James D. Irish

Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea

A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.

Geometry, migration, and evolution of wave orbital ripples at LEO‐15

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 laboratory evaluation of the laser in situ scattering and transmissometery instrument using natural sediments

Abstract The LISST (Laser In Situ Scattering and Transmissometery) instrument was designed by Sequoia Scientific to measure the particle size distributions and concentrations of sediment suspensions in the field environment. To understand the LISSTs performance with natural sediments from a variety of marine sources, several experiments were performed to compare the LISSTs results to traditional sieving, filtering and weighing techniques. The LISST was able to correctly locate the peak of a unimodal particle size distribution and resolve the two peaks of a bimodal distribution if they are separated by at least 1φ for sediment sizes of 5 to 250 μm. Unlike a single frequency scattering sensor for sediment concentration, which require sediment size specific calibration constants, the LISST is able to accurately measure sediment concentration with a single calibration constant for varying size distributions within the size range of 5 to 250 μm. The LISST was also found to adequately represent the particle volumetric size distribution for two different samples from marine environments.

Determining suspended sediment particle size information from acoustical and optical backscatter measurements

Abstract During the winter of 1990–1991 an Acoustic BackScatter System (ABSS), five Optical Backscatterance Sensors (OBSs) and a Laser In Situ Settling Tube (LISST) were deployed in 90 m of water off the California coast for 3 months as part of the Sediment Transport Events on Shelves and Slopes (STRESS) experiment. By looking at sediment transport events with both optical (OBS) and acoustic (ABSS) sensors, one obtains information about the size of the particles transported as well as their concentration. Specifically, we employ two different methods of estimating “average particle size”. First, we use vertical scattering intensity profile slopes (acoustical and optical) to infer average particle size using a Rouse profile model of the boundary layer and a Stokes law fall velocity assumption. Secondly, we use a combination of optics and acoustics to form a multifrequency (two frequency) inverse for the average particle size. These results are compared to independent observations from the LISST instrument, which measures the particle size spectrum in situ using laser diffraction techniques. Rouse profile based inversions for particle size are found to be in good agreement with the LISST results except during periods of transport event initiation, when the Rouse profile is not expected to be valid. The two frequency inverse, which is boundary layer model independent, worked reasonably during all periods, with average particle sizes correlating well with the LISST estimates. In order to further corroborate the particle size inverses from the acoustical and optical instruments, we also examined size spectra obtained from in situ sediment grab samples and water column samples (suspended sediments), as well as laboratory tank experiments using STRESS sediments. Again, good agreement is noted. The laboratory tank experiment also allowed us to study the acoustical and optical scattering law characteristics of the STRESS sediments. It is seen that, for optics, using the cross sectional area of an equivalent sphere is a very good first approximation whereas for acoustics, which is most sensitive in the region ka ∼ 1, the particle volume itself is best sensed. In concluding, we briefly interpret the history of some STRESS transport events in light of the size distribution and other information available. For one of the events “anomalous” suspended particle size distributions are noted, i.e. larger particles are seen suspended before finer ones. Speculative hypotheses for why this signature is observed are presented.

Estimates of suspended-sediment flux and bedform activity on the inner portion of the Eel continental shelf

Energetic waves, strong bottom currents, and relatively high rates of sediment discharge from the Eel River combined to produce large amounts of suspended-sediment transport on the inner continental shelf near the Eel River during the winter of 1995–1996. Bottom-boundary-layer (BBL) measurements at a depth of ∼50 m using the GEOPROBE tripod showed that the strongest near-bottom flows (combined wave and current speeds of over 1 m/s) and highest sediment concentrations (exceeding 2 g/l at ∼1.2 m above the bed) occurred during two storms, one in December 1995 and the other in February 1996. Discharge from the Eel River during these storms was estimated at between 2 and 4×103 m3/s. Suspended-sediment flux (SSF) was measured 1.2 m above the bed and calculated throughout the BBL, by applying the tripod data to a shelf sediment-transport model. These results showed initially northward along-shelf SSF during the storms, followed by abrupt and persistent southward reversals. Along-shelf flux was more pronounced during and after the December storm than in February. Across-shelf SSF over the entire measurement period was decidedly seaward. This seaward transport could be responsible for surficial deposits of recent sediment on the outer shelf and upper continental slope in this region. Sediment ripples and larger bedforms were observed in the very fine to fine sand at 50-m depth using a sector-scanning sonar mounted on the tripod. Ripple wavelengths estimated from the sonar images were about 9 cm, which compared favorably with photographs of the bottom taken with a camera mounted on the tripod. The ripple patterns were stable during periods of low combined wave–current bottom stresses, but changed significantly during high-stress events, such as the February storm. Two different sonic altimeters recorded changes in bed elevation of 10 to 20 cm during the periods of measurement. These changes are thought to have been caused principally by the migration of low-amplitude, long-wavelength sand waves into the measurement area.

Fish cage and mooring system dynamics using physical and numerical models with field measurements

Abstract As wild fish stocks decline, marine aquaculture is expected to play an increasing role in satisfying the global need for seafood. Since the expansion of near-shore aquaculture is becoming more difficult because of multi-use issues and environmental impact concerns, the feasibility of moving aquaculture into the open ocean is being studied. To enable the optimum design and evaluation of fish cage and mooring performance in the energetic open ocean, physical and numerical modeling techniques are being utilized. To validate these methods, the dynamics of a fish cage and mooring system deployed in the Gulf of Maine are simulated with physical and numerical models and compared with field observations. Assuming that the system can be modeled as a linear system, a stochastic approach was used to analyze the motion response (heave, surge and pitch) characteristics of the fish cage and the load (tension) response in an anchor line to wave forcing. Transfer functions were calculated from field observations for storm events and used to understand the system dynamics and to validate the models for the deployed system. Fish cage heave and surge motions were found to be overdamped, while pitch exhibited a resonance at low frequencies (less than 0.1 Hz). Transfer functions for anchor line tension were consistent with observations over most of the wave frequencies. The physical model clearly revealed the pitch resonance, while the numerical model was better at predicting mooring line tension. Results also provided insight concerning dynamical processes that require further study, including fluid–net panel interaction, transfer function amplitude dependence and the nonlinear relationship between steady current and wave fluid velocity on drag and its effect on system geometry and therefore response.

The annual evolution of geostrophic flow in the Gulf of Maine : 1986-1987

Abstract The annual evolution of the geostrophic flow structure in the Gulf of Maine was studied with a combined set of moored pressure time-series measurements and five hydrographic surveys from August 1986 through September 1987. A series of quasi-synoptic dynamic height maps depicts a gulf flow structure whose typical spatial scales decrease from order 100 km during the winter to about half that in the summer, when the evolution of surface, intermediate, and deep water masses is most rapid and complex. The unusually large amount of freshwater in the gulf during 1987 was partially responsible for the establishment of a north–south across-gulf front during the summer. Year-long time series of bottom pressure and internal pressure (derived from temperature and conductivity measurements in Georges and Jordan basins) have been differenced with coastal synthetic subsurface pressures (SSP) to yield a history of the basin-scale geostrophic flow variability. The basin-scale geostrophic transport was dominated b...

Barotropic tide in the northeast South China Sea

A moored array deployed across the shelf break in the northeast South China Sea during April-May 2001 collected sufficient current and pressure data to allow estimation of the barotropic tidal currents and energy fluxes at five sites ranging in depth from 350 to 71 m. The tidal currents in this area were mixed, with the diurnal O1 and K1 currents dominant over the upper slope and the semidiurnal M2 current dominant over the shelf. The semidiurnal S2 current also increased onshelf (northward), but was always weaker than O1 and K1. The tidal currents were elliptical at all sites, with clockwise turning with time. The O1 and K1 transports decreased monotonically northward by a factor of 2 onto the shelf, with energy fluxes directed roughly westward over the slope and eastward over the shelf. The M2 and S2 current ellipses turned clockwise and increased in amplitude northward onto the shelf. The M2 and S2 transport ellipses also exhibited clockwise veering but little change in amplitude, suggesting roughly nondivergent flow in the direction of major axis orientation. The M2 energy flux was generally aligned with the transport major axis with little phase lag between high water and maximum transport. These barotropic energy fluxes are compared with the locally generated diurnal internal tide and high-frequency internal solitary-type waves generated by the M2 flow through the Luzon Strait.

The Nantucket Shoals Flux Experiment (NSFE79). Part II: The Structure and Variability of Across-Shelf Pressure Gradients

Abstract The Nantucket Shoals Flux Experiment (NSFE) was a collaborative effort to measure the alongshelf transport of mass, heat, salt and nutrients from March 1979 through April 1980 with a dense army consisting of moored current, temperature and bottom pressure instruments in an across-shelf and upper-slope transect south of Nantucket Island. The pressure component of that experiment is described here. Bottom pressure recorders were deployed at stations N1 (46 m), N2 (66 m), N4 (105 m), and N5 (196 m) during two half-year periods. spring–summer 1979 (SUMMER) and fall–winter 1979/80 (WINTER). A synthetic subsurface pressure (SSP) record was formed from atmospheric pressure and sea level observations at Nantucket Island. The low-pass filtered (periods > 36 h) or subtidal pressures were used for the subsequent analysis. It was found that Nantucket SSP and BP are very nearly equivalent for fluctuation periods less than about 50 days if steric changes in sea level, due to density changes above the seasonal ...

Moored fish cage dynamics in waves and currents

Recent work in the area of open ocean aquaculture system dynamics has focused separately upon either the response of fish cages in waves or the steady drag response due to ocean currents. In reality, however, forcing on these open ocean structures is a nonlinear, multidirectional combination of both wave and current profiles. At the University of New Hampshire-operated Open Ocean Aquaculture site, data were collected from a wave measurement buoy and a downward-looking Acoustic Doppler Current Profiler to characterize the surface elevation and water velocity profiles during an extreme northeast storm event. In addition to waves and currents, fish cage motion response in heave, surge, and pitch was inferred from accelerometer measurements during the same storm. The environmental data sets obtained during the peak of the storm were processed, analyzed, and used as input to a dynamic finite-element model. Simulations were performed using three load case scenarios: 1) in both waves and currents; 2) in waves only; and 3) in currents only. Model motion response results in both the time and frequency domain were compared with data obtained in situ . In addition to the motion response tests, the wave and current forcing influencing the mooring line tension response was also investigated. Analysis shows that in this case, the currents do not severely influence the oscillatory motion response, but do cause the cage to tilt, layback, and sink. The wave and current interaction effect did, however, influence the anchor line loads with a portion being attributed to nonlinear effects.