D. Heffler

Bio: D. Heffler is an academic researcher from Dartmouth College. The author has contributed to research in topics: Beach morphodynamics & Seabed. The author has an hindex of 1, co-authored 1 publications receiving 8 citations.

RALPH-a dynamic instrument for sediment dynamics

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.

Sediment transport processes at the head of Halibut Canyon, Eastern Canada margin: An interplay between internal tides and dense shelf water cascading.

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.

Wave-current bedform scales, orientation, and migration on Sable Island Bank

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.

Sediment Stability and Dispersion at the Black Point Offshore Disposal Site, Saint John Harbour, New Brunswick, Canada

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...