Grace M. Cartwright

A test of the ADV-based Reynolds flux method for in situ estimation of sediment settling velocity in a muddy estuary

Under conditions common in muddy coastal and estuarine environments, acoustic Doppler velocimeters (ADVs) can serve to estimate sediment settling velocity (ws) by assuming a balance between upward turbulent Reynolds flux and downward gravitational settling. Advantages of this method include simple instrument deployment, lack of flow disturbance, and relative insensitivity to biofouling and water column stratification. Although this method is being used with increasing frequency in coastal and estuarine environments, to date it has received little direct ground truthing. This study compared in situ estimates of ws inferred by a 5-MHz ADV to independent in situ observations from a high-definition video settling column over the course of a flood tide in the bottom boundary layer of the York River estuary, Virginia, USA. The ADV-based measurements were found to agree with those of the settling column when the current speed at about 40 cm above the bed was greater than about 20 cm/s. This corresponded to periods when the estimated magnitude of the settling term in the suspended sediment continuity equation was four or more times larger than the time rate of change of concentration. For ADV observations restricted to these conditions, ADV-based estimates of ws (mean 0.48±0.04 mm/s) were highly consistent with those observed by the settling column (mean 0.45±0.02 mm/s). However, the ADV-based method for estimating ws was sensitive to the prescribed concentration of the non-settling washload, Cwash. In an objective operational definition, Cwash can be set equal to the lowest suspended solids concentration observed around slack water.

IN SITU CHARACTERIZATION OF ESTUARINE SUSPENDED SEDIMENT IN THE PRESENCE OF MUDDY FLOCS AND PELLETS

Observations are presented from a benthic observatory in the middle reaches of the York River estuary, VA, USA, that show evidence for both muddy flocs and pellets in the lower 1 m of the water column. This study combines in situ time series estimates of (i) volume concentration and particle size distribution from a Laser In Situ Scattering Transmisometer (LISST) (for 2.5-500 μm) and a high-definition particle camera (for 20 μm to 20 mm), and (ii) water velocity, turbulent stress, mass concentration and settling velocity derived from an Acoustic Doppler Velocimeter (ADV). Mass concentration, mass settling velocity and the abundant 88 μm size class are in phase with velocity and stress, consistent with suspension of relatively dense, rapidly settling and resilient ~90 μm pellets. Volume concentration of the abundant 280 mm class peaks well after stress and velocity begin to decrease, consistent with the formation of lower density, slowly settling and fragile ~300 μm flocs.

Sediment Suspension and Deposition Across Restored Oyster Reefs of Varying Orientation to Flow: Implications for Restoration

The eastern oyster, Crassostrea virginica, is a prominent ecosystem engineer, whose reefs exhibit strikingly consistent morphologies at multiple spatial scales throughout its North American range. These distinct morphologies are thought to form by interactions of nascent reef structures with hydrodynamics. We carried out two field studies to determine if historical reef configurations applied in a restoration context would improve reef persistence and restoration outcomes. We collected seabed and water column observations across constructed reefs of three orientations representative of those found historically throughout the oyster’s range: parallel or perpendicular to tidal currents or circular. Areas adjacent to reefs were sites of fine sediment trapping, with lower flow velocities, evidence of particle settling, and more fine sediments on the seabed relative to off-reef reference sites. The water column above the reef crest exhibited higher acoustic backscatter, higher flow velocities, and larger particles in suspension, consistent with local erosion of flocculated fine sediment from the reef crest. Perpendicular reefs produced conditions that were more conducive to reef persistence and improved oyster performance, including high flow velocities and enhanced resuspension of sediments from the reef, compared to parallel or circular reefs. Particle trapping in areas between reefs has the potential to inhibit reef growth between existing reef structures, providing support for hypotheses of landscape-scale reef pattern formation. Oyster reef restoration efforts can benefit from this improved understanding of biophysical interactions arising from reef orientation that contribute to sediment dynamics on constructed oyster reefs.

Ultrasonic Scattering Measurements of Dispersed Oil Droplets in the Presence of Gas

ABSTRACT To help minimize the effects of oil spills on marine environments, chemical dispersants are used to disperse the oil in the water column so the oil can be consumed by naturally occurring bacteria. During the Deepwater Horizon incident, 1.1 million gallons of dispersant were injected directly into the flowing plume of oil and natural gas over 1500 meters deep. Dispersants main effect is to decrease the surface tension at the oil-water interface causing the oil to form droplets smaller than ~70 microns so they can remain in the water column. Currently the efficacy of aerial applied dispersants on surface slicks is determined by measuring the droplet size decrease using a Laser In-Situ Scattering Transmissometer (LISST) or by detecting the oil in the water column using fluorometers. LISST instruments are limited to dilute mixtures, below ~500 ppm, because the LISST signal saturates for concentrated mixtures, and their windows can become occluded by oil and biofilms. Fluorometers only measure oil co...

Acoustic scattering to measure dispersed oil droplet size and sediment particle size

The use of sound waves in oceanographic environments is well established including active sonar applications for mapping seafloors, for submarine detection, and for passive listening. Acoustic waves have been used for many years to study the ocean, including detecting and identifying objects in the water column, and the measuring the seafloor and sub seafloor properties. One of the key parameters of the acoustic field is the amplitude of the wave which scatters from the seafloor or from objects in the water column. The amplitude, time of flight, and frequency response can be used to map the seafloor, measure current flow, or to detect and classify objects in the water column. In addition to these standard uses, acoustics can also be used to size particulates including sediment, oil droplets and gas bubbles in the water. Our particular application for this work is to detect, classify, and size sediment particles and separately, oil droplets suspended in the water column using knowledge of the acoustic backscattering and attenuation. Specifically, we have measured and separated the absorption, single scattering and multiple scattering contributions to attenuation measurements. Our results show that the absorption dominates the attenuation at low ka values <;<; 1 and multiple scattering and particle-particle interactions dominate at higher ka values when ka >;~1 with a transition between theses ranges depending on the concentration of the suspensions. The physics has been proven out on silica particles in water and work is ongoing on suspended sediment and suspensions of oil droplets.

Cruise: YR130612, Stations: S5127- S5138, York River Estuary and Pamunkey River, Virginia, MUDBED Longitudinal Profiler Station Survey bracketing a Flood Tide.

During each station in the survey, while anchored, a profile time series was collected with a suite of instrumentation mounted on the CHSD profiler including: a YSI 6600 CTD, a Sequia LISST 100X, PICS floc camera system, a Nortek Vector, and a Sontek ADVOcean. The raw data of profile stations are processed to provide a smooth profile of data throughout the water column and a series of between 2 to 5 minute bursts from various heights in the water column. Total Suspended Solids (and fixed solids) were sampled from depth to calibrate the acoustic backscatter. Additional water samples were collected and analyzed for Chlorophyll A. Simultaneously, at each station, a burst was collected with a bow mounted, downward looking, RDI 1200 KHz ADCP. After each deployment of the profiler, a Trios RAMSES hyperspectral radiometer was deployed while drifting across the station location. The “logbook” is the hand written field notes and instrument setup documents. The “Profiler Set up” is a log of the location and serial number of the instruments mounted on the profiler. The “Consecutive Station Log” is an excel spreadsheet of the metadata associated with each station in the survey. Excel spreadsheet “Averaged Data” contains burst averaged data and statistics from the water column and bottom bursts. Raw and processed data from each instrument are zipped in a folder, or series of folders, identified by the type and serial number of the instrument. All times are Eastern Standard Time (EST).

Cruise: YR080415, Stations: S4461- S4488, Clay Bank, York River Virginia 6-hour MUDBED Calibration Survey bracketing an Ebb Tide

Dataset consists of profile and water column burst data and bottom burst data collected as part of a 6-hour anchor station survey in support of an Acoustic Doppler Velocimeter (ADV) tripod deployed in nearby location. Description of Data: During each station in the survey a profile or bottom time series was collected with a suite of instrumentation including: a YSI 6600 CTD, a Sequoia LISST 100X, RDI 1200 kHz ADCP and a Sontek ADVOcean. The raw data of profile stations are processed to provide a smooth profile of data throughout the water column and a series of between 2 to 5 minute bursts from various heights in the water column. Bottom bursts are time series collected when the profiler was resting on the seafloor. Total Suspended Solids (and fixed solids) were sampled from depth to calibrate the acoustic backscatter. The “logbook” is the hand written field notes and instrument setup documents. The “Profiler Set up” is a log of the location and serial number of the instruments mounted on the profiler. The “Consecutive Station Log” is an excel spreadsheet of the metadata associated with each station in the survey. Excel spreadsheet “Averaged Data” contains burst averaged data and statistics from the water column and bottom bursts. Raw and processed data from each instrument are zipped in a folder, or series of folders, identified by the type and serial number of the instrument. All times are Eastern Standard Time (EST). Funding sources: NSF grants OCE-0536572 and OCE-1061781 Publication Type: Data Related Material: Subject

Dual use of a sediment mixing tank for calibrating Acoustic backscatter and direct Doppler measurement of settling velocity

While the Acoustic Doppler Velocimeter (ADV) is designed to determine fluid velocity, it is important to recognize that it is actually the velocity of the scatterers themselves that is measured. Thus in a calibration tank designed to relate sediment-induced backscatter to sediment concentration, the vertical velocity registered by an ADV at a given point is actually the true fluid velocity plus the sediments settling velocity. And absent net vertical volume flux, the average vertical velocity registered by an ADV across a horizontal plane is equal to the mean sediment settling velocity. For this study, a series of ADV calibrations were run in a 118-liter re-circulating tank for six sand sizes between 63 and 150 microns. A grid of ADV measurements distributed in a horizontal plane across the tank revealed that the mean vertical velocity registered by the ADV in each case was indeed consistent with each grain sizes settling velocity as independently measured by a “rapid sand analyzer” laboratory settling tube. In addition, a systematic increase in the proportionality between sand concentration and backscatter was observed with increasing grain size.