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Floc fraction in the waters of the Po River prodelta

2004, Continental Shelf Research

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

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 ARTICLE IN PRESS 1700 J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 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 ARTICLE IN PRESS J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 1701 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. ARTICLE IN PRESS 1702 J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 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 ARTICLE IN PRESS J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 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. ARTICLE IN PRESS J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 1704 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. ARTICLE IN PRESS J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 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 1705 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 ARTICLE IN PRESS J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 1706 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. ARTICLE IN PRESS J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 1707 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 ARTICLE IN PRESS J.M. Fox et al. / Continental Shelf Research 24 (2004) 1699–1715 1708 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 ARTICLE IN PRESS (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. ARTICLE IN PRESS 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 ARTICLE IN PRESS 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 ARTICLE IN PRESS 1712 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 ARTICLE IN PRESS 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 ARTICLE IN PRESS 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. 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