Flocs of increasing size Suspended Sediment Size Distribution in a Numerical Sediment Transport Model for a Partially-Mixed Estuary Danielle R.N. Tarpley, Courtney K. Harris, Carl T. Friedrichs EP33A-0976 Cohesive Properties Objective & Questions Conclusions Surface charge on clay particles leads to: Flocculation and variations in settling velocity. Consolidation on the seabed and reduced erodibility (Fig. 2) At elevated suspended concentrations, sediment-induced stratification can limit sediment entrainment. Sediment transport models often neglect these processes. Figure 1: Representative images of flocculated particles using Environmental Scanning Electron Microscopy (ESEM) techniques (Garcia-Aragon et al., 2011). Objective: Use a numerical model of an idealized, partially- mixed estuary to examine the distribution of different sized sediment classes in the estuary. Research Question: Do cohesive processes (sediment-induced stratification and bed consolidation) impact the vertical and lateral distributions of various sediment sizes differently? Suspended sediment size distributions vary between the estuary turbidity maximum (ETM) region and further down stream in the idealized estuary. In the ETM: Higher applied bed stress and lower consolidation time allow more sediment and larger sizes to be suspended. Sediment-induced stratification likely influences the height above the bed of the peak in suspended sediment concentration (SSC). Down Stream: Primary particles and small flocs are winnowed from the bed but the primary particles remain in the water column. Lower tidal average of applied bed stress reduces SSC allowing the bed to consolidate and further reduce SSC Since the different size classes sort themselves, aggregation and disaggregation may change the vertical and lateral suspended sediment size distributions. Figure 2: Postma diagrams of thresholds for erosion and deposition according to average particle size (Grabowski et al., 2011) Model Design Sediment Specifications Figure 3: Top: Grid for the idealized quasi 2-dimensional estuary. Blue dot represents the location of the model data used to calculate ETM estimates. Bottom: Salinity structure for idealized two-dimensional estuary with the location of the estuarine turbidity maximum (ETM) marked. ETM Distance Along-Estuary (km) 40 vertical layers 10 bed layers Less Erodible Sept τceq=1.0m0.62 April τceq=0.4m0.55 More Erodible Figure 4: Average (dashed lines) and assumed equilibrium (solid lines) critical stress profiles for April and September, 2007. Equilibrium profiles obtained by a power-law fit to the observed values (Rinehimer, 2008). Symbols show observed erodibility data for the York River from Dickhudt et al. (2009). Scaled similar to York River Estuary, Virginia. 500 m along estuary resolution Idealized, 12 hour tidal period Depth-varying critical shear stress ( ) Sanford (2008) Primary Particles Flocs of increasing size 1 2 3 4 5 6 Di (μm) 15 40.5 109.5 296 800 ρ (kg m-3) 2650 1340 1134 1058 1030 1019 ws (mm s-1) 0.008 0.040 0.109 0.293 0.792 2.140 Future Work Figure 10: Cycle of deposition and resuspension of cohesive sediment involved in particle aggregation and breakup (Maggi, 2005). Include aggregation and breakup of flocculated particles (Fig. 10) FLOCMOD: population size class model. Use logarithmically spaced size classes ranging from 3-1000 μm Density (ρf) and Settling Velocity (ws) where nf = 2.0 (Verney et al., 2011) Results Capture the dynamics of the Secondary Turbidity Maximum (STM) Full 3-dimensional model of the York River estuary (Rinehimer, 2008; Fall et al., 2014; Fig. 11) Use observations to inform the representation of flocs Longitude Latitude Figure 11: The three-dimensional York River estuary model grid, each square represents 5 model grid cells. Figure 5: Top: The tidal average of along channel velocity u (solid line) and applied bed stress (dashed line) from the mouth (0 km) to the head (100 km). Bottom: The tidal average of cumulative bed thickness in meters throughout the estuary. ETM Down Stream Animation Animation 1: Modeled suspended sediment concentration (color), along channel velocities (arrows) along the idealized estuary and salinity (black contours) for the size class 1 (top; smallest), size class 3 (middle), and size class 6 (bottom; largest). Hydrodynamics (Top): Tidal averaged u-velocity Peak at the ETM (Fig. 5) Lower near the bed (Fig. 6a) Applied bed stress: Tidal average peak near the ETM (Fig. 5) Extremes higher down stream (Fig. 7a). Sediment Bed: Deposition occurs near the ETM region (Fig. 5) Erosion occurs upstream of the ETM region (Fig. 5) Suspended mass: Primary particles and small flocs dominate the suspended mass in the ETM (Fig. 7b). Less material suspended down stream and primary particles dominate the suspension (Fig. 7c). Figure 6: (a) Tidal average of the along channel velocity u for the upper, middle, and lower water column in the ETM region over the 12 hour tidal cycle. (b) Tidal average of the depth integrated suspended mass for each size class in the ETM region and (c) further down stream (right) over the 12 hour tidal cycle. a b c Figure 8: Vertical profiles of the tidal average of suspended sediment for each size class in the ETM region (left) and further down stream (right) over the 12 hour tidal cycle. Sediment Distribution: Peak Tide: Primary particles and small flocs are suspended (Fig. 6) Slack Tide: Primary particles and small flocs are deposited (Fig. 6) Flood Tide: Larger flocs are suspended (Fig. 6 & 9) Lateral segregation of size classes ETM: SSC peaks ~2 mab (Fig. 8) Concentrated primary particles and small flocs (Fig. 10) Down Stream: Primary particles are vertically distributed (Fig. 8) Near bed: Larger flocs are constrained to the near bed (Fig. 10). References Dellapenna, T.M., Kuehl, S.A., Schaffner, L.C., 2003. Estuarine, Coastal and Shelf Science, 58(3), 621-643. Dickhudt, P.J., Friedrichs, C.T., Schaffner, L C., Sanford, L.P., 2009. Marine Geology, 267(3), 128-140. Fall, K.A., Harris, C.K., Friedrichs, C.T., Rinehimer, J.P., Sherwood, C.R., 2014. Journal of Marine Science and Engineering, 2(2), 413-436. Garcia-Aragon, J., Droppo, I.G., Krishnappan, B.G., Trapp, B., Jaskot, C., 2011. Journal of Soils and Sediments, 11(4), 679-689. Rinehimer, J.P., Harris, C.K., Sherwood, C.R., Sanford, L.P., 2008. Estuarine and Coastal Modeling, Proceedings of the Tenth Conference, 5-7. Sanford, L.P., 2008. Computers & Geosciences, 34(10), 1263-1283. Traykovski, P., Geyer, R., Sommerfield, C., 2004. Journal of Geophysical Research: Earth Surface, 109(F2). Verney, R., Lafite, R., Brun-Cottan, J.C., Le Hir, P., 2011. Continental Shelf Research, 31(10), S64-S83. Woodruff, J.D., Geyer, W.R., Sommerfield, C.K., Driscoll, N.W., 2001 Marine Geology, 179(1), 105-119. Figure 7: (a) The applied bed stress in the ETM region (solid line) and further down stream (dashed line). (b) The the vertically integrated suspended sediment from the ETM region of each size class. (c) The the vertically integrated suspended sediment from the further down stream. a b c Figure 9: The lateral distribution near peak flow of primary particles (top), microflocs (middle), and macroflocs (bottom) in color, salinity contours in black, and velocity magnitude and direction for the arrows. Acknowledgments Thanks to Julia Moriarty for assistance with data analysis. Thanks the IT team maintaining the HPC (Sciclone). Thanks to Alfredo Alretxabaleta at USGS for his useful feedback. This work was funded by NSF Grant OCE-1459708 Abstract 186924; AGU Fall Meeting 2016; San Francisco, CA; November 2016.