Frazil ice entrainment of sediment: large

Frazil ice entrainment of sediment: large
Journal of Glaciology, Vol. 47, No. 158, 2001
Frazil-ice entrainment of sediment: large-tank
laboratory experiments
Lars Henrik Smedsrud
Geophysical Institute, University of Bergen, Allëgaten 70, N-5007 Bergen, Norway
ABSTRACT. Laboratory experiments that simulate natural ice-formation
processes
^1
and sediment entrainment in shallow water are presented. A 10^30 cm s current was
forced with impellers in a 20 m long, 1m deep indoor tank. Turbulence in the flow maintained a suspension of sediments at concentrations of 10^20 mg L^1 at 0.5 m depth. Low
air temperatures (*^15³C) and 5 m s^1 winds resulted in total upward heat fluxes in the
range 140^260 W m^2. The cooling produced frazil-ice crystals up to 2 cm in diameter
with concentrations up to 4.5 g L^1 at 0.5 m depth. Considerable temporal variability with
time-scales of 51min was documented. A close to constant portion of the smaller frazil
crystals remained in suspension. After some hours the larger crystals, which made up
most of the ice volume, accumulated as slush at the surface. Current measurements were
used to calculate the turbulent dissipation rate, and estimates of vertical diffusion were
derived. After 5^8 hours, sediment concentrations in the surface slush were normally close
to those of the water. After 24 hours, however, they were 2^4 times higher. Data indicate
that sediment entrainment depends on high heat fluxes and correspondingly high frazilice production rates, as well as sufficiently strong turbulence. Waves do not seem to
increase sediment entrainment significantly.
1. INTRODUCTION
Sediment can be found inside or on top of sea ice in all parts of
the ice-covered Arctic Ocean (Barnes and others,1982; NÏrnberg and others,1994; Landa and others,1998). Levels of sediment in the ice are often patchy in nature, usually in the range
5^500 mg L^1, but vary both horizontally and vertically down
to the centimetre scale. The entrainment processes appear to
be governed by episodic events (Eicken and others, 2000).
Although in situ field observations of sediment entrainment are very rare, many researchers assume that much of
the entrainment is due to formation of frazil ice in shallow
water with sediment in suspension (NÏrnberg and others,
1994). This process is termed suspension freezing (Reimnitz
and others, 1992) and probably takes place over shallow shelf
areas during ice formation in autumn, or in flaw leads and
polynyas during winter. High entrainment rates of the process
have been documented in experiments in small tanks with a
short duration (51hour), but the turbulence and heat flux
have not been quantified (Kempema and others, 1993; Reimnitz and others,1993; Ackermann and others,1994).
This paper presents results of four laboratory experiments
Fig. 1. A sketch of the experimental tank at HSVA as seen from
above. Positions of the instruments during experiments are
indicated.
performed in a 20 m long, 1m deep tank. Measurements of
currents, frazil-ice concentration and crystal size, and
sediment concentration and particle size are given and
discussed with reference to earlier laboratory experiments
and natural conditions. The aim of the discussion is to understand how efficiently frazil crystals can entrain sediment into
the surface ice layer.
2. SET-UP
Experiments were performed in the Arctic Environmental
Test Basin, a cold room with a 1m deep tank at the
Hamburg Ship Model Basin (HSVA), Germany. Data from
two different experimental periods are presented here:
Interice I (November 1996^February 1997; Eicken and
others, 1999) and Interice II (December 1998; Haas and
others,1999).The first basic results on sediment entrainment
into ice from Interice I were presented by Smedsrud (1998)
and Lindemann and Smedrud (1999). The present paper is
based on 3 weeks of experiments, mostly from Interice II.
The flow in the tank was 3 m wide (Fig. 1) and driven by
four impellers. The tank was equipped with a wave machine
capable of generating waves with a 10 cm amplitude, and with
wind propellers producing winds of up to 5 m s^1 over the
water surface. The air temperature in the cold room can be
regulated through a cooling system with a capacity of 69.7 kW,
having cooling ribs in the ceiling. During experiments the air
temperature was maintained below ^10³C, and most of the
time temperatures were between ^14³ and ^18³C.
In the beginning of each experiment, the test section was
cleared of slush and ice. At the impeller side of the tank a
*20 cm thick, solid layer of ice persisted through the
experiments.
461
Journal of Glaciology
Table 1. Overview of physical conditions in the four laboratory experiments
A
B
Experiment
C
D
Air temperature Ta (³C)
Current speed (cm s^1)
Wave height (cm)
Duration (h)
^10.43
32.03
0
25
^17.17
25.05
0
24
^13.85
32.12
10
24
rms fluctuation q (cm s^1)
Turbulent dissipation rate (W kg^1)
Turbulent diffusion K (m2 s^1)
Heat flux Qf (W m^2)
9.3
4.3610^4
3.4610^3
160.2
7.4
3.5610^4
1.7610^3
257.2
10.3
3.7610^4
5.9610^3
190.4
6.8
3.7610^6
111.4610^3
135.4
Water cooling rate Tw =t (³C h^1)
Salinity increase rate Sw =t (psu h^1)
Frazil-ice concentration Ci (g L^1)
Frazil-ice production rate Ci =t (g L^1 h^1)
^1.9610^3
29.2610^3
1.1
0.75
^1.7610^3
43.8610^3
0.8
1.17
^3.6610^3
34.7610^3
0.9
0.88
^5.0610^3
22.0610^3
3.0
0.63
Silt and sand
12.3
44.3
Silt and sand
18.1
198.5
Silt and sand
16.3
21.8
Clay and silt
12.0
21.2
Sediment type
Initial sediment conc. SPM (mg L^1)
Max. sediment conc. in ice IRS (mg L^1)
^14.11
9.12
10
6
Note: Values are averages over the duration of experiments unless otherwise stated.
The water had a salinity of 36^38 psu (practical salinity
units; *%) initially, and was always close to the freezing
point because of the permanent ice cover. Levels of supercooling were typically 0.01³C, and persisted through the
entire experiment. Sediment was added to the tank,
resulting in mass concentrations in the range 10^20 mg L^1.
Key parameters of each experiment are given in Table 1 for
later reference. Key features of experiment D are presented
in Smedsrud (1998), and a more detailed description of all
experiments is given in Smedsrud (2000).
3. METHODS
Currents were measured with either an ADV (acoustic
Doppler velocity meter; Nortek, Norway) sampling at
25 Hz, or a UCM (ultrasonic current meter; NE Sensortek,
Norway) sampling at 2 Hz. Both instruments measure three
components of current velocity with an accuracy of þ5
mm s^1. Currents in the ``test section'' were sampled 5 min
every hour during experiments, and, unless otherwise
mentioned, at 0.35 m depth in the positions shown in Figure 1.
A mini CTD (conductivity-, temperature- and depthmeasuring device; Meerestechnik Elektronik, Germany)
was used to measure water temperature and salinity once
every minute. The temperature sensor was calibrated
2 weeks before the experiments to an accuracy of þ0.001³C,
while the conductivity sensor was accurate to 0.01psu.
Water samples were taken twice every day to check the
calibration of the conductivity measurements with a
Portasal 8410-A salinometer with an accuracy of
0.003 psu at the University of Bergen, Norway. Air
temperatures were sampled every 10 min with a Tiny-talk
temperature sensor, with a 0.1³C resolution and accuracy.
Video images of frazil crystals were recorded using a
Canon Hi8 camera and a macrolens filmed through
polarized glass. The crystals were scooped out of the water
using a 9 cm diameter glass plate. Still images were
processed from the video, and the numbers of crystals in
different size groups were counted directly on the monitor
to determine crystal-size distributions. Frazil crystals down
to 100 m were resolved, and a total of 435 crystals were
counted at three different stages of experiment D. Samples
462
of the congealed surface slush (solid ice) were taken to make
thin sections and to document the crystal structure.
Suspended frazil concentration was determined by
holding a 5 L water bottle, open at both ends, in the water
column at various depths. Slush (frazil ice and water) was
collected by draining the bottle contents through a commercial sieve (mesh width 0.5 mm). The slush was allowed to
melt in clean boxes, and the salinity of the melt was determined using a laboratory conductometer WTW LF 191
(Wissenschaftlich Technische Werkstatten, accuracy þ0.1
psu). The slush volume was determined using a graduated
cylinder (accuracy þ 1mL). The true frazil-ice concentration was then calculated, assuming zero salinity for the ice,
and bulk salinity of the tank for the water.
The second method of measuring suspended frazil concentration was with an a-3 absorption meter(Wetlabs,
Oregon, U.S.A.) modified to measure light absorption at
900 and 975 nm wavelengths at 5.8 Hz. The meter was used
vertically, with water flowing between the source and detector units, and all values are therefore vertical averages over
these 10 cm. It was calibrated in a laboratory test with freshwater frazil ice, and has a noise level that translates into an
error of 0.1g L^1 (Pegau and others,1996). At 975 nm there is
a peak in the absorption spectrum of water, which makes it
possible to calculate the changes in absorption due to the
change in frazil-ice concentrations. A pure water sampling
done during the experiment serves as a reference frazil
concentration of zero.
Sediment concentrations were determined either by
filtering sample water directly through pre-weighed 0.45 m
membrane filters, or by melting ice samples and filtering the
resulting meltwater. Mass concentrations were calculated by
subtracting the initial filter weight from the sedimentcontaining filters. Due to the low sediment content the weighing was performed on a sensitive micro-balance.
Size distribution of the sediment suspended in water was
determined from the bulk sample added to the tank using a
standard sieving and sedigraph technique. First the original
sediment sample was dried to get the initial dry weight, and
further wet-sieved to distinguish the sand fraction (463 m).
The coarse fraction was dry-sieved to get the weight of the
different coarse grain-size fractions (63^125 m,125^250 m,
Smedsrud: Frazil-ice entrainment of sediment
Fig. 2.Values for the rms velocity q and the turbulent dissipation
rate , during the experiments. Experiments A(*) and B(+)
are forced with a 30 cm s^1 current, C() with 30 cm s^1
current and 10 cm waves, and D(&) with a 10 cm s^1 current
and 10 cm waves.Values showing the horizontal variation with
the same forcing as experiment C are shown as ^.
250^500 m, 500 m^1mm and 41mm). The grain-size of
the silt and clay fraction (563 m) was determined by their
settling velocities. The accuracy for a standard sample is
þ0.02 mg L^1.
Size distribution of the sediment in the ice was determined
using a scanning electron microscope (SEM). The samples
were filtered on 0.22 m Millipore filters, and gold^palladium
was damped on. The different grain-sizes were divided into
groups, and the number of grains in each group was then
estimated by counting along the filter using magnifications
of 100^4000.
4. RESULTS
4.1. Currents and turbulence
Horizontal mean velocities during the four experiments were
in the range 9.1^32.1cm s^1, as given in Table 1. Before the
experiments started, the UCM was used to determine variations within the test section, both across the flow and with
depth. For the mean speed, horizontal variations across the
flow were typically þ3 cm s^1, and in the vertical þ1cm s^1.
The flow was clearly turbulent, and eddies were easily
observed at the surface. Turbulence caused four important
features in the experiments: (i) downward diffusion of frazil,
(ii) upward diffusion of sediment, (iii) collision and aggregation between frazil and sediment, and (iv) diffusion of heat
and salinity.
The turbulent strength may be indicated by q2 ˆ
0 2
…u † ‡ …v0 †2 ‡ …w0 †2 , the rms value of the velocity fluctuations. u is the speed in the direction along the tank and with
the flow, v is the speed across the tank, and w is the vertical
speed.The fluctuating part is u0 ˆ u u.The value of q had
an overall range of 4^12 cm s^1 for all experiments (Fig. 2).
In experiment D the UCM was logging at different depths,
and q varied significantly with depth, from 10.7 cm s^1 at 0.15
m depth to 4.2 cm s^1 at 0.5 m depth (Fig. 2; depths are not
shown). In addition, the increasing surface slush layer
seemed to dampen q in the near-surface layer (0.15 m) quite
efficiently, from 10.7 to 5.3 cm s^1 during 3 hours.
Calculated values of q from the four experiments are
Fig. 3. PSD from 0.35 m depth at the beginning of experiment
A, u (dashed line) and w (solid line) components.The error
bar represents the 95% confidence interval.The straight line
is the theoretical f ^5/3 spectrum, using the measured and the
calculated .
shown in Figure 2 together with the turbulent dissipation
rate . Mean values for each experiment are shown inTable
1. expresses how much energy is being transferred into
heat at the molecular level by friction. In a stationary
process, such as the experimental tank with constant energy
input from the impellers, also indicates how much energy
is being transferred down through the different turbulent
scales, from the largest eddies of the flow down to the
smallest ones (Tennekes and Lumley,1994).
As the main focus is vertical diffusion, and the geometry
of the acoustic sensors on the current meter allows
minimum instrument noise on the vertical component w,
dissipation is calculated using the power spectral density
(PSD), S…fk †, of each time series of w0 :
2=3
q
5=3
2=3
2=3
p
…fk †
fk
;
…1†
S…fk † ˆ 2
3
where fk is the frequency after a Fourier transformation of
the time series (Tennekes and Lumley,1994). With Equation
(1) a value for …fk † on each frequency is obtained. This way
of calculating the dissipation rate was termed ``single-value
method'' (Stiansen and Sundby, in press), and showed
values in close correspondence with other methods.
A typical PSD plot is shown in Figure 3. It shows that the
5=3
energy distribution follows closely a fk
relationship.
Variations over the frequency range for are small, so the
dissipation rate used from each data file in Figure 2 is simply
the mean, . The variation of during each experiment was
small, and all values from experiments A^C are in the
narrow range 2.8 10^4 to 6.7610^4 W kg^1. The horizontal
variation of cannot be ignored, and decreases to *0.2
10^4 close to the tank walls, as shown in Figure 2.
The mean value for experiments A^C is ˆ 3.8610^4. If
is increased to 15.0610^4, and q ˆ 8.0 cm s^1 in Equation (1),
the calculated theoretical PSD corresponds closely to the
upper 95% confidence interval of Figure 3. Likewise a value
of ˆ 1.0610^4 corresponds to the lower confidence level,
hence ˆ h 1.0610^4,15.0610^4i with 95% confidence.
The calculated value for can be compared with the total
input of energy by the impellers. The four impellers were
measured to use 12.528 kWon the maximum speed, resulting
in a current of 30 cm s^1. This is an absolute upper limit for ,
assuming all this electrical energy was transformed into
463
Journal of Glaciology
Fig. 4.Water temperature (solid line), salinity (dashed line)
and freezing point (dash-dot line) from experiment B. The
data gaps (straight line) are caused by frazil crystals blocking
the conductivity sensor.
kinetic energy without any loss in the impellers. If the kinetic
motion dissipated evenly into heat everywhere in the 93 m3
of tank water, ˆ 0.13 W kg^1. The calculated is *0.3% of
this value. Some energy is lost at the impellers, of course, and
the local dissipation around the impellers is probably higher
than in the test section.
Values for in the ocean cover many orders of magnitude,
from 10^8 to 10^3 W kg^1 (Anis and Moum, 1995). Therefore
experiments A^C can be characterized as strongly turbulent,
while experiment D has a value in the middle of the natural
range.
4.2. Temperature and salinity
The CTD was kept in one fixed position in the test section of
the tank, at 0.5 m depth, Y ˆ 2.3 m and X ˆ 13.5 m, throughout the experiments (Fig. 1).
The change in salinity for the water is used to calculate
the mass of frozen ice, because most salt is expelled during
ice formation. However, during the experiments frazil
crystals tended to get stuck in the conductivity sensor, and
the salinity calculated by the CTD software was too high,
as pure sodium chloride (NaCl), and not the usual mix of
sea salts, was used in the tank. Equations assuming the
water is on the freezing point for NaCl water were therefore
used to give the salinity.The absolute salinity values have an
accuracy better than þ0.5 psu (Bodnar,1993).
As soon as the water reached the freezing point, small
ice crystals started to appear close to the surface, visible as
a faint shimmering at first. The temperature (Tw ), salinity
(Sw ) and freezing point (Tf ) from experiment B are shown
in Figure 4.The data are unfiltered, and the small variations
in Tw are greater than instrument noise.
Because all experiments were run with similar air
temperatures and winds, the evolution of Tw and Sw are
similar between experiments, but due to the difficulties with
the conductivity, it is hard to determine the exact level of
supercooling. For experiment B, Tw decreased by *0.04³C
in 24 hours, and Sw increased by about 1 (Fig. 4). For the
other experiments see Table 1.
4.3. Frazil ice
The initial ice formed in turbulent water bodies is frazil-ice
crystals formed in supercooled water. Formation starts
464
Fig. 5. A typical frazil crystal filmed during experiment D;
the diameter of the glass plate is 90 mm.
when the supercooled water is seeded with ice crystals from
the air above, and the suspended crystals increase in
number and size with time. The heat flux from the crystals
to the surrounding water increases with the level of supercooling and with increasing turbulence. When frazil
crystals collide, new microscopic crystals are produced,
and thus more and more crystals form and grow, as long as
heat is removed from the water. Turbulent diffusion
transports these crystals downwards into the water column.
The distribution of frazil in the water column depends on
the balance between downward diffusion and crystal-rise
velocity, which is a function of crystal size.
Frazil crystals resembled snowflakes during all experiments. Figure 5 shows a typical frazil crystal. Sometimes the
dendritic twigs break off and form a new crystal, and many
different crystal habits were observed. No (qualitative)
change in the dendritic form was observed. The largest
crystals observed were about 2 cm in diameter, and the thickness was 10^50 times smaller than this. During the experiments frazil crystals remained separate crystals and did not
seem to form frazil flocs.
A frazil-crystal size spectrum was calculated by
estimating the number of frazil crystals in five chosen size
groups. The size classes and corresponding rise velocities
are given inTable 2 (shown later). The ``Small''group could
not be resolved with the video image, and the concentration
here remains unknown. No significant change was found
between the different classes over time, so an overall size
distribution was calculated (Fig. 6).
The distribution of the frazil sizes at the surface was
calculated assuming the crystals are perfect discs, with a
mean diameter for each class, and a thickness of 1/30 the
diameter. The thickness was not measured directly, only
qualitatively estimated as ti *51mm, for the crystals with
diameter *20 mm. Although about 60% (by number) of
the crystals are ``Fine'', their volume is only 0.006% of the
total, while the 2.5% ``Large'' crystals make up about 59%
of the volume. This makes it hard to talk of an average
crystal size, but an average diameter of 2 mm was chosen
in the early simplified approach (Smedsrud,1998).
Frazil-ice concentration at different depths was determined
in two ways. The 5 L water bottle gave an instantaneous measurement where the frazil was ``captured'' and could be further
analyzed, while the absorption meter gave a time evolution of
the concentration and therefore a good average value.
Smedsrud: Frazil-ice entrainment of sediment
Fig. 6. Average frazil crystal-size distribution in the surface
during experiment D. Distribution is plotted as volume distribution (solid line) and number distribution (dashed line).
The ``true'' mass of frazil ice is defined as the water in a
melted bottle sample of slush that has zero salinity, where
slush contains an unknown portion of salt water as well as
frazil ice. The slush salinity, Ss, was in the range 20^28 psu,
while salinity of the water, Sw, was close to 38 psu. This
implies a water content of the drained slush of about 60%.
The concentration of suspended (pure-water) frazil, Ci, was
then calculated using the relation;
Ss
Ci ˆ w Cs 1
:
…2†
Sw
The slush concentration, Cs, is the (melted) mass of the slush
divided by the 5 L bottle volume, and the water density is
w *1030 kg m^3. Results of the water-bottle measurements
were in the range 0.1^1.8 g L^1 for experiments A^C, while
in experiment D Ci increased to about 4 g L^1, and remained
at this level at all depths to the end of the 6 hour experiment
(Smedsrud,1998, Fig. 1).
Figure 7 shows calculated frazil concentration from
absorption-meter data that document changes of about 100%
from the mean value over short time-scales. The arel (975)
values are systematically lower than the arel (900) values, due
to the presence of frazil ice between the source and the detector of the meter, since ice absorbs less light at 975 nm.
The differences between the absorption-derived mean
concentrations and the instantaneous water-bottle values
are up to 1g L^1. The absorption-meter data are usually
higher than the corresponding water-bottle values for all
experiments. Just before the absorption sampling in Figure
13 was done, a water-bottle value showed 0.87 g L^1.
Although this is only 62% of the mean, it lies clearly within
the observed variation in time.
Figure 8 shows Ci derived from the water-bottle and
mean absorption data from experiment C. During the first
few hours the difference between the two methods is quite
large (*1g L^1), but still within the observed variation in
Figure 7 and the other absorption-meter data. Later in the
experiment the two methods agree fairly well, so the level of
Ci is confirmed to be close to 1g L^1. The average values of
Ci from the absorption meter at 0.5 m depth (Fig. 8)
decreased slightly with time despite the growing thickness
of the surface slush layer. After 24 hours the slush-layer
thickness was approximately 20 cm, although not distributed evenly everywhere in the tank.
Experiments A^C had similar levels of Ci from the
absorption meter, with average values in the range 0.2^
1.5 g L^1. After the first few hours, no increase with time below
0.25 m could be seen. Means of Ci from all depths during
experiments are given inTable 1.
4.4. Sediment
The sediment added to the tank came from a sand pit near
Kiel, Germany. A major fraction of the mass was quartz.
Significant volumes of magnesium (Mg), aluminium (Al),
potassium (K) and iron (Fe) also existed, indicating a
mixture of different minerals.
When sediment was added to the tank, the whole water
column changed to a greyish colour. After a few hours with a
steady current, a bedload of larger grains was observed on
the bottom of the tank in experiments A^C. In experiment
D no bedload was observed, and the sediment concentration
was observed to be homogeneous. In experiments A^C the
same sediment was used. The grain-size analysis of the bulk
sample introduced to the tank (Fig. 9) shows that most of the
Fig. 7. (a) Low-pass filtered relative absorption coefficients, arel (900) (solid line) and arel (975) (dashed line). (b) Resulting
calculated frazil-ice concentration 3 hours into experiment B at 0.25 m depth.The average value, Ci, is plotted as *.
465
Journal of Glaciology
Fig. 8. Measured frazil concentrations, Cw during experiment
C. Water-bottle measurements are open symbols; average
absorption data are filled symbols. Measurements are from
0.25 m (3), 0.5 m (^) and 0.75 m depth (&).
Fig. 9. Cumulative mass frequency for the different size classes
of the sediment used in experiments A^C. Sizes are from sediment data (^*^), with the five-class approximation shown
as ^ ^.The counted IRS size distribution is plotted as ö.
sediment (65%) is very fine or fine sand (63^250 m). In
experiment D, much finer sediment was used. The median
size was 2.5 m, and all of the grains were silt or clay.
All of the sediment added to the tank water will from
now on be termed suspended particulate matter (SPM).
The grains that are sampled attached to or within sea ice
will be termed ice-rafted sediment (IRS).
Although sediment can have a variety of shapes, a rough
generalization is that they are spherical down to the clay
fraction. When counting the individual grains from the
IRS samples on the SEM screen, it was obvious that this
assumption has its limitations. Figure 10 shows a SEM
image of a coarse silt particle. The grain-size of the IRS is
also plotted in Figure 9. The IRS grain-size is deduced from
the two samples in experiment B with the maximum IRS
concentration of the slush ice (Fig. 11).
The percentage of mass in each size class is calculated
from a mean diameter of the class, assuming all particles are
spheres and have the same density. The total mass of the
grains in each class depends on the cube of the radius, r3s . So
even when the number of grains in the fine-sand fraction is
only *30 on a given area of the filter, these particles hold
63% of the mass. In the same area the clay fraction has
*36105 particles, holding only 0.28% of the mass.
Figure 11 shows the increasing IRS concentrations
during experiment B, and the high variation observed.
Suspended frazil-ice had high IRS concentrations early in
the experiment. After some time, many of the free-floating
frazil-ice crystals gathered in the surface as slush, and the
maximum sediment concentration in the slush at the end of
the experiment was 163 mg L^1.
For the 24 h experiments A^C, much of the surface slush
consolidated into granular ice during the night when observations could not be performed. In the morning much of the
slush ice and the granular ice was discoloured by sediment.
The granular ice was sampled and divided into an upper
and lower sample, to give an indication of the IRS concentration resulting from this consolidation. This ice contained the
greatest IRS concentration of all the experiments,199 mg L^1
(Fig. 11). This maximum IRS concentration was from the
lower sample which represents slush that consolidated fairly
close to the time of sampling (24 hours). The upper sample of
this ice consolidated during the night, so this IRS concentration is moved to 14 hours on the x axis in Figure 11.
Experiment A showed similar increases in IRS concentration with time, and after about 5 hours the SPM concentration had decreased to less than half. As the turbulence was
constant during experiment A, sediment did not settle to the
bed in a large degree, and much of the sediment mass had
thus been entrained into the ice. In experiment C the maximum IRS concentration of frazil in suspension was
33.9 mg L^1 at 8 hours at 0.25 m depth (mean over 2 hours is
22.83, n ˆ 4). Maximum IRS concentrations as well as initial
SPM for all experiments are given inTable 1.
466
5. DISCUSSION
Although horizontal differences of the flow cannot be
totally ignored, the discussion of the experiment will be
one-dimensional. The only feature that showed large
horizontal differences was the slush layer. This horizontal
heterogeneity of the slush was due to the ice edge at the end
of the test section, but this had a minor influence on the flow.
5.1. Turbulent diffusion
The calculated values of in section 4.1 are in the lower to
middle range for earlier small-tank experiments with frazilice formation (0.07^137.5610^3 W kg^1 (Daly 1994)). The
experiment of Carstens (1966) had a mean velocity of
50 cm s^1 in a 20 cm wide, 30 cm deep flume, and an of the
same magnitude, 1.3610^3 W kg^1 . Eddy viscosities K are
commonly used to quantify the turbulent diffusion and are
often parameterized by
K ˆ SM lq ;
…m2 s 1 †
…3†
where SM ˆ 0.32, an empirical constant derived from
laboratory experiments for a non-stratified water column
(Mellor and Yamada, 1982; Melsom, 1996). The turbulent
length scale is l, and q is the rms fluctuation velocity of the
three directions. Following Mellor and Yamada (1982)
again, a calculation of the length scale, l, is found from
lˆ
q3
;
16:6
…4†
where is the turbulent rate of dissipation. To estimate the
Smedsrud: Frazil-ice entrainment of sediment
Fig. 10. SEM image of a sediment grain from the coarse-silt
size class (diameter 20 m), surrounded by smaller grains.
vertical eddy viscosity, l is substituted for from Equation (4),
and
q4
' 3:4 10 3 ;
…5†
16:6
with the calculated means from experiment A inTable 1. For
experiments B and C, Equation (5) yields similar values for
K; in contrast, the value from experiment D is much higher
(Table 1).
Towards the walls in the tank, decreases to around
0.4610^3 with the 30 cm s^1 current, while q stays fairly
constant (Fig. 2). This indicates that there was a stronger
vertical diffusion close to the tank walls than in the middle
of the flow in general and K*18.5610^3 at 0.2 m from the
wall.
K ' 0:32
5.2. Heat fluxes
As horizontal differences are omitted in the approach to
describe the experiment, the equation for the temperature
variation takes the following form:
DTw @ Tw
@ Tw
@
@ Tw
K
ˆ
‡w
ˆ
…6†
‡ GT ;
@z
dt
@t
@z
@z
where the overbars indicate time averages. The measured
decrease in water temperature in Figure 4 corresponds to
ˆ 0, @ Tw [email protected] is a balance
@ Tw [email protected] in Equation (6). With w
between the turbulent vertical diffusion and the source of
heat, GT, from the ice production. There is an insulating
bottom boundary, and an upward heat flux, constantly
lowering the temperature in the surface. The time series of
Tw and Sw are used to calculate two parts of the total
upward heat flux QT.
The flux of heat that results in freezing can be calculated as
Qf Lw w Sw Vtank
ˆ
;
…7†
t
Sw t
where Lw ˆ 3.346105 (J kg^1) (Yen, 1981), the latent heat of
freezing (for fresh water, and fresh frazil crystals). Calculating
the average flux from experiment B again, Sw ˆ 1.05 psu,
t ˆ 24 hours, and Qf ˆ 10.03 kW, or Qf ˆ 257.2 W m^2 for
the open-water area.
A part of the heat flux from the water to the air causes the
water to cool down as salinity increases, but this turns out to
be as low as 273 W, or 7.0 W m^2, for the average heat fluxes
over 24 hours. Here the added heat from the molecular
Fig. 11. Sediment concentrations from experiment B. SPM at
0.5 m depth (), and IRS from frazil ice in suspension (+),
surface slush (3) and granular ice (^).
dissipation is also taken into account. This part of the heat
flux can be ignored as Qf covers 498% of the total flux.
Heat fluxes derived from Sw =t are given inTable 1.
The total upward heat fluxes are in the middle of the
range for observed heat fluxes for natural Arctic conditions
(120^400 W m^2) (BrÏmmer,1996), but are in the lower part
of the range for smaller laboratory experiments on frazil ice
(Daly,1994).
As short- and longwave radiation as well as evaporation
are probably negligible in the cold room, the results can be
compared with straightforward bulk sensible-heat-flux parameterizations obtained by dimensional arguments (Gill,1982):
QS ˆ cH a Cpa Ua …Tw Ta † ;
…8†
where cH is a dimensionless coefficient, and a value of cH ˆ
1.1610^3 is representative of an open-water situation
(Simonsen and Haugan,1996), Cpa ˆ1004 (J ³C^1 kg^1) is the
specific heat for air at constant pressure, a ˆ 1.225 kg m^3 is
the air density, and Ua is the wind speed. Using values from
experiment B, Ua ˆ 5.0 m s^1, and Tw Ta ˆ 14.87³C yields
QS ˆ 100.6 W m^2. In comparison QT was 265 W m^2.
Using the observed values of QT to calculate a mean
tank value for the dimensionless coefficient yields cH ˆ
2.73610^3. This is comparable to suggested values for cH
over leads and thin ice in the Arctic (Simonsen and Haugan,
1996). The increase for cH in the tank, compared to an openwater situation, is probably caused by the unstable conditions for the cold-air flow in the tank, as it is being cooled
from the ceiling and therefore will convect down towards
the water continuously.
5.3. Frazil ice
With the time-scale of several hours in the experiment, the
crystals are expected to continue to grow, compared to the
small average sizes and total range reported by Daly and
Colbeck (1986). As frazil crystals grow in size, their rise
velocity also increases, simply because the buoyancy force
is proportional to the crystal volume, while the retarding
drag force is proportional to the crystal area.
Rise velocities based on Gosink and Osterkamp (1983)
are given in Table 2, together with chosen size classes for
the crystals. A steady-state situation was reached in 0.2 s or
less for all crystal sizes. For the ``Small''crystal class the Reynolds number fell outside the valid range, so the value given
is obtained by linear interpolation between zero and the
``Fine''class.
467
Journal of Glaciology
Table 2. Frazil-ice classes (diameter di ), and calculated rise
velocities based on equations (1^3) in Gosink and Osterkamp
(1983)
Table 3. Sediment-size classes and sinking velocities calculated
after Neilsen (1992)
Class
Class
Small
Fine
Medium
Coarse
Large
Range
Median size
mm
mm
0^0.05
0.05^0.5
0.5^5.0
5^10
10^20
0.025
0.25
2.5
7.5
15
Rise velocity
mm s
Clay
Fine and med. silt
Coarse silt
Fine sand
Med. and coarse sand
0.005
0.054
6.3
21.2
38.8
…9†
Using values for experiment B and t ˆ 24 h (Fig. 4),
Ci =t ˆ 1.17 g L^1 h^1, and the total mass of ice grown
during the experiment is 2.626103 kg. Values for the other
experiments are given inTable 1.
It is a good assumption that all ice freezes as frazil ice
since the only other type of ice that was observed to grow
was a layer along the tank walls from the surface and 5 cm
downwards. At the end of the experiment, this layer had
grown to a thickness of *3 cm, giving 36 kg of ice only.
Even with small time increments in Equation (9),
Sw =t is close to constant for all experiments (see Fig. 4
for experiment B data). This means that the ice-production
rate is close to constant, and that the total volume of ice
increases linearly with time.
In contrast, the direct measurements of Ci in suspension
during experiment A^C show fairly constant levels, i.e.
52 g L^1. This indicates that there is a balance between the
downward diffusion and upward rise velocity for a constant
portion of the frazil crystals. The increasing volume accumulates in the surface as slush. The Ci in suspension must be the
smaller crystals, and as these grow they rise to the surface. But
new, small crystals are produced continuously, so there is
always a portion of these in suspension.
Experiment D is different, and has values of Ci up to
4 g L^1. With the relatively large K, indicating a stronger
vertical diffusion, larger crystals were probably kept in
suspension than in the other experiments. At the end, slush
also gathered on the surface, but more of the ice volume
stayed in suspension throughout the experiment.
468
Median size
Settling velocity
m
m
mm s^1
0.3^2
2^20
20^63
63^250
250^1000
1.15
11.0
41.5
156.5
625
^1
According to Martin (1981) and Daly (1984), circular
discs up to a few mm in size are usually observed in the early
stages of fresh-water experiments. Such circular discs were
not observed at all in these experiments, confirming the
results of Hanley (1984). Their absence is probably caused
by the fact that salt, as well as heat, has to be removed when
a crystal grows in salt water. In addition, the size of the saltwater disc could be significantly smaller than 1mm. The
frazil crystal shape is probably a complicated product of
supercooling and turbulence, and no systematic observation
of frazil-ice shape and size during growth in salt water has
been found in the literature.
Frazil ice is here defined as pure fresh-water crystals.The
thin film of salt water between crystals (the interstitial
water), or around each crystal, is defined as a part of the
water. The production rate of frazil crystal mass per volume
Ci can then be calculated exactly by a modified version of
Equation (7):
Ci w Sw
ˆ
:
t
Sw t
Range
^0.0005
^0.05
^0.7
^10.7
^171.9
5.4. Sediment
With the constant turbulent forcing in the tank, a balance is
reached between the upward diffusion and the downward
advection of sediment grains, depending on their size and
the corresponding settling velocity. The size classes given
inTable 3 are the ones used when counting individual grains
of the IRS samples. All sizes are given as a diameter of the
grains. These five classes approximate the real size distribution as shown in Figure 9.
The sinking velocities (Table 3) are Stokes settling
velocities (Neilsen, 1992) using Tw ˆ ^2.0 and Sw ˆ 35 psu,
a sediment density of 2650 kg m^3 and a depth of 1m, and
they span the rise velocities of the frazil ice (Table 2). So
when it is observed that the surface Ci consisted of mostly
``Large'' crystals, it is also expected that the ``Medium and
Coarse Sand''grains stay at the bottom.
The real ``non-spherical'' form of the grains is of
importance for the aggregation process, and the likelihood
for bonding between a colliding grain and a crystal. A
perfect sphere should have less chance to aggregate than a
particle with an irregular surface. As the forcing was
constant within each experiment, it is expected that the
entrainment process operates at a constant rate as well.
The frazil crystals in suspension, or in the surface slush,
collide with sediment grains, and it is obvious that sediment
aggregates to the crystals in some way, and that the concentration of IRS increases with time.
The size distribution of the IRS from Figure 9 shows that
no grains larger than 250 m became aggregated to frazil ice.
This is consistent with Arctic field observations, where sand
fractions of IRS are usually in the range 5^10%, a significantly lower value than for the local bed sediment (NÏrnberg
and others,1994), indicating a preferential entrainment of the
fine fraction.
The fine-sand fraction (63^250 m) is almost constant
between the SPM and the IRS, decreasing from 65% to
63%. This difference is insignificant given the uncertainties
when counting a given size class of particles.
The coarse-silt fraction is constant between the IRS and
the SPM, with a difference of only 0.4%. The largest change
is for the fine-silt fraction, showing an increase from 8.17% to
19.41%. This is the compensation for the ``lost'' sand fraction,
and confirms that the sediment entrainment process is selective. Smaller sediment grains aggregate more easily than the
larger ones.
The clay fraction is reduced from 1.17% of the SPM to
0.28% for the IRS, against expectations. This could be a
result of the uncertainties in estimating the mass by counting
the particles. For instance, if most of the grains in the size
class have a larger diameter than the average used in the
Smedsrud: Frazil-ice entrainment of sediment
calculations (1.15 m), a larger percentage by mass would be
the result, up to *2% when using rs ˆ 2 mm. Another
explanation could be that the clay is smeared out on the
filter. The clay may not look like separate particles, but
rather like the ``porridge'' visible in Figure 10.
The preferential entrainment of smaller particles can be
caused by the aggregation process itself. In a 1.9 m high
cylinder, individual frazil crystals tended to become
unbalanced and tilted over when loaded by sand grains
from the top (Reimnitz and others, 1993). In the tank, the
equivalent of this would be that even though the sand grains
collide with the frazil crystals, they do not aggregate, but
tend to separate after the collision.
In Nature, the selective entrainment of small grains is
usually understood to be an indication that suspended
sediment is entrained (NÏrnberg and others, 1994). Only
the fine fraction will usually be suspended outside the strong
flow of the delivering river, unless wind and tidal mixing are
very strong. In the tank experiment, the equivalent to this
would be that the turbulence is not strong enough either to
diffuse the larger grains significantly above the tank bottom
or to diffuse the frazil crystals down to the bed load.
5.5. Entrainment and aggregation
An entrainment factor between the initial or average SPM
concentration in the water and the resulting IRS concentration of the formed ice can be defined as
IRS
XE ˆ
:
…10†
SPM
Here the initial SPM value of each experiment (12.0^
18.1mg L^1) is used. The maximum value of all experiments
for the IRS was 198.5 mg L^1. This was from granular ice in
experiment B (Fig. 11), making XE ˆ 11.0 at 24 hours. For the
slush ice, the maximum XE was 9.2 at 23 hours in experiment
B, and for frazil ice in suspension the maximum was XE ˆ 8.3
after 4 hours in experiment A. All available measurements of
IRS are plotted in Figure 12 as entrainment factors, XE, and
maximum values from surface ice (slush and granular) are
given inTable 1.
Figure 12 shows that the entrainment process works over
long time-scales and that one cannot expect high IRS
concentrations if the ice-growth process lasts only a few
hours. It is clearly seen that the nature of the process is
patchy and that large variations are common, even in a tank
with constant forcing.
There is a tendency that IRSfrazil > IRSslush after a few
hours in the experiments. This indicates that the suspended
frazil scavenges sediment, but, as time passes, these crystals
also become a part of the surface slush, bringing the sediment
grains along.
A sum of the entrainment process is found in the
decreasing SPM concentrations of the tank water, shown at
7 and 22 hours in Figure 12. Since the turbulence was fairly
constant it is unlikely that more sediment had been
deposited on the tank bottom, and thus the lost SPM was
transformed into IRS.
Note that the first surface slush ice that formed had an
IRS concentration of zero, and not the SPM concentration
of the water, as indicated in Figure 12. It takes about 5^10
hours to reach the initial SPM concentration (XE ˆ 1.0).
After 15 hours all measurements document an enrichment
of the ice, i.e. XE 41.0. It has earlier been assumed that
frazil or slush will entrain sediment up to the local SPM con-
Fig. 12. Sediment entrainment factors, XE from experiments
A^D.The initial and end SPM is plotted as *. XE are from
frazil ice in suspension (+), surface slush (3) and congealed
granular ice (^).
centration (Sherwood, 2000), so XE *1 after the entrainment process has taken place. These experiments shows that
values as high as XE *10 can be reached within 24 hours,
and that XE * 5 in about 30% of the samples. Comparable
earlier laboratory experiments report different entrainment
factors as outlined below:
Two lead experiments from the same tank were described
by Lindemann and Smedsrud (1999). The current was
19 cm s^1, and the experiments lasted 16 hours. A layer of
1^5 cm of granular ice formed in the lead, and XE ˆ 4.0.
Reimnitz and others (1993) used a vertical tank (1.9 m
high, 12 cm wide) filled with sea water, stirred at the
bottom for 20^40 min in their experiments. For 12 experiments XE ˆ 1.48 with varying SPM concentrations of silt
and clay (10.6^5881mg L^1), and frazil-ice formation. The
range of XE was 0.7^2.13. Entrainment showed no particle
dependency on the clay and silt fraction.
In a 1.2 m long tank, with 17 cm water depth, Kempema
and others (1993) found efficient entrainment (XE in the
range 2^8). This was shortly after frazil formation. They
found a decrease of the IRS concentration with time down
to XE ˆ 1.0 after 2 hours. Experiments were conducted
with salt water, and a 70 cm s^1 current. Sediment was clay
and silt, with SPM concentrations *100 mg L^1. This was
interpreted as frazil being ``sticky''only during the time of
maximum supercooling, *0.1³C for 5^10 min. The authors
claim that frazil loses its ability to bond to sediment when
Tw increases to the equilibrium temperature, *0.02³C
below Tf.
Ackermann and others (1994) report on experiments with
high SPM concentrations of fine silt (*10 g L^1). Here
XE *0.7 until maximum supercooling was reached.
Experiments were conducted with fresh water, in a 1.15 m
high, 13.8 cm wide tank, stirred from below. The surprising results were that when sand was used as SPM, much of
the frazil crystals sank due to their load, indicating efficient entrainment for this grain-size.
Together these experimental results confirm that frazil
ice can aggregate to sediment and can result in surface
slush containing sediment.
The cited small-tank experiments likely had much higher
turbulence levels than may occur in nature, and also higher
469
Journal of Glaciology
than during experiments A^D. This could result in an
artificially effective entrainment process, creating the
XE * 0.7^8 on the time-scale of minutes, and with the frazil
in a``sticky'' mode during the maximum supercooling period.
If the experiments of Ackermann and others (1994) were
continued for a longer time, they should then have given
higher XE 's. An explanation for the efficient aggregation
between the sand grains and frazil ice could be that the
probability of collision between the two is proportional to
the cube of the radius of the grains (Smedsrud, 1998). As
the frazil loses its ``stickiness'', these larger grains may disaggregate, as there are no sand grains observed in the IRS
in experiments A^D.
The initial stage of maximum supercooling is mostly due
to the laboratory set-up, where cooling is constant and the
water is seeded with ice crystals after it has reached supercooling. In nature this probably happens rarely, as there will
often be some snow crystals falling into the sea surface when
air temperatures are below freezing. During strong wind, sea
spray will also freeze and fall back into the water.
Experiments A^D did not have these maximum supercooled minutes in the beginning, and Tw is normally
*0.01³C below the freezing point. Often this is called the
equilibrium temperature. As in nature, this is caused by
the snow falling from the ceiling, and also the presence of
sea ice around the test section.
The first efficiently entrained sediment might drain out
(Kempema and others, 1993) and XE might come down to
unity over a period of time. This may then be comparable to
the 5^8 hours in our experiments, with more moderate
turbulence forcing.This is consistent with the relatively high
XE 's from frazil ice in suspension at *4 hours (Fig. 12).
At longer time-scales, results presented here indicate that
entrainment again becomes effective, and that SPM
aggregates with frazil ice in the surface (slush), or in
suspension. The term ``aggregation'' is preferred here, and
includes any process that leads to bonding between sediment
and frazil after a collision. In this way, discussion of weather
``scavenging'' by frazil crystals, or ``filtration processes'' by the
surface slush as the dominant mechanism (Osterkamp and
Gosink, 1984) is avoided. Measurements presented here
indicate a continuous flux of frazil from the suspended state
to the surface, and there may also be a significant exchange
between the two states.
In experiments A^D, frazil ice in the surface slush
appeared to remain as individual crystals, and did not form
frazil flocs, confirming the findings of Hanley and Tsang
(1984). At some stage this slush congealed into granular ice,
but this was usually when the experiments ran unattended
during night-time. The idea of the surface slush as a more or
less rigid ``filter'' may therefore only be applied for a
maximum period of 12 hours. More likely this should be
around 6 hours, as the sampled granular ice was firm and
solid. As the lower part of the granular ice had higher levels
of IRS, this also indicates that there is no efficient ``filtration
process'' taking place within the solid ice.
Osterkamp and Gosking (1984) noted that a ``filtration
process'' will be most efficient when waves splash over the
ice to produce vertical flow, but the experiments presented
here show maximum XE's in experiments A and B with no
waves. This indicates that such a``filtration process'' was not
the dominant process. On the contrary, the lower IRS in
experiment C compared to experiment B may indicate that
waves actually make the entrainment process less effective,
470
as experiments B and C were quite similar apart from the
presence of waves. For the moment, the efficiency of the
``filtration process'' remains unknown.
Experiment D has very low , and should thus give lower
aggregation according to Smedsrud (1998), because of the
lower probability of collisions between sediment and frazil.
But it also has high Ci as a consequence of high K, and these
appear to cancel out, and produce XE's around unity, as in
the other experiments, around 6 hours.
The highest IRS values are found in experiment B,
together with the highest initial SPM, the coldest Ta, the
largest heat flux and therefore also the highest Ci =t.
This indicates that there is an important link towards ice
growth and SPM level in the entrainment and aggregation
process. Differences in turbulence are quite small for
experiments A^C, but experiment B has the lowest q and
K, indicating that such differences are not important for
the entrainment process.
A new version of the numerical model developed in
Sherwood (2000) is presently being developed by the author,
incorporating the frazil- and sediment-size spectra and the
aggregation process presented in Smedsrud (1998). This work
will give further insight into the entrainment process.
As these experiments resemble natural settings better
than those reported above, it can be expected that an
aggregation process is important in explaining how sediment is entrained into Arctic sea ice.
6. CONCLUSION
Four experiments are presented that simulate natural
freezing processes in shallow Arctic seas. The experiments
are comparable to natural conditions for several
parameters: sediment size and concentrations, heat fluxes
with their associated frazil-ice formation rates, turbulent
rms velocities and turbulent dissipation rates.
As the frazil-ice maximum diameter is observed to be
around 2 cm, and the sediment is much smaller, the spatial
scale of the tank should allow for natural interaction
between the two types of particles. The time-scale in the
experiment (24 hours) is also comparable to natural timescales for ice formation in Arctic leads or polynyas.
The experimental data show that the mass of sediment
entrained into the sea-ice cover formed with a constant level
of turbulence increases with time.The ice formed during the
experiment had up to 11 times higher concentrations of sediment than the water. Average values were between 2 and 10
in 24 hours. The entrainment depends on: (i) sediment
concentration of the water, (ii) low air temperature leading
to high heat fluxes and corresponding frazil-ice production,
and (iii) strong enough turbulence. Waves do not appear to
increase entrainment by frazil ice significantly.
The data presented here should be adequate for verification of numerical models for sediment entrainment into
growing ice by aggregation between sediment and frazilice crystals in the surface or in suspension.
ACKNOWLEDGEMENTS
The author would like to thank the HSVA for technical
support and professional execution of the test programme
in the ARCTECLAB, and the Commission of the European
Communities (TMR-Program ``Access to Large-Scale
Smedsrud: Frazil-ice entrainment of sediment
Facilities'') for financial support and the opportunity to carry
out the experiment. H. Eicken and C. Haas (Alfred Wegener
Institute) were excellent coordinators of the experiments, and
G. Kuhlman and D. Dethleff (Geomar) provided valuable
help with the sediment. Thanks also to S. Pegau (Oregon
State University) and J. E. Stiansen (Institute of Marine
Research, Bergen) for lending the absorption meter and the
ADV, E. Kempema for all his suggestions for improvements
as a referee, and the scientific editor M. Lange. This work
was supported by the Norwegian Research Council, under
contract No.71928/410.
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MS received 29 August 2000 and accepted in revised form 27 June 2001
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