Annualmeeting:2017 CSDMS meeting-126

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Flocculation and bed consolidation in a partially mixed estuary: an idealized numerical sediment transport model

Courtney Harris, Virginia Institute of Marine Science Gloucester Point Virginia, United States.
Sarah Danielle Tarpley, Virginia Institute of Marine Sciences Gloucester Point Virginia, United States.
Christopher Sherwood, US Geological Survey Woods Hole Massachusetts, United States.
Carl Friedrichs, Virginia Institute of Marine Science Gloucester Point Virginia, United States.

[[Image:|300px|right|link=File:]]Particle settling velocity and bed erodibility impact the transport of suspended sediment to the first order, but are especially difficult to parameterize for the muds that often dominate estuarine sediments. For example, fine grained silts and clays typically form loosely bound aggregates (flocs) whose settling velocity can vary widely. Properties of flocculated sediment such as settling velocity and particle density are difficult to prescribe because they change in response to several factors, including salinity, suspended sediment concentration, turbulent mixing, organic content, and mineral composition. Additionally, mud consolidates after deposition, so that its erodibility changes over timescales of days to weeks in response to erosion, deposition, dewatering, and bioturbation. As understanding of flocculation and consolidation grows in response to recent technical advances in field sampling, numerical models describing cohesive behavior have been developed.

For this study, we implement an idealized two-dimensional model that represents a longitudinal section of a partially mixed estuary that mimics the primary features of the York River estuary, VA; and accounts for freshwater input, tides, and estuarine circulation. Suspended transport, erosion, and deposition are calculated using routines from the COAWST (Coupled Ocean-Atmosphere-Wave-and-Sediment Transport) modeling system. Here we evaluate the impact that bed consolidation and flocculation have on suspended sediment dispersal in the idealized model using a series of model runs. The simplest, standard model run neglects flocculation dynamics and consolidation. Next, a size-class-based flocculation model (FLOCMOD) is implemented. The third model run includes bed consolidation processes, but neglects flocculation; while the last model run includes both processes. Differences in tidal and daily averages of suspended load, bulk settling velocity and bed deposition are compared between the four model runs, to evaluate the relative roles of the different cohesive processes in limiting suspension in this partially mixed estuary. With an eye toward implementing these formulations in a realistic-grid model, we also consider the computational cost of including flocculation and consolidation.