Controls of Suspended Sediment Particle Size in the York River Estuary

Transcription

1 Controls of Suspended Sediment Particle Size in the York River Estuary A thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Geology from the College of William and Mary in Virginia, by Benjamin D. Lewis Williamsburg, Virginia May,

2 Table of Contents Abstract...3 Introduction..4 Background Information..6 Methodology...9 Results.11 Expected..11 General.12 Discussion...17 Conclusions..20 Acknowledgments References Cited..22 Figures

3 Abstract Until recently, observations of suspended sediment particle size have been a difficult prospect. Optical instruments now allow suspended sediment sizes to be observed in-situ, without disturbing the delicate flocs. In this study, suspended sediment particle size data were gathered at two sites along the York River estuary. The Gloucester Point site is characterized by higher biologic activity while the Clay Bank site is characterized by higher physical activity and intense sediment transport. It was found that as water velocity and stress along the bed increased, suspended sediment particle size decreased. This relationship was most prominent in the larger particle size classes and at the physically dominated Clay Bank site. The smallest size particles seemed to have little correlation with water velocity and stress, suggesting they are either much more cohesive than the larger particles or are individual grains. Water temperature had no noticeable effect on particle size, suggesting that it may not be an appropriate proxy for organic activity or that increased organic activity may not lead to larger observed particles in the York River estuary. 3

4 Introduction Though many of the equations for suspended sediment transport are well known, many of their inputs and variables are not well defined (Lynch et al, 1994; Hill et al, 1998). This is due in part to the difficulty of analyzing suspended sediment. Water samples can be gathered and analyzed but doing so disturbs the natural turbulence the sediment is exposed to and thus may destroy some of the aggregates present in situ. The LISST (Laser In-Situ Scattering and Transmissometry) device uses laser diffraction to obtain particle size distributions and volume concentrations in-situ without disturbing any of the natural variables present. Other instruments such as optical backscatterance sensors require potentially imprecise calibrations which may skew measured data (Ludwig and Hanes, 1990; Downing and Beach 1989). The York River extends southeastward across the Virginia coastal plain into the Chesapeake Bay and forms a partially mixed estuary. The upper York estuary is dominated by physical controls, such that current velocity, turbulence and sediment concentrations increase with distance up the estuary (Schaffner et al. 2001). These physical controls which dominate the upper estuary are not as prominent in the lower estuary which is more strongly influenced by biological processes (Figure 1). The goal of this research was to analyze data gathered from the LISST instruments which have been and currently are being deployed on tripods located in two parts of the York River estuary. One tripod is located at a site called Clay Bank located 4

5 in middle to upper portion of the York where physical processes often dominate. The other tripod is in the lower estuary adjacent to the Virginia Institute of Marine Science (VIMS) at Gloucester Point where physical processes are less dominant. The tripods are deployed for periods of one to two months at their respective sites, during which time the LISST and other instruments gather time data directly from the lower water column. After a period of one to two months, the tripods are retrieved from the water and taken back to VIMS where the instruments are removed, cleaned and the data in them are extracted for analysis. The process of leaving tripods in the water column for a long period of time allows for data to be gathered under the influence of not only the daily tidal cycle, but the monthly spring and neap tidal cycles also. Since the tripods are deployed throughout much of the year, the data sets can be compared based on seasonal changes as well. In addition, permanent tripod deployment allows for data from periodic storm events to be gathered. Due to the randomness involved with large storm events, there are relatively little data available for analysis from them; however, long term tripod deployments allow data to be gathered from the water column during these large storm events. The LISST is a device designed for in-situ particle size determination introduced by Sequoia Scientific Inc. (Pottsmith and Bhogal, 1995). Using the principles of laser diffraction, the LISST can measure particle size spectra between 1.25 and 250 microns. Particle size spectra can be determined using video imaging as well, but video imaging can only measure particle sizes on the scale of 100 s to 1000 s of microns (Eisma et al. 1996). The LISST emits a laser that travels through the water and is diffracted by 5

6 suspended sediment particles. The LISST measures the angle at which the laser is diffracted by the sediment and classifies it into one of thirty-two size categories. One important assumption the LISST makes is that all measured particles are perfect spheres. This makes the calculations involved in classifying sediment fairly simple. Of course, most fine-silt and clay sized sediments are not perfectly spherical, but a model that does not assume spherically shaped sediments would be more complicated and is currently not available. So while the LISST data are all off by a little, it is as close as is currently possible. Sequoia Scientific Inc is currently working to address this issue and incorporate randomly shaped particles into their model for measuring sediment size by laser diffraction (Agrawal et al., 2008). Background Information Understanding fine sediment transport is critical to better understanding coastal, estuarine, and shallow marine environments. Fine sediment transport plays an important role in coastal eutrophication (Boesch et al. 2001) and nutrient contamination (Lee and Wiberg 2002). This is due to the tendency of nutrients, such as nitrogen and phosphorous, to adsorb onto fine grained sediment. Eutrophication is an overabundance of nutrients in an ecosystem, which leads to a very high level of primary production. Eventual decay of high primary productivity can lead to water hypoxia, a lowering of water quality, and can have many harmful effects on not only fish and other marine species, but humans as well. As nutrient and waste run off from human societies are a 6

7 primary contributor to eutrophication, a better understanding of fine grained sediment transport will not only be of academic importance, but may also lead to a better understanding of the process of eutrophication. The physical and biological conditions found in the York River estuary and its surrounding watershed are similar to many other estuaries and coastal environments around the world. The York contains benthic biological activity and suspended particle properties similar to many other locations around the globe. The strong gradient in the roles of physically dominated processes and biologically dominated processes in the York are in many other muddy shelves worldwide, including the East China Sea and Amazon shelf (Aller 1998). Thus, results found from a detailed study of the York River s fine sediment transport processes can be applied to many other systems around the United States and around the globe, not just the Chesapeake Bay. Biology plays an important role in influencing sediment transport (Fugate and Friedrichs 2002). All else being equal, suspended sediment in estuaries with a strong biologic benthic community tends to be of larger size and of a lower concentration than estuaries without a strong benthic community. This is due in part to the tendency of organic matter to hasten flocculation of suspended sediment particles, increasing their size in the process, and also biological pelletization. Pelletization is the process by which estuarine organisms feed on sediment laden with organic matter and excrete said sediment. This process compacts sediment and increases its cohesiveness which contributes to lower sediment concentrations found in the water columns of biologically controlled environments. 7

8 The York River flows for 50 kilometers from the confluence of the Mattaponi and Pamunkey rivers all the way into the Chesapeake Bay. The York s watershed is bounded to the north by the Rappahannock River watershed and to the south by the James River watershed (Figure 2). The depth of the York River varies between 20 meters in the lower estuary near Gloucester Point and 6 meters in the upper estuary near the Mattaponi and Pamunkey. The mean width of the York River is 3.8 kilometers (Nichols et al., 1991). The sea bed in the York can be differentiated based upon location along the York River (Figure 3). The sea bed in the physically dominated upper estuary is characterized by physical striations aligned with the flow of the river. This allows layering to be preserved and for relatively little macrobenthic activity. The sea bed in the lower estuary is characterized by an abundance of macrobenthos, and the sediment in it tends to be reworked by the benthic organisms such that little physical layering is distinguishable (Schaffner et al. 2001). The York is a microtidal estuary (tidal range<2m) with a tidal range of approximately 1 meter (Friedrichs, 2009) (Figure 4). Even though the tidal range is relatively small, the tidal currents in the upper estuary are strong enough to cause significant sediment suspension (Schaffner et al., 2001). The salinity of the York varies between 6 parts per thousand (ppt) at the confluence of the Mattaponi and Pamunkey in the upper estuary and 25 ppt in the lower estuary near Gloucester Point. For reference, typical sea water has a salinity of 35 ppt. The water column in the lower estuary is also more stratified than the upper estuary due to the fact that the upper estuary is shallower and has stronger currents than the lower estuary, both of which tend to disrupt salinity stratification. 8

9 Most of the sediments in the York fall into the category of mud (<4 microns) and over 80% of the material on the bed is classified as mud. The sediment concentrations in the lower water column of the York are influenced heavily by tidal current strength. This implies that the sediment concentrations at Clay Bank in the upper estuary should be on average higher than the sediment concentrations at Gloucester Point in the lower estuary because of Clay Bank s higher tidal velocity and shallower depth. The York contains two estuarine turbidity maximums (ETMs) (Figure 5). These are locations at which the water is most turbid, or contains the most amount of suspended solids. The more turbid water is, the muddier it will look. Both ETMs in the York are located in the mid to upper estuary (Friedrichs 2009). Methodology The tripods from the Clay Bank and Gloucester Point sites have been deployed intermittently for the past three years. Seven sets of usable LISST data have been recovered during this time interval. Gloucester Point has data sets from three time intervals, from 7/26/07 to 8/14/07 (Summer 07), from 12/05/07 to 4/05/07 (Winter 07), and from 4/02/08 to 6/09/08 (Spring 08). Clay Bank has data sets from four time intervals, from 8/31/07 to 10/26/07 (Fall 07), from 12/05/07 to 2/05/08 (Winter 07, 08), from 2/08/08 to 2/10/08 (Winter 08), and from 6/23/08 to 8/22/08 (Summer 08). However, much of the LISST data from the aforementioned sets are not usable due to biological fouling that builds up on the instrument after a certain amount of time. 9

10 The LISST requires a clear pathway so that its laser can be fully transmitted and diffracted. If this pathway is blocked, the LISST will not record any data. In many of the data sets after a period of about two weeks, the instrument becomes coated in barnacles and other benthic epifauna such that laser transmission, and thus sediment size classification is impossible. VIMS is currently working to solve this problem by setting up a real time observation system that transmits data directly from the in-situ tripods to VIMS. This would allow researchers to recognize when the LISST data are beginning to go bad due to fouling, and react accordingly. Once the time intervals over which the LISST data are corrupted have been removed, then burst averages are taken to reduce noise associated with random data fluctuations, cut down on the total amount of data, and for ease of analysis. The LISST takes measurements in bursts of Hz samples every 15 minutes. This process reduces the amount of data needed to be analyzed by 100 times. At this point the data from the LISST are synchronized with data gathered from the Acoustic Doppler Velocimeter (ADV), an instrument deployed along with the LISST on the tripods. The ADV emits sound waves that reflect off suspended sediment particles and then detects the amount of reflected energy. The more energy returned the more sediment there is in the water column. In addition, water velocity is determined by the Doppler frequency shift of the reflected sound. This process allows another measurement of sediment concentration and also water velocity. With the LISST and ADV synchronized such that they provide information over the same time interval, suspended sediment size and concentration gathered from the LISST can be compared with suspended sediment concentration and water velocity gathered from the ADV. 10

11 Synchronizing the LISST and ADV data allow for not only analysis on the size of suspended sediment but also serves as quality control of sorts. Sediment concentration measured optically from the LISST can be compared to sediment concentration measured acoustically by the ADV for practical information on the nature of acoustic versus optical measurements. Results Expected Results: Knowing that the mid to-upper portion of the York River estuary is more dominated by physical conditions and the lower portion of the estuary is more dominated by biological conditions, it was expected that there would be clear correlations within the data sets. It was expected that there would be a strong negative correlation between current velocity and suspended sediment particle size at the Clay Bank site. As the velocity of the current increases, the shear stress acting on suspended sediment clumps increases, shearing them into smaller clumps. The correlation was expected to be weaker in the biologically controlled portion of the estuary, and stronger in the physically controlled mid to-upper portion of the estuary. In addition, it was expected that there would be a strong positive correlation between water temperature and particle size at the Gloucester Point site. As water temperature increases, so does organic activity, and as such, more organic matter should 11

12 favor flocculation, making particle aggregates larger. The correlation was expected to be weaker in the physically dominated portion of the estuary and stronger in the biologically controlled lower portion of the estuary. Along the same lines, it was expected that the largest particles would be found in the summer months due to the higher temperatures and more biologic activity, leading to larger sediment clumps. Conversely, it was expected the smallest particles would be found in the winter months due to a reduction in organic material necessary for flocculation. General Results: In general, there was not nearly as much data retrieved from the LISST devices as was hoped. As mentioned above, this was largely due to biologic fouling that corrupted the measured data. The tripod at the Clay Bank site for the August 2007 data set was deployed from August 31 st 2007 until October 26 th During this time the LISST gathered reliable data from August 31 st 2007 until September 9 th The tripod at the Clay Bank site for the December 2007 data set was deployed from December 4 th 2007 until February 5 th 2008 and gathered reliable LISST data from December 4 th 2007 until December 22 nd The tripod at the Clay Bank site for the June 2008 data gathered was deployed from June 23 rd 2008 until August 22 nd 2008 and gathered reliable LISST data from June 23 rd 2008 until July 5 th The tripod at the Gloucester Point site for the July 2007 data set was deployed from July 26 th 2007 until August 14 th 2007 and during this time gathered reliable LISST 12

13 data from July 31 st 2007 until August 2 nd The tripod at the Gloucester Point site for the December 2007 data set was deployed from December 5 th 2007 until April 5 th 2008 and gathered reliable LISST data from December 5 th 2007 until December 11 th The tripod at the Gloucester Point site for the April 2008 data set was deployed from April 2 nd 2008 until June 9 th 2008 and gathered reliable data from April 2 nd 2008 until April 4 th On average, the LISST gathered reliable information for a much longer period of time at the Clay Bank site than it did at the Gloucester Point site. The high turbulence and current velocity at the Clay Bank site may act as a shield for the LISST s optical sensors, preventing organic matter from accumulating on the sensitive lenses and corrupting the data. There may also be less organic matter present in general near Clay Bank. The biologically dominant Gloucester Point site was only able to gather reliable data over a short time interval, perhaps because of more organic matter being present and the lack of strong physical conditions allowing organic matter to quickly foul the instrument. After gathering the LISST data and synchronizing it with ADV data over the same time interval, data from the LISST and ADV, including sediment size, sediment concentration, water velocity, and water temperature were analyzed in order to establish relationships on the controls of suspended sediment size. Suspended sediment concentrations (measured in ul/l) were gathered by the LISST and compared with water velocity measurements gathered by the ADV (Figure 6). Suspended sediment concentrations at the Clay Bank site show a clear correlation with water velocity. As water velocity in the York increases, so does the amount of sediment 13

14 in suspension. This relationship stays consistent through all the data sets. The suspended sediment concentration varies between approximately 100 and 400 ul/l, depending on the water velocity. Suspended sediment concentrations at the Gloucester Point site show no clear correlation with water velocity. As the water velocity increases, the concentration varies little, if at all. This trend is consistent seasonally for the most part and sediment concentration stays consistently near 100 ul/l. Suspended sediment concentrations gathered by the LISST were also compared with time (Figures 7,8,9). This allowed daily tidal effects to be observed at the Clay Bank and Gloucester Point sites. As an optical instrument, the LISST requires a laser to be transmitted through the water in order to measure concentration. Figure 7 shows the suspended sediment concentration through time and the percent transmission through time. As the percent transmission decreases the suspended sediment concentration measurements increase, and as the percent transmission increases the suspended sediment concentrations decrease. As suspended sediment concentrations increase in the water column, more lasers will be blocked, and not fully transmitted. As suspended sediment concentrations decrease in the water column, lasers will have a clearer path, and thus transmission will increase. Figure 8 shows the suspended sediment concentrations through time for the data sets at the Clay Bank site. The first plot shows the concentrations for the August 2007 data set. It is clear that the daily tidal cycle has an effect on suspended sediment concentration. The daily high value of sediment concentration averages approximately 700 ul/l with the daily low value averaging approximately 100 ul/l. The second plot shows the concentrations for the December 2007 data set with a daily high concentration 14

15 average value of approximately 500 ul/l and a daily low value of approximately 50 ul/l. The third plot shows the concentration values for the June 2008 data set with a daily high concentration average value of 600 ul/l and a daily low value of approximately 120 ul/l. Figure 9 shows the suspended sediment concentrations through time for the data sets at the Gloucester Point site. Due to the biological dominance at the Gloucester Point site, the time intervals over which the LISST gathered usable data were quite short. The instrument gathered seven days worth of good data in December, but only two to three days worth of good data in April and July. The first plot shows the concentrations for the July 2007 data set. The daily high value of sediment concentration was approximately 800 ul/l, and the daily low value averages approximately 50 ul/l. The second plot shows the concentration values for the December 2007 data set. The daily high value of sediment concentration averages approximately 100 ul/l, and the daily low value averages approximately 50 ul/l. The third plot shows the concentrations for the April 2008 dat set. The daily high value of sediment concentration averages approximately 100 ul/l, and the daily low value averages approximately 30 ul/l. The suspended sediment particle sizes measured in microns from the LISST were compared with the water velocity measurements gathered by the ADV (Figure 10). The particle sizes are split into three classifications, large (D84), median (D50), and small (D16). Large and median sized particles at the Clay Bank site show a clear correlation with water velocity. As the water velocity increases, the suspended sediment particle sizes decrease. There is no apparent correlation between water velocity and sediment size for the small particles, as velocity increases they do not change. The data sets at the 15

16 Gloucester Point site show the same sign correlations, but the correlations themselves are not as strong. Depending on velocity, large sediment particles range between 150 and 400 microns and median particles range between 50 and 150 microns. The smaller particles consistently stay around 20 microns no matter the velocity. The gathered suspended sediment particle sizes from the LISST were also compared with water temperature (Figure 11). Again, the particles were split into large, median, and small size classifications. At the Clay Bank site, all three particle size classes, small, median, and large show no apparent correlation with temperature. At the Gloucester Point site, the December 2007 and April 2008 data sets show no clear correlation with temperature, but the largest particles in the July 2007 data set decrease in size as the temperature increases. Suspended sediment particle sizes were also compared with time (Figure 12). It is clear that the daily tidal cycle plays an important role in the control of suspended sediment particle size. All three particle size classes contain four daily size maximums and four daily size minimums. This is consistent with the twice-daily flood and ebb tides observed in the York River estuary. 16

17 Discussion Much of the data have been gathered under less than ideal conditions, with tripods rarely being deployed at both site at the same time, and with biological fouling accumulating rapidly on the LISST corrupting the data. This makes interpretation of gathered data a somewhat difficult prospect. While direct comparison of data between the Clay Bank and Gloucester Point sites is still possible, it is important to take into account the fact that the Gloucester Point data foul much more rapidly than the Clay Bank data. This rapid fouling makes it such that there is far less reliable data to be analyzed at Gloucester Point and may account for some of the interpretational problems with the data from that site. One of the major points of interest from this research was that the observed suspended sediment concentrations at the Clay Bank site were much higher than the concentrations at the Gloucester Point site. Also, suspended sediment concentrations at the Clay Bank site increased as water velocity increased, but increasing water velocity seemed to have no effect on suspended sediment concentrations at the Gloucester Point site. Friedrichs et al. (2008) suggests that high suspended sediment concentration is correlated to high erodibility. High erodibility at the Clay Bank would account for the high concentrations observed at that site. Since the bed is easily eroded, increasing water velocity would generate increasing lift, suspending particles into the water column. A low erodibility at the Gloucester Point site would account for the low sediment 17

18 concentrations observed and for the fact that sediment concentrations are not increasing with an increasing velocity. Even water moving as fast as half a meter a second is not able to lift much sediment into suspension at Gloucester Point. The low erodibility of the Gloucester Point site may be correlated to the benthic epiflora living along the bed. Their coverage of the bottom and processing of the sediment may act as a buffer against suspension, binding together sediment and preventing it from being easily resuspended. As expected, suspended sediment particle size decreased with increasing velocity. The largest particles (D84) decreased in size rapidly with an increasing velocity, suggesting the forces binding them together are not very strong. The median particles (D50) also decreased in size with increasing velocity, but not as rapidly as the largest size particles, suggesting the forces holding them together are stronger than the forces binding the largest sized particles. Increasing velocity had no effect on the smallest particles (D16). Even at maximum velocity, the smallest particles stayed the same size, suggesting that the cohesiveness of small particles (20-40 microns) is quite high. The forces binding these particles together are probably much stronger than the forces binding the median and large sized particles together. The larger particles may be as large as they are due to flocculation. Flocculation is the tendency of loose organic matter to aid in binding together sediment clumps, increasing their size. These loosely bound clumps would easily be torn apart at higher velocities, decreasing their size. The smallest particles would contain few, if any flocs, and thus may not break up easily. One of the most interesting observations from this research was that water temperature seemed to play no detectable role in suspended sediment particle size. It was 18

19 expected that water temperature would be a proxy for organic activity, and that higher organic activity would lead to more flocculation and thus larger observed suspended sediment sizes. Figure 11 shows that there was no correlation between water temperature and particle size. In general, increasing temperature had no effect on the size of particles at both the Clay Bank and Gloucester Point sites. The range of measured temperatures over each data set is quite small however. Perhaps if the LISST had gathered data over a longer time interval in each data set, then a broader temperature range would have been observed, which may have led to a correlation developing. In the July 2007 data set at Gloucester Point, particle size decreases with increasing temperature. This is opposite of the expected results and observations from all other data sets, suggesting an external factor may have led to this decrease over the July 2007 Gloucester Point data set. In general, it was found that the particles were larger in the December 2007 data sets than they were in all other data sets, suggesting that temperature has no positive effect on particle size. One possible explanation for this is that the particles are torn up during the winter and aggregate together over the spring, summer, and fall. In both data sets, the usable LISST data end in the beginning-middle of December, and winter does not begin until the end of December, suggesting that the observed particles in the December data sets may be the largest particles of the year just before they begin to break up over the winter. The July 2007 Gloucester Point data set proved to be a bit strange as it followed none of the trends observed in the other Gloucester Point data sets. One possible explanation for the inconsistencies observed for this data set is that a storm event 19

20 occurred during this time and advected sediment from upstream into Gloucester Point, interfering with observations gathered during this time. Conclusions Suspended sediment concentration increased with increasing water velocity at the Clay Bank site. This was expected due to the high erodibility characteristic of the Clay Bank site. Velocity had no impact on sediment concentration at the Gloucester Point site, most likely due to the low erodibility found at that site. Suspended sediment particle size decreased with increasing water velocity at both the Claybank and Gloucester Point sites. This relationship was strongest at the Clay Bank site due to its dominant physical conditions and high erodibility, and weaker at the Gloucester Point site due to its dominant biological condition and low erodibility. At both sites, water temperature had no effect on the size of suspended sediment particle size. Once a real time observation system that transmits data directly from the in-situ tripods to VIMS is completed, future work will be easier and faster to do. Currently, a LISST device is deployed for days, but usually only gathers a few days worth of usable data. With real time data transmission in place, observers will be able to recognize when the LISST data begin to go bad and repair it such that reliable data can be gathered over a much longer period of time. This will allow long term seasonal and tidal effects to be much more readily observed. 20

21 Acknowledgements This work was funded by The National Science Foundation Grant OCE Assistance and guidance in data collection and analysis was provided by Grace Cartwright, Pat Dickhudt, and Carl Friedrichs. 21

22 References CBNERRVA A site profile of the Chesapeake Bay Nation Estuarine Research Reserve in Virginia. Version September Special Scientific Report. K.A. Moore and W.G. Reay (eds.). The Virginia Institute of Marine Science, College of William and Mary. Gloucester Point, Va. 204p. Agrawal, Y. C., A. Whitmire, O. A. Mikkelsen, and H. C. Pottsmith (2008), Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction, J. Geophys. Res., 113, C Aller, R.C., Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors. Marine Chemistry, 61: Boesch, D.F., R.B. Brinsfield, and R.E. Magnien, Chesapeake Bay eutrophication: scientific understanding, ecosystem restoration, and challenges for agriculture. Journal of Environmental Quality, 30: Dellapena, T.M., S.A. Kuehl and L. Pitts, Transient, longitudinal, sedimentary furrows in the York River subestuary, Chesapeake Bay: furrow evolution and effects on seabed mixing and sediment transport. Estuaries, 24: Downing, J.P., Beach, R.A., Laboratory apparatus for calibrating optical suspended solid sensors. Mar. Geol. 86, Eisma, D., Bale, A.J., Dearnaley, M.P., Fennesy, M.J., Van Leussen, W., Maldiney, M.- A., Pfeiffer, A., Wells, J.T., 1996 Intercomparison of in-situ suspended matter (floc) size measurements. J. Sea Res. 36 (1-2), Friedrichs, C.T., York River physical oceanography and sediment transport. In: K.A. Moore and W.G. Reay (eds.), A Site Profile of the Chesapeake Bay National Estuarine Research Reserve, Virginia. Journal of Coastal Research, Special Issue No. 57, in press. Friedrichs, C.T., G.M. Cartwright, and P.J. Dickhut, Quantifying benthic exchange of fine sediment via continuous, non-invasive measurements of settling velocity and bed erodibility. Oceanography, 21(4): Fugate, D.C., and C.T. Friedrichs, 2003a. Controls on suspended aggregate size in partially mixed estuaries. Estuarine Coastal and Shelf Science, 58:

23 Hill, P.S., Syvitski, J.P., Cowan, E.A., Powell, R.D., In situ observations of floc settling velocities in Glacier Bay, Alaska. Marine Geology 145, Lee, H.J., and P.L. Wiberg, Character, fate, and biological effects of contaminated, effluentaffected sediment on the Palos Verdes margin, southern California: an overview, Continental Shelf Research, 22 (6-7), Ludwig, K.A., Hanes, D.M., A laboratory evaluation of optical backscatterance suspended solids sensors exposed to sand-mud mixtures. Mar. Geol. 94, Lynch, J.F., Irish, J.D., Sherwood, C.R., Agrawal, Y.C., Determining suspended sediment particle size information from acoustical and optical backscatter measurements. Continental Shelf Research 14 (10-11), Pottsmith, H.C., Bhogal, V.K., In situ particle size distribution in the aquatic environment. Paper presented at 14 th World Dredging Congress, November 1995, Amsterdam, The Netherlands. Schaffner, L.C., T.M. Dellapenna, E.K. Hinchey, C.T. Friedrichs, M. Thompson Neubauer, M.E. Smith and S.A. Kuehl, Physical energy regimes, sea-bed dynamics and organism-sediment interactions along an estuarine gradient. In J.Y. Aller, S.A. Woodin and R.C. Aller (eds), Organism-Sediment Interactions. University of South Carolina Press, Columbia, SC. Pp

24 FIGURE 1 Observed floc size as a function of the Komogorov microscale, which is the size of the smallest energetic eddies. In the upper York River estuary, increased turbulence (indicated by a smaller sized energetic eddies) reduces floc size, presumably be ripping them apart. The opposite pattern is seen in the lower Chesapeake Bay where turbulence presumably does not effectively rip apart flocs. (Fugate and Friedrichs, 2003) 24

25 FIGURE 2 -York River watershed highlighting county borders. (Figure 1.4 from CBNERRVA) 25

26 FIGURE 3 -Side scan and profile camera images collected in the York River estuary (Dellapenna et al and Schaffner et al. 2001) 26

27 FIGURE 4 -Comparison of tidal range along the York, Pamunkey, and Mattapoini (Figure 3.2 of CBNERRVA) 27

28 FIGURE 5 -Mean salinity map of York River and general locations of primary and secondary ETM (Figure 4.2 of CBNERRVA) 28

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30 FIGURE 7 -Figure showing sediment concentration through time and LISST percent transmission through time for Clay Bank December

31 FIGURE 8 -Figure showing sediment concentrations through time at the Clay Bank site for August 2007, December 2007, and June 2008 data sets 31

32 FIGURE 9 -Figure showing sediment concentrations through time at the Gloucester Point site for July 2007, December 2007, and April

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