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of about forty-five special committees which assist the National Research Council
in its work. Formed in 1945 to deal with an urgent wartime problem involving
soil and snow, the Committee is now performing its intended task of co -ordinating Canadian research studies concerned with the physical and mechanical properties
of the terrain of the Dominion. It does this through subcommittees on Snow and Ice,
Soil Mechanics, Muskeg, Permafrost, and Pipeline and Land Use Technology in
Northern Terrain. The Committee consists of about twenty-five Canadians
appointed as individuals and not as representatives, each for a three -year term. Enquiries will be welcomed and should be addressed to: The Secretary, Associate Committee on Geotechnical Research, c/o Division of Building Research, National Research Council, Ottawa, Ontario.
This publication is one of a series being produced by the Associate
Committee on Geotechnical Research of the National Research Council. It may
the refore be reproduced, without amendment, provided that the Division is told in advance and that full and due acknowledgment of this publication is always made. No abridgment of this report may be published without the written authority of
NATIONAL RESEARCH COUNCIL OF CANADA
ASSOCIATE COMMITTEE ON GEOTECHNICAL RESEARCH
WORKSHOP ON THE ACTION OF ICE ON STRUCTURES
November 12 -13, 1970 Division of Building Research
National Research Council of Canada, Ottawa
Sponsored by
Snow and Ice Subcommittee
Associate Committee on Geotechnical Research
TECHNICAL MEMORANDUM NO. 101
OTTAWA APRIL 1971
PREFACE
Page L. W. Gold
PLENAR Y SESSION
Movement of Ice - E. R. Pounder 1
Interaction Between Ice and Structures - B. Michel 2
Properties of Ice - L. W. Gold 5
Shore Fast Sea Ice - E. F. Roots 8
STUDY SESSIONS
Session 1 - Movement of Ice
Session 2 - Properties of Ice, Deformation
and Strength
Session 3 - Action of Ice on Structures
WORKSHOP RECOMMENDATIONS
WORKSHOP SUMMARY - J. Hnatiuk
LIST OF PARTICIPANTS 21 40 53
77
81 85PREFACE
It is still not possible to establish with confidence the m.axim.um. force
that ice at a given site will exert on structures. The need to be able to do
this has greatly increased in the past few years because of the requirem.ent
for structures in ice infested waters of the Arctic. Such structures will be
exposed to som.e of the m.ost severe ice conditions in the world, and there m.ust be very little possibility of their failure.
The Snow and Ice Subcom.m.ittee of the Associate Com.m.ittee on
Geo-technical Research has kept this problem. under continuous review. It convened
a special m.eeting on it in 1964, and a conference on ice pressures in 1966. The proceedings of the conference provides a good sum.m.ary of the current
"'_
state of knowledge to that date ." In 1968 the Subcom.rnittee set up a Working Group with the objective to establish the basis for predicting ice pressures. Further discussions concerning the interaction between ice and structures
セ\J
took place at a conference on snow and ice held in 1969.
During discussions with the Arctic Petroleum. Operators Association, it becam.e clear that it would be useful to have a review and dis cussion of the
problem. with particular reference to Arctic conditions. In response to the
interest of the APOA, the Working Group on Ice Pressures organized a
Workshop on the Interaction Between Ice and Structures. About 40 experts
on ice and ice pressures were invited to partici pate. The subject was
dis-cus sed in depth in three concurrent study ses sions dealing with m.overnent of
ice, properties of ice, and the interaction between ice and structures. This
Technical Mem.orandum. contains the record of these deliberations and the recom.m.endations that were developed concerning needed research.
The Workshop was initiated in a plenary session with three papers on the subjects to be discussed. These papers are reproduced in this report along with a comprehensive review of knowledge concerning shore fast sea ice subm.itted by Dr. E. F. Roots.
The Workshop produced a particularly useful statem.ent on the cur rent state of knowledge concerning the interaction between ice and structures. The Associate Com.mittee on Geotechnical Research, and its Subcornm.i ttee on Snow and Ice, wis h to express their appreciation to all those who
partici-pated in the Workshop. It wishes also to express its appreciation to Miss
J.
Butler for her assistance to Messrs. Goodrich, William.s and Frederkingin the preparation of this Technical Mem.orandum..
L. W. Gold
Chairm.an, Working Group on Ice Pressures *Ice Pressures Against Structures, Tech. Mem.. 92, Assoc. Com.m.. on Geotech. Res. National Research Council of Canada, Ottawa, 1968.
**Proceedings of Conference on Ice Engineering and Avalanche Forecasting and Control, Tech. Mem. 98, Assoc. Com.rn. on Geotech. Res. National Res earch Council of Canada, Ottawa, 1971.
MOVEMENT OF ICE
Summary of Introductory Remarks by
E. R. Pounder, Department of Physics, McGill University
An obvious but important classification of the mo vement of ice covers is "small- scale" and "large- scale ". Small- scale occur s in "shore fast" ice as a result of stresses from moving ice impinging on the fast ice
or of thermal expansion or contraction. These may result in horizontal
displacements of up to a few feet. Tidal effects may cause similar or
larger vertical movements. Displacements of this size are of importance
in the design of bottom-mounted structures. They must be investigated
locally because of their dependence on geography, topography, and local climate.
Largescale movement of floating ice results from body forces -weight, buoyancy and Coriolis force, from fluid stresses - wind stress driving the ice, water stress acting as a drag, and from so-called "ice
stressII which is the integrated effect of the jostling of other floes on the
one in question. In general, body forces are well understood and
predict-able, except that the sea surface is normally tilted (by a fraction of a second of arc) so that there is a component of weight downhill, and the ice
and supporting water move in consequence. This effect is predictable if
enough is known about the currents and tides in the area. Dr. J. R. Weber
of the Dominion Observatory has developed a levelling technique for direct observation of sea-surface tilt from an ice floe.
Fluid stresses on an ice floe have been investigated by several
groups (e. g. in the Gulf of St. Lawrence). The Gulf of 51. Lawrence studies
were undertaken after a series of winter experiments carried out for several years by the McGill Ice Physics group in collaboration with
Defence Research Establishment Ottawa, Atlantic Oceanographic Labora-tory, and other Government agencies.
Ice stress is the least understood, most difficult to measure and, in
some ways, the most important force on ice floes. Convergent ice
move-ment may damage shipping or at least immobilize it. Predictive ability is
badly needed. The difficulty is that this stress is the result of integration
of wind and current fields over areas of up to hundreds of miles across.
Local observations at one point tell little. It is hoped that a better
under-standing of the stress that can develop in ice covers will be obtained from the proposed AIDJEX (Arctic Ice Dynamics Joint Experiment) programme,
which envisages the study of ice movement in a square area about 100
kilo-meters to the side in the Beaufort Sea in 1973 (1).
(1) Arctic Ice Dynamics Joint Experiment, Division of Marine Resources,
University of Washington, Seattle, Washington 98105, Joseph O. Fletcher,
2
-INTRODUCTION TO THE ICE PRESSURES PROBLEM AND INTERACTION BETWEEN ICE AND STRUCTURES
B. Michel
Depa rtem ent de Genie Civil, U'n i ve r site Laval, Quebec
This introductory presentation will try to delineate the current state
of knowledge on the question of ice action on structures. Not much has been
done or published in this area in the past, but there has been quite an
increase of research recently, which will be reported only if it is published. The subject divides itself logically into static ice pressures, dynamic ice impact and other effects of the interaction between ice and structures. Static ice pressure
Static ice pressure may develop in a number of ways. A buildup of
internal horizontal pres sures can occur in a solid ice sheet when there is
thermal expansion. Internal pres sures may also be set up by the shear
forces of water flow or wind, or any mechanism that causes slow movement of an ice sheet relati ve to a structure.
In this domain of slow deformation, ice deforms plastically. It may
fail like a ductile material or it may undergo considerable plastic deforma-tion at the lowest strain rates without rupture.
The problem receiving the greatest attention has been the thermal
thrust of ice on a dam. Unfortunately this problem has not been sol ved
completely. Empirical values now used by enginners are as good as any
existing theory. The distribution of temperature inside the ice sheet can be
predicted quite accurately from meteorological conditions. The
correspond-ing stresses set up in the ice have been determined in some cases for
uni-axial expansion. But it has not been possible yet to relate the uniaxial
deformation behaviour of different ice structures to the biaxial condition existing in the field.
There are some measurements on the horizontal movement of lake ice sheets under various creep conditions and also some pressure
measure-ments by gauges set on the face of darns. Little success was achieved in
correlating movements and pressures to pertinent meteorological parameters. There appears to be no published information on the internal stresses
set up by wind or water current in a solid ice sheet. One extensi ve study of
ice movement on Wamplers Lake concluded that no appreciable movement of
the ice sheet was caused by the wind. This certainly would not apply to the
Another case of horizontal static ice pressure is that of an uncon-solidated ice field, an ice jam or a hummocked ice cover under stress by water or wind drag forces, and held by a retaining structure, generally a
boom. This question has been covered for a uniform strait channeL but
seems too complex for analytical computation when there are variations in shape in plan.
Dynamic ice pressure
Many bridge piers and other structures have failed because of ice impact, usually at breakup time in rivers.
In North America the practice has been generally to use the code of
the American Association of State Highway Officials. It simply says that
one has to apply a horizontal component of force to the pier equal to the projection of the ice area perpendicular to the axis of the flow, multiplied by the crushing strength of ice, which is recommended to be taken as 400 psi. Under impact when the velocity of incoming ice is more than a few tenths of foot per second, the ice behaves like a brittle material and its strength is constant for increasing rates of impact.
The force exerted by ice on a pier has been computed to a first
ap-proximation, and included in the U .R.S.S. code for a long time. It takes into
account different conditions such as complete or partial indentation on a vertical face of various forms, splitting of ice floes and failure of ice on inclined piers. The Russianmethods certainly provide a rough approximation
of the pressure if the basic ice properties are known. Most of the empirical
factors which are used, however, such as the contact factor and the forrn
factor, have to be more accurately determined. One of the biggest
weak-nesses of the method is the determination of the effect of biaxial restraint. A magnification factor of 2. 5 is used to determine the maximum force from results obtained for uniaxial compression. This value is certainly in the range of those obtained from the theory of rigid perfectly plastic solids restrained in three directions, but it does depend on the ratio of the width of the indentor to the thickness of the ice sheet, and probably also on the
crystal structure of the ice. Little is known about these effects.
The impact force of ice on complex structures other than single pIer is unknown. This is particularly so when one part of the structure interacts with other parts, as in the case of a multi-legged structure.
Impact forces can also be imposed by hummocked ice or a pressure
ridge. No information is available in the literature on these cases.
Interaction between ice and structure
One of the effects of ice on structures that has often been neglected is the vertical force transmitted by an ice sheet during water level fluctua-tions. With a rise in water level this force may lift piers, caissons, pile
4
-be subject to serious uplift forces following the usual lowering of a reser-voir level during winter operation.
The determination of the vertical force exerted by ice on a structure
is quite similar to that of the bearing capacity of an ice sheet. This latter
pro blem is well co vered in the literature either with the elastic or the ideal rigid plastic theory. The main problem remains in finding out which theory applies in each case.
The tidal movements along marine structures produce ice caps on
the upright surfaces of structures. These formations have not received
much attention. The fall of such ice caps in spring recently destroyed an
important wharf.
Ice pieces moving along structures have definite abrasive action by their repetitive local impacts on the surface.
A moving ice field may hit an engineering structure and produce an important ice accumulation while dissipating its energy and slowing down. These accumulations may have harmful effects on works merely because of
their bulk. This would particularly be the case if they accumulate on the
face of a wharf in the Arctic and prevent berthing of incoming ships. On the other hand, a structure may form a lee in moving ice, like the track of an icebreaker, which could be used as a protected area of a harbour to receive ships in Arctic sea operation.
Conclusion
Most problems of ice pres sure and ice action on structures are
amenable to more or less standard computations of applied mechanics. Little advance has been recorded on these questions simply because of the ignorance of the basic mechanical and other physical properties of the various types of
ice. Plastic. ductile and brittle behaviour of the material have to be
per-fectly delineated and quantified for each type. Surface properties, contact
coefficients and angles of internal friction in accumulation have to be meas-ured. When these basic data are known there will be very fast progress in
practical applications to sol ve ice pres sure problems. M any technique s are
available to do that including mathematical and physical modeling. References
"Ice Symposium, Reykjavik" - International As sociation for Hydraulic
Research. Preprints of Papers. July 1970.
"Ice Pres sure on Engineering Structures" - B. Michel, U.S.A. CRREL. Monograph III-BIb, June 1970-71 pages.
"Ice Pressure against Structures" - Proc. Co nf, N.R.C .. Tech. Mem. 92, 247 pages.
PROPERTIES OF ICE L. W. Gold
Division of Building Research, National Research Council, Ottawa
In this brief introduction to the study session on the properties of ice, attention will be given to the role that knowledge concerning these
properties plays in design. Consideration will also be given to the
infor-mation that is required concerning these properties. Role in Design
When designing a structure that must stand up to the forces due to ice, the engineer must be able to:
1. specify the most serious ice condition to which the structure
will probably be exposed;
2. establish the mode of failure or state of stress in the ice
that is associated with this condition;
3. predict the force that will be exerted against the structure.
These questions involve the interaction between ice and structures, which
is the subject of one of the study sessions. It is necessary to consider
them in order to appreciate the information that is required concerning the properties of ice.
In order to answer these questions, the engineer must establish:
1. the characteristics of the ice cover for the extreme condition
(evg , thickness, type of ice, structure of the cover, salinity,
temperature, e tc .}:
2. the movement and rate of movement that should be assumed
for the design condition.
He must do this to establish the mode of failure or state of stres s that will determine the maximum load, or if he wishes to design the
structure so that it will control or predetermine the critical load condition.
It is at this point that information is required on the strength and
defor-mation properties of ice.
It should be appreciated that this is not always the case. There are
relati vely few structures that ha ve not been built because of lack of
knowledge concerning ice (e.g. dams, bridges, harbour facilities). The
current concern and interest is due mainly to the potential need to con-struct major con-structures and operate in areas having severe ice conditions,
and for which there is very little past construction experience. There is
also a need to establish with greater accuracy the design ice loads that should be used for those situations where they significantly affect costs (e.g. bridge piers).
6
-the properties of ice. This has not always been appreciated by individuals
studying ice, or even by the design engineer. Many of the investigations
that have been undertaken have had for motivation the challenge of the
material, and this is still true to a large extent. These studies have
demon-strated the unique strength and deformation characteristics of ice, and
provide a useful appreciation of these properties. On the other hand,
engineers and others have undertaken measurements of properties required for specific application without sufficient appreciation of the deformation behaviour of ice, and because of this their tests have failed to provide them
with the information required. Neither the engineer nor the ice scientist
ha ve given sufficient attention to clearly defining the information concerning ice that is required for design.
Information Required
The design ice condition may not be determined primarily by the properties of the ice per se , but rather by the characteristics of the cover
(e.g. thickness, size of ridges). For those situations where the properties
of the ice have a significant effect on the load, particular attention must be given to those ice types that offer the greatest resistance to deformation. This requires an appreciation of the effect of crystal type. crystal orienta-tion, grain size, density, purity or salinity and temperature on deformation behaviour and strength.
Although ice covers are usually composed of various types of ice, from the design point of view it may be necessary initially to consider that
it is composed entirely of the strongest. Valid reasons may subsequently
be established for accepting lower strength values at particular sites. Because of the marked dependence of the strength and deformation behaviour of ice on type, close attention must be given to clearly describing
it in any test program. The information concerning deformation that is
required for each type is:
1. its elastic properties;
2. its behaviour during creep (i.e. time dependence of the strain
under constant load conditions);
3. its stress-strain behaviour for constant rate of strain
condi-tions;
4. the influence of temperature.
The above information should be obtained for uniaxial, biaxial and triaxial stress conditions over the full ductile and brittle range of behaviour. Static Ice Forces
In order to develop a thrust on a structure, an ice cover must move
relative to it. The forces causing the cover to move may be due to wind,
water currents, change in water level, change in temperature, or a
restraining area of the shore plus the structure is sufficiently large that the thrust that can develop is not serious from a design point of view. Such
a situation exists for many dams and bridge piers. At other sites the rate
of movement that can occur is sufficiently small that the ice will deform
in a viscous manner. This is probably the situation for most cases where
the load imposed on the structure by the ice is due to change in water l e ve l . For these situations there is probably now sufficient information on the uniaxial elastic and vis cous properties of both fresh water and sea ice to allow considerable progress to be made in establishing proper design
practice. A particularly urgent need for design, however, is the
formula-tion of the appropriate mathematical models for describing the behaviour of the ice when in contact with structures under such conditions.
A similar situation exists for shore fast ice frozen to a structure and the thrust exerted increases rapidly to the failure condition (e.g.
buckling, general crushing). Again, what is required now is the appropriate
mathematical model to describe the interaction between the ice and the
structure. The mathematical models will help establish if information is
needed concerning the strength and deformation behaviour of ice for biaxial
and triaxial stress conditions. They will also be of assistance in
establish-ing the appropriate constitutive equations for describestablish-ing the stress, strain, temperature and time dependence of the deformation of ice.
Dynamic Loads
During conditions of impact there is not continuous and uniform
contact between the ice and the structure. Because of this, uncertainty
exists that prevents, at this time, the determination of the design condition from a knowledge of the properties of the ice and the characteristics of
the structure. It has been customary to measure such forces directly on
appropriate structures (e.g. Cooke Inlet drilling platform investigations,
Manhattan project). Information on the dynamic strength and deformation
properties of ice, and the appropriate mathematical models are required to establish if such expensive studies are necessary, and to properly design and interpret them if they are.
Concluding Remarks
The Session on the properties of ice is to consider initially the needs
of the design engineer for information 'concerning ice. It will then review
the current state of this information, particularly that on deformation and
strength. Attention will also be given to test methods including those
8
-SHORE FAST SEA ICE E. F. Roots
Department of Energy, Mines and Resources
Definitions
Most concepts associated with ice carry a confusing and often colourful series of terms as men from different backgrounds and engaged in a wide range of activities have tried to describe the various characteristics
of this deceptively simple and familiar substance. The idea of shore fast
sea ice is no exception. The idea is self-evident: - sea ice fastened along
the shore. But what is it? - a kind of ice? - any kind of ice found in certain
locations? - fastened to what, for how long, how "fast"? - what is the
"shore"? A check on polar and ice literature and glossaries will show that different people have found it useful to distinguish shore fast sea ice in different ways for different purposes.
After a tremendous amount of argument, and r evisions , the World
Meteorological Organization has achieved a sort of running truce on
terminology for types of sea ice and sea ice features. This has been a
great step forward in the systematic description and understanding of ice. We should acknowledge the important role played by Canada, and in particular by Mr. Markham and Miss Dunbar, in developing a glossary of sea ice terms that has been accepted internationally and which is becoming standard both among practical seamen and oil drillers and among theoretical scientists. But the glossary, which deals with sea ice types and sea ice features, does
not define "shore fast sea ice". Maybe it is not a type or a feature. Perhaps
it's just an occurrence.
We have to go to the navigators to find written definitions of this
occurrence. In doing so we 'enter the very chaos that the W.M.O. glossary
was drawn up to avoid. All the English-language Arctic Pilots are more or
les s in agreement:
"Shore fast ice (or shore ice) is a basic form of fast ice, representing a compact cover attached to the shore and in shallow waters, also grounded. Shore ice may be found several hundreds of miles offshore. " (Pilot of
Arctic Canada, 1960 edition, Vol. I, p. 79).
So we check on what is fast ice. Here the mariners and the
----W. M.O. are in general agreement, with varying degrees of detail. The pilots state that fast ice is:
-"Sea ice which remains fast, generally in position where originally
formed, and which may attain a considerable thickness. It is found along
coasts, where it is attached to the shore, or over shoals, where it may be held in position by islands, grounded icebergs, or grounded multi-year ice. "
The W. M.O. would add that:
"Vertical fluctuations may be observed during changes of sea level. Fast ice may be formed in situ from sea water or by the freezing of pack ice of any age to the shore, and it may extend a few meters or several
hundred kilometers from the coast, Fast ice may be mo re than one year
old. " If it is thicker than about 2 m above sea level it is called an ice
shelf. " (Dunbar, 1960, P: 107)
If we look up pack ice we find that it is any accumulation of sea ice
other than fast ice. If we look up ice shelf we encounter a new field of
definitions that could lead us far into glaciology, but which contain the general idea that all or most of the shelf is made from snow or fresh-water glacier ice and not from the freezing of sea fresh-water.
So where are we? Shore fast sea ice is ice that is grounded, or
attached to shore, but it can move up and down with sea l e ve l . It may be
found a long way from the shoreline. If it becomes thick, it is called
something which by definition is not normally sea ice.
If now we check the glossaries for other terms that pertain to sea ice fastened to the shore, we encounter two that are widely used:
Bay ice is not in the W. M.O. terminology (again, it is an occurrence, not a type or kind of ice or a feature), but it is widely used by mariners and polar non-scientists to describe (Pilot of Arctic Canada definition);
"Level or fast ice of more than one winter's growth, which has remained unhumrnocked and is also nourished by surface layers of snow. Thickness of ice and snow up to 2 m above sea level. "
One has to be careful here; in British naval tradition of the pre-Franklin days, bay ice was used to describe grey young ice that was not
safe for men to walk upon. It seems to have been the whalers who made
bay ice safe.
An icefoot is, according to the Pilot (Vol. 1, p.76) "ice ... attached
to the coast, unmoved by tides and remaining after the fast ice has m.oved away. "
So it would appear that the only kinds of shore-fast sea ice that have any secure fastening or permanence are either not fast ice (e.gc ice foot) or
not all sea ice (evg. bay ice, ice shelf). It will be noted that none of these
definitions states how long the ice must be stationary, or how immobile it must be, to qualify for its particular category.
This brief excursion into the confusion of sea ice nom.enclature is not meant to be facetious, or to ridicule the sincere attempts at bringing
10
-elusi ve are the phe norn e na that we are dealing with, and how very hard it is
to 0 btain useful information about - or even make a useful and
understand-able description of - a fairly common and very important part of the Arctic environment.
For the purposes of this meeting, and to get on with the discussions of what I think our chairman has in mind, I propose that we consider shore fast sea ice, in its strict sense, to be that sea ice which is so firmly resting on the ground, or attached to the shore, that it does not move appreciably in any direction (laterally or vertically) over periods of a few days of normal weather. (Normal weather would include the usual winter storms, but not exceptionally severe ones.) This, in effect, limits shore fast sea ice,
sensuo stricto, to sea ice on the landward side of the tide crack, if there is
one. For all practical purposes, this kind of ice is stationary ice.
It should be remembered that in large parts of the Arctic, in both
hemispheres, where sea ice information is important today, the tidal range
is very small. This is in distinct contrast to the North Atlantic region
where much of the sea ice terminology was developed, and where tidal phenomena are very apparent.
There is another category of ice, however, that can be loosely grouped with the strict shore fast sea ice, and it is of much greater areal extent. This is the ice which is not immobilized, but which for one reason or another does not move laterally a significant distance during the time
under consideration. It is most of the fast ice and the bay ice, the セ ice
of the mariners, and, as long as there is not bodily drifting of the pack, it includes the young ice and the winter ice of the WMO classification provided it is adjacent to the shore and not fractured, rafted or hummocked. Whether or not a satisfactory term can be found for this category of ice, and indeed whether one is desirable, I do not know. Perhaps the mariner's descriptive term level ice comes closest, but that term does not seem suitable for previously drifting pack ice, often very rough, that has become part of the fast ice, and ice that is level can be found anywhere in the ocean, drifting
at any speed. The Arctic Pilot defines level ice as:
"Ice with a flat surface, which has never been hummocked; typical with regard to bays, gulfs, straits, archipelagoes, and shallow waters, where the ice formation occurs under undisturbed conditions."
To keep within the context of our program, I propose to call this
category near-shore constrained ice. (This is .!lQl a formal proposal for
a new term!)
We therefore have three categories of sea ice, with respect to their freedom of bodily movement:
a) Stationary: ice in which the mo vement in any direction is negligible
b) Near-shore constrained ice: ice that moves freely in a vertical direction with the tides and normal changes of sea le vel, and which may not be physically fastened to the land, but whose bodily lateral movement is rarely more than a few meters over periods of days in normal weather;
c) Pack ice: floating ice whose translation under wind or current
forces is such as to enable it to move bodily into or out of the area under consideration over periods of days.
In the past, when the main operational reasons for distinguishing between the loosenes s or fastnes s of sea ice was how well it could be sailed through or how satisfactory it was to walk on, the re was little practical value in distinguishing between ice that is essentially immobile and ice that
stays essentially in place but is free to move a few meters. If one was
hunting seal or travelling on the ice, the difference, except for the tide
crack, was unimportant. Today, however, whether the ice is essentially
immobile, or whether it is free to move a few feet, is of vital importance to the design and operation of coastal and offshore structures, transportation and communication facilities, to the effect of community and industrial activities on the shoreline and marine environment, and to the prediction
and cost analysis of northern development as a whole. It is therefore of
real practical as well as scientific value to consider what we know and what
we need to know about these two categories of non-freely drifting ice. The
following is not a review of our state of knowledge, but an incomplete sketch of some aspects that should be considered; I am sure that other papers in this conference will deal with some of these in detail.
Distribution
1. Stationary sea Ice
a) Typical locations: If the definition given above is accepted, sea ice
that is essentially stationary will be restricted to:
i) ice foot. This is ice mechanically fastened to or resting on the
shore. In part it is grounded or frozen to the shore; the floating part,
if any, accommodates to changes of water level by flexure or multiple "en e c he lon " fractures, and is not free to move independently from the shore;
ii) ice continuous with ri ve r mouths where there is little tide or
cur-rent. Smooth, stationary floating ice of this type is a characteristic
feature of the western Arctic coast of North America in the winter. In
a sense it may be considered a special type of ice foot. It is grounded
along the shore and becomes progressively anchored over large areas as the water level in rivers and deltas drops during the winter. The ice occurs typically as an unbroken sheet, strong enough to resist displace-ment by storms and currents, often extending tens of miles without a
12
-iii) ice in lagoons or bays in areas of little tide. Where the coastline is fringed by an offshore bar, or where bays are dotted with islands or guarded by shallows across their rn ou th s , the ice on the shoreward
side of these obstructions rnay be protected fr orn the stresses of
cur-rents and irn pa ct of drifting ice, and survive as a lightly anchored unbroken and essentially stationary sheet.
b) Geographical distribution: Stationary sea ice is not extensive in
the eastern sub-arctic or arctic Canada or in the Canadian arctic
archi-pelago. The ice foot is ubiquitous, and Inany srn a.Il bays and fiords have
areas of essentially stationary ice; but although these are vital to the travel of the Es kim o , they fo r m an insignificant proportion of the sea ice of the Eastern Arctic, and there are few places where present or foreseeable industrial de ve Io prn e nt s in this region can be expected to be influenced by stationary ice.
On the other hand, stationary ice appears to be no rrn al.l y quite extensive in the western Canadian arctic and off the north slope of Alaska,
including rnany areas considered likely for industrial or engineering
activity in the fo rrn of shore-based or off-shore structures. A practical
e xarn pl e of how "fast" is the stationary ice in lagoons is the sight at SOIne Alaskan s e ttl ern ent s of native boats frozen in fOT the winter, and not hauled up on shore out of ha r m ' s way as in the other parts of the arctic. The tidal
range is sm al.l in rn o st of the western Canadian arctic, and it is probable
that change of sea level due to strong winds (the so-called wind set-up) is m o r e effective than the tides in l im it ing the areas and duration of shore-fast ice. Wind set-up can, of course, have an effect on sea ice or shore ice only during periods of partial ice cover.
Stationary sea ice appears to be very l irnite d in extent in Hudson's
Bay, being confined to river rn o uths , although there are SOIne areas of J'arn e s Bay where the ice appears to be essentially irnrno bi.l e for rnu c h of the winter.
2. Constrained Sea Ice
a) Typical locations: Constrained sea ice, according to the above
definition, will include rn o st areas of winter ice (one -year ice) adjacent to
shores during winter and early spring. It bounds m o st of our coasts where
there was open water the previous a uturnn , until "break-up". It includes
a lrn o st all the ice in bays and channels that freeze over solidly during the
winter, except for those that are so well anchored as to be "stationary ice".
It also includes certain l irnite d areas in the high arctic where, because of
unusual cir curn s tarice s of protection or lack of stresses, the ice, while free
to rn o ve , does not drift bodily away and so r ern a ins throughout rnu ch or all of the year.
Although some constrained ice will be grounded or frozen to the shore, in most cases the "constraints" that hold this ice more or less in place are mainly geometrical - the shape of the shoreline, the restriction by a grounded pressure ridge, or the presence off-shore of immobilized pack-ice - so that the sheet of ice is held in position like a piece in a jig-saw puzzle, and is protected from external forces that otherwise might break it up.
The spring "break-up" which is such a typical and dramatic event in the yearly cycle of northern Canada is usually observed from shore, and what is seen, at most northern settlements, is the breaking of the constraints of the constrained ice, so that the constrained ice joins the pack ice.
b) Geographical distribution: Normally, constrained ice is found in a
belt up to 30 miles wide fringing the Beaufort Sea. In most years in this
area, the constrained ice includes a certain amount of two-year and
multi-year ice. Although there must be many places where the transition between
stationary ice and constrained ice is gradual in this region, the traveller is often impressed by a sharp line, a pressure ridge or a repeatedly re - broken fracture, between the immobile ice and the ice that stays more or less in
place but moves a little. This sharp boundary is often attributed to
ground-ing of the ice; there are probably a variety of reasons that apply indifferent locations, and it is suggested that we know too little about this ice to make general conclusions about the reasons for its behaviour.
Constrained ice is found in most of the bays and many of the straits
of the Canadian arctic archipelago. For varying periods, it covers
exten-sive areas of Hudson Bay, James Bay, many fiords and inlets of Labrador and Newfoundland, and parts of the St. Lawrence estuary and the coasts of New Brunswick, Prince Edward Island and Nova Scotia.
Duration
Shore fast sea ice is, by its very nature, only an occurrence of sea ice, or a term to describe its position and degree of freedom of motion at
a given moment. In the definitions suggested above we have assumed that
the condition or occurrence must stay relatively constant for several days - at least through the passage of normal weather systems and a number of
tidal cycles. In practice, in most parts of Canada, shore fast sea ice lasts
from a few weeks (as in the Strait of Belle Isle area) to several years (as in parts of Hecla and Griper Bay); and if one extends the definition to ice shelves, it may persist for decades (Ward Hunt Ice Shelf).
Thickness
1. Stationary Sea Ice: The ice of the ice-foot, or stationary sea Ice
14 -Reasons for this are:
i) the ice is subjected to an essentially one-way shove - on shore - so
there is a progressive accumulation and compaction of material;
ii) over some or much of its area, the ice is grounded on permafrost
and thus lacks the source of heat or heat transfer that gives floating ice a comparatively higher temperature-depth gradient;
iii) stationary ice tends to accumulate a relati vely permanent snow cover, which while retarding the growth of ice early in the winter, serves to insulate and protect the ice later in the year, and which in some places tends to become incorporated with the sea ice to form a mixed deposit of considerable thickness.
2. Constrained Sea Ice: The sea ice thicknesses measured
system-atically in the Canadian arctic (as at the Joint Arctic Weather Stations) are
typically measured in one-year (winter) constrained ice. The thickness
developed under these conditions is a direct function of the num be r of degree-days, the depth of snow cover and the timing of snowfalls, modified by the effecti venes s of water circulation in transporting heat to or from
the lower surface of the ice. By and large the thickness of constrained
ice, although less than that of stationary ice, represents the maximum thickness likely to be developed in any floating ice in that particular region
in a given time interval. Over any area of a few square kilometers the
growth of freely-floating constrained ice shows the closest approach to the theoretical ice potential for the area as indicated by the seasonal or annual heat balance of the region.
It is interesting to note that the thickness of freely-floating
con-strained sea ice is remarkably uniform throughout the Canadian arctic and
from year to year. The main cause of variation is irregularity in the
timing of snowfall; - heavy snow early in the season delays the growth of
ice and results in thinner ice during most or all of the season. Figure 1
(from Bilello, 1960) and Figure 2 (from Lindsay, 1969) indicate typical
thicknesses of constrained shore fast ice in the Canadian arctic archipelago.
3. Variations in thickness: Local complications or variations in
thickness of shore fast sea ice may be introduced:
i) near tide cracks, where cold air can penetrate into the ice sheet,
or where tidal "pumping" can produce surges of heat from below;
ii) near points of grounding, where there can be a difference in
thermal conducti vity and heat capacity of the substrata, mechanical disturbance, or irregular horizontal distribution of heat due to impeded water circulation;
iii) by irregularities in snow deposition (see measurements by
Watmore and Cooper reported below). These irregularities may be
due to the normal pattern of scour and drift developed by wind on a flat surface, or they may be caused by the topographic irregularity of the shore, or of ice-pushed ridges, etc;
iv} by variations in water salinity, with consequent effects on the freezing characteristics of the water: this factor is probably of very minor importance in the Arctic but may be of significance in certain fiords in the sub-Arctic.
There appear to be few careful studies of the variations of thickness of shore fast ice from place to place, or of changes in thickness or profile during the season. Common experience suggests that a typical general cross-section profile during much of the winter, near shore and away from mouths of rivers, is similar to Figure 3; however, preliminary surveys by Lindsay and Seifert (personal communications) of the spatial and temporal variations in thickness profile and height above water level of the constrained ice in Mould Bay, Prince Patrick Island, show that the apparently simple situation may be deceptively complex.
Several studies have shown that for seasonal shore fast ice, both stationary and constrained, the total thickness of snow and sea ice, away from the shoreline itself, remains remarkably constant, despite considerable
variation in snow depth. In Kugmallit Bay, southeastern Beaufort Sea,
P. F. Cooper (Polar Continental Shelf Project, work in progress) has meas-ured the thickness of sea ice in snow- covered areas and in adjacent wind-scoured bare spots, with the following results (the sites reported are several miles apart but at each site the measurements are only a few feet apart):
Site
Snow depth, inches (average of several local measurements)
Thickness of sea ice, inches
Total thickness, snow plus ice, inches
1 12 0
37
5549
55 2 14 0 34 51 48 51 3 12 0 44 50 56 50 The Imperial Oil Company and Arctic Petroleum Operators Asso-ciation have kindly released data from surveys by Mr. T. Watmore in southern Beaufort Sea which show the same uniformity of total snow plusice thickness. In 24 miles of instrument-controlled survey on shore fast
ice, Mr. Watmore measured the thickness of snow plus ice at regular intervals (there was no local selection of the station with respect to snow
topography or depth).
a
ver this distance the sea ice thickness ranged from52 to 87 inches, but the total ice-pIus-snow thickness ranged only from 66
to 87 inches. For the 12 mile portion of this survey farthest from land, the
depth of snow cover varied from 0 to 17 inches, but the total thickness of ice-pIus-snow varied only six inches, remaining between 81 and 87 inches at all points measured.
The measurements by Cooper and Watmore tend to suggest that:
i} wind-blown bare spots and snowdrifts stay essentially stationary
16
-ii) the bulk thermal conductivity of wind-packed arctic snow is not
greatly different from that of the local sea ice; and
iii) the shore fast ice must have a subsurface profile approximately as irregular as the upper snow-surface profile, for by and large the upper surface of the ice remains nearly smooth and flat under snowdrifts. Generalized Characteristics and Behaviour
It has been pointed out that shore fast sea ice is not a unique kind
of ice; it is a mode of occurrence of significant but limited duration. The
physical and chemical characteristics of the ice are not distinctive. Shore
fast sea ice does appear, in bulk and on an average, to have some charac-teristics, in addition to the lack of movement and generally level upper surface, that differ from average pack ice of the same age in the same
area. These differences, if substantiated, may be important in considering
whether given measurements or obser vations are representati ve , or in
evaluating the action of shore fast ice on fixed or mobile structures. Some
such apparent characteristics, based on empirical observation and not on measurement, and thus subject to error or exaggeration, are listed below.
1. Stationary Sea Ice
i) tends to be fresher than the average first-year sea ice, due to the
admixture of snow and river water, and because it is typically formed in areas where the water is well stratified with a relatively fresh upper layer;
ii) tends to be stronger than adjacent other ice; - if an ice floe, even a
thick one, crashes against an ice foot, it is nearly always the floe that crumples;
iii) stationary ice tends to protect beaches and shores from erosion.
If an arctic shoreline, seen in summer, has a fringe of delicate sandbanks,
spits and deltas, or a shore of boulder-laden talus fans, there is a good
chance that stationary ice is typically present in the winter. On the other
hand, a shore with ice-pushed beach ridges, truncated deltas, beaches
pock-marked from kettle holes, or smooth scoured bedrock, is an indication of the typical absence of stationary ice, or even of any kind of shore fast ice;
iv) tends to last longer than other ice in the vicinity. The ice foot often
remains relatively intact, except at the mouths of streams, after other ice
has disappeared. Some traditional migration routes of the Eskimo to their
early summer hunting grounds depend on this phenomenon;
v) the area in which stationary ice is found often tends to be the area
locally richest in marine flora and invertebrates, largely because of the effect of characteristics (iii) and (iv) above.
2. Constrained Sea Ice
rare floes of one-year ice formed on the open ocean which may ha ve drifted near the shore in late winter;
ii) tends to be mechanically the weakest ice in the area, but being
structurally uniform and in large smooth plates, it can develop enormous momentum within its small range of movement;
iii) in regions where pack ice is present the year round, constrained sea ice tends to occupy the areas where there is the greatest extent of open water in the autumn. There are obvious exceptions to this generalization at the mouths of streams and in channels of strong current;
iv) the shore lead, bounding the constrained ice, is typically the first open water in the spring, with consequent atmospheric and biological consequences.
REFERENCES
Bilello, M. A., "Formation, Growth and Decay of Sea Ice in Canadian Arctic
Archipelago", U. S. Army Snow, Ice, and Permafrost Research
Establishment, Research Report HO.
65, 1960.
Dunbar, M., "A Glossary of Ice Terms (WMO Terminology)", Ice Seminar,
Canadian Institute of Mining and Metallurgy, Special Volume No.
10,
pp.
10 5 -
110, 1969.
Lindsay, D. G., "Ice Distribution in the Queen Elizabeth Islands ", Ice
Seminar, Canadian Institute of Mining and Metallurgy, Special
Volume No.
10,
pp.45-60, 1969.
Pilot of Arctic Canada, Canadian Hydrographic Service, Ottawa, Vol. I,
18 -275 ...--,...,...,...-..,...-.,...-...-...-,-..,...-.,...,...,....,...
...,....,...--r--,...,....,...
NNNLNNNNNNNNMセNLNNNNNLNNNNNLNNNNLNMNNNNNNNLNNNNNN 275 250 225 200 175 150 セ u 125 15 NOV 30 15 DEC I ! ! 15 JAN I ! ! 15 FEB 28 ! ! ! 15 MARCH 31 ! , ! 15 APRIL 250 225 200 175 150 125 100 75 50From "Formation, Growth and Decay of Sea Ice in the Canadian Arctic Archipelago" by M.A. Bilello
U.S. Army Snow Ice and Permafrost Research Establishment.
Research Report No. 65, July 1960
0 - 0 ICE THICKNESS ISACHSEN 1949-50
.-.
ICE THICKNESS RESOLUTE 1952-536.--6 SNOW DEPTH RESOLUTE 1952-53
.-.
SNOW DEPTH ISACHSEN 1949-50FIGURE 1
SEA ICE THICKNESS AND DEPTH OF SNOW COVER ON CONSTRAINED
ICE AT TYPICAL LOCATIONS NEAR SHORE IN THE CANADIAN ARCTIC
Churchill Thule
---40 Point Barrow 80
r-
Penny Strait 30 Lancaster Sound - - - - 70 Jones Sound ----20 Wellington Channel 60 セ 1 0
g
2
Eureka Sound 50 Pr. Regent In I.=!..セセss
V)セ
w u, 0f!!
{.: 40 Pr. ofwoャ・GPセ
...セセ
J: 0 U Norwegi2n_Bay Lgセ Z -10 Peel Sound 30 -20 Barrow Strait 20 M 'Clintock Chan. -30 the ParryセエAゥ・N⦅ 10 -40 0 M A M J J A S 0 N D J F M A FIGURE 2RELATIONSHIP OF THE MEAN AIR TEMPERATURE, THE MEAN ICE
THICKNESS AND THE AVERAGE LENGTH OF THE NAVIGATION
SEASON IN SELECTED AREAS AND IN SPECIFIC CHANNELS OF
CONSTRAINED ICE
SEA LEVEL
J r
-WATER
STATIONARY ICE (ICE FOOT)
POSITION OF ICE FOOT WALL DEPENDS ON PERMAFROST LINE
FIGURE 3
IDEALIZED CROSS-SECTION OF CONSTRAINED ICE AND STATIONARY
ICE ON SLOPING SHORELINE (NOT TO SCALE, THICKNESS AND SLOPES
EXAGGERATED)
Study Session No.1
Chairman, E. Pounder Recorder, G. P. William.s
The main objectives of this study session were: (a) to sum.m.arize the present state of knowledge concerning the movement
of ice covers for the design of structures, and (b) to make recornm.endations how the information still required can be obtained.
This record contains the prepared statements on the needs and
present state of knowledge on various aspects of ice movement. The
main recommendations and conclusions arising from the statements are contained in the conference recornm.endations placed together with the recommendations from the other two sessions.
22
-ICE DATA REQUIREMENTS OF THE OIL INDUSTRY
J. Hnatiuk
Arctic Petroleum Operators Association
The increasing demand for crude oil necessitates exploration in
Canada's more hostile frontier areas. These sedimentary basins include
the Beaufort Sea, Hudson Bay and the East Coast, which are covered with infested waters, and the Arctic Islands, which are surrounded by
ice-covered waters. The potential recoverable reserves under Canada's
Continental Shelf and ice affected areas have been estimated to be 80 billion
barrels of crude oil and 480 trillion cubic feet of raw gas. By comparison
the reserves proven to date, including those already produced, are 13
billion barrels of oil and 82 trillion cubic feet of gas, representing approxi-mately 10% of Canada's potential reserve.
Permits totalling nearly 700 million acres have been issued for
these ice affected areas, carrying with them work commitments of $2 billion if retained for the duration of their terms. The expenditure on these lands
will likely be several times this amount if oil is found in commercial
quantities. The cost of offshore drilling and production and crude oil
transportation may be five times that for comparable operations on land in
southern Canada. Reliable and comprehensive ice data are essential to
minimize these high expenditures and safeguard offshore facilities. Exploratory drilling in the Beaufort Sea will likely commence in 1973 from a barge during the ice-free period, or from artificial islands
year-round. On the East Coast two wells have been drilled in areas
menaced by icebergs and sea ice and two rigs are expected to commence
drilling next year, one off Labrador and one on the Grand Banks. One
partial well has been drilled in the Hudson Bay and twel ve on the Arctic
Islands. Following discovery of oil, multi-well bottom-founded platforms,
pipelines and possibly offshore loading terminals and tankers will be required.
The ice forces on offshore structures, drilling barges, pipelines, supply vessels and tankers, represent the most formidable problem in
exploiting ice-infested waters. A knowledge of ice conditions, distribution,
thickness, age, strength, movement, growth, decay and physical properties is required to establish the forces which facilities must withstand.
The oil industry has recently conducted field research including large-scale ice strength tests using steel legs frozen into sea ice, Brazil tests, movement of landfast ice, salinity and temperature profiles, ice
thickness and current measurements. Aerial reconnaissance of sea ice
and icebergs has been conducted occasionally in the Beaufort Sea and East Coast. One pressure ridge profile was obtained last winter.
Ice Data Requirements
Future projects in vol ving ice, currently under consideration, are studies of exploratory drilling systems, large-scale ice strength tests, pressure ridge studies, ocean floor scouring investigations, iceberg move-ments, and pack ice studies.
More specifically the Oil Industry's needs relating to Arctic sea ice include:
1) Extent of ice and annual variations
2) Movement patterns and velocity
3) Thickness
4) Pressure ridge properties
5) Breakup patterns
6) Freezeup and breakup dates
7) Floe sizes and characteristics
8) Strength, age, salinity and temperature
9) Ice island frequency, tracks and scouring.
On the East Coast the needs relating to icebergs include:
1) Population and distribution
2) Mass, dimensions and shape
3) Movement (paths and rates)
4) Sea-air environment (winds, currents, temperature, and presence
of sea ice)
East Coast requirements regarding sea ice are similar to tho se for the Arctic.
In Hudson Bay, more information is required regarding:
1) Breakup and freezeup times
2) Ice Thickness buildup
3) Ice Pack size, movement and properties
4) Knowledge of conditions in Hudson Strait, as these will affect
duration of operations in Hudson Bay.
In summary, the highest priority must be given to the collection of data which will help establish the magnitude of ice forces necessary for the design of effecti ve and safe facilities for exploration, production and
trans-portation of crude oil. Apart from icebergs and islands, the maximum
forces will likely be imposed by pressure ridges in multi-year floes. The
occurrence and distribution of ice and ice features will also be of prime interest during design and operating phases.
24
-SHORE FAST SEA ICE E. F. Roots
SUMMARY':'
The sea ice commonly found near the shore and not freely drifting is not a unique type of ice, but tends to exhibit distincti ve characteristics
that are related to its freedom of bodily movement. Sea ice in general
may be divided into three categories, with respect to its freedom of move-ment: (i) stationary ice, in which the movement in any direction is nor-mally less than a meter over periods of a few days; (ii) constrained ice, which may mo ve freely in a vertical direction, but whose bodily lateral movement is rarely more than a few meters over periods of days in normal weather; (iii) pack ice, whose translation under wind or current forces is such as to enable it to move bodily into or out of the area under
considera-tion over periods of a few days. Stationary ice is widely distributed but
negligible in extent in the eastern Canadian Arctic, but relatively extensive
in the western Arctic. Constrained ice is widespread and may be extensive,
but confined to certain geographical settings, many of which are important
to northern and wintertime industrial activities. Stationary ice tends to be
thicker than constrained ice, which in turn is usually thicker than one-year
pack ice. Local variations in thickness may be greater than the
decep-tively smooth surface might suggest, and there is evidence that the total ice-pIus-snow thickness may be more uniform than the ice thickness. Stationary ice tends to be fresher than average first-year ice, and to be mechanically stronger; it protects beaches, lasts longer than other ice,
and provides local comparatively fertile biological habitats. Constrained
ice has relatively high salinity, is mechanically weak, occupies areas of late-season open water, and is associated with the first open water in the spring.
THE MOVEMENT OF CONSTRAINED SEA ICE (Measurements by P. F. Cooper)
E. F. Roots
Introduction
There have been very few careful studies of the movement of
unbroken "land-fast" or constrained sea ice. Among the few is the
con-tinuing series of measurements carried out by Dr. P. F. Cooper on the one-year ice of Kugmallit Bay, at the east edge of the Mackenzie River
delta. Each year since the winter of 1965-66, as circumstances and other
duties have permitted, Dr. Cooper has observed the fracture pattern,
measured the movement of ice at intervals during the winter, and recorded meteorological or other influences that may have influenced the behaviour
of the ice. This slight but valuable study has produced unique information
about the behaviour of a large homogeneous sheet of constrained ice. The
following notes are based on information presented informally to the Polar Continental Shelf Project; Dr. Cooper is in the Arctic and has given per-mission for this information to be presented at this meeting, but has not seen these notes nor had an opportunity to verify their content or
conclusions.
The study of the constrained ice of Kugmallit Bay began as a private investigation resulting from Dr. Cooper's concern about why the shoreline at the village of Tuktoyaktuk should sometimes be marked by an open crack and sometimes by a persistent pressure ridge, apparently regardless of
the local wind direction. This difference of behaviour was of considerable
significance to the local inhabitants who used the ice for motorized or dog sled travel and sometimes found access from the shore very difficult. Part of the study was supported by the Department of Indian Affairs and Northern Development, and since 1968, by the Polar Continental Shelf Project.
Setting
Kugmallit Bay, at the mouth of the East Channel of the Mackenzie Ri ve r distributary system, is a nearly square indentation, about 15 x 15
kilometers, in the shoreline of Beaufort Sea. The sides of the bay are
relatively straight, and except for two islands in the western middle part
the outline of the bay is uncluttered and simple. The Mackenzie River
(East Channel) enters at the southwest corner through a maze of shallow
sandbars, and the town of Tuktoyaktuk is near the southeast corner. The
Bay is shallow, nowhere more than seven meters deep except in Tuktoyaktuk harbour; the bottom is silt, smoothly undulating, and the depth increases
gradually seaward with no marked bar, threshold, or channel. The Bay is
26
-sheet of uniform thickness, reaching about 130 centimeters by late May. There is usually a belt of rough ice, commonly thought to be grounded,
about ten kilometers off the mouth of the Bay. Several shoals and small
islands offshore, east and west of the mouth of the Bay, apparently protect it from the direct influences of the westward-drifting pack ice of the
Beaufort Sea. Kugmallit Bay is thus an excellent place to observe the
behaviour of a large simple sheet of sea ice in a protected setting, con-strained 'by land on three sides and open to the ocean, but possibly grounded
on the fourth. The only characteristic not representative of most
season-ally ice-covered seas in Arctic Canada is that the surface waters of
Kugmallit Bay are typically fresh (salinity less than 10 / 0 0 ) in late winter
and spring, with the salinity increasing to 300 / 0 0 or more in late summer
(Ince, 1962; Barber, 1968). This wide seasonal range is probably greater
than that of most Arctic surface waters.
Measurements and Description of Ice Behaviour
Dr. Cooper has mapped the main structural features developed in the ice of Kugmallit Bay, surveyed repeated traverses across the ice to determine the displacement of markers, and measured the opening and
offset of active cracks. The depth and distribution of snow cover has been
observed, and the temperature and salinity within the ice has been
deter-mined. All available meteorological data have been collected. The work
will be described elsewhere; the following is a brief summary of some pertinent observations and results.
1965-66 - Pressure ridges up to 4 meters high grew along the east side of
the bay between late February and late March. There were
apparently similar ridges (unmeasured) along the west side, but
none at the south end. By mid-April the ice on the bay side of
the east ridge was displaced 1. 3 meters south, parallel to the ridge, compared to its position in February, but there was no obvious displacement at the south end.
1966-67 - No pressure ridge developed; there were persistent cracks in the same position that ridges had been measured the previous year. The crack at the east side, near Tuktoyaktuk, opened 0.5 m during February, and continued to open, although less
rapidly, during the rest of the winter. A surveyed line 6.6
kilo-meters long showed a deflection of less than 3 kilo-meters, parallel
to the crack, in four months (January - May). The slippage
parallel to the crack appeared to be similar in magnitude, but in reverse direction, to that observed in 1965-66.
1967-68 - No pressure ridges developed; there were cracks in the same
location as in the previous year. The displacement along the
mid-May was less than the measurement error. An attempt was made to measure the ri ver current at the head, and the tidal current in the middle of the bay, to determine the order of magnitude of the stresses on the ice from these sources. As far as could be deter-mined, the river current was negligible; during the winter most of the water entering the bay from East Channel appears to flow through the sands beneath the river bed. The maximum tidal current found in mid-bay was less than O. 35 knots.
1968-69 - Pressure ridges developed in early January along the sides of the bay, as in 1966, and continued to grow all winter. A more thorough survey was carried out, between 20 January and 20 March. The observedmovement is summarized in Figure 1. The sheet of ice on the bay appears to have expanded laterally, with a net "take-up" of ice on the east side of at least 4 meters. There was no evidence of movement along the south shore. The stakes on a surveyed line across the mouth of the bay showed a consistent deflection toward the south, greatest (up to 7 meters) in the middle of the bay
(Figure 2). During this period there was no open crack in the 25 kilometers or so observed directly offshore from the mouth of the bay, but an offshore crack just to the east of the tip ofthe eastern-most pressure ridge opened 3.4 meters and a north-south crack developed near the west edge of the bay mouth, opening to 2 meters with the east (bayward) side sliding one meter south past the west in the two-month interval (Figure 1). Repeated travels across the ice and observations of persistent vehicle and dog-sled tracks,etc., confirmed the observation that except for the fractures mapped, the ice of Kugmallit Bay remained smooth and free from any other major fractures.
1969-70 - A crack developed along the east side of the bay in December; this changed to a small pressure ridge in January, which grew slowly until March, and then grew rapidly. Numerous measure-ments were taken, which are not immediately available to the
writer. They will be presented in detail elsewhere.
Tentative Conclusions
The interpretation of these measurements and the conclusions to be drawn concerning the behaviour of a large sheet of constrained ice will be
presented by P. F. Cooper in due course. Dr. Cooper has, however,
indi-cated that he feels the evidence is sufficient to indicate that:
i) Wind, tidal current, river current, or pres sure from drifting pack
ice do not contribute in any important way to the movement observed;
ii) Temperature changes within the ice sheet itself are sufficient to
pro-duce the expansion and contraction required to gi ve movements of the same order of magnitude , anddirections, as those observed, provided the ice sheet is periodically anchored by grounded hummocks off the mouth of the bay.
28
-A theoretical check of the movement to be expected under the changes of temperature encountered in the Arctic is difficult. The bulk coefficient of expansion of sea ice is imperfectly known; it varies with temperature and salinity, and changes from positi ve to negati ve within the range of natural
winter conditions at Kugmallit Bay. The temperature profile of Kugmallit
Bay has been determined in several places and at several times, but local variations, and differences in rate of change of profile due to changing air temperatures, are imperfectly known and depend upon local variations in snow cover, textural and salinity changes due to vagaries of the history of freeze-up and the timing of snowfalls. Thus, not only the trend of tempera-ture at the moment, but also the history of the ice sheet, and the history of the fluctuations of air temperature can determine whethe r at any gi ve n time the Ice is expanding or contracting.
If these deductions are correct, Dr. Cooper has suggested that:
i) The ice sheet in Kugmallit Bay is. in the years observed. an
essen-tially unfractured sheet, acting as a single plate;
ii) The horizontal movement of ice due to thermal processes alone under
the conditions 0 bserved may be as much as 10 meters in a bay 15 x 15 kilometer s .
From this it would appear that temperature changes in a normal winter in the Beaufort Sea area could cause a creep or displacement of up to 60 centimeters per kilometer of floating ice, and that this displacement could be cumulative over as much as fifteen kilometers. The implications for bottom-mounted structures, or for long structures such as pipelines mounted on the ice, are obvious.
Continuing Work
The study of the movement of the unbroken constrained ice is con-tinuing. Equipment has been devised to record the movement on a continuous
basis, and thus to observe whether the movement at fractures and 111 the
middle of the bay is gradual, or by a series of sudden incrernental rn o ve rn e nts .
A group from the Sea Ice Research Laboratory at Hokkaido University made initial measurements of the strain, and rate of change of strain, near a growing pressure ridge in 1970, and plan to continue theirwork. The Arl,til' Petroleum Operators As sociation, in cooperation with Dr. Cooper. IS unde r-taking a series of measurements of the strength of the one-year ice and its ability to transmit a continuous horizontal load.
REFERENCES
Barber, F. G., 1968. On the Water of Tuktoyaktuk Harbour. Canada, Dept. of Energy, Mines and Resources. Marine Sciences Br., Manuscript
Report Series, No.9, 32pp.
Iric e , S., 1962. Winter regime of a tidal inlet in the Arctic and the use of
air bubblers for the protection of wharf structures. Eighth
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