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).

Partnership in Coastal Zone Management

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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 subma-rine 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:

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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 subma-1970s, 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 backscat-ter 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 informa-tion 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. Super-imposed 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,

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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. Super-le 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,

Multibeam Bathymetry to Determine Seabed Impacts

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Figure 3 Oblique 3-D view, illuminated from the bottom right, looking southwest towards the head of Argentia Harbour A terrace at A is believed to be composed of glacial sediments; its surface consists of boulder gravel and patches of muddy sand. Superimposed on the terrace is a submerged, mid-Holocene gravel spit stretching from B to D. The spit was dredged; the landward limit of dredging is marked by the 4.5 m high escarpment at B (for scale, this escarpment is 500 m long). In the dredged area (C), which has a mean depth 8 m, the sea floor is predominantly fine to medium gravel. The undredged, trailing end of the former spit is shown at D. The water depth in the left of the image increases to almost 50 m. Sidescan sonar data suggest that dredge spoil was dumped on the south side of harbour (extreme left). The spoils probably triggered a slump (E) that descended into the basin

trailing end of the former spit is shown at D. The water depth in the left of the image increases to almost 50 m. Sidescan sonar data suggest that dredged spoil was dumped on the south side of harbour (extreme left). The dumping probably triggered a slump that descended into the deepest part of the basin (E) forming a mound that is 4 m high. The soft Holocene muds in these deeper parts of the harbour, and in shallow areas close to the wharves, was a good background against which anthropogenic debris could be imaged on sidescan sonograms. Further evidence of dredging is evident in Figure 4, which shows part ofPlacentia Sound (right) and Argentia Harbour (left). This vertical view of the multibeam data has been shaded from the upper left. The shoal area at B was dredged and swept to a depth of 6 fathoms (11 m) according to old hydrographic charts. The northeast-southwest imprint of dredging is clearly seen on the multibeam data. The terrace at B is highly reflective on sidescan sonograms and multibeam

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1ts; its surface consists of sed on the terrace is a D. The spit was dredged; igh escarpment at B (for a (C), which has a mean 1 gravel. The undredged, th in the left of the image :Jredge spoil was dumped 3bly triggered a slump (E)

pth in the left of the hat dredged spoil was e dumping probably e basin (E) forming a e deeper parts of the d background against onograms. 1ows part ofPlacentia ew of the multibeam .t B was dredged and rographic charts. The the multibeam data.

rams and multibeam

Multibeam Bathymetry to Determine Seabed Impacts

Figure 4 Vertical view of multibeam imagery from Placentia Sound, illuminated from the northwest (upper left) A large dredged area (A) has northeast-southwest lineations. The surface of the terrace (B) is at the level of the postglacial sea-level lowstand (-17 m), and is strewn with gravel and boulders. To the southeast of the terrace the sea floor drops steeply into a deep muddy basin with a maximum water depths of 98 m. The most striking feature in this basin is an area of small mounds (C) that is believed to consist of spoil from the dredged area

backscatter images; its surface is a veneer ofboulder gravel. Against this background it was difficult to discriminate between the boulders and small anthropogenic targets. The terrace drops off steeply to a deep (98 m) muddy basin. The most striking feature of the basin is an area of irregular mounds (C) that is interpreted as dredge spoil (presumably removed from A). Only a few aspects of the Argentia multibeam data have been described here. Other notable features in the harbour include the large rectangular hole that marks the site of the former floating dry dock, and aligned mounds on the sea floor along the position of the former submarine nets. The backscatter data show the distribution of sediment types, and provide an important interpretative tool, especially when used in conjunction with sidescan-sonar data (which has higher resolution).

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Discussion Hopefully these examples show that multibeam data are useful for understanding marine geology. Nevertheless, multibeam does not render more conventional remote sensing tools redundant. Sub-bottom profilers remain essential for map-ping the thicknesses of the various surficial sediment units, and sidescan-sonar systems are vital for resolving bottom types and for identifying small and medium targets of human origin. For example, the multibeam system revealed extensive areas of apparently smooth sea floor in Argentia Harbour. Sidescan-sonar data showed that the sea floor, consisting of soft silty mud, was not smooth, but was heavily imprinted with intersecting anchor-drag furrows. The presence of furrows is important, because it indicates a mechanism whereby debris can be repeatedly buried and uncovered by vessels arriving at the wharves. (New multibeam systems such as the Simrad EM-3000 system, presently on trial in Halifax Harbour, Nova Scotia, are capable of resolving anchor furrows). The multibeam data collected in Argentia showed other areas that appeared smooth. Although the associated backscatter data showed that the sea floor was reflective (i.e. gravelly), sidescan sonograms showed that the gravel actually contained boulders (which are difficult to distinguish from small anthropogenic targets). In summary, digital multibeam bathymetry is fast becoming the Geological Survey of Canada's primary marine geology mapping tool, but interpretation of geology and identification of small targets requires use of more traditional techniques such as sub-bottom profiling and sidescan-sonar surveys.

References Cardoulis, J.N. 1990. A Friendly Invasion: the American Military in Newfoundland,

1940-1990. Breakwater, St. John's, Newfoundland, Canada. 224 pp. Canadian Hydrographic Service. 1989. Chart 4841, Cape St. Mary's to Argentia.

Department of Fisheries and Oceans Canada, Ottawa, Ontario, Canada. Fader, G.B., King, L.H. &Josenhans, H.W. 1982. Sutficialgeology of the Laurentian Channel

and the western Grand Banks of Newfoundland. Paper 81-22, Geological Survey of Canada, Ottawa, Ontario, Canada. 37 pp.

Houlihan, E. 1992. Uprooted! The Argentia Story. Creative Publishers, St. John's, Newfoundland, Canada. 81 pp.

Morton, H.V. 1944 Atlantic Charter Meeting. Methuen and Company, London, UK. 160 pp. Neu, H.J.A. 1982. 11-year deep-water wave climate of Canadian Atlantic waters.

Canadian Technical Report of Hydrography and .Ocean Sciences No.13. Department of Fisheries and Oceans, Ottawa, Canada. 41 pp.

Shaw, J., Johnston, L. & Wile, B. 1989. Navicula Operations in Placentia Bay, Newfoundland. Open File Report 2029, Geological Survey of Canada, Ottawa, Ontario, Canada. 70 pp.

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