Use of Multibeam Bathymetry to Determine Seabed ...

Partnership in Coastal Zone Management. J. Taussik & J. Mitchell (eds) 1996, Samara Publishing Limited, Cardigan, ISBN 1 873692 09 9

Use of Multibeam Bathymetry to Determine Seabed Impacts at the Argentia Naval Base, Newfoundland

J. Shawa, D.R. Parrotta & J. Hughes-Clarkeb a Geological Survey of Canada (Atlantic),Be4ford Institute

of Oceanography,

Box 1006, Dartmouth B2Y 4A2, Nova Scotia, Canada b Department of Geodesy and Geomatics Engineering, University

of New Brunswick,

Fredericton E3B 5A3, New Brunswick, Canada

Abstract: A geological framework for investigations offshore from a former military base at Argentia, Newfoundland was established using multibeam bathymetry. In the inner harbour, bathymetric data were collected using a boom-mounted sweep system. In the outer harbour and in offshore areas, data were obtained using a Simrad EM-1 000 system. The seabed was also mapped using digital sidescan sonar systems, and high and low resolution sub-bottom profilers. Targets identified from sidescan sonar and multibeam bathymetry data were investigated by ROV and divers. Shaded relief bathymetry images of the harbour reveal natural features such as a submerged (mid-Holocene) spit, wave cut platforms, and deep muddy basins. Evidence of large-scale human impacts included dredged areas and an underwater slide triggered by spoil dumping. Sidescan sonar data provided information on sediment distribution, small-scale human disturbance of the sea bed, and the presence of debris. Areas of naturally-occurring boulder gravel were mapped, in which it was difficult to distinguish anthropogenic targets. Muddy areas were found to be extensively furrowed by anchor dragging. Digital multibeam bathymetry is becoming the Geological Survey of Canada's primary marine geology mapping tool for reconnaissance surveys. For interpreting geology and identifying small targets the data should be used in conjunction with data provided by sub-bottom profilers and sidescan-sonar systems. Key words: multibeam, Argentia, dredging, Newfoundland

Introduction The Argentia Peninsula (Figure 1), located in Placentia Bay, Newfoundland, was the site of a number of thriving communities early in this century (Houlihan, 1992), but as a result of the Lend-Lease Deal of 1940 the United States leased the peninsula and surrounding areas from the United Kingdom for use as a naval air station, and the civilian population was resettled. For the remainder of the war Argentia functioned as a centre for military operations, including convoy escort, anti-submarine air patrols, and weather patrols (Houlihan, 1992; Cardoulis, 1990). On 10 August 1941 the Ship Harbour anchorage was the site of the historic Atlantic Charter meeting between President Roosevelt and Prime Minister Churchill (Morton, 1944).

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Figure 1

Map showing the Argentia Peninsula in Placentia Bay, Newfoundland, showing areas used as anchorages: Argentia Harbour, Placentia Sound and Ship Harbour

The Argentia Peninsula was the site of runways, aircraft parking areas, seaplane slipways, aircraft hangers, fuel tanks, ammunition stores, and many other facilities. Installations built at the coast included a fleet dock, ship repair wharf, floating dry dock, and various small piers. The coastal waters encompassed by the base- Argentia Harbour, Placentia Sound, and Ship Harbour - were used as anchorages for ships and seaplanes. Argentia Harbour and Placentia Sound were protected by a submarine net. The peninsular part of the base was closed in the 1970s, leaving a small area in military control until1995, when it too was closed. The port of Argentia is still active however, and is a summer terminus for the ferry to Nova Scotia.

Objectives Between 1940 and 1995 the seafloor was modified by a range ofactivities,including dredging, spoil dumping, and cable-laying, and materials were deposited in a deep-water about 400 m) dump site about 25 km west of Argentia. Recently various Canadian government agencies have been attempting to assess the scale and impact of these activities. The principal aims of marine geological surveys were: 500

Bay, Newfoundland, r, Placentia Sound and

arking areas, seaplane many other facilities. air wharf, floating dry by the base - Argentia s anchorages for ships >rotected by a subma1970s, leaving a small 'he port of Argentia is to Nova Scotia.

of activities, including were deposited in a mtia. Recently various ss the scale and impact urveys were:

Multi beam Bathymetry to Determine Seabed Impacts

a)

to understand the marine geology of the harbour, deep-water dump site, and the connecting corridor;

b) to determine the nature of impacts to the sea floor, particularly the location of anthropogenic materials. In this paper we report on one aspect of these efforts, namely the use of multibeam data.

Physical setting Offshore, bedrock is overlain by Quaternary sediments described by Fader et al. (1982): till, glacial-marine mud, and postglacial mud. The sediments are thickest in basins, and bedrock is exposed in shallower areas. In coastal areas, relative sea level dropped to about -17 m in the early Holocene, partly exposing moraines. The resulting erosional terraces were submerged by subsequent sea-level rise, resulting in platforms in Ship Harbour, Argentia Harbour, Placentia Sound, and north of the Argentia Peninsula at depths of 15 to 18 m. Tidal ranges are 1.6 m for mean tides and 2.5 m for large tides (Canadian Hydrographic Service, 1989). The largest significant wave height for one year at the entrance to Placentia Bay is 8 m (Neu, 1982)

Methods A geological framework was established using multibeam bathymetry. In Argentia Harbour bathymetric data were collected with a Navitronics sweep bathymetry system consisting of a boom-mounted array of vertical incidence transducers. Twelve transducers, at a 1.2 m separation, were deployed from a 9. 5 m hydrographic survey launch owned and operated by Public Works and Government Services Canada. A daily coverage rate of about 1 km2 can be achieved with this equipment. In Placentia Sound, Ship Harbour, and in offshore areas bathymetry and backscatter data were obtained using a Simrad EM-1000 multibeam system. The system was deployed from the Canadian Hydrographic Service vessel Frederick G. Creed, a small water area twin-hull (SWATH) vessel, which surveys at speeds of25 km/h. The EM-1 000 uses a multi-element transducer to provide up to 60 determinations of water depth and backscatter per ping in a swath of up to 7.4 times the water depth (in shallow water). The combination of high survey speed and wide swath coverage allowed about 110 km2 to be surveyed in five days. Cell size (area resolved on the sea floor) varied according to water depth, and ranged upwards from about 2m; vertical datum was accurate to within about 1% of water depth and horizontal positioning to within 2-5 m. Survey lines were spaced to provide overlapping coverage of the survey area. The bathymetry data were processed to remove the effects of tides and vessel motion, integrated with navigation to produce a geographically referenced data set, and imported into the Geographic Resources Analysis Support System (GRASS) developed by the United States Army Corps of Engineers. Bathymetry data were then combined with various maps and aerial photographs of the area. Shaded relief images created from digital bathymetry provided detailed information on sea-floor morphology. Sidescan sonar data and various types of sub-bottom profiler data were collected during the surveys, and use was made of data previously collected (Shaw et al., 1989).

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Corridor to the deep-water dump site The deep-water dump site and the corridor to Argentia are shown in the multibeam bathymetry collected by the SWATH vessel (Figure 2). Directly north of the the Argentia Peninsula the sea floor is a gravelly, seaward-deepening platform (A) with mean depths of25 m. The corridor that extends west to the deep-water dump site crosses several deep channels: Eastern Channel, which is more than 250 m deep (B) and Central Channel, more than 150 m (C). These contain glacial-marine mud overlain by postglacial mud. The corridor also crosses shoals off Red Island that are only 12 m deep in places. The backscatter data show that much of the irregular bottom around Red Island is highly reflective (D), and is either bedrock, or till with a boulder-gravel veneer. The deep-water dump site, located immediately southwest of Merasheen Island, is a saddle-shaped basin with a maximum depth of 425 m, and with steep (25•) bedrock sidewalls. The smooth bottom in the centre of the dumpsite (E) has low reflectivity on the backscatter images, and is interpreted as postglacial mud. However, an area with slightly darker tone on Figure 2 (F) is interpreted as glacial-marine mud; the backscatter data show higher reflectivity here, due to the presence of a thin surficial gravel lag. Targets were located on the muddy bottom in the dump site using sidescan sonar, and were investigated using an ROV. Argentia Harbour, Ship Harbour, and Placentia Sound The coast of Argentia Harbour (Figure 1) was the location of wharves, docks, piers, seaplane slipways, and other facilities. Placentia Sound and Ship Harbour were used as anchorages, and Ship Harbour was site of an ammunition handling berth. Various kinds of debris were deposited on the sea floor, which has also been intensely turbated by anchor dragging. Figures 1 and 2 show that the sea floor in these areas includes a series ofbasins, namely inner Ship Harbour (maximum depth 48 m), outer Ship Harbour (44 m), inner Argentia Harbour (30 m), outer Argentia Harbour (58 m), inner Placentia Sound (44 m), middle Placentia Sound (98 m), outer Placentia Sound (48 m). These basins contain soft, Holocene mud. Where the mud is more than about 5 m thick the acoustic stratigraphy is masked by shallow gas. The areas between the basins are flat-topped terraces that formed during the mid-Holocene sea-level lowstand (-17 m). Their surfaces consist of bouldery gravel. Figure 3 is an oblique 3-D view towards the southwest (illuminated from bottom right) showing the shoal that extends into Argentia Harbour. Part of this shoal (A) consists of glacial deposits, probably eroded somewhat during the sea-level lowstand. The sea floor on this platform is littered with large boulders that are difficult to distinguish from anthropogenic targets on sidescan sonograms. Superimposed on the glacial deposits is a submerged mid-Holocene spit stretching from B to D. The spit is composed of wave-transported pebble-cobble gravel, and targets identified on sidescan sonar records were assumed to be anthropogenic. According to old hydrographic charts, the former spit was dredged and swept to just below 4 fathoms (7.3 m). The escarpment (B) which marks the northern limit of dredging is 4.5 m high. (For scale, the escarpment is 500 min length). In the dredged area (C) the sea floor is imprinted by scalloped troughs oriented about 350•, with depths averaging 0.7 m and widths of about 10 m. The undredged, 502

Multibeam Bathymetry to Determine Seabed Impacts :I

a are shown in the 1re 2). Directly north :!-deepening platform est to the deep-water which is more than 1 (C). These contain lor also crosses shoals catter data show that reflective (D), and is

of Merasheen Island, and with steep (25.) dumpsite (E) has low as postglacial mud. (F) is interpreted as :ivity here, due to the n the muddy bottom . using an ROV

ld n of wharves, docks, :d and Ship Harbour mmunition handling r, which has also been 'I that the sea floor in our (maximum depth 30m), outer Argentia :entia Sound (98 m), olocene mud. Where graphy is masked by terraces that formed ~ir surfaces consist of

ninated from bottom . Part of this shoal (A) during the sea-level rge boulders that are m sonograms. Superle spit stretching from He gravel, and targets ;hropogenic. iredged and swept to rks the northern limit ) m in length). In the mghs oriented about ) m. The undredged,

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