RALPH-a dynamic instrument for sediment dynamics
23 Sep 1996-Vol. 2, pp 728-732
TL;DR: A seafloor instrument, RALPH, has been developed and used at the Geological Survey of Canada (Atlantic) as discussed by the authors, which is capable of continuous monitoring for several months, and has been deployed in the intertidal zone and on the continental shelf.
Abstract: A seafloor instrument, RALPH, has been developed and used at the Geological Survey of Canada (Atlantic). RALPH was designed to study sediment dynamics and the seafloor processes involved. Over the past 17 years, it has grown from a small tripod with a crude tape recorder to a system comprising 5 computers, 800 Mbytes of disk storage and many sensors, RALPH has been deployed in the intertidal zone and on the continental shelf. It is capable of continuous monitoring for several months. It logs time series of waves, tides, and near bed flows. It takes time lapse photographs of the seabed to monitor bedform migration and also images the seafloor acoustically. Suspended sediment is monitored acoustically and optically.
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TL;DR: In this article, an integrated approach of combining multibeam bathymetry survey, seabed sampling, sidescan/seismic surveys and seafloor instrumentation measurements was used to characterize the surficial geology and morphology of offshore sand ridges on Sable Island Bank, Scotian Shelf, and evaluate the relationship between sand ridge morphology, migration and the storm processes on this storm-dominated outer shelf bank.
73 citations
TL;DR: In this paper, the bottom boundary layer quadrapod RALPH was deployed at 276m depth in the West Halibut Canyon (off Newfoundland) during winter 2008-2009.
37 citations
01 Apr 2013
TL;DR: In this article, the bottom boundary layer quadrapod RALPH was deployed at 276m depth in the West Halibut Canyon (off Newfoundland) during winter 2008-2009.
Abstract: To investigate the processes by which sediment is transported through a submarine canyon incised in a glaciated margin, the bottom boundary layer quadrapod RALPH was deployed at 276-m depth in the West Halibut Canyon (off Newfoundland) during winter 2008–2009. Two main sediment transport processes were identified throughout the deployment. Firstly, periodic increases of near-bottom suspended-sediment concentrations (SSC) were recorded associated with the up-canyon propagation of the semidiurnal internal tidal bore along the canyon axis, carrying fine sediment particles resuspended from deeper canyon regions. The recorded SSC peaks, lasting less than 1 h, were observed sporadically and were linked to bottom intensified up-canyon flows (~ 40 cm s − 1 ) concomitant with sharp drops in temperature. Secondly, sediment transport was also observed during events of intensified down-canyon current velocities that occurred during periods of sustained heat loss from surface waters, but were not associated with large storm waves. High-resolution velocity profiles throughout the water column during these events revealed that the highest current speeds (~ 1 m s − 1 ) were centered several meters above the sea floor and corresponded to the region of maximum velocities of a gravity flow. Such flows had associated low SSC and cold water temperatures and are interpreted as dense shelf water cascading events channelized along the canyon axis. Sediment transport during these events was largely restricted to bedload and saltation, producing winnowing of sands and fine sediments around larger gravel particles. Analysis of historical hydrographic data suggests that such gravity flows are not related to the formation of coastal dense waters advected towards the outer shelf that reached the canyon head. Rather, the dense shelf waters appear to be generated around the outer shelf, where convection during winter is able to reach the sea floor and generate a pool of near-bottom dense water that cascades into the canyon during one or two tidal cycles. A similar transport mechanism is likely to occur in other submarine canyons along the eastern Canadian margin, as well in other canyoned margins where winter convection can reach the shelf-edge.
30 citations
TL;DR: In this article, field observations of wave-current bedforms on Sable Island Bank show that medium to large bedforms were generally aligned with the wave direction, and did not follow the rotating tidal current.
Abstract: [1] Field observations of wave-current bedforms on Sable Island Bank show that medium to large bedforms were generally aligned with the wave direction, and did not follow the rotating tidal current. Normalized bedform heights and wavelengths were larger than predictions by Nielsen (1992), but agreed well with predictions by Khelifa and Ouellet (2000) which includes current effects. Maximum observed bedform wavelengths of 1.9 m were larger than those predicted for bedforms in wave-dominated nearshore conditions, but this may be expected as the water depths are larger (20–42 m) and currents are present. Measured bedform migration rates had higher vector correlation amplitudes when compared to significant wave velocity than with current velocity or skewness. Migration rate predictions from three presently available models were not able to predict net migration rate and direction in all cases.
21 citations
TL;DR: In this paper, the stability and dispersion of dredged material at an offshore disposal site were studied using integrated multibeam-geophysical surveys, sampling, in situ monitoring, and modelling techniques.
Abstract: The stability and dispersion of dredged material at an offshore disposal site was studied using integrated multibeam–geophysical surveys, sampling, in situ monitoring, and modelling techniques. The disposed material at the disposal site undergoes significant reworking by strong tidal currents and superimposed waves: 84% of the dumped material, mainly fine-grained sediments, was transported away from the disposal site and only 16%, mainly coarser material, remained near the disposal site centre to form an irregularly shaped spoil mound with ∼1 km radius and approximately 14 m height at the disposal centre. A large slump, 1.4 by 1.4 km in size, developed south of the spoil mound because of the failure of the dumped material. Tidal currents up to 60 cm s−1 were measured and can cause sediment mobilization over more than 50% of a tidal cycle. The total-load transport rate calculated using measured currents can reach 0.16 kg m−1 s−1. Superimposition of 1-year storm waves could enhance this by one to t...
9 citations
Cites background from "RALPH-a dynamic instrument for sedi..."
...RALPH is an autonomous, bottom mounted instrumentation platform for long-term in situ measurements of waves, currents, and sediment transport processes in marine environments (Heffler, 1996)....
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TL;DR: A benthic annular flume (Sea Carousel) has been developed and tested to measure in situ the erodibility of cohesive sediments as mentioned in this paper, which is inferred from the rate of change in suspended sediment concentration detected in the annulus.
Abstract: A benthic annular flume (Sea Carousel) has been developed and tested to measure in situ the erodibility of cohesive sediments. The flume is equipped with three optical backscatter sensors, a lid rotation switch, and an electromagnetic (EM) flow meter capable of detecting azimuthal and vertical components of flow. Data are logged at rates up to 10·66 Hz. Erodibility is inferred from the rate of change in suspended sediment concentration detected in the annulus. The energy-density/wave number spectrum of azimuthal flow showed peaks in the energy spectrum at paddle rotation wave numbers (k) of 14 and 7 m −1 (macroturbulent time scales) but were not significant. Friction velocity ( U *), measured (1) at 1 Hz using a flush-mounted hot-film sensor, and (2) derived from measured velocity profiles in the inner part of the logarithmic layer gave comparable results for Ū * −1 . At higher values of U*, method (2) underpredicted by up to 20%. Method (1) showed radial increases in Ū * in the annulus for Ū y > 0·32 m s −1 . Radial velocity gradients were proportional to ( Ū y − 0·32 m s −1 ). Maximum radial differences in U * were 10% for Ū y = 0·5 ms −1 . Suspended sediment mass concentration ( S ) in the annulus resulted in a significant decrease (10·5%) in Ū * derived by method (1) over the range 0 S −1 . These decreases were not evident in method (2). Method (1) may, therefore, be subject to changes in stress sensor calibration with changes in S . Subaerial deployments of Sea Carousel caused severe substrate disturbance, water losses, and aeration of the annulus. Submarine deployments produced stable results, though dispersion of turbid flume water took place. Results clearly demonstrated the existence of ‘Type I’ and ‘Type II’ erosion documented from laboratory studies.
170 citations
TL;DR: In this paper, an analysis of an 18-day time-lapse film record of shoreface ripple development, with concurrent measurements of near-bottom flow and surface waves, provides new insight on equilibrium bedform conditions, adjustment of ripple planform to variable hydrodynamics, and ripple migration behavior.
Abstract: Analysis of an 18-day time-lapse film record of shoreface ripple development, with concurrent measurements of near-bottom flow and surface waves, provides new insight on equilibrium bedform conditions, adjustment of ripple planform to variable hydrodynamics, and ripple migration behaviour. The study was conducted in approximately 10 m water depth, 1 km off Martinique Beach on the Atlantic coast of Nova Scotia (Canada), under low-energy summer wave conditions. Significant wave-height and peak period during the study averaged 0–7 m and 8 s, respectively, with extremes up to 1–7 m and 11 s during passage of three weak weather disturbances. Six mutually exclusive ripple types have been defined: (1) short-wavelength regular ripples; (2) variable bifurcated ripples; (3) variable terminated ripples; (4) short-crested ripples; (5) long-wavelength regular ripples; and (6) chaotic ripples. Ripple wavelength ranged from 0–07 m to 0–24 m and displayed a strong Reynolds number dependence. Together with other published field data, the results suggest a lower limit of γ=0–06 m for the wavelength of wave ripples in ocean shoreface environments. Ripple orientation ranged through 38° and responded rapidly to changes in wave approach direction, but did not conform to the orientation of the adjacent shoreline. Ripples were observed to migrate both on- and off-shore (with and against the wave advance direction) at rates up to ±0–1 m h-1, associated with net flows other than wave-induced onshore asymmetry and mass transport. Migration (mainly of ripple types 1 and 2) occurred during the peak of storm events, but showed no obvious correlation with measured near-bottom flow magnitude or direction. Ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight-crested rippie types 1 and 5 are present. Disequilibrium in orientation or dimensions is expressed by increasing sinuosity, bifurcation and crest termination in types 2,3,4 and 6.
72 citations
TL;DR: In this paper, near-bed measurements of waves, currents, seabed responses and ripple migration rates were obtained using an instrumented tripod at a water depth of 39 m on the Scotian Shelf during the winter of 1992/93.
62 citations
01 May 1984
TL;DR: The U.S. Army Corps of Engineers, Coastal Engineering Research Center, Waterways Experiment Station, Vicksburg, MS, under contract DACW/2-82-C-0014; work was initialized with funding from the NOAA National Oceanographic National Oceanic and Atmospheric Association (NOAA) grant under grant number NA80-AA-D-00077; the Coastal======Research Center of the Woods Hole Qceanographic Institute provided support as discussed by the authors.
Abstract: Prepared for the U.S. Army Corps of Engineers, Coastal
Engineering Research Center, Waterways Experiment Station,
Vicksburg, MS, under contract DACW/2-82-C-0014; work was
initiated with funding from the NOAA National Office of Sea
Grant under grant number NA80-AA-D-00077; the Coastal
Research Center of the Woods Hole Qceanographic
Institution provided support.
20 citations