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Ice behaviour around a small Arctic island Frederking, R. M. W.; Sanderson, T.; Wessels, E.; Inoue, M.

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The Seventh International Conference on Port and Ocean Engineering under Arctic Conditions. POAC 83, 2, pp. 875-887, 1983

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ICE REHAVIOUR AROUND A SMALL ARCTIC ISLAND

by R. Frederking, T. Sanderson, E. Wessels and M. lnoue

Appeared in VTT Symposium 28 The Seventh International Conference on Port and Ocean Engineering under Arctic Conditions Helsinki, Finland, 5 - 9 April, 1983 Volume 2, p. 875 - 887

Reprinted with permission Technical Research Centre of Finland (VTT) , . p2-i- !

I DBR Paper No. 1149 i3:T,. , .- --. , . , .t--*- 1 Division of Building Research ?.,-,.i ,, I;;-:- I -;l,,b;, TJ . - - >.--.-.-,I-,-

Price $1.25 OTTAWA NRCC 22804

R. Frederking, Division of Building Research, National Research Council of Canada, Ottawa. Canada KIA 0R6. T. Sanderson, BP Petroleum Development (Overseas) Ltd. London ECZY 9BU. England. E. Wessels, Hamburgische Schiffbau-Versuchsanstalt (HSVA), Hamburg. West Germany. M. Inoue, Nippon Kokan K.K., Japan. (Visiting scientist at DBR, NRC .)

ICE BEHAVIOUR AROUND A SMALL ARCTIC ISLAND

Abstract

Adams Island, 200 m in diameter and about 3 km offshore in Lancaster Sound, has been established as a site for studying ice interaction processes. Preliminary surveys of ice and environmental conditions over the winter 1981/82 shoved ice features reflecting a short-term pile-up as well as long-term thrust on the island. Associated ice pressures were in the range 40-400 Wa. Information on first-year ice cover, icebergs and multi-year floes was also collected.

1. INTRODUCTION

A small island can be taken as representative of a man-made offshore structure. Observations of ice interactions with the island are relevant to predicting ice forces on structures. provided there are suitable ice conditions and movements. This approach was used in an investigation of impact of large multi- year floes on /6/. Similar studies have been made of first-year sea ice rubble formations at Fairway Rock /5/. Adams Island, about 200 m in diameter, is located at the intersection of Lancaster Sound and Navy Board Inlet in the Canadian Arctic. Ice conditions in the area include level first-year sea ice, pressure ridges, multi-year ice, pack ice and icebergs. Both dynamic and land-fast ice conditions prevail over the course of the winter.

A joint project, to extend over three winter seasons, with participants from several countries, was initiated to observe and measure ice interaction processes and environmental driving forces around Adams Island in order to characterize the processes and develcp analytical models for predicting ice forces. Over the winter 1981182, periodic site visits, including several by a contractor and a 2-week major field visit in March, were made to survey ice and environmental conditions. A more extensive presentation of these results appears elsewhere 131.

2. SITE DESCRIPTION

The area of investigation is at the intersection of Lancaster Sound and Navy Board Inlet (Fig. 1). Lancaster Sound, the

FIGURE 1 LOCATION MAP FOR ADAMS ISLAND AREA eastern entrance to the Northwest Passage, extends to the west from into the islands of the Canadian Arctic. Ice moves generally eastward through Lancaster Sound, under the action of wind and current. Navy Board Inlet is a fiord-like feature which connects through Eclipse Sound and Pond Inlet to Baffin Bay. Water depth in Lancaster Sound reaches 800 m, and 400 m in Navy Board Inlet. A sill at their intersection does not exceed 200 m in depth and for the most part is less than 100 m deep. The tidal range is about 1.5 m.

Adams Island is situated about 3 km offshore from the Borden Peninsula (Fig. 1 insert). Borden Station, with accomodatioa buildings and a 300 m air-strip, provided a base for operations in the area. A detailed map of the island and local bathymetry are shown in Fig. 2. The island is rocky, with steep cliffs on

Fig. 2

Adams Island sharing profiled features, ice movements and bathymetry (water depth in m)

0 SHORI-TERM ICE MOVEMENT LlARKERS LOhG-ICRM ICf MOVChlENT SIIRVEY POLES

IlOliEhlLNI SCALE m i- > h!AD SCALE m 0 I92 IOC the north side. Maxinum elevation (20 m) is on the northern side, with gradual slopes to the east, south and west. The south shore is also nearvertical, with an escarpment about 4 m high. The beaches are rocky and relatively steep 171. There is no possibility of landing even a small boat on the island, except on a mobile spit on the southwest corner. As can be seen in Pig. 2, the slope of the sea bottom is gradual to the north and west. The steepest submarine slope is to the southeast. The sea bottom is very rocby.

For a normal season, ice first forms in the middle of Navy Board Inlet in October. This ice becomes land-fast and progresses up the inlet, reaching Lancaster Sound in December. The island is generally at the boundary between level and ridged ice. Shear and pressure ridges form a zone of ridged ice which becomes land- fast in mid-winter to about 2 Ian north of the island. During the open water season, multi-year ice from the moves through Lancaster Sound to be discharged into Baffin Bay. Similarly, icebergs from the glaciers of Greenland and circulate in the Sound. Some of these bergs pass into Navy Board Inlet and are frozen into the ice cover while others ground on the sill at the entrance to the inlet. Typically there can be 5 to 10 large icebergs within 10 km of Adam Island.

Multi-year ice is also frozen into the ice cover and may reach - coverages of 1 to 5%. The pack in Lancaster Sound is generally dynamic throughout the entire winter although on occasion it has been reported as completely land-fast and static /I/.

3. ICE PROFILES

Ice interactions in the immediate vicinity of Adams Island were characterized by measuring vertical profiles along several lines extending out from the island. The results of two representative lines, one to the southwest believed to represent a long term thrust (buckle) feature, and the other to the southeast representing a pile-up (Fig. 2 gives locations) are presented in - 879 -

Pigs. 3 and 4 respectively. The profiles were constructed from measurements of surface elevation, ice thickness, snov depth and water depth at 4 m intervals. These measurements were made during the March field visit.

3.1 Long term thrust (buckle) feature

The profile illustrated in Fig. 3 is typical of a feature which extended for about 150 m along the southwest side of the island, roughly parallel to the shore. The ice thickness in this feature (2-3 m) is greater than the adjacent level ice (1.3 m). This is -f 5

TIDAL -

- .I

MEASURED POINTS //5 SNOW WXY ROCK

--la

FIGURE 3 PROFILE OF SOUIHWEST ICE FEATURE AOAMS ISLAND MARCH 1982 most likely due to water flooding onto the surface at high tide and subsequently freezing. Tidal flooding was observed during the course of the two-week field visit. Tidal cracks were also noted and seen to change, some becoming inoperative and new ones forming. A similar tidal-related thickened ice zone has been observed in Strathcona Sound around a wharf 121.

The near-vertical ice feature at 20 m was produced by a progressive tilting up of the ice cover. This rotation is shown by the curved arrow in Fig. 3. Further evidence was a set of snowmobile tracks, clearly visible on this nearvertical surf ace; they had been made during a February reconnaissance to the island. At some positions along the feature the tilt-up was not as great and in other cases the rotation was more than 90'. As yet there is no definite explanation for this tilting. It is most likely due to the combined effects of buckling, resulting from horizontal movement of the ice cover towards the island, and tidal cycling.

3.2 Pile-up feature

The profile of the pile-up on the southeast side of the island (Fig. 4) is representative of this feature, which extended for a length of almost 50 m along the shore. A significant part of the pile-up is above the mean sea level and it extends about 10 m

niEASURED POINTS I HARD ICE ' SOFi ICE = GAP. SHOWING SIZI - GAP. 5 cm OR LESS ,"',SNOW ;YA-+ R 0 C K 11 ACTIVE CRACK ------z 0: POSITION, m <>

POSITION. n

FIGURE C PROFILE OF PILE-UP OY SOUTHLAST SIDE OF AOAMS ISLn'rD hlARCH 1982

inland. Visually there appeared to be both first-year and multi- year ice in the pile-up. The designations of "hard" and "soft" ice are qualitative and related to the ease of drilling through the pile-up. The hard'' ice out to the 12 m position was extremely hard, probably due to the low temperature of the ice and the presence of multiyear ice fragments. Some ice blocks on the surface of the pile-up measured from 0.3 to 0.65 m thick. with an average of 0.45 m. The lengths and widths were from 3 to 4 times the thickness. Based on the lengths of voids encountered while drilling through the pile-up (-lo%), a volume porosity value of 0.3 was established. The pile-up probably occurred in December, shortly before freeze-up, when first-year ice 0.3 to 0.4 m thick was present.

The shore in the area was quite rocky, with a slope of about 1:4 (Fig. 4), similar to the slope of the sea bottom in Fig. 2. The slope of the pile-up between 0 and the 5 m position was about 1:l. between the 5 m and 13 m positions, about 1:4. and between 13 m and 17 m. about 1:l. Taking the porosity value of 0.3. an 80 m movement of 0.45 m-thick ice would be required to form the observed feature. From the cross-sectional shape of the pile-up, it is possible that an equilateral-triangularshaped pile-up of slope about 1:l formed at the point of maximum shoreward intrusion, reaching a lid ting height. With continuing ice movement, the pile-up would continue to grow to seaward until the ice movement ceased. At the point of maxi~rmm landward ice intrusion there was no obvious shore obstruction which would have limited ice movement.

Kovacs and Sodhi 151 have proposed an approach-for calculating the force required to add gravitational potential energy to a shore pile-up. The resulting equation has the form

where F = force per unit width a = a factor which depends on the shape of the pile-up and the slope of the shore (value in range 1 to 2) pi = density of ice g = gravitational acceleration Hs = height of ice pile-up t = ice thickness.

For the geometry of the pile-up observed (He = 5 m, t = 0.45 m. a = 1.5). the force per unit width required to build the pile-up was calculated to be 15 kN/m or, in terms of ice pressure for the given ice thickness, 35 kPa. That does not include frictional effects so the loads estimated are a lwer bound. An upper bound can be determined by estimating the resistance to sliding offered by the rubble. The sliding resistance per unit area is

where u = coefficient of static friction between the ice and shore on = vertical pressure on shore.

The vertical pressure can be,calculated from simple buoyancy and gravity considerations,

where H = sail height, above waterline or shore - keel depth, below waterline pi, 0,- densities of ice and water respectively cs. ck = porosities of sail and keel respectively.

The value of the vertical pressure, on, will vary with position relative to shore. Vertical pressures calculated at the drill profile locations out to the 15 m position (with cs = 0.3. ck - 0) are plotted on Fig. 4. The average vertical pressure was 18 Wa. Using this value in equation (2), with a friction coefficient of 0.7, yielded a sliding resistance of 196 kN/m or an equivalent pressure of 430 kPa in 6.45 m-thick ice. The two cases evaluated establish the upper and lower bounds for the force required to develop the pile-up. 4. ICE UOVEIIENTS

4.1 Short-term movements

An array of seven markers was established on the ice at distances of 200 to 400 m from the island (see Pig.2 for actual locations). The positions of the markers were sqrveyed using theodolite intersection from a base network of three fixed stations on the island. The precision of a position fix waa estimated as 25 mm in the tangential direction and 100 mm in the radial direction. The measurements to marker "S" were unreliable due to the small angle of intersection. Three complete surveys were made, on March 14, 17 and 20. The results are presented in Pig. 5.

MAR 17-20

Fig. 5

Short term ice movements, Adams Island. Harch 14-20 1982. ICE MOVEMENTS

c---4 MARCH 17-20

I XALI. s

A 0 510 I000 MA? SCALE. - During the first time interval, March 14-17, there were high winds from the east and south, with a maxinum velocity of 21 m s-l. These winds resulted in the ice movements shown as solid vectors in Fig. 5 and also gave rise to an open fracture to the north of marker "E". The root mean square wind vector is plotted for comparison with the ice movement vectors. The movements are about a metre, well above the resolution of the measurements. The directions of movement are not as might be expected. Although markers to the north and west of Adams Island shov a roughly coherent movement to the north, markers "S" and "SE" show movement to the south, against the predominant wlnd direction. This velocity field is not consistent with a simple model in which a uniform slab of ice is driven against an obstacle and creeps steadily around it. In such a model, motion cannot occur contrary to the driving force; ice immediately upwind of the island should be moving tcuards it and undergoing compressive strain. The velocity field observed shows very little evidence of compressive strain. It is likely that new or existing fractures are facilitating movement and that large areas of ice are moving as blocks, undergoing translations and rotations under the forces exerted by wind drag and the reaction of the island. A gross block rotation may lead to movements contrary to the driving force.

During the second time interval, March 17-20, winds were from the west with a maximum velocity of 9 m s-l. These movements are shown by broken vectors in Fig. 5. They are considerably smaller than those of the previous period and are only barely significant. Again, movements shown are broadly divergent from the island and not consistent with uniform creep around it.

4.2 Long-term movements

Additional ice movement information is available for the near- island interaction zone to the west of Adams Island. A series of survey poles was installed in the ice during February, and measurements were made of distances between them and of distances from fixed points on the island. See Fig. 2 for locations of these poles. The survey line was initially lined up visually.

The line was remeasured in March and June. The changes in measured distance from the island provide radial movement information, and the distance moved out of line provides tangential movement inf orration. Also, changes in distance between on-ice survey poles provide strain information. The movement vectors of the two poles are indicated by arrows in Fig. 2. Movements were of the order of 1 to 2 m per month and in an easterly direction (towards the island). Most of this movement is accommodated in the thrust (buckle) feature. The first-year ice does not appear to be moving towards the island by simple compression creep; instead it is failing in bending and moving as a block into the failure zone. From the movement vectors, average compressive strains between the two poles of 3.3 x and 4.5 x were determined for the February-March and March-June periods, respectively. The corresponding average strain rates would be s-l and 5 x 10-10 s-1. This compares reasonably well with average strain rate of 10-lo s-I estimated for the first-year ice cover on Strathcona Sound 121. Assuming an average ice temperature of -lO°C and steady state creep /&I, the uniaxial compression stresses required to produce strain rates of 10-9 s-1 and 5 x 10-10 6-1 are 180 and 140 kPa, respectively. This method of estimation yields an average (lower bound value). Peak stress would certainly be higher.

5. ICE PROPERTIES

Extensive measurements wete made of the properties of the various ice types found around Adam Island. Because of space limitations, only selected results will be discussed here. Almost 30 measurements of thickness, sncw depth and freeboard in a 40 by 80 m area of level first-year ice adjacent to the island were made in mid-March. The mean and standard deviation of depth was 0.10 2 0.09 m and freeboard, 0.13 2 0.09 m. This demonstrates the large spatial variability in snaw depth and freeboard, which is common for first-year sea ice. Temperature measurements in level first-year ice yielded linear gradients with mean ice cover temperatures of -10 to -12°C. In one of the flooded ice zones (about 1.8 m thick) adjacent to the thrust feature, ice temperature increased from -17'C at the surface to -5°C at 0.3 m depth and remained constant at this temperature to the bottom surface. Such high ice temperatures would be consistent with an easily deforraable ice cover, as evidenced by the tilted-up feature in Fig. 3.

A number of multi-year floes were frozen into the ice cover immediately adjacent to Adam Island; the ice movement markers "NU" and "S" were located on multi-year floes. The dimensions of several floes were measured. They averaged 7 m thick and about 40 m in diameter, with an estimated weight of 10 tonnes. The floes were not grounded and there was no evidence that they were moving with respect to the surrounding ice. It was apparent, hwever, that in the latter stages of freeze-up, first-year ice had failed against them, building up small rubble accumulations which, in some cases, encroached on the top surface of the floe.

In March detailed surveys were made of two icebergs located in the entrance to Navy Baard Inlet. Their above-water dimensions were estimated from a combination of radar fixes and photographs. A visit to the icebergs established that they were grounded, i.e., tidal hinges were observed in the ice cover around them. Water depth measurements were made adjacent to each berg. In one case, water depth was 118 m and in the other it exceeded 139 m.

The estimated mass of each berg is 2 x lo6 tonnes.

A time lapse camera system-based on a Super 8 movie camera was used to record the evolution of ice conditions as well as the progression of break-up. The camera was placed in an insulated box, which also contained a heater and a thermostat to control the temperature at -lO°C ('2°C) to ensure sufficient film flexibility. A timer activated the camera to take one frame every 30 minutes, thus allowing one film cassette to last over two months. A fully charged 230 amp-hour lead-acid battery successfully operated the camera system for over two months at ambient temperature of about -30°C. Altogether eight months of film record was obtained between two cameras. The logistic support provided by Polar Continental Shelf Project is greatly appreciated, as is the use of the facilities at Borden Station, owned by the Canadian Dept. of Fisheries and Oceans. Technical support was provided by the Arctic Research ~stablishmentof Pond Inlet. Financial support of the German Uinistry of Resources and Technology made possible the participation of HSVA in the project.

REFERENCES

I. DEY. Balaram B., Shipping routes, ice cover and year-round navigation in the Canadian Arctic. Polar Record, Vol. 20, No. 129, 1981, pp. 549-559.

2. FREDERKING. R., Ice action on Nanisivik Wharf, Strathcona Sound, N.W.T. Winter 1978-79. Canadian Geotechnical Journal, Vol. 7, No. 3, September 1980, pp. 558-563.

3. FREDERKING, R., INOUE, M., SANDERSW, T., and WESSELS, E., Adams Island Project - Data Report Winter 1981-82. TO be published as NRC report, 1983.

4. GOLD, L.W., Activation energy for creep of columnar-grained ice. (In Whalley, E. et al.. eds. Physics and Chemistry of ice: papers presented at the Symposium on the Physics and Chemistry of Ice, held in Ottawa, Canada, 14-18 August 1972.) Royal Society of Canada, Ottawa, 1973, pp. 362-364.

5. KOVACS, A.. and SODHI, D.. Sea ice piling at Fairway Rock, Bering Strait, Alaska : Observations and theoretical analysis. 6th Int. Conf. on Port and Ocean Engineering under Arctic Conditions (POAC), Quebec 1981. Vol. 11, pp. 985-1000.

6. METGE, M.. DANIELEWZCZ, B., and HOARE. R., On measuring large scale ice forces; Hans Island 1980. 6th Int. Conf. on Port and Ocean Engineering under Arctic Conditions (POAC), Quebec 1981. Vol. 11, pp. 629-642.

7. STELTNER, H.A.R., Director, Arctic Research Establishment. Pond Inlet, N. W.T., personal comnunication 1982. This publication is being distributed by the Division of Building Research of the National Research Council of Canada. It should not be reproduced in whole or in part without permission of the original publisher. The Di- vision would be glad to be of assistance in obtaining such permission. Publications of the Division may be obtained by mail- ing the appropriate remittance (a Bank, Express, or Post Office Money Order, or a cheque, made payable to the Receiver General of Canada. credit NRC) to the NationalResearch Council of Canada, Ottawa. KIA OR6. Stamps are not acceptable. A list of all publications of the Division is available and may be obtained from the Publications Section, Division of Building Research, National Research Council of Canada, Ottawa. KIA 0R6.