Sediment Transport and Erodibility in the York River Estuary: A Model Study

A cohesive sediment bed model was implemented in the Community Sediment Transport Modeling System (CSTMS) to examine processes influencing sediment erodibility and suspended sediment concentrations. Estimates of eroded mass from the sediment bed model were calibrated and verified with erosion chamber measurements from the York River, Virginia, a tidally-dominated environment. A constant erosion rate parameter combined with depth-varying critical shear stress was sufficient to model erosion observations of depth-limited sediment cores. Sensitivity of total eroded mass to seasonal variations in erodibility and changes in consolidation time scale was evaluated during spring-neap variations in bottom stresses. Differences were greatest during spring tide and varied by as much as a factor of 2.5. Consolidation created an asymmetry between the spring-to-neap and neap-to-spring transitions with more sediment being eroded during the decreasing phase of maximum tidal stress. Consolidation time scales controlled the magnitude of this asymmetry with larger asymmetries occurring when slower consolidation time scales were assumed. Eroded mass estimates were potentially as sensitive to uncertainties in the consolidation time scale as they were to observed seasonal variability in critical stress. The cohesive sediment bed model was then implemented within a numerical model of the York River Estuary to examine feedbacks between sediment ux convergence and erodibility. Model results show the development of a highly erodible pool of sediment near the ETM location. Even when sediment convergence processes were diminished, suspended sediment concentrations remain high due to high sediment erodibility. Sediment concentrations and erodibility exhibited high spatial variability in both the along and across channel directions. As opposed to the results of the one-dimensional model, sediment concentrations and erodibility estimates were less sensitive to variations in the consolidation rate than to the initial bed conditions. Model calculations of sediment concentrations and erodibility showed similar patterns to observational data

Acoustic and sedimentological investigations of seabed conditions and related bio-geological parameters in a tidally energetic, fine-grained environment: York River Estuary, Virginia

The transport and fate of fine-grained sediments is a critical factor affecting the physical, chemical, and biological health of estuaries, coastal embayments, riverine, lacustrine, and continental shelf environments. A geophysical and sedimentological study of the York River as a part of the NSF Multi-disciplinary Benthic Exchange Dynamics (MUDBED) project was conducted to determine: 1) the primary drivers of sediment erodibility within a fine-grained system, 2) if these drivers can be accurately measured through sedimentological and acoustic information, and 3) the spatial and seasonal variability of erosion within the estuary. Previous studies indicate that increased erodibility within the York River Estuary is mainly due to recent ephemeral deposition, whereas lower erodibility is associated with eroded or biologically reworked conditions. By studying key physical and biological parameters in the York River estuary, we can more generally apply knowledge gained on relationships among sediment facies, seabed erodibility, and the recent history of deposition, erosion, consolidation, and biological reworking. Three different experiments were conducted to look at erosion, deposition, consolidation, and biological reworking in the Clay Bank region of the York River Estuary, each highlighting varying scales of temporal change. The first experimental approach utilized an Imagenex 881A rotary sonar for one- to three-month deployments to examine surficial changes of the seabed, from hourly to monthly timescales, and allow scientists to track movement of sediment in and out of the system using sonar imagery. Optimized parameters were determined for cohesive sediment environments and a real-time observing rotary sonar was created to analyze the seabed on an hourly basis. In the second experiment, cores were collected on a weekly basis to investigate relationships between sediment properties and erodibility during the post-freshet dissipation of the mid-estuary turbidity maximum as well as over the spring-neap cycle. Grain size, water content, abundance of resilient pellets, the occurrence of 7Be, and x-radiographs were analyzed and compared to the results of Gust microcosm erosion tests to further constrain the controls on erodibility. The third experimental approach utilized seven high-resolution bathymetric surveys conducted between September 2008 and August 2009 within a 3.75 km 2 region at Clay Bank. Seabed height was shown to vary both spatially and temporally in association with the spring freshet, likely related to the presence and migration of a local secondary turbidity maximum

Sediment Deposition and Reworking: A Modeling Study using Isotopically Tagged Sediment Classes

A sediment transport model within the Regional Ocean Modeling System (ROMS) was used to examine how repeated cycles of deposition, erosion, and bioturbation influence flood and storm event bed character offshore of a significant fluvial source. Short-lived radioisotopes Beryllium-7 (7Be) and Thorium-234 (234Th) can be used as tracers of deposition and reworking on the continental shelf, and modeled profiles of these radioisotopes, along with simulated profiles of sediment bed grain size distributions, were analyzed for various model runs.The presence of an atmospherically derived radionuclide,7Be, in seafloor sedimentindicates terrestrial (riverine derived) sediment deposition offshore of a fluvial source.In contrast,234Th naturally occurs in seawater through the decay of its generally conservative parent, 238U, and its presence in the seabed indicates the recent suspension of sediment in oceanographic water. Simulated profiles of 7Be and 234Th weredirectly related to the flood and storm sequences used as model input.The model results showedthat the radioisotopic profiles are sensitive to the timing of 7Be input, phasing of wave and current energy, and intensity of bioturbation; complicating the relationship between simulated profiles andmodel input of flood and hydrodynamic forcing. Sediment grain size and geochronological tracers were used as markers of event beds for flood and storm deposition scenarios

Cohesive and mixed sediment in the Regional Ocean Modeling System (ROMS v3.6) implemented in the Coupled Ocean-Atmosphere-Wave-Sediment Transport Modeling System (COAWST r1234)

We describe and demonstrate algorithms for treating cohesive and mixed sediment that have been added to the Regional Ocean Modeling System (ROMS version 3.6), as implemented in the Coupled Ocean-Atmosphere-Wave- Sediment Transport Modeling System (COAWST Subversion repository revision 1234). These include the following: floc dynamics (aggregation and disaggregation in the water column); changes in floc characteristics in the seabed; erosion and deposition of cohesive and mixed (combination of cohesive and non-cohesive) sediment; and biodiffusive mixing of bed sediment. These routines supplement existing noncohesive sediment modules, thereby increasing our ability to model fine-grained and mixed-sediment environments. Additionally, we describe changes to the sediment bed layering scheme that improve the fidelity of the modeled stratigraphic record. Finally, we provide examples of these modules implemented in idealized test cases and a realistic application

Relationships among Fine Sediment Settling and Suspension, Bed Erodibility, and Particle Type in the York River Estuary, Virginia

In order to understand the processes controlling the temporal variability in settling velocity (Ws) and bed erodibility (ε), in the middle reaches of the York River estuary, VA, the relationships between the hydrodynamics and particle types were investigated with a near-­‐bed Acoustic Doppler Velocimeter (ADV) and the York River 3-­‐D Hydrodynamic Cohesive Bed Model. ADV observations of the flow characteristics that occurred over a strong temporal transition period indicated that Ws and ε were characterized by two distinct regimes with contrasting sediment and water column characteristics: (i) a physically-­‐dominated regime (Regime 1) which was a period dominated by flocculated muds (flocs), and (ii) a biologically-­‐influenced regime (Regime 2) which was a period dominated by biologically formed pellets mixed with flocs. During Regime 1, Ws averaged about 0.5 mm/s, and ε averaged about 3 kg/m2/Pa. In contrast, during Regime 2 average Ws increased to 1.5 mm/s, and average ε dropped to 1 kg/m2/Pa. The change between these two regimes and the transition in Ws and ε were linked with the arrival and departure of a seasonal density front. Comparison between ADV observations and the results from the York River 3-­‐D Hydrodynamic Cohesive Bed Model suggested that the current model version was not conducive to examining the temporal variability in settling velocity associated with the transition of the distinct sediment regimes. The existing model version estimated realistic values for current speed and concentration and resolved the daily variation associated with in current speed, bed stress, concentration, and settling velocity. However, model estimates of bed stress, current speed, settling velocity, and erodibility did not suggest the presence of two distinct sediment regimes. The model did a poor job of predicting peak bed stresses and settling velocities. Both were over estimated by a factor of 2 throughout most of the study period. Possible modifications to create a version that is able to simulate the bed stresses and sediment properties (i.e. erodibility and settling velocity) during each regime with more accuracy are: (1) define finer sediment classes in the model that are more representative of the water column and not just the seabed, (2) use a consolidation time scale of 5 days rather than 24 hours to allow more sediment to be suspended at lower bed stresses, (3) further reduce hydraulic roughness, and (4) turn on sediment induced stratification

A Hydrodynamic and Sediment Transport Model for the Waipaoa Shelf, New Zealand: Sensitivity of Fluxes to Spatially-Varying Erodibility and Model Nesting

Numerical models can complement observations in investigations of marine sediment transport and depositional processes. A coupled hydrodynamic and sediment transport model was implemented for the Waipaoa River continental shelf offshore of the North Island of New Zealand, to complement a 13-month field campaign that collected seabed and hydrodynamic measurements. This paper described the formulations used within the model, and analyzed the sensitivity of sediment flux estimates to model nesting and seabed erodibility. Calculations were based on the Regional Ocean Modeling System—Community Sediment Transport Modeling System (ROMS-CSTMS), a primitive equation model using a finite difference solution to the equations for momentum and water mass conservation, and transport of salinity, temperature, and multiple classes of suspended sediment. The three-dimensional model resolved the complex bathymetry, bottom boundary layer, and river plume that impact sediment dispersal on this shelf, and accounted for processes including fluvial input, winds, waves, tides, and sediment resuspension. Nesting within a larger-scale, lower resolution hydrodynamic model stabilized model behavior during river floods and allowed large-scale shelf currents to impact sediment dispersal. To better represent observations showing that sediment erodibility decreased away from the river mouth, the seabed erosion rate parameter was reduced with water depth. This allowed the model to account for the observed spatial pattern of erodibility, though the model held the critical shear stress for erosion constant. Although the model neglected consolidation and swelling processes, use of a spatially-varying erodibility parameter significantly increased export of fluvial sediment from Poverty Bay to deeper areas of the shelf

Efficient computational models for shallow water flows over multilayer erodible beds

Purpose: The purpose of this paper is to present a new numerical model for shallow water flows over heterogeneous sedimentary layers. It is already several years since the single-layered models have been used to model shallow water flows over erodible beds. Although such models present a real opportunity for shallow water flows over movable beds, this paper is the first to propose a multilayered solver for this class of flow problems. Design/methodology/approach: Multilayered beds formed with different erodible soils are considered in this study. The governing equations consist of the well-established shallow water equations for the flow, a transport equation for the suspended sediments, an Exner-type equation for the bed load and a set of empirical equations for erosion and deposition terms. For the numerical solution of the coupled system, the authors consider a non-homogeneous Riemann solver equipped with interface-tracking tools to resolve discontinuous soil properties in the multilayered bed. The solver consists of a predictor stage for the discretization of gradient terms and a corrector stage for the treatment of source terms. Findings: This paper reveals that modeling shallow water flows over multilayered sedimentary topography can be achieved by using a coupled system of partial differential equations governing sediment transport. The obtained results demonstrate that the proposed numerical model preserves the conservation property, and it provides accurate results, avoiding numerical oscillations and numerical dissipation in the approximated solutions. Originality/value: A novel implementation of sediment handling is presented where both averaged and separate values for sediment species are used to ensure speed and precision in the simulations

Effects of hard clam (Mercenaria mercenaria) density and bottom shear stress on cohesive sediment erodibility and implications for benthic-pelagic coupling

The interacting effects of little neck hard clam (Mercenaria mercenaria) density and bottom shear stress on cohesive sediment erodibility were investigated. Short-term stepwise erosion experiments in 30 and 40 cm diameter Gust microcosms over a range of 0.0083 to 0.1932 Pa were performed using sequential 20-minute constant shear stress steps while sampling turbidity regularly. In addition, sediment erodibility was monitored in two one-month long ecosystem experiments with tidal resuspension and 0, 10, and 50 hard clams in 1 m3 shear turbulence resuspension mesocosms (STURM) with an initial stepwise erosion experiment (0.313 to 0.444 Pa). In short-term erosion experiments, a low density of hard clams did not significantly affect sediment erodibility, but a high density of hard clams destabilized muddy sediments through significantly decreased critical shear stresses and higher erosion rates, resulting in higher cumulative suspended mass (CSM). In long-term erosion experiments, the sediment stabilized over time between treatments and decreased to a CSM of approximately 60 g m–2 with different densities of hard clams. This was likely due to development of microphytobenthos, mediated by the filter-feeding clams, bottom shear stress and increased light. Bioturbation by a dense bed of hard clams in interaction with infrequent high bottom shear due to storms may increase CSM in the water column, with subsequent direct and indirect effects on the ecosystem. However, more controlled longer-term erosion studies to determine the interacting effects of long-term exposure to high bottom shear stress, benthos, and microphytobenthos on sediment erodibility and benthic-pelagic coupling are needed

Investigating the importance of sediment resuspension in Alexandrium fundyense cyst population dynamics in the Gulf of Maine

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 103 (2014): 79–95, doi:10.1016/j.dsr2.2013.10.011.Cysts of Alexandrium fundyense, a dinoflagellate that causes toxic algal blooms in the Gulf of Maine, spend the winter as dormant cells in the upper layer of bottom sediment or the bottom nepheloid layer and germinate in spring to initiate new blooms. Erosion measurements were made on sediment cores collected at seven stations in the Gulf of Maine in the autumn of 2011 to explore if resuspension (by waves and currents) could change the distribution of over-wintering cysts from patterns observed in the previous autumn; or if resuspension could contribute cysts to the water column during spring when cysts are viable. The mass of sediment eroded from the core surface at 0.4 Pa ranged from 0.05 kg m−2 near Grand Manan Island, to 0.35 kg m−2 in northern Wilkinson Basin. The depth of sediment eroded ranged from about 0.05 mm at a station with sandy sediment at 70 m water depth on the western Maine shelf, to about 1.2 mm in clayey–silt sediment at 250 m water depth in northern Wilkinson Basin. The sediment erodibility measurements were used in a sediment-transport model forced with modeled waves and currents for the period October 1, 2010 to May 31, 2011 to predict resuspension and bed erosion. The simulated spatial distribution and variation of bottom shear stress was controlled by the strength of the semi-diurnal tidal currents, which decrease from east to west along the Maine coast, and oscillatory wave-induced currents, which are strongest in shallow water. Simulations showed occasional sediment resuspension along the central and western Maine coast associated with storms, steady resuspension on the eastern Maine shelf and in the Bay of Fundy associated with tidal currents, no resuspension in northern Wilkinson Basin, and very small resuspension in western Jordan Basin. The sediment response in the model depended primarily on the profile of sediment erodibility, strength and time history of bottom stress, consolidation time scale, and the current in the water column. Based on analysis of wave data from offshore buoys from 1996 to 2012, the number of wave events inducing a bottom shear stress large enough to resuspend sediment at 80 m ranged from 0 to 2 in spring (April and May) and 0 to 10 in winter (October through March). Wave-induced resuspension is unlikely in water greater than about 100 m deep. The observations and model results suggest that a millimeter or so of sediment and associated cysts may be mobilized in both winter and spring, and that the frequency of resuspension will vary interannually. Depending on cyst concentration in the sediment and the vertical distribution in the water column, these events could result in a concentration in the water column of at least 104 cysts m−3. In some years, resuspension events could episodically introduce cysts into the water column in spring, where germination is likely to be facilitated at the time of bloom formation. An assessment of the quantitative effects of cyst resuspension on bloom dynamics in any particular year requires more detailed investigation.Research support to Donald M. Anderson and Bruce A. Keafer provided through the Woods Hole Center for Oceans and Human Health; National Science Foundation Grants OCE-0430724 and OCE-0911031; and National Institute of Environmental Health Sciences Grant 1-P50-ES012742-01; the ECOHAB Grant program through NOAA Grants NA06NOS4780245 and A09NOS4780193; the MERHAB Grant program through NOAA Grant NA11NOS4780025; and the PCMHAB Grant program through NOAA Grant NA11NOS4780023. Research support to all other authors was provided by U.S. Geological Survey