Document Type

Presentation

Department/Program

Virginia Institute of Marine Science

Publication Date

9-7-2015

Sponsorship/Conference/Institution

13th International Conference on Cohesive Sediment Transport Processes (INTERCOH), Leuven, Belgium

Abstract

Appropriate parameterization of time-dependent erodibility of muddy seabeds is a significant barrier to improved understanding and accurate modelling of sediment dynamics in estuaries and other coastal regions. In an effort to better understand controls on muddy seabed erodibility, bed erodibility and associated bed sediment properties have been measured by our group on cores collected on dozens of cruises over the last decade in the York Estuary (e.g., Dickhudt et al., 2009, 2011; Kraatz, 2013). We have also inferred time-varying erodibility indirectly in the York Estuary over several years by vertically integrating observations of tidally-varying suspended sediment concentration (e.g. Friedrichs et al., 2008; Cartwright et al., 2009; Fall, 2012). This paper synthesizes the results of these long-term observations in this partially-mixed estuary, whose seabed is similar to that of many other moderately energetic, muddy tidal estuaries. Seabed erodibility was measured directly on bed samples from the York Estuary utilizing a dual core Gust erosion microcosm. This device uses a rotating disc with central suction to impose a nearly uniform shear stress on the surface of 10cm diameter sediment cores. After collection of box cores in the York River Estuary, sub-cores for use in the microcosm were carefully transported to the Virginia Institute of Marine Science (VIMS) located nearby on the banks of the York. To minimize consolidation effects, erodibility measurements were generally underway in our sediment lab within about 2h of core collection. Erodibility measurements consisted of a sequence of seven steps of approximately 20-min duration, each with a consecutively increasing shear stress (0.01, 0.05, 0.1, 0.2, 0.3, 0.45 and 0.6Pa) applied to the sediment surface. Bed erodibility was also measured indirectly in the York Estuary utilizing Acoustic Doppler Velocimeters (ADVs). Although an ADV cannot control shear stress, a bottom-mounted ADV still documents time-varying bed stress by measuring near-bed turbulent velocity, and ADV backscatter can be calibrated for suspended sediment concentration. Estimating the vertical integral of suspended sediment concentration then gives an estimate of eroded mass as a continuous function of stress, analogous to the data provided by a Gust microcosm.

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