using the method described by Gibbs et al.
(1971).
Deployment site
k (s)1)
Smax (mg L)1)
Ws (ms)1)
ds (mm)
C1
C3
C4
C5
C6
Mean
)1100
)653
)1010
)1029
)661
890 (± 193)
2904
1809
2615
2742
1887
2391 (± 453)
0Æ00064
0Æ00094
0Æ00194
0Æ00183
0Æ00131
0Æ00133 (± 0Æ0005)
0Æ040
0Æ040
0Æ049
0Æ051
0Æ027
0Æ042 (± 0Æ008)
W1
W2
W3
W4
Mean
)910
)769
)745
)704
782 (± 77)
1304
1733
1373
1128
1384 (± 220)
0Æ00186
0Æ00133
0Æ00158
0Æ00150
0Æ00156 (± 0Æ0002)
0Æ039
0Æ036
0Æ029
0Æ041
0Æ036 (± 0Æ004)
OIP1
OIP2
OIP3
OIP4
Mean
)736
)743
)779
)583
710 (± 75)
1403
1202
1255
1065
1231 (± 121)
0Æ00180
0Æ00121
0Æ00151
0Æ00130
0Æ00145 (± 0Æ0002)
0Æ064
0Æ034
0Æ036
0Æ047
0Æ045 (± 0Æ011)
Fig. 6. The mean decay constant (k)
of suspended mass in Hamilton
Harbour as a function of maximum
sediment concentration (Smax) at the
onset of settling. A linear trend is
evident, which is in continuity with
previously published values of k
from the marine environment.
Notice that the trend is linear across
a concentration range to
7000 mg L)1, which suggests that
free settling take place over this
range.
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
160
C. L. Amos et al.
diameters of the changing population of settling
sediment varied from medium sand at the initial
stages to coarse silt within the first 60 s of
settling, which is considerably coarser than the
disaggregated size population of 4–6 lm. Chlorophyll a showed a systematic increase from a
background of 7 lg L)1 to maxima of over
200 lg L)1. These increases were synoptic with
S. Particulate organic carbon showed similar
trends to S varying from 1 to 2 mg L)1 to over
200 mg L)1 as a result of bed erosion.
The effects of water injection
The time series plots did not appear to be
significantly different from the control despite
physical disturbance and fluidization of the sediment. Well-defined peaks in erosion rate (Ep)
were evident at the onset of the first five increments of flow (above threshold), i.e. type I
erosion. Thereafter, constant (type II) erosion
prevailed under turbulent rough flow. The transition in erosion type took place at a lid speed of
about 1 ms)1 (Uy ¼ 0Æ57 ms)1). The erosion
threshold from method 1 was similar to that at
the control site (0Æ5 ± 0Æ07 Pa). However, this
threshold was well above those derived using
methods 2 and 3 and supports the earlier finding
that method 1 is a poor index of sc(0). The highly
variable results are diagnostic of a spatially
variable substrate, which may reflect disruption
by the injection method. The mean erosion
threshold derived from method 2 was 0Æ23
(±0Æ08) Pa, r2 ¼ 0Æ59, whereas that for method 3
was 0Æ33 (±0Æ12) Pa, r2 ¼ 0Æ96, which demonstrates a greater degree of site heterogeneity.
The erosion thresholds at this site showed a
systematic increase in strength with time. The
site-averaged mean erosion rate (Em) and S
approximated a power relationship with bed
shear stress. The friction coefficient was much
less (4Æ5 ± 2Æ3) than at the control site and showed
an increase with time.
The mass settling velocities were 20% higher
than at the control site with a site-averaged value
of 0Æ00156 (±0Æ0002) ms)1. Mass settling velocities between sites were within the scatter and
therefore trends could not be inferred. The
site-averaged still water-settling rate was approximated by: S(t) ¼ )782 log10(t) + 2282. The mean
sedimentation diameter was 36 (± 4) lm and so
was within the medium silt size range. Ws and ds
appeared to decrease throughout the period of
settling, although considerable scatter was evident. The sedimentation diameters varied from
fine sand during initial settling to coarse silt
during the latter stages (Fig. 5B). The cumulative
plot was similar to the control, indicating a
similar size population of suspended aggregates.
The effects of oxidant injection
Time series plots were similar to those from the
control and water injection sites. Erosion was
dominated by type I erosion. The mean erosion
thresholds for the three methods were 0Æ4 (±0Æ07)
Pa, 0Æ19 (±0Æ04) Pa, r2 ¼ 0Æ65, and 0Æ28 (±0Æ07) Pa
r2 ¼ 0Æ97. Results from methods 2 and 3 were
about 30% less than those for the control. Friction
coefficients (5Æ2 ± 2Æ2) were positive and showed
monotonic increases with depth, diagnostic of a
normal consolidation profile, as well as an
increase with time (Table 1); however, these
coefficient values were lower than the controls,
indicating a reduction in strength. Erosion rates
were power functions of applied stress and were
similar in value to the control and water injection
sites. The mean value of settling rate (Ws) was
0Æ00145 (±0Æ00023) ms)1 and was intermediate
between the other sites. The median sedimentation diameter was 45 (±11) lm, which was not
significantly different from the control. Chlorophyll a and POC showed values that overlapped
those at the control and water injection sites.
DISCUSSION
Changes in bed stability with time
The period of this study was 17–25 August 1995.
During this time, the air temperature exceeded
33 C, and the water temperature was 25 C. A
relatively constant sc(0) ¼ 0Æ34 Pa was apparent
at the control site (Fig. 7A). However, significant
changes were detected within the disrupted sites
with no apparent differences between water and
oxidant injection. The disrupted sites were
ploughed completely twice about 6 h before the
Sea Carousel deployment on 21 August. sc(0)
shows minima immediately after ploughing
(0Æ17 Pa) but, after 2 days, the bed strength had
returned to the control mean value and, after
3 days, had exceeded the strength of the
control (Fig. 7A). The bed strengthening appeared
to be linear with time: sc(0,t) ¼ 0Æ16 + 0Æ076(Dt),
r2 ¼ 0Æ63. The friction coefficient showed similar
trends (Fig. 7B); the control site / was generally
constant with time (12), whereas the disrupted
sites showed a reduction in / to zero (fluidized)
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
161
Fig. 7. Temporal changes in (A)
erosion threshold and (B) friction
coefficient for the three sites. The
time series from the control site
shows constant values, whereas the
disturbed sites show initial weakening followed by systematic
strengthening over 3 days.
and a linear increase with time after disruption
with the form: /(0Æ01, t) ¼ 2Æ12(Dt); r2 ¼ 0Æ97
(where 0Æ01 m is the depth of the estimation of
/). No differences were evident between the two
disrupted sites, although there was a slight
increase in chlorophyll a that may reflect the
erosion of a developing biofilm.
Bed treatment appears to enhance stability, as
is evident in increases in bed strength with time
for both the erosion threshold and the internal
friction coefficient. This effect is likely to increase
with time because of the oxidation of organic
matter, which bonds the sediment aggregates
(Droppo & Amos, 2001). Oxidation, according to
Murphy et al. (1996), is completed in about
6 days, whereas the present measurements
spanned 3 days.
Comparison with marine settings
The erosion threshold (method 3) was compared
with other values derived by Sea Carousel from
Miramichi Bay, New Brunswick (Amos & Gibson,
1994), Lunenburg Bay, Nova Scotia (Sutherland,
1996; Sutherland et al., 1998b), the Humber
estuary, UK (Amos et al., 1998a), Manitounuk
Sound, Quebec (Amos et al., 1996b), the Fraser
River delta, British Columbia (Amos et al., 1997),
and Venice Lagoon, Italy (Amos et al., 2000). All
these examples are marine settings of varying
salinities. Nevertheless, the erosion thresholds
from these sites showed a positive trend with
sediment wet bulk density (Fig. 8). The results
from this study fall within the scatter of this trend
and suggest that the freshwater sites behave
similarly to the saline ones at similar bulk
densities. The present results contrast sharply
with those of Villaret & Paulic (1986), Williamson
& Ockenden (1996) and Mitchener & Torfs (1996)
that were conducted under laboratory conditions
(see Lavelle et al., 1984). Their results overlap
those here within the region 1000 > qb >
1400 kg)3 but diverge at higher bed densities,
with the laboratory results predicting much
higher values. Cappucci (2002) monitored bed
stability on intertidal mudflats in Venice Lagoon
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
162
C. L. Amos et al.
Fig. 8. The surface erosion thresholds measured in this study plotted against surface sediment wet bulk density
based on CT scanner analyses of syringe cores taken at each site (Amos et al., 1996a). The results follow a positive
linear trend, which is in continuity with previously published examples from the marine environment and with
those of Cappucci (2002). The near-surface sediment density in Hamilton Harbour was buoyant in places (arrow),
indicating the presence of a lift force resulting from the presence of interstitial gas trapped within the structure of a
surface biofilm (Droppo & Amos, 2001). The results of other laboratory studies are also shown for comparison:
Villaret & Paulic (1986), Berlamont et al. (1993), Williamson & Ockenden (1996) and Mitchener & Torfs (1996).
using the cohesive strength meter of Paterson
(1989) and showed trends similar to those described here, as did Tolhurst et al. (1997) using an
impeller-type erosion device called EROMES.
The buoyant nature of the biofilm was observed
by Sutherland et al. (1998b), who found buoyant,
gassy bottom sediments within a mid-latitude
microtidal estuary (Nova Scotia, Canada). Hamilton Harbour sediments are highly gas charged as a
result of their anoxic state, and gas bubbles were
observed to vent continuously from the bed
throughout the erosion experiments. These bubbles create a lift force, which reduces the geostatic
load (submerged weight). Once the sediment bulk
density is lower than the density of water, the
geostatic load is negative (lift), and the sediment
is held in place by cohesive strength and biostabilization. The magnitude of the lift force/unit
area is a measure of the upward-directed effective
stress. For a 2 mm thick biofilm (evident in CTscanned syringe cores) at the minimum density
shown in Fig. 8 (950 kg)3), the lift force is
0Æ98 Pa. This value is near the maximum strength
of biostabilized sediments recorded by Sea
Carousel shown in Fig. 8. The inference is that
buoyant biofilms can be held in place by adhesion
and organic binding to the bed provided that they
are no thicker than 2 mm. Thicker buoyant
biofilms would possess upward effective stresses
larger than the biofilm strength and hence would
break free and float to the surface. Within the
context of this study, the lift force (Fl) per unit
area acts to overcome the surface bed strength
(sb(0)). The ‘true’ critical shear stress for erosion
(sc(0)) is the addition of these two forces (per unit
area):
sc ð0Þ ¼ sb ð0Þ ¼ F1 þ s0
ð10Þ
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
Hence, the applied fluid bed shear stress (s0) at
the onset of bed erosion is an ‘apparent’ critical
value, which may represent only 30% of the true
bed strength.
Buoyant aggregates have been observed in
northern Venice Lagoon (Amos et al., 1998b) in
large quantities during periods of high temperatures and strong light and appear to contribute
significantly to the sediment budget of the region.
The bed of Hamilton Harbour is composed of a
monoculture of Oligochaetes reflecting highly
reducing conditions; the gas bubbles are thus
probably caused by bacterial reduction and
respiration.
The similarity in results of mass settling in still
water between freshwater and marine sediments
is dramatic and defined by @S=@t ¼ Ws S=hs ¼
k logðtÞ=Dt. As t approaches unity, then Ws ¼
)khs/S. As –k is proportional to S (Fig. 6), then it
follows that Ws is a function of the water depth at
which S was measured (hs) and thus independent
of S. Thus, changes in Ws with time denote
changes in sedimentation diameter, as evident in
Fig. 5B. Figure 6 shows the decay constant for
settling
for
four
marine
settings
for
0 < S < 7000 mg L)1. The data from this study
fit the linear trend over a range 1000 < S <
3000 mg L)1 and are therefore in continuity with
the marine examples. The inference of this is that
salinity changes are unimportant to the process of
mass settling of freshly eroded aggregates, such as
those of the Sea Carousel, and that aggregate size
is the important parameter.
163
Three methods have been used to monitor
aggregate size: two direct measurements on
pumped (disturbed?) samples and the third estimated as described earlier. The aggregates were
analysed for the control site only during the
erosion phases of C1, C3 and C5. Although not
directly comparable, the assumption was made
that the aggregates eroded from the bed during
flow are those that settle during subsequent still
water (neglecting corrasion). The results are plotted in Fig. 9. No trends were found in the median
diameter (d50) and sorting (vertical bars) through
the erosion phase. The Malvern 2600C yielded
values of 40 > d50 > 60 lm, whereas the Optimax
V showed a lower range of 20 > d50 > 50 lm.
The site-averaged median sedimentation diameters also show ds to fall midway between the
results of the two laboratory methods. Fennessy
et al. (1997) found that the Malvern 2600C gave
good results in settling experiments using an in
situ settling column. However, as the two laboratory analyses were undertaken on the same
samples, differences can only be the product
of the instrument, sample preparation errors or
a smaller sample population analysed by the
Optimax V.
Bedload transport vs. suspended sediment
transport
High-8 video observations made of the bed during
the period of erosion show that eroded aggregates
move initially as bedload (traction) followed by a
Fig. 9. The median aggregated
diameter derived from pumped
sample analyses using the Malvern
2600C and the Optimax V for
increments of flow velocity during
deployments C1, C3 and C5. These
are compared with the sedimentation diameter (ds) determined
from still water-settling trends in S
at the end of each in situ experiment. The results from the Malvern
2600C are larger than ds, whereas
the Optimax V results are lower.
The two laboratory methods were
undertaken on the same samples.
Thus, the divergence in the results
of the two laboratory methods
results from sample preparation or
instrument error.
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
164
C. L. Amos et al.
period of saltation and then suspension. A full
report on these analyses is given by Amos &
Droppo (1996). The videos have been used to
determine the mass fluxes and diameters of
eroded aggregates in the three modes of transport
(Fig. 10). Not all experiments are presented because of varying image quality. However, it is
clear that aggregate motion as bedload is the first
manifestation of the erosion process, and that this
begins for 0Æ15 > U > 0Æ20 ms)1. The flux of aggregates in traction increases with current speed,
and saltation begins for U > 0Æ3 ms)1. For
U > 0Æ4 ms)1, the aggregates decrease in number
as they move out of the field of view and into full
suspension; for U > 0Æ5 ms)1, no aggregates are
visible because of high turbidity. Using the bulk
density values of the surface sediment (derived
from the CT scanner) together with the measured
mean diameters (2–4 mm), a dry mass transport
rate is derived (Fig. 10B), which parallels the
trends in aggregate numbers. There is a wide
variation in results with no obvious clustering of
the three experimental sites.
The suspended flux in the flume (Fig. 10C;
SUHw ¼ 0Æ045SU, where H is flume height and w
is flume width), shows a consistent trend with
flow speed for all experimental sites, and the
magnitude of the flux is several orders of magnitude larger than the bedload flux. The ratio of the
suspended flux:bedload flux (Fig. 11) shows the
highest value to be immediately after the onset of
the erosion process, but it never exceeds 1%. The
ratio decreases systematically with increasing
current speed as more material is taken into
suspension. The inference of the above is that the
mechanism of erosion and mode of transport are
strongly dependent on flow intensity: for intense
events, suspension will dominate, whereas for
weakly eroding events, bedload transport will
probably play a major role in cohesive bed
evolution.
Conclusions
A series of in situ measures of erodibility were
undertaken in Hamilton Harbour, Lake Ontario,
using the benthic flume Sea Carousel. The measurements were made on a site of low-density,
homogeneous natural mud (C) and on two sites
artificially disrupted through ploughing. One
disrupted site was injected with water (W), the
second with a strong oxidant (OIP), calcium
nitrate. The results from the three sites were
compared to determine the effects of ploughing,
bed fluidization and chemical amendment against
measures made in marine settings. Several conclusions can be drawn from this study.
(1) The control (undisturbed) erosion thresholds showed a range of values (0Æ27 < sc(0) <
0Æ5 Pa) that were similar to marine counterparts of
similar bulk density (950–1500 kg)3). Surface
buoyant biofilms, similar to those described
by Sutherland et al. (1998b), were found in
the topmost 2 mm of the bed and appear to
contribute 70% of the mean force required for
bed erosion.
(2) The erosion thresholds of the (freshwater)
sites appeared to fall within the scatter of a
linear relationship sc(0) ¼ 5Æ85 · 10)4(qb) ) 0Æ37
Pa, which is largely derived from estuarine
settings. This suggests that salinity plays no
direct role in controlling this index of sediment
erodibility.
(3) Mass settling (expressed as a decay constant
k, s)1) after induced bed erosion showed a strong
linear correlation with peak sediment concentration Smax, which is similar to four other examples
from marine settings. The inference is that flocculation of freshly eroded aggregates (such as
were found within the Sea Carousel during these
tests) does not take place, and that settling is
dominated by the primary size and density of the
eroded aggregates.
(4) The aggregate size determined from still
water mass settling at the end of each erosion
test showed no differences between the control
and the water- and oxidant-injected sites. Mass
settling rates were within those of marine
counterparts and yielded normal size spectra
and median sedimentation diameters in the very
fine sand size to coarse silt classes. The median
sedimentation diameter (ds) falls midway
between the median diameter measured by the
Malvern 2600C and the Optimax V. Systematic
trends in aggregate size with flow speed were not
evident.
(5) The effect of ploughing of the lakebed was
short lived. The bed strength recovered linearly
with time (sc(0,t) ¼ 0Æ16 + 0Æ076(Dt)) to the mean
value of the control site (0Æ34 Pa) in 2Æ4 days.
Consolidation (expressed in terms of the friction
coefficient, /) showed trends of fluidization during ploughing (/ ¼ 0) and linear recovery with
time (/(0Æ01,t) ¼ 2Æ12(Dt)). The predicted time
interval needed for a return to natural conditions
(/ ¼ 12) was 6 days, which is also the time required for full oxidation to take place.
(6) Bedload transport of aggregates began at the
onset of bed erosion. Aggregates 2–4 mm in
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
165
Fig. 10. High-resolution video
observations of the size, flux and
transport mode of aggregates near
the bed throughout the erosion process: (A) aggregate flux (min)1) as a
function of current speed (Uy); bedload is seen to be greatest for
Uy ¼ 0Æ4 ms)1 and decreases as the
flow approaches the suspension
threshold. The diameter of aggregates was between 2 and 4 mm and
appeared not to change with flow
speed; (B) dry mass transport rate of
aggregates as bedload vs. current
speed; note the transition from
traction to saltation at
Uy ¼ 0Æ35 ms)1; and (C) the dry
mass transport rate of material in
suspension as a function of current
speed. Notice the similarity in
results from all sites and the onset of
full aggregate suspension at an
extrapolated Uy ¼ 0Æ6 ms)1.
diameter moved by surface creep and then in
saltation. The bedload flux never exceeded 1%
of the suspended flux and decreased throughout
the erosion process.
(7) There appears to be no negative long-term
impact of remediation on lakebed sediment stability. In later studies, oxidation of sediments
using this approach took at least 6–10 days. Thus,
future work could measure sediment sulphide
concentrations of ambient S as a guide to resuspension by natural events.
ACKNOWLEDGEMENTS
If this project is considered a success, it is due
largely to the efforts of several key people. In
particular, we wish to thank: A. Robertson who
helped mobilize and ship the GSCA Sediment
Geodynamics container to Burlington; R. Murphy
(GSCA) for technical support to C.L.A. throughout the survey period; H. Don (NWRI) for boat
handling and general field logistics while on site
in Hamilton Harbour; Dr B. Krishnappan (NWRI)
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
166
C. L. Amos et al.
Fig. 11. The ratio of bedload flux to
suspended flux as a function of flow
speed for all experiments. Notice
that the bedload contribution is
greatest at the onset of erosion and
decreases thereafter. It is no greater
than 1% of the suspended flux and
thus contributes very little to the
sediment mass balance.
for provision of the settling chamber and microscope used in the particle settling analyses;
C. Jaskot for analyses of suspended sediments
for IGD; and D. Clattenburg (GSCA) for the
detailed grain-size analyses. This project was
funded by Environment Canada and by RODAC
funds to C.L.A. Finally, thanks go to Professor
J. L. Best and two anonymous reviewers who
significantly improved this manuscript.
NOMENCLATURE
b
Cd
D
d50
ds
E
Ef
Em
Ep
Fl
h
hs
H
k
M
P
P¢
R
S
Smax
Sea Carousel duct width (m)
drag coefficient
net bed mass deposition (kg)
median diameter of disaggregated particles
(m)
sedimentation diameter of aggregates (m)
net bed mass erosion (kg)
floc. erosion rate (kg m)2 s)1)
mean bed erosion rate (kg m)2 s)1)
peak erosion rate under type I erosion
(kg m)2 s)1)
lift force at bed due to buoyancy (Pa)
eroded depth (m)
depth of measurement of S
Sea Carousel duct height (m)
settling decay constant
dry mass of sediment in suspension with
Sea Carousel (kg)
sediment bed porosity
sediment porewater pressure (Pam)2)
radial speed of Sea Carousel lid (ms)1)
suspended sediment concentration (kg m)3)
maximum suspended sediment concentration at onset of settling (kg m)3)
ambient nearbed suspended sediment concentration (kg m)3)
t
time (s)
Dt
time increment in the evaluation of erosion
rate (s)
U
azimuthal (tangential) mean current velocity (ms)1)
index tangential velocity measured Sea
Uy
Carousel (ms)1), where y ¼ 0Æ18 m
U* friction velocity (ms)1)
U*s turbidity corrected friction velocity (ms)1)
V
radial mean current velocity (ms)1)
sample volume (m3)
Vl
Vs
sediment volume (m3)
Vsc Sea Carousel flume volume (0Æ218 m3)
w
Sea Carousel flume width (m)
W
vertical mean current velocity (ms)1)
Ws sediment mass settling rate (ms)1)
Wg suspended sediment weight (kg)
x
azimuthal (tangential) distance (m)
Dy
spatial scale in Sverdrup equation (m)
y
radial distance (m)
z
vertical distance (m)
a
Sea Carousel bed area (m2)
q
ambient seawater density (k.m)3)
sediment density (2650 kg)3)
qs
qo
clear-water density (kg m)3)
sediment bulk density (kg m)3)
qb
fluid-transmitted bed shear stress (Pa)
so
sc(z) critical erosion threshold stress at depth z
(Pa)
sediment yield strength (Pa)
sb
l
absolute fluid viscosity (m2 s)
r¢
sediment effective stress (Pa)
e
diffusion coefficient in the Sverdrup equation
/
internal friction coefficient (degrees)
So
2003 International Association of Sedimentologists, Sedimentology, 50, 149–168
Bed stability in Lake Ontario
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