Keath Borg

The capabilities of Doppler current profilers for directional wave measurements in coastal and nearshore waters

The adaptation of Doppler current profilers to measure directional wave spectra has provided a new instrumentation approach to coastal and nearshore oceanographic studies. Past studies have shown favorable comparisons between Doppler current profiler wave instruments with bottom mounted PUV (pressure-velocity) sensors sampling at wave frequencies and wave buoys. In this paper, we examine the capabilities and limitations of two different Doppler current profilers for directional wave measurements in shallow coastal waters of 0-25 m water depth. Data collection programs using Doppler current profilers for wave measurements have been conducted for one month long periods in the early spring of 2002, 2003 and 2004 on Roberts Bank in the Fraser River foreslope region of the Strait of Georgia, British Columbia, Canada. In 2004, an RD Instrument ADCP along with the newly-released 1000 kHz Nortek AWAC current profiler and wave instrument were co-located in 7 m water depth at a different site on the edge of Roberts Bank. Inter-comparisons between these bottom mounted instruments are used to examine the capabilities of the directional wave spectral parameters, in terms of: resolvable frequencies for directional and nondirectional wave spectra; wave directional resolution and reliability, and limitations arising from the use of linear wave theory. For a preliminary assessment of the capability of Doppler wave spectra in deeper waters of 20-25 m depths, in particular for very long wave periods, some experiences derived from a long-term measurement program being conducted off the west coast of Africa are presented

Sediment Plume and Deposition Modeling of Removal and Installation of Underwater Electrical Cables on Roberts Bank, Strait of Georgia, British Columbia, Canada

A three dimensional, integrated numerical model, COCIRM-SED, was applied to predict the sediment plumes and deposition resulting from the removal and installation activities of existing and proposed underwater transmission cables across the Strait of Georgia, British Columbia, Canada. In the model, a total of six sediment categories from fine silt to medium sand were classified and simulated together in terms of sampled sediment characteristics. The model results were obtained with a trenching rate of 300 m/h and two trench sizes, a wide trench of 1.0 m wide  1.0 m deep and a narrow trench of 0.2 m wide  1.0 m deep. The amount of sediment that is suspended above the trench was taken to be 30% of the total volume for the wide trench and 25% for the narrow trench. The detailed model results of total suspended sediment values, plumes and depositions are reported in this paper.

MUD, the next generation in acoustic backscatter sediment measurements

Detection and classification of sediments within rarely occurring turbidity flows is challenging, in part because of the acoustic absorption and scattering within the dense suspended sediments. A new calibrated acoustic backscatter echosounder, the Multifrequency Ultrasonic Device (MUD™), was deployed from 13 to 16, May 2018, at about 120 m water depth within Bute Inlet, Canada. The MUD prototype (1.2 MHz, 769 kHz and 200 kHz) was deployed in a tautline mooring which included a passive acoustic recorder, an ADCP, CT sensor and OBS sensor. The frequencies for the prototype were selected to allow for a compromise between good acoustic range and penetration into dense flows with the potential for particle size discrimination. Preliminary analysis of the data indicates the presence of three turbidity flows over a two-hour period with speeds of up to 2.5 m/s. As anticipated, the lower frequencies did better in penetrating through the dense turbidity flow. This suggests that it will be possible to use inversion...

Investigating Propagation of Short-Period Ocean Waves Into the Periphery of Arctic Pack Ice Using High-Resolution Upward-Looking Sonar

Abstract Springtime fetch in the Cape Bathurst Polynya System may present opportunities for winds to generate waves capable of propagating into the thick pack ice formed over the winter. A waves-in-ice event at a study site located on the Canadian Shelf in the southern Beaufort Sea that occurred 22–23 May 2011 is presented and analyzed for wave energy attenuation and dissipation characteristics. The event was monitored near the ice edge and, therefore, presents information on attenuation of waves from the ice edge into the pack. Waves of T = 5 s, λ = 37.5 m were observed up to approximately 143 m and approximately 77 m away from the ice edge during two separate observation periods of ice edge wave propagation. We estimated reflection coefficients of 53% and 52% and wave attenuation coefficients of α = 2.4 × 10−2 m−1 and α = 5.4 × 10−2 m−1, respectively, for the two periods. Estimated attenuation rates are an order of magnitude greater than in comparable studies and are inconsistent with previous findings of a “rollover” effect in attenuation rates for short-period waves.

Estimation and Validation of Floe Size Distribution from Upward Looking Sonars

Moored upward looking sonars (ULS) have been used extensively for over twenty years to measure sea ice dra thicknesses and ice keel widths. They have rarely been used to analyze ice floe sizes. In 2015, Statoil Canada, Arc cNet, the Research & Development Corpora on of Newfoundland and Labrador (RDC) and Husky Energy partnered in an offshore research expedi on, a component of which was Ice Profiler Sonar (IPS) and Acous c Doppler Current Profiler (ADCP) measurements in waters off Newfoundland. This provides an excellent opportunity to develop methods to es mate floe size distribu ons in the marginal ice zone. IPS data is typically analyzed for ice dra and for the presence and absence of sea ice. ADCP bo om tracking data during periods of high ice concentra ons provides direct measurement of ice dri . Deriving these ULS-based parameters in the low concentra ons and o en energe c wave environment of the marginal ice zone is difficult. A six-day period of rela vely low wave energies was analyzed for ULS derived ice floe sizes. Over 1000 floes were detected with most of the detected widths being less than 30 m and a peak in the distribu on at less than 10 m. Ice concentra ons and ice dri s as derived from the ULS were similar to those reported by Canadian Ice Service daily ice charts. Analysis of both theore cal and natural ice floe shapes suggests that the average of the ULS determined ice floe widths is typically about 70 to 80% of the equivalent diameter and about 55% of the typical maximum horizontal extent. Thus, much of the ULS detected floes were likely smaller than the resolu on of satellite imagery. As the ULS moorings measure ice dra every one or two seconds and ice speeds every one minute, es mates of average floe mass, momentum and energy of ice features observed during the six-day analysis episode were possible. Contact Informa on Todd Mudge ASL Environmental Sciences Inc. 1-6703 Rajpur Place Victoria, B.C. V8M 1Z5 Canada m: +1-778-977-3385 | t: +1-250-656-0177 x 116 | f: +1-250-656-2162 e: | w: tmudge@aslenv.com www.aslenv.com Estimation and Validation of Floe Size Distribution from Upward Looking Sonars Todd Mudge; Keath Borg; Kaan Ersahin; Ed Ross; Dawn Sadowy; Jessy Barrette; Nikola Milutinovic. ASL Environmental Sciences Inc., Victoria, BC, Canada.