Date Awarded

2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Carl T. Friedrichs

Abstract

As part of this dissertation work, a long term observing station at Clay Bank on the York River in Virginia has been established and maintained since 2006, and was used to gain a better understanding of sediment processes in a muddy estuary and in muddy coastal environments in general. While data from this NSF-funded Multi-Disciplinary Benthic Exchange Dynamic (MUDBED) observing system has and will be used by other students for this general purpose, this dissertation focuses specifically on better understanding and interpretation of the data collected by key instrumentation regularly deployed at the observing station, especially the acoustic Doppler velocimeter (ADV). Chapter 1, the introduction to this dissertation, provides an overview of the setting for the MUDBED observing system, namely the York River Estuary, Virginia, and briefly discusses some of the scientific and societal issues that motivate the ongoing study of this environment. Background is provided into the history of the MUDBED observing system and into the properties and operation of the ADV and other key instruments applied in this dissertation, including the Laser In Situ Scattering Transmissometer (LISST) and two particle cameras. In the context of describing these instruments, the science papers associated with the dissertation (Chapters 2 through 6) are introduced. Chapter 2 describes use of SonTek ADVs within the real-time components of the MUDBED observing system and findings based on ADV observations through 2009. ADVs deployed at Clay Bank, and also at a more biologically-dominated down-river site, provided long-term estimates of water velocity, bottom stress, suspended sediment concentration, sediment settling velocity (ws), and bed stress under spatially and seasonally variable conditions. Bed erodibility and ws were found to be inversely correlated in both time and space, but both tended to remain more consistent in time at the biological site. at the physical site the erodibility increased and ws decreased following seasonal increases in river discharge. Chapter 3 reports on dual use of a mixing tank for calibrating SonTek ADV acoustic backscatter (ABS) and for direct Doppler measurement of w s. This study utilized the fact that, absent net vertical volume flux, the average vertical velocity registered by an ADV across a horizontal plane is equal to the sediment's mean ws. A series of calibrations were run for sand sizes between 63 and 150 mum . A grid of ADV measurements revealed that the mean vertical velocity registered by the ADV was indeed consistent with each grain size's ws as independently measured in a settling tube. Also, a systematic increase in the proportionality between sand concentration and ABS was observed with increasing grain size. Chapter 4 compares ABS from five 6-MHz Nortek ADVs versus five 5-MHz SonTek ADVs to examine the relative roles played by inter-vendor, intra-vendor, and sediment variability in determining their ABS response. Significant ABS offsets were found for both vendors' ADVs. Before offset correction, ABS was more consistent among Nortek or SonTek units which had consecutive serial numbers. Sand calibrations indicated that the higher frequency Norteks were more susceptible to attenuation. For well-mixed silty-mud in the lab, calibration slopes for both vendors were close to the theoretical value for a constant grain-size suspension. In the field, however, a clearly different slope suggests a change in the acoustic properties of suspended particles with concentration. Chapter 5 characterizes suspended sediment at Clay Bank in the presence of both muddy flocs and pellets through use of an ADV for bulk ws, pump samples for mass concentration, and a LISST plus a high definition (non-video) particle camera for size distribution. Mass concentration, bulk ws and an abundant ---90 mm size class were found to be in phase with velocity and stress, consistent with the suspension of relatively dense, rapidly settling and resilient pellets. Volume concentration of an abundant ---300 mm class peaked well after stress and velocity began to decrease, consistent with the formation of lower density, slowly settling and fragile flocs. Chapter 6 builds on Chapter 3 by utilizing two separate ADV methods to measure ws and comparing both to observations from settling tubes. as well as direct Doppler measurement of sand, ws for mud was measured by assuming a Rouse balance between upward Reynolds flux and downward settling. Rouse-balance ADV estimates of ws were collected at Clay Bank for muddy flocs and confirmed in situ by a high-definition video settling column. Observations suggested that, in the absence of significant particle aggregation/disaggregation, (i) measurement of ws and (ii) ws itself are both relatively insensitive to the local intensity of fluid turbulence for ws up to several cm/s.

DOI

https://dx.doi.org/doi:10.25773/v5-52qd-1490

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