Measurements of seafloor ripples under wave‐dominated conditions from the LEO15 site and the Martha’s Vineyard coastal observatory were used to develop a time‐dependent model for ripple geometry. The ...measurements consisted of backscatter imagery from rotary side‐scan sonars, centimeter resolution bathymetric maps from a two‐axis rotary pencil‐beam sonar, and forcing hydrodynamics. During moderate energy conditions the ripple wavelength typically scaled with wave orbital diameter. In more energetic conditions the ripples reached a maximum wavelength of 0.8 to 1.2 m and did not continue to increase in wavelength or decrease in height. The observations showed that the relict ripples left after storms typically had wavelengths close to the maximum wavelength. The time‐dependent model is based on an equilibrium model that allows the ripples to maintain wavelength proportional to wave orbital diameter until a suspension threshold determined by wave velocity and grain size is reached. The time‐dependent model allows the ripple spectra to follow the equilibrium solution with a temporal delay that is based on the ratio of the ripple cross‐sectional area to the sediment transport rate. The data was compared to the equilibrium model, a simplified version of the time‐dependent model (where the ripples were assumed to follow the equilibrium model only when the bed stress was sufficient to move sediment), and the complete time‐dependent model. It was found that only the complete time‐dependent model was able to correctly predict the long wavelength relict ripples and that the other approaches underpredicted relict ripple wavelengths.
Sand waves of approximately 2 m in height were observed to migrate nearly 40 m with counterclockwise rotation between two bathymetric surveys performed three months apart near the southeastern corner ...of Martha's Vineyard, Massachusetts. The region is characterized by strong tidal currents, intermittent energetic surface wave events, and shallow water with local depth ranging from 2 to 7 m. This study uses the process-based model, Delft3D, with a three-dimensional approach to examine the sand wave dynamics by incorporating surface waves, winds, currents, and bathymetric observations. The model successfully simulates sand wave migration in comparisons to observations. Model sensitivity analyses show that the sand wave migration reduces by 65% with the absence of the surface waves. The modeled sand wave migration speed is correlated with the tidal current Shields parameter, and sharp increases in migration speed occur when the wave-driven Shields parameter increases in response to energetic surface wave events. The combined effect of tides, surface waves, and bathymetry is the origin of the rotational aspect of the sand wave, using the Shields parameter as an indicator of tidal currents and surface wave influence on sand wave dynamics.
•Sand wave rotation and high migration speed are observed with strong tidal currents and intermittent energetic wave events.•Model indicates that sand wave behavior is associated with currents, surface waves, bathymetry, and the Shields parameter.•Model shows that shallow depths with wave events are primarily responsible for periods of rapid migration and rotation.
Wave‐supported gravity‐driven mudflow has been identified as a major offshore fine sediment transport mechanism of terrestrial sediment into the coastal ocean. This transport process essentially ...occurs within the wave boundary layer. In this study, wave‐supported gravity‐driven mudflow is investigated via a wave‐phase‐resolving high‐resolution numerical model for fluid mud transport. The model results are verified with field observation of sediment concentration and near‐bed flow velocities at Po prodelta. The characteristics of wave‐supported gravity‐driven mudflows are diagnosed by varying the bed erodibility, floc properties (fractal dimension), and rheological stresses in the numerical simulations. Model results for moderate concentration suggest that using an appropriately specified fractal dimension, the dynamics of wave‐supported gravity‐driven mudflow can be predicted without explicitly incorporating rheological stress. However, incorporating rheological stress makes the results less sensitive to prescribed fractal dimension. For high‐concentration conditions, it is necessary to incorporate rheological stress in order to match observed intensity of downslope gravity‐driven current. Model results are further analyzed to evaluate and calibrate simple parameterizations. Analysis suggests that when neglecting rheological stress, the drag coefficient decreases with increasing wave intensity and seems to follow a power law. However, when rheological stress is incorporated, the resulting drag coefficient is more or less constant (around 0.0013) for different wave intensities. Model results further suggest the bulk Richardson number has a magnitude smaller than 0.25 and is essentially determined by the amount of available soft mud (i.e., the erodibility), suggesting a supply limited condition for unconsolidated mud.
Francis, H. and Traykovski, P., 2021. Development of a highly portable unmanned surface vehicle for surf zone bathymetric surveying. Journal of Coastal Research, 37(5), 933–945. Coconut Creek ...(Florida), ISSN 0749-0208. This study reviews the design and subsequent effectiveness of a prototype autonomous survey vehicle built to collect data specifically in the surf zone. The breaking wave transitional zone between ocean and land is an important location to survey due to its impact on human infrastructure and vulnerability to the effects of climate change. However, this environment is notoriously difficult to survey due to its shallow depth and the turbulence of waves and currents. Three distinctive design choices were made at the beginning of the project with the goal of operating in the surf zone: First, the surface vehicle is light (15 kg) and fast (up to 7 m/s), both characteristics intended to enable one person to deploy it quickly and easily into the surf zone. Second, an electric motor that is connected to a jet drive eliminates a combustion engine's air intake, which can be contaminated with seawater and sand. The jet drive also removes any danger of spinning propellers and allows the vessel to run in very shallow water. Finally, the vessel has a foam bulb hatch cover that is watertight and allows the vessel to right itself if capsized by a wave. The outcome of this development effort is an unmanned vessel that has the maneuverability and power sufficient for surf zone operations and is self-righting. It runs off the waypoint based Ardupilot Mavlink program, which allows rapid transitions from autonomous modes to remote controlled modes and has a runtime of approximately 1.5 hours. The vessel has initially been used with a single beam echosounder and precision GPS to create highly detailed shallow water bathymetric maps. This study demonstrates this technique as a highly efficient method of creating bathymetric maps in coastal environments.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Data has been collected on two field days to characterize underwater acoustic conditions in small shallow-water spaces in the Fall River Harbor area of Taunton River, MA. This work is part of a ...project to develop sound characterization and modeling capabilities for constrained spaces. First, a bathymetric sonar from an uncrewed surface platform was used to accurately measure the seabed and boundary contours. Next, point-to-point sound propagation characteristics were measured, including data transmission throughput. Propagation characteristics and boundary reflection properties in the harbor were collected with source tracks of more than 8300 meters, transmitting thousands of rapid LFM pulses for quantifying the echo environment. Simultaneously, more than 30025-kHz data packets were transmitted at ranges of 20 to 800 m from the receivers, and more than 700 10-kHz data packets were transmitted at ranges of 20 m to 2200 m from the seven receiving hydrophones. The project goal of improving modeling of sound within ports and harbors can in turn improve the effectiveness of model use in the design of underwater acoustic systems. For example, such modeling could be used to understand causes of system performance variations or to make performance predictions.
A large flood of the Eel River, northern California, created a thick sediment deposit between water depths of 50 and 70 m in January 1997. The freshwater plume, however, confined sediment delivery to ...water depths shallower than 30 m. Mechanisms proposed to explain the apparent cross‐shelf transport include dispersal by oceanographic currents, resuspension by energetic waves, and gravitationally forced transport of a thin layer of fluidized mud. Field observations indicate that these processes were all active but cannot determine their relative significance or whether these mechanisms alone explain the location, size, and timing of deposition. Approximately 30% of the sediment delivered by the Eel River is accounted for in the midshelf mud bed and inner shelf, but the fate of the remaining 70% is uncertain. A three‐dimensional, hydrodynamic model was used to examine potential mechanisms of sediment transport on the Eel River shelf. The model includes suspended sediment transport and was modified to account for a thin, near‐bed layer of fluidized mud. It was used to simulate flood dispersal on the Eel River shelf, to compare the relative importance of transport within the near‐bed fluid mud layer to suspended sediment transport, and to evaluate sediment budgets for floods. Settling properties of fine‐grained sediment, both within the flood plume and the fluid mud layer, critically impact depositional patterns. To a lesser degree, wind‐driven ocean currents influence the volume of sediment that escapes the shelf, and wave magnitude affects the cross‐shelf location of flood deposits. Though dilute suspension accounts for a large fraction of total flux, cross‐shelf transport by gravitational forcing appears necessary to produce a midshelf mud deposit similar in volume, location, and timing to those seen offshore of the Eel River.
A 9 month time series of tripod‐mounted optical and acoustic measurements of sediment concentration and bed elevation was used to examine depositional processes in relationship to hydrodynamic ...variables in the Hudson River estuary. A series of cores was also taken directly under and adjacent to the acoustic measurements to examine the relation between the depositional processes and the resulting fine‐scale stratigraphy. The measurements reveal that deposition occurs as a result of sediment flux convergence behind a salinity front and that the accumulation rates are sufficient to deposit up to 25 cm of new high‐porosity sediment in a single ebb‐tidal phase. Subsequent dewatering and erosion reduces the thickness of the initial deposit to several centimeters. These depositional events were only observed on spring tides. Ten depositional events during two spring tidal cycles produced a seasonal deposit of 18 cm, consistent with estimates of seasonal deposition from cores. A proxy for near‐bed suspended grain size variations was estimated from the combined acoustic and optical measurements, implying that the erosional processes resuspend only the finer‐grained sediments, thus leaving behind silt and very fine grained sand beds. The thickness of the deposited homogenous clayey silt beds, and the vertical separation between beds interlaminated with silt and very fine sand, are roughly consistent with the acoustic measurements of changes in bed elevations during deposition and erosion. The variability in individual bed thickness is the result of variations of processes over an individual tidal cycle and is not a product of variations over the spring neap fortnightly timescale.
The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term deposition of sediment occurs during and following the spring freshet. Water-column measurements of ...currents and suspended sediment were performed during the spring of 1999 to determine the rate and mechanisms of sediment transport and trapping in the estuary. The net convergence of sediment in the lower estuary was approximately 300,000 tons, consistent with an estimate based on sediment cores. The major input of sediment from the watershed occurred during the spring freshet, as expected. Unexpected, however, was that an even larger quantity of sediment was transported landward into the estuary during the 3-mo observation period. The landward movement was largely accomplished by tidal pumping (i. e., the correlation between concentration and velocity at tidal frequencies) during spring tides, when the concentrations were 5 to 10 times higher than during neap tides. The landward flux is not consistent with the long-term sediment budget, which requires a seaward flux at the mouth to account for the excess input from the watershed relative to net accumulation. The anomalous, landward transport in 1999 occurred in part because the freshet was relatively weak, and the freshet occurred during neap tides when sediment resuspension was minimal. An extreme freshet occurred during 1998, which may have provided a repository of sediment just seaward of the mouth that re-entered the estuary in 1999. The amplitude of the spring freshet and its timing with respect to the spring-neap cycle cause large interannual variations in estuarine sediment flux. These variations can result in the remobilization of previously deposited sediment, the mass of which may exceed the annual inputs from the watershed.
Observations of the temporal evolution of the geometric properties and migration of wave‐formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements. Six ...weeks of bedform observations were taken at the sandy (medium to coarse sized sand) LEO‐15 site located on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed. The predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3/4 of the wave orbital diameter. Although orbital diameters become larger than 130 cm during the maximum wave event, it is unclear if a transition to non‐orbital scaling is occurring. Ripple migration is found to be directed primarily onshore at rates of up to 80 cm/day. Suspended transport due to wave motions, calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements, are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration. Thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near‐bottom suspended transport. Bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates. Sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction, as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave‐to‐current velocities.
A Reynolds-averaged two-phase Eulerian model for sediment transport, SedFoam, is utilized in a two-dimensional domain for a given sediment grain size, flow period, and mobility number to study the ...asymmetric and skewed flow effects on the sediment transport over coarse-sand migrating ripples. First, the model is validated with a full-scale water tunnel experiment of orbital ripple driven by acceleration skewed (asymmetric) oscillatory flow with good agreement in the flow velocity, net sediment transport, and ripple migration rate. The model results showed that the asymmetric flow causes a net onshore sediment transport of both suspended and near-bed load (the conventional bed load and part of the near-bed suspended load, responsible for ripple migration). The suspended load transport is driven by the “positive phase-lag” effect, while the near-bed transport is due to the large erosion of the boundary layer on the stoss flank, sediment avalanching on the lee flank, and the returning flux induced by the stoss vortex. Together, these processes result in a net onshore transport rate. In contrast, for an energetic velocity skewed (skewed) flow, the net transport rate is offshore directed. This is due to a larger offshore-directed suspended load transport rate, resulting from the “negative phase-lag” effect, compared to the onshore-directed near-bed load transport rate. Compared to the asymmetric flow, the onshore near-bed load transport (and migration) rate is limited by the larger offshore directed flux associated with returning flow on the lee side, due to a stronger lee vortex generation during the onshore flow half-cycle. In the combined asymmetric-skewed case, the near-bed load and migration rate are higher than in the asymmetric flow case. Moreover, the offshore-directed suspended load is much smaller compared to the skewed flow case due to a competition between the negative (due to velocity skewness) and positive (due to acceleration skewness) phase-lag effects. As a result, the net transport rate is onshore directed but slightly smaller than the asymmetric flow case.
•Vortex generation-ejection processes lead to different phase-lag effects for asymmetric and skewed flows.•The onshore ripple migration rate is limited (enhanced) by the lee (stoss) vortex induced returning flux for skewed (asymmetric) flows.•Combined asymmetric-skewed flow has the highest onshore migration rate (near-bed load).
PDF
