ARTICLE IN PRESS
Continental Shelf Research 24 (2004) 1699–1715
www.elsevier.com/locate/csr
Floc fraction in the waters of the Po River prodelta
J.M. Foxa,, P.S. Hilla, T.G. Milliganb, A.S. Ogstonc, A. Boldrind
a
Department of Oceanography, Dalhousie University, Halifax, NS, B3H 4J1, Canada
Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, NS, B2Y 4A2, Canada
c
School of Oceanography, University of Washington, Seattle, WA, 98195-7940, USA
d
Istituto di Scienze Marine—ISMAR, Biologia del Mare—C.N.R., Castello 1364/A, 30122 Venice, Italy
b
Received 2 June 2003; received in revised form 5 April 2004; accepted 14 May 2004
Available online 19 August 2004
Abstract
Three independent methods of estimating the proportion of suspended material packaged within flocs, termed the
floc fraction (f), were employed using hydrographic and suspended sediment data, and core and tripod-based
observation data collected from the waters and sediments of the Po River prodelta throughout 2001. Using a floc size
versus settling velocity relationship established in this study, floc fraction estimates were derived as follows: (1) by
calculation of floc concentration in in situ images versus total suspended concentration using knowledge of floc effective
density via Stokes’ approximation; (2) by parameterization of disaggregated inorganic grain size (DIGS) distributions
of bottom sediments to infer floc fraction in suspension necessary to produce the observed flux to the seabed; and (3) by
calculating a mean representative floc fraction estimate for the system derived from estimates of floc bulk density.
Calculated estimates of floc fraction are near unity at the river mouth, with the bulk of floc deposition occurring by the
8 m-isobath. Seaward and to the south of the river mouth, the suspension is not highly flocculated (f 0:08).
Subsequent transport and removal of deposited sediment from the prodelta, as suggested in previous study, is believed
to occur within the bottom boundary layer.
r 2004 Elsevier Ltd. All rights reserved.
Keywords: Aggregation; Delta; Effective density; Floc; Floc fraction; Po River; Sedimentation; Settling velocity
1. Introduction
Fine particle marine aggregates, or flocs, are
ubiquitous in the ocean (Shanks, 2002) and are
considered responsible for the rapid flux of the
majority of fine particles to the seabed (McCave,
Corresponding author. Fax: +1-902-494-3877.
E-mail address: jfox@phys.ocean.dal.ca (J.M. Fox).
1975; Drake, 1976; Shanks and Trent, 1980;
Syvitski et al., 1995). Past studies of river
sedimentation along continental shelves (Gibbs
and Konwar, 1986; Boldrin et al., 1988; Kineke et
al., 1991; Geyer et al., 2000) invoke flocculation as
necessary and responsible for the observed sedimentation rates, yet the quantitative mass percentage of flocculated material in suspension, or floc
fraction (f ), in suspension remains poorly known.
0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.csr.2004.05.009
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Knowledge of floc fraction is essential for accurate
predictions of sedimentation, because deposition
can change appreciably with even a small variation
in floc fraction due to the substantial difference
between floc and single grain settling velocities in
river plumes (Hill et al., 2001). With a method to
quantify floc fraction in suspension, a fuller
understanding of the sedimentary dynamics of a
given system would emerge. However, flocs are
fragile, difficult to sample, and have physical
properties yet to be fully resolved, complicating
the task of estimating the floc fraction.
The goal of this paper is twofold: to apply three
methods of floc fraction determination to the
sedimentary dynamics of the Po River prodelta
and to assess the validity of those methods. The
method that has been applied previously in the
literature uses Stokes’ approximation for the
settling rate of a sphere to calculate excess density
based on floc size, which, in turn, provides the
knowledge needed to calculate mass bound within
flocs of a given size (Syvitski et al., 1995; Dyer and
Manning, 1999; Curran et al., 2002a). Floc
concentration can be calculated using size distributions of flocs from in situ images provided the
total suspended sediment concentration is known.
Estimates can also be achieved by examining
sediment deposits. Parameterization of bottom
sediment disaggregated inorganic grain size
(DIGS) distributions allows estimates of the mass
fraction of material deposited within flocs to be
made (Kranck and Milligan, 1991; Kranck et al.,
1996; Curran et al., 2004). In addition to these
established methods, a new approach to floc
fraction estimation based on the technique of
Mikkelsen and Pejrup (2000) is applied. Using an
in situ laser particle sizer, Mikkelsen and Pejrup
(2000) calculated bulk density estimates of material in suspension, generating a bulk size versus bulk
settling velocity relationship representing the field
site as a whole. When this technique is applied to
in situ photographs and corresponding concentration data, a mean representative estimation of floc
fraction is possible. Essential to all these techniques is a size versus settling velocity relationship of
flocs in the waters of the Po prodelta which can be
applied directly to the Stokes’ and bulk density
methods, and from which a mean representative
floc settling velocity is applied to the DIGS
method.
Following a large-scale flood of the Po River in
October 2000, extensive coring, hydrodynamic
profiling, including suspended sediment profiling
of the receiving basin, and the deployment of a
tripod for long-term observation of bottom
boundary layer dynamics were undertaken as part
of the Office of Naval Research EUROSTRATAFORM program. Although abundant flocs were
observed in the Po River and its prodelta
environment (Fox et al., 2004), floc fraction could
not be determined due to lack of a size versus
settling velocity relationship in the system. Previous studies of floc settling suggest a similar
settling rate (on the order of 1 mm s1 ) from
regions of varied geographical and energetic
conditions (McCave, 1975; Kranck et al., 1992;
Fennessy et al., 1994; ten Brinke, 1994; Syvitski et
al., 1995; Hill et al., 1998; Dyer and Manning,
1999; Sternberg et al., 1999). Determination of floc
settling rates in a marginal sea such as the
Northern Adriatic would be a welcome addition.
By equipping the tripod with a settling column/
video apparatus (Sternberg et al., 1996), all the
elements necessary for estimation of floc fraction
were available.
2. Methods
2.1. Overview
The Po River dominates sediment input to the
Northern Adriatic Sea. Fed by the Alps to the
north and west and by the Appenines to the south,
the Po delivers an estimated 20 millon tonnes of
sediment annually (Nelson, 1970). In addition to
natural inputs, the Po receives anthropogenic
inputs from Italy’s most industrialized regions.
These large inputs allow the Po to influence the
budgets of dissolved and particulate materials of
the entire Adriatic sea (Matteucci and Frascari,
1997). The Po delta is a product of centuries of
human manipulation and flow re-direction and
today is the site of substantial sediment deposition
and accumulation. The present-day delta has five
major distributaries, with 60% of the flow passing
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Fig. 1. Site Map of the study area. Station points are subdivided into those profiled from on board Mysis (circles) and Sarom VIII
(squares). In the full domain view, every station from which a Bulk Settling Velocity estimate was made is marked as well as Bridge
stations (B1, B2). In the magnified view, station transects are marked as North, Central, E-line, Southern, and I-line as well as Delta
stations (D1–D3) from within the Po river. The tripod location (T) is marked with an open square.
through the Pila mouth located at the apex of the
delta (Fig. 1).
Data collection was conducted in 2001 during
cruises in January, June, and October. A tripod
assembly was deployed January 25 and June 5
south of the delta at 14 m depth (Fig. 1), with
retrieval occurring in June 5 and October 10,
respectively. A settling column/video apparatus
was affixed to the tripod at 2 m above the seabed,
logging video of settling aggregates and collecting
them in a trap at the base of the column (Sternberg
et al., 1999). Box core samples were collected at the
tripod location during each deployment and
retrieval. Suspended sediment surveys took place
in June and October involving collection of in situ
suspended sediment photographs, water samples,
transmissometer profiles, and surficial sediment
samples as described in Fox et al. (2004). From
June 4–7, a total of fourty-four offshore stations
were profiled from the Istituto di Biologia del
Mare–Consiglio Nazionale delle Ricerche (CNRIBM) research vessel Mysis and the Micoperi
vessel Sarom VIII. This investigation concentrated
on five transects in the vicinity of the Pila mouth
and the tripod location (Fig. 1). On October 14,
two stations upstream of the mouth were visited
for collection of in situ photographs and water
samples. On October 16, three stations were
profiled inside the delta with the addition of
current meter readings from on board the Mysis.
Disaggregated inorganic grain size (DIGS) distributions were measured with a Coulter Multisizer IIe for all core subsamples, settling column
trap subsamples, surficial sediment samples, and
suspended sediment samples with methods described previously (Milligan and Kranck, 1991).
Measurements were conducted over a relatively small range of environmental conditions.
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Suspended particulate matter (SPM) was generally
less than 15 mg l1 , current speeds were generally
less than 0:5 m s1 , and the water column was
highly stratified (Fox et al., 2004). As a result,
turbulent kinetic energy dissipation rates, although
unmeasured, were likely low. Because floc fraction
depends on concentration as well as turbulent
kinetic energy dissipation rate, floc fractions in
turbid energetic environments likely would differ
from those measured here.
2.2. Size versus settling velocity
The instrumentation of Sternberg et al. (1996,
1999) was used to collect size versus settling
velocity data. The assembly consisted of a baffled
settling trap with one transparent side, to which a
Sony Hi8 Video Camera in a pressure housing was
attached. Video was recorded for 10 s every 6 h
over the course of each tripod deployment. The
video was transferred to Sony DVcam tapes,
generating a full digital data log of 720 480
pixel images with no signal loss. The camera used
different powers of zoom for each deployment,
resulting in a maximum resolution of 66 and
98 mm in January and June, respectively. Images
from a resolvable burst of video were offloaded to
a PC at intervals of 15 or 30 frames, depending on
the speed of the settling flocs. A MATLAB script
was executed to isolate the moving flocs in the
image set, track the particle settling, and output
independent estimates of diameter as equivalent
spherical diameter (ESD), which considers the
projected surface area of each floc to be circular,
and elliptical nominal diameter (END), which
considers projected surface area to be elliptical
having a minor and major axis, and settling
velocity (wf ). There were three periods where
video proved to be unresolvable. On January 27,
waves with a significant height of 4 m stirred the
fluid in the settling column vigorously. On January
30, a downwelling-favorable Bora wind event
generated enough resuspension to coat the transparent viewing surface and disabled particle
detection for the remainder of the deployment.
During the June deployment, sediment accumulation was sufficient to fill the trap, eliminating a
working view by July 13.
2.3. In situ image analysis
At every station shown in Fig. 1, a Benthos 373
plankton silhouette camera cast was made through
the water column in concert with other hydrographic instruments. Photographs were shot every
4 s. In situ suspended sediment photographs were
analyzed as outlined in Fox et al. (2004). Prior to
analysis, all photographs, having a depth of field
of 4:0 cm, were duplicated using a Fuji FinePix S1
Pro digital SLR generating high resolution (3040
2016 pixel) grayscale image files. Binary thresholding, particle counting, and particle sizing were
performed using Image Pro Plus (Media Cybernetics) PC image analysis software following the
procedure of Curran et al. (2002b). The lower
detection limit of particles within the images is
125 mm, and as such, does not reflect a full
complement of material in suspension. Median
floc
size (d 50 ) of each image was calculated as
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð4=pÞA50 , where A50 is the median floc area.
Large particle volume concentration (LPVC) is the
total volume of suspended material divided by the
full volume represented in an image provided all
the identified material is considered to be solid
and spherical, with individual floc volumes a
function of ESD. LPVC is expressed as parts per
million and provides a representation of abundance of large particles in suspension (Milligan
et al., 2001).
3. Determination of floc fraction
3.1. Stokes’ approximation
Floc fraction estimates via Stokes’ approximation have been made in previous studies (Syvitski
et al., 1995; Dyer and Manning, 1999; Curran et
al., 2002a). A fundamental assumption necessary
to employ this method is to consider aggregates to
exist in an environment where viscous forces of the
surrounding fluid dominate the inertial forces of
the aggregate. Calculation of Reynolds number
(Re ) quantifies this relationship:
Re ¼
wf dr
;
m
ð1Þ
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where wf is floc settling velocity (m s1 ), d is the
aggregate diameter (m), r is the density of
seawater (kg m3 ), and m is dynamic viscosity
(kg m1 s1 ).
In a viscous regime (Re 51), Stokes’ approximation for a settling sphere can be rearranged to solve
for effective density, Dr (kg m3 ):
18mwf
;
gd 2
Dr ¼
ð2Þ
where g is gravitational acceleration (m s2 ).
Application of the size versus settling velocity
data with corresponding viscosity data determined
from temperature and salinity measurements,
which we obtained from the tripod CTD, result
in a size versus effective density relationship
that can be applied to processed in situ still images
to determine the mass bound within each floc
image:
Sf ¼
Dr
;
rs r
ð3Þ
p 3
d rs Sf ;
ð4Þ
6
where Sf is the solid fraction of a given floc, rs is
the density of quartz (kg m3 ), and M f is mass of a
given floc (kg).
Each in situ image is representative of a volume
of water. By dividing the sum of floc mass by the
image volume, floc concentration is calculated. To
complete determination of floc fraction, suspended
particulate matter concentration at the depth of
the image is necessary. This can be achieved by
calibrating transmissometer data to known concentrations from water samples. The formal
equation for floc fraction (f ) is simply:
Mf ¼
Cf
;
ð5Þ
SPM
where C f is floc concentration (kg m3 ), and SPM
is suspended particulate mass concentration
(kg m3 ).
f ¼
3.2. DIGS Parameterization
Consider that all material deposited on the
seabed arrives as either single grains or flocs, and
their relative proportions are representative of the
1703
environmental conditions of deposition. This is the
fundamental assumption behind the bottom sediment DIGS parameterization (Kranck et al., 1996;
Milligan and Loring, 1997; Curran et al., 2004). If
concentration of grain size class i (CðiÞ) is
represented by the following equation:
m
di
^2
eðd i =dÞ
ð6Þ
CðiÞ ¼ Q
d0
where d 0 is the reference diameter (m), Q
represents the concentration of the reference
diameter (kg m3 ), m represents the distribution
of source material, d i is the diameter of size class i
(m), and d^ is the diameter whose relative
concentration is 1=e of its relative concentration
in the source distribution (m), a non-linear fit of
the size distribution is made to an equation for
total flux:
2 !
di
JðiÞ ¼ wf fCðiÞ 1 þ
;
ð7Þ
df
where JðiÞ is the total flux of the ith size class to the
seabed (kg m2 s1 ) and d f is the diameter at
which the flux to the seabed is equal for singlegrain and floc deposition. This parameter is
termed the floc limit (m).
^ and d f are made from the
Estimates for m, d,
fit of Eq. (7) to a given distribution. To accommodate a proper fit, particle counts in the coarsest
of size classes that reflect processes other than
deposition from suspension (i.e. bedload) are
excluded from the fit routine. These classes appear
as a distinct mode or bulge in the coarsest part
of the size distribution (Kranck and Milligan,
1991). The excluded points, however, are included
in the integration calculation to obtain the fraction of mass deposited to the seabed within flocs
(K f ):
P
Jðf Þ
Kf ¼ P
;
ð8Þ
JðtÞ
where Jðf Þ is flux bound within flocs and JðtÞ
is total flux. A graphical representation of the
non-linear fit and parameters pertinent to this
study can be found for a low and high K f scenario
in Fig. 2.
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Now consider single-grain and floc flux written
in terms of floc fraction:
J s ðiÞ ¼ ws ðiÞCðiÞð1 f Þ;
Alternatively,
f ¼
ð9Þ
J f ðiÞ ¼ wf CðiÞf ;
where J s ðiÞ and J f ðiÞ are single-grain and floc flux
for the ith size class respectively (kg m2 s1 ), ws ðiÞ
is the single-grain settling velocity of the ith size
class (m s1 ), wf is the mean floc settling velocity
(m s1 ), CðiÞ is the total concentration of the ith
size class, and f is the floc fraction.
By solving for single-grain and floc flux of d f ,
Eqs. (9) and (10) are set equal and rearranged to
solve for f, that being the floc fraction necessary in
suspension to generate the given flux:
f ¼
1
3.3. Bulk density estimates
Mikkelsen and Pejrup (2000) made bulk density
calculations of material in suspension using
volume concentration measurements from a
LISST-100 in situ laser particle sizer paired with
directly measured SPM values based on filtration
of a 2 l water sample on 0:45 mm membrane filters:
Drb ¼
ð11Þ
ð13Þ
Kf =0.74
df =24 µm
df = 6 µm
Relative Abundance
SPM
VC
where Drb is the bulk effective density of material
in suspension (kg m3 ) and VC is the volume
10-1 K =0.42
f
10-2
10-3
df
df
10-4
100
(a)
ð12Þ
A size versus settling velocity relationship is
required to provide a mean representative floc
settling velocity.
ð10Þ
ws ðd f Þ
wf
:
w ðd Þ
þ swf f
ws ðd f Þ
:
ws ðd f Þ þ wf
101
Diameter (µm)
102
100
(b)
101
Diameter (µm)
102
Fig. 2. Application of Disaggregated Inorganic Grain Size (DIGS) parameterization is displayed for selected 01/01-Trap (a) and 06/01Core (b) sediment subsamples. The non-linear fit (solid line) of the full size distribution (asterisks) is fractionated into flux bound
within flocs (circles and dashed line) and as single grains (squares and dotted line). Coarser material in the distribution is excluded from
the fit routine as their arrival is from processes other than deposition from the water column. The area underneath the floc curve
divided by the area under the total flux (asterisks) represents the fraction of material deposited to the seabed as flocs (K f ). The point of
intersection of the floc and single-grain curves represents the diameter at which single-grain and floc fluxes are equal, termed the ‘floc
limit’ (d f ). It is through the use of d f that a calculation of the floc fraction in suspension necessary to generate the observed DIGS can
be made.
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concentration of material in suspension. The
particle detection limit of the LISST-100 is
1.25–250 mm, allowing for the inclusion of fine
single grain particles in suspension.
Given that volume concentration will be taken
from still images as opposed to a laser sizer in our
analysis, Drb is re-defined in terms of SPM from a
water sample and LPVC calculated from a
collocated in situ image:
Drb ¼
SPM
:
LPVC
ð14Þ
Given a median size estimate from the size
distribution, a bulk estimate of settling velocity
can be calculated using Stokes’ approximation and
the median floc size d50 calculated from the same
image for which LPVC was determined:
g
wsB ¼
Drb ðd50Þ2
ð15Þ
18m
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intervals of the respective data sets to overlap
(Tables 1 and 2), allowing for a pooling of the data
sets. The result is a data set of 409 independent
estimates of size versus settling velocity (Fig. 3).
Velocities ranged from 0.08 to 8 mm s1 , with a
mean settling velocity of 1:2 mm s1 based on
number of flocs. For the smallest flocs analyzed, a
pixelation effect is observed on a given image
where arbitrary increases in floc size are represented by changes to a small number of pixels (o 4).
By bin-averaging the data set into 13 F size classes,
mean settling velocities emerge each associated
with a logarithmic range of sizes, eliminating
concerns about pixelation and serving to elucidate
the relationship between mean size and settling
velocity (Syvitski et al., 1995; Hill et al., 1998).
Regression of the bin-averaged data provides
expressions for settling velocity (m s1 ):
wf ¼ 86:95ðESDÞ1:33
ð16Þ
1
where wsB is bulk settling velocity (m s ) and d50
is the median size of material in suspension (m).
The bulk size versus settling velocity relationship generated by this method assumes implicitly
that all material in suspension is bound within the
flocs visible in each image (f ¼ 1). This relationship can then be compared to the established
relationship from video analysis. The set of bulk
estimates will overestimate floc settling rates as it is
known that the images are not representative of
the full suspended load. By that rationale,
provided the slopes of the respective size versus
settling velocity relationships are not significantly
different, the offset of the intercept is directly
proportional to the bulk density overestimate, and
a floc fraction representative of the system as a
whole can be determined. For example, if the bulk
settling velocity relationship is 6 larger than the
trend observed in the settling column, the floc
fraction of the system would be f ¼ 16 0.17.
4. Results
Resolvable video spanned January 26–29 and
June 6–July 12, generating 209 and 200 estimates
of floc size and settling velocity respectively.
Regression analyses showed the 95% confidence
with r2 =0.97 where ESD indicates equivalent
spherical diameter (m).
The same bin-averaging procedure was performed on the calculations of effective density
(kg m3 ):
Dr ¼ 0:07ðESDÞ0:76
ð17Þ
with r2 =0.88.
The results for equivalent spherical diameter
and elliptical nominal diameter were closely
Table 1
Regression results: equivalent spherical diameter
Time period W s Expression (m s1 ) Dr Expression ðkg m3 )
Data
r2
r2
January
Raw
January
Bin Avg.
June
Raw
June
Bin Avg.
Jan+Jun
Raw
Jan+Jun
Bin Avg.
21.39(ESD)1:22
0.57
81.19(ESD)1:35
0.93
36.08(ESD)1:24
0.55
31.79(ESD)1:19
0.95
78.01(ESD)1:35
0.63
86.95(ESD)1:33
0.97
0.03(ESD)0:83
0.37
0.12(ESD)0:70
0.75
0.03(ESD)0:85
0.36
0.06(ESD)0:81
0.86
0.04(ESD)0:80
0.38
0.07(ESD)0:76
0.88
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Table 2
Regression results: elliptical nominal diameter
Time period W s Expression (m s1 ) Dr Expression (kg m3 )
Data
r2
r2
0.03(END)0:84
0.39
0.14(END)0:69
0.81
0.04(END)0:83
0.38
0.06(END)0:80
0.95
0.05(END)0:79
0.40
0.10(END)0:74
0.94
Effective Density (kg m-3)
12.61(END)1:16
0.56
35.37(END)1:26
0.92
19.85(END)1:17
0.54
38.95(END)1:23
0.97
43.76(END)1:29
0.61
77.60(END)1:33
0.97
Settling Velocity (mm s-1)
January
Raw
January
Bin Avg.
June
Raw
June
Bin Avg.
Jan+Jun
Raw
Jan+Jun
Bin Avg.
related (Tables 1 and 2) as a majority of the sample
population was approximately spherical.
Application of Stokes’ approximation to calculation of f for in situ images yields estimates
greater than unity (Figs. 4–6). The largest floc
fraction estimate (f=2.92) resides at 2:5 m at the
4 m station of the C-line directly offshore of the
Pila mouth. Seaward and to the south of the 10 m
station on the C-line, SPM and LPVC decrease
considerably, with floc fractions remaining high
(f 4 0.6) throughout.
Parameterization of DIGS distributions at the
tripod location provide floc fraction estimates in
the range 0.04 to 0.10 (Table 3). The fraction of
mass deposited to the seabed within flocs (K f ) was
higher in the settling trap than in the seabed, yet
1
100
10
0.1
Settling Velocity (mm s-1)
1000
1
100
(b)
Effective Density (kg m-3)
100
(a)
1000
100
10
0.1
100
(c)
100
1000
Diameter (m)
(d)
1000
Diameter (m)
Fig. 3. The size (ESD) versus settling velocity relationship of Po prodelta flocs was obtained through video observation of 409
aggregates (a) in a settling column fixed to the tripod. Calculation of aggregate effective density was based on Stokes’ Law (b). Binaveraging of the data into 1/3F size intervals was performed (c,d), yielding more robust relationships.
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Fig. 4. Contours of suspended particulate matter (SPM), floc
volume concentration (LPVC), and floc fraction (f ) calculated
via Stokes approximation are observed along the C-line off the
mouth of the Pila mouth. Data points are marked in gray.
SPM, LPVC, and floc fraction are highest at 2:5 m depth at the
4 m station, with all variables decreasing considerably by the
10 m station. Concentration increases at the 15 m-isobath,
where bottom sediments likely become entrained in the southward drift current. Note that many floc fractions are well above
unity, indicating that an assumption in the calculation is in
error.
Fig. 5. Contours of suspended particulate matter (SPM), floc
volume concentration (LPVC), and floc fraction (f ) calculated
via Stokes approximation are observed along the E-line off the
mouth of the Pila mouth but in deeper water than the C-line.
Sampling points are marked as gray dots. The highest
concentrations are observed at the 10 m station and elevated
concentration at the 15 m-isobath. Floc fractions are lower than
observed on the C-line, suggesting that most material has been
removed prior to the 10 m-isobath, beyond which bottom
boundary layer transport is suggested to be responsible for the
majority of sediment flux.
floc fraction was low and similar to that of the core
sediments (Table 3). Surficial sediment DIGS
parameters from the N, C, and S-lines indicate
high floc fraction and a large proportion of floc
deposition directly offshore of the Pila mouth
between 6 m and 8 m (f 0.50) with floc fraction
decreasing (f o 0.1) seaward and to the south of
the 10 m-isobath (Fig. 7). Elsewhere, high K f
values coupled with comparatively low f values
reflect a suspension that is predominantly fine
single grain particles. In such a suspension, low
single grain settling velocities limit the single-grain
flux to the seabed despite low floc fractions
allowing for relatively high K f values (Fig. 7).
The northern and southern transects both exhibit
low floc fractions, with the northern transect
receiving a greater proportion of floc-deposited
material due to its proximity to a distributary
channel to the North (Figs. 1 and 7). It is
noteworthy that K f values for the three transects
converge at 15 m.
Sixty-three data points from 42 stations (Fig. 1)
were available for bulk density estimates. The
regression of bin-averaged bulk settling velocity
estimates on bin-averaged diameters provides a
slope (m=1.26) close to that generated by settling
video observation (m=1.33). The intercept, however, is a factor of 4 larger for the bulk density
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5. Discussion
Fig. 6. Contours of suspended particulate matter (SPM), floc
volume concentration (LPVC), and floc fraction (f) calculated
via Stokes approximation are observed along the I-line in the
vicinity of the tripod. Sampling points are marked as gray dots.
Concentration is highest in shallow water near the seabed where
floc fraction is also high. LPVC is low throughout the profile.
As a result, floc fraction estimates beyond 12:5 m
(SPMo4 mg l1 ) are prone to error.
Table 3
DIGS parameterization: tripod station
Sample
No. subsamples Mean d f (mm) Mean K f Mean f
01/01-Core 9
01/01-Trap 8
06/01-Core-A 11
06/01-Core-B 12
06/01-Trap 18
10/01-Core 8
8
10
11
11
12
12
0.31
0.52
0.37
0.43
0.62
0.42
0.04
0.07
0.08
0.09
0.09
0.10
estimates than the direct observations of settling
velocity, suggesting a bulk floc fraction estimate of
0.25 for the system as a whole (Fig. 8).
The size versus settling velocity relationship
obtained in this study resembles those from
previous studies of size and settling from locations
of varied energy and geography (Fig. 9) (McCave,
1975; Kranck et al., 1992; Fennessy et al., 1994;
Syvitski et al., 1995; Sternberg et al., 1999). The
slope of the trendline is slightly greater than those
from studies in estuaries (Kranck et al., 1992;
Fennessy et al., 1994; Syvitski et al., 1995), and
slightly less than results from the exposed continental shelf (Sternberg et al., 1999) and the deep
sea (McCave, 1975). The relationships established
for the Po prodelta improve the precision of floc
fraction determinations.
A wide range of floc fraction estimates emerge
for the system from the three methods applied
(0:04 p f p 2:92), will all overestimates associated
with the Stokes’ approximation method. Estimates
that fall within a region of low SPM (o4 mg l1 )
and low LPVC (o 50 ppm) are prone to error
because they derive from the ratio of two small
numbers, confining the focus here to waters
landward of the 15 m-isobath (Figs. 4–6). When
estimates are based on two parameters with low
values, the range of results increases and the
confidence is diminished. Aggregates observed in
regions of low SPM and LPVC are primarily of
large irregular and comet-like shapes, indicative of
a high proportion of organic material which are
not of relevance to the present study (Kranck and
Milligan, 1991). The calibration of the transmissometer (r2 =0.56) was difficult to establish as a
large range of turbidity readings existed for low
concentration waters, likely reflecting variability in
sediment size and composition (Baker and Lavelle,
1984) and the stronger bearing of aggregate size on
suspended concentrations than on measured values of light attenuation (Mikkelsen and Pejrup,
2000). However, elimination of offshore data
points still does not rectify the high floc fraction
estimates made in the nearshore of the central
transect where the conditions (SPM and LPVC)
for estimation are considered most favorable and
reliable (Fig. 4). Perhaps it is the assumptions
necessary to apply Stokes’ approximation which
are problematic. For example, Li and Logan
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(1997) showed that Stokes’ Law underestimates
settling velocities of aggregates. They attributed
this result to reduced drag exerted due to
channelling of fluid through pore spaces of the
aggregate. The factor of underestimation ranges
from 2.182 to 2.962 (Li and Logan, 1997). The
50
df (µm)
40
Bulk Settling Velocity (mm s-1)
J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715
1709
10
1
30
0.1
20
100
1000
Diameter (µm)
10
0
0
5
10
15
20
25
1
Fig. 8. Bin-averaged bulk settling velocity estimates are shown
assuming all material visible in in situ photographs is bound
within flocs (f=1) (triangles), and divided by an offset of 4
(f=0.25) (squares). The established size versus settling velocity
relationship (dashed) shows the corrected bulk settling velocity
estimates match the observed results.
0.8
Kf
0.6
0.4
0.2
0
0
5
10
15
20
25
0.5
f
0.4
0.3
0.2
0.1
0
0
5
10
15
Bottom Depth (m)
20
25
location of highest abundance of flocs at 2:5 m
depth at the 4 m C-line station (Fox et al., 2004) is
the same location where the highest floc fraction
estimate is reported (f=2.92). If it is assumed that
floc fraction actually was unity at this location,
floc density was overestimated by a factor of 2.9.
By dividing all estimates of f by this factor, a more
reasonable range of values results (Fig. 10). This
correction factor lies within the range of Li and
Logan (1997), and it provides values for f more
Fig. 7. Profiles of floc limit (d f ), the fraction of mass deposited
within flocs (K f ), and floc fraction (f) of surficial sediment
samples are shown with depth along the N-line (dashed), C-line
(solid), and S-line (dotted). The highest values of all three
parameters occur along the C-line at depths of 6 and 8 m, where
flocs deposit rapidly from suspension offshore of the river
mouth. Along the N-line, intermediate K f values are observed
due to its proximity to a distributary channel to the north. The
floc fractions along the N-line, however, are low. Along the Sline, K f and f are extremely low in the nearshore, reflecting
efficient removal of material in suspension along the C-line and
the lack of a distributary channel in its vicinity. It is noteworthy
that despite the contrasting dynamics of each transect, K f
values converge at 15 m where it has been suggested that
bottom transport carries sediment southward.
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J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715
1710
St
K
Settling Velocity (mm s-1)
1
F
Sy
P
0.1
0.01
0.001
Mc
1
10
100
1000
Mc
Effective Density (kg m-3)
F
P K
100
Fig. 10. Corrected f profiles for the C-line, E-line, and I-line: A
correction factor of 2.9 is applied to the f profiles for the C-line
(Top), E-line (Middle), and I-line (Bottom) following the results
of Li and Logan (1997). Values are now within an acceptable
range (f o1) with floc fraction dropping off immediately
offshore of the river mouth (a,b), and low floc fractions to
the south (c).
St
10
1
Sy
1
10
100
1000
Diameter (µm)
Fig. 9. The established size versus settling velocity relationship
is compared to those determined from varied locations and
energies. Studies are marked as follows: Mc= (McCave, 1975)
from the Deep Sea, K=(Kranck et al., 1992) from San
Francisco Bay, F=(Fennessy et al., 1994) from the Tamar
Estuary, Sy=(Syvitski et al., 1995) from Halifax Inlet,
St=(Sternberg et al., 1999) from the California Shelf, and
P=present study. Excluding Syvitski et al. (1995), these studies
show that on the average, flocs settle on the order of 1 mm s1 ,
regardless of the sedimentary input and the dynamics of the
system.
consistent with the DIGS results. Maximum floc
fraction reported by the surficial sediment DIGS
( 0.50) matches the corrected profile along the Cline (Fig. 10 top). Along the I-line at the tripod
station, corrected Stokes’ approximation floc
fractions are between 0.15 and 0.2, which are
higher than the core and trap DIGS estimates (f
0.08). As the SPM and LPVC are relatively low at
the tripod location, the I-line values may be
overestimates.
The Stoke’s Law underestimate of settling
velocity (Li and Logan, 1997) also reflects an
overestimate of effective density. Thus, the correction factor should also be applied to the bulk
settling velocity estimates from the Mikkelsen/
Pejrup method (Mikkelsen and Pejrup, 2000). By
applying the correction factor used for Stokes
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J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715
1711
15
15
GMD (µm)
f= 0.053
10
10
f= 0.055
5
0
10 m
0.02
0.04
0.06
0.08
f
0.1
5
0
0.8 mAB
0.02
0.04
0.06
0.08
0.1
f
Fig. 11. From samples collected in June, Geometric Mean Diameters (GMD) of expected flux from the 10 m water sample above the
settling trap and the 0:8 m AB water sample between the trap and bottom sediments are plotted as a function of floc fraction (solid
line). The mean GMD determined from the trap and the core (filled circles) are plotted with the 10 m sample and 0:8 m AB sample,
respectively. The floc fraction necessary to match the observed GMD in the deposited sediments is 0.055 ð10 mÞ and 0.053 (0:8 m AB)
showing that floc fraction remains the same even though the GMD increases from 7 mm to 12 mm as the suspension becomes
increasingly more coarse closer to the seabed.
approximation estimates, the intercept of the
settling velocity regression line based on bulk
density is a factor of 12 larger than the
established relationship from direct observations.
The corresponding floc fraction required to
reconcile these two expressions is 0.09 which is
consistent with the DIGS estimates of f and
broadly reconciles the results of the three estimation methods.
Floc fraction is high immediately seaward of the
river mouth, where deposition occurs rapidly and
removes the bulk of flocculated material from
suspension by 8 m. A mid-water density interface
impinges on the prodelta seafloor, serving to
resuspend the deposited material and retain
turbidity in the bottom boundary layer, where
nearshore wave and current action are believed to
transport material seaward along the bottom (Fox
et al., 2004). A re-emergence of turbidity at 15 m is
associated with a southward current which removes the material away from the prodelta (Fox et
al., 2004). Seaward and to the south of the C-line,
floc fractions are low, as are suspended concentrations and visible floc abundance (i.e.: LPVC) in the
in situ images.
At the tripod station, the fraction of mass
deposited to the seabed within flocs (K f ) is
higher in the sediment trap 2 m above the seabed than in the bottom sediments (Table 3).
The trap sediments also have a higher mean
d f and overall lower proportion of coarser grains,
yet values of f remain relatively consistent
throughout (Table 3). If the suspension coarsens
as the seabed is approached, K f would be expected
to decrease as the flux of coarser single grains
to the seabed would rise while not necessarily
altering the proportion of material bound within
flocs in suspension. However, it has been suggested
that as flocs approach the bottom boundary layer,
shear stresses increase accordingly, initiating
nearbed floc break-up (McCave, 1985; Hill
et al., 2001). In an effort to establish the dominant cause of the difference between material
settling to the trap versus the seabed, a simple
calculation was performed using suspended size
distributions from 3 m above the sediment trap
ð10 mÞ and in between the trap and the bottom
sediments (0:8 m AB) collected in June. The flux
to the seabed from each size class over a range
of floc fractions can be calculated by summing
Eqs. (9) and (10) to solve for total flux in size
class i, JðiÞ:
JðiÞ ¼ CðiÞ½ws ðiÞð1 f Þ þ wf f
ð18Þ
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J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715
where wf is the mean floc settling velocity
(1:2 mm s1 ), and f is a pre-assigned floc fraction
within the desired range which is being considered.
The total flux is the sum over i of Eq. (18). Bottom
sediment size distributions are calculated by
dividing the flux in each size class by the total
flux. Geometric mean diameter equals
Pnclass
GMD ¼ 2 i¼1 FðiÞJðiÞ 1000
ð19Þ
0.01
D1
1
Concentration (ppm)
expressed in micron where FðiÞ ¼ log d med ðiÞ=
log2 with d med ðiÞ as the median diameter of size
class i. GMD can be compared for various values
of f.
Geometric mean diameter (GMD) of the 06/01Trap and 06/01-Core DIGS distributions were
calculated and compared to the GMD of the
resultant synthetic flux distributions calculated
with Eq. (18) for the 10 m and 0:8 m AB samples
respectively (Fig. 11). The floc fraction for which
the GMD from the deposited sediments and the
synthetic flux GMD are equal is considered the
floc fraction necessary in suspension to generate
the observed deposit. Critical floc fractions are
0.055 and 0.053 for the waters above and below
the trap respectively. Although a coarsening of the
suspension occurs as the seabed is approached, the
general uniformity of f in the DIGS parameterization results is supported, suggesting that resuspension, not floc break-up is responsible for the
reduction of K f at the tripod location as material
approaches the seabed. The possibility that flocs
are resuspended along with a coarse single grain
population exists as previous observations suggest
resuspended material from the Po prodelta to be
highly flocculated (Matteucci and Frascari, 1997).
Also, the minor increase in f at the tripod station
from January to October might reflect the gradual
removal of flood sediments from the prodelta.
During flood events, the sediments would be
generally coarser with more single-grain material
exiting the river, reflecting deposition due to much
greater energetic forcing than those at the time of
sampling.
Fox et al. (2004) observed flocs far upstream of
the river mouth in salinities of 0:2 ppt. At the
upstream bridge stations, in situ photographs and
water samples were taken in the surface waters of
0.1
10
0.1
0.01
D2
1
10
0.1
0.01
D3
1
10
Diameter (µm)
Fig. 12. Suspended DIGS distributions of surface water (solid
lines) and 0:5 m AB water (dashed lines) samples are plotted for
stations D1, D2, and D3 in the river. The water column is
relatively well mixed at D1 and D2 as the two samples exhibit
similar concentrations with the nearbottom sample having a
naturally coarser end. At D3, concentration for both surface
and bottom samples increase relative to D1 and D2 with the
addition of a coarse mode to the nearbottom sample reflecting
more extensive bottom resuspension.
the river. Corrected calculations of floc fraction
were made with SPM values from the respective
water samples, yielding floc fraction estimates of
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J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715
1713
0
1
Depth (m)
2
3
4
5
6
7
8
0
10
20
30
40
0
SPM (mgl-1)
100
200
300
400
LPVC (ppm)
0
1
Depth (m)
2
3
4
5
6
7
8
0
0.2
0.4
0.6
Corrected f
0
0.5
1
1.5
Current Speed (m)
Fig. 13. Water column profiles of SPM (mg l1 ), LPVC (ppm), corrected f, and current speed (m s1 ) are plotted for stations D1
(dashed), D2 (dotted), and D3 (solid). As the river mouth is approached at D3, current speed increases twofold and SPM increases up
to fourfold. Floc fraction at D3 has a maximum at 2 m and decreases towards the seabed. Floc fraction at D2 and D3 show variation
down column even though current magnitude and concentration are relatively constant.
0.51 and 0.55 for stations B1 and B2, respectively.
At the stations within the delta, suspended size
distributions show a water column that is well
mixed at D1 and D2 as the surface and 0:5 m AB
size distributions show a similar concentration and
shape, with the near-bottom distribution having a
naturally coarser end (Fig. 12). Where SPM and
current speed are uniform through the column for
D1 and D2, corrected floc fraction estimates are
shown to fluctuate, with maximum values 40.40
(Fig. 13). As the flow is constricted through the
more shallow waters at D3, current speed increases
twofold and nearbed SPM increases fourfold, with
LPVC and floc fraction decreasing nearbed
(Fig. 13). The suspended DIGS at D3 show not
only an increase in concentration, but the near
bottom sample is enriched with a modal resuspended single grain population from the seabed
(Fig. 12), suggesting that the decrease in floc
fraction is caused by bottom resuspension of
coarse single grains and not by floc break-up. In
comparison, if the difference between D3 and D2
DIGS distributions is examined (dCðiÞ ¼ CðiÞD3
CðiÞD2 ), resuspension is further supported. Not
only is the input of coarser single grains at D3
made clear, enrichment in fines is observed in the
surface sample as the erosion and transport of
smaller sized single grains from the seabed extend
beyond that of progressively larger and heavier
single grains (Fig. 14). The act of last-minute
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J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715
1714
6. Conclusions
Concentration (ppm)
0.1
0.01
0.001
1
10
Diameter (µm)
Fig. 14. The difference in DIGS distributions between D2 and
D3 are plotted for the surface (solid line) and nearbottom
(dashed line) water samples. The suspension at D3 is enriched in
resuspended material relative to D2. The bottom sample
receives resuspended input of all size classes including a coarse
single-grain population seen in the modal distribution. The
surface sample receives resuspended input void of coarse
material as the liberation of fine grains from the seabed extends
beyond that of increasingly coarser material.
mixing of a mature floc population with a
resuspended single grain population, and the
intruding salinity of the sea may further flocculate
the suspension leading to the observed rapid
removal. The mature flocs from far upstream
appear to reach the nearshore prodelta intact,
where floc sedimentation is rapid and nearly
complete. Coarse single grains also sink rapidly
to the seabed. The suspension remaining after
initial deposition has a low degree of flocculation
(f 0.08) and is relatively fine-grained. During
data collection periods, bottom boundary layer
processes dominated sediment transport beyond
the C-line.
By implementing the correction factor as per the
findings of Li and Logan (1997), three methods for
estimating floc fraction yield similar estimates in
the waters of the Po prodelta. Future studies of
floc dynamics would be enhanced by having
collocated measurements of floc size, size versus
settling velocity, DIGS, and sediment flux.
The goals of this study were to apply three
methods of floc fraction determination and assess
the validity of each method in interpreting the
flocculation dynamics of the Po River prodelta
system. Essential to all methods, the aggregate
size versus settling velocity relationship was first
determined ðwf ¼ 86:95 ðESDÞ1:33 Þ on a sample
set of 409 aggregates. The relationship, having
floc settling velocities on the order of 1 mm s1 ,
was consistent with results from studies in
locations of varied geography and local energetics (Hill, 1998). With a correction factor in
place to account for the effective density overestimate of Stokes’ approximation, all three
methods of floc fraction determination produce
similar results. Flocs formed in the river deposit
directly offshore of the river mouth, where the
highest floc fraction of the system is observed
(f 1). The bulk of material is removed from the
water column by the 8 m-isobath, where subsequent transport occurs within the bottom boundary layer. The remaining material in suspension
is, for the most part, not highly flocculated (f
0.08). A large improvement of such analyses
would be to collocate all applicable instruments,
making multiple localized studies simple and
effective.
Acknowledgements
This study was supported by the U.S. Office
of Naval Research contracts N00014-97-1-0160
and N00014-99-1-0113 and by EU project Eurodelta (Concerted Action n. EVK3-CT-200120001). Special thanks are given to ISMAR/
BM and also to Ali Khelifa, for his thoughtprovoking input. Thanks are also given to Brent
Law and Paul Macpherson in the Particle Dynamics Laboratory at the Bedford Institute of
Oceanography for conducting particle size analysis. Preparation of the transect figures was
carried out using Ocean Data View software
designed by Reiner Schlitzer (http://www.
awi-bremerhaven.de/GEO/ODV2001).
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