A frequent inconvenience in the use of water power in cold climates is
that peculiar form of ice called anchor or ground ice. It adheres to
stones, gravel, wood, and other substances forming the beds of streams,
the channels of conduits, and orifices through which water is drawn,
sometimes raising the level of water courses many feet by its
accumulation on the bed, and entirely closing small orifices through
which water is drawn for industrial purposes. I have been for many years
in a position to observe its effects and the conditions under which it
is formed.
The essential conditions are, that the temperature of the water is at
its freezing point, and that of the air below that point; the surface of
the water must be exposed to the air, and there must be a current in the
water.
The ice is formed in small needles on the surface, which would remain
there and form a sheet if the surface was not too much agitated, except
for a current or movement in the body of water sufficient to maintain
it in a constant state of intermixture. Even when flowing in a regular
channel there is a continued interchange of position of the different
parts of a stream; the retardation of the bed causes variations in the
velocity, which produce whirls and eddies and a general instability in
the movement of the water in different parts of the section--the result
being that the water at the bottom soon finds its way to the surface,
and the reverse. I found by experiments on straight canals in earth and
masonry that colored water discharged at the bottom reached the surface
at distances varying from ten to thirty times the depth.[1] In natural
water courses, in which the beds are always more or less irregular, the
disturbance would be much greater. The result is that the water at the
surface of a running stream does not remain there, and when it leaves
the surface it carries with it the needles of ice, the specific gravity
of which differs but little from that of the water, which, combined with
their small size, allows them to be carried by the currents of water in
any direction. The converse effect takes place in muddy streams. The mud
is apparently held in suspension, but is only prevented from subsiding
by the constant intermixture of the different parts of the stream; when
the current ceases the mud sinks to the bottom, the earthy particles
composing it, being heavier than water, would sink in still water in
times inversely proportional to their size and specific gravity. This,
I think, is a satisfactory explanation of the manner in which the ice
formed at the surface finds its way to the bottom; its adherence to the
bottom, I think, is explained by the phenomenon of _regelation_, first
observed by Faraday; he found that when the wetted surfaces of two
pieces of ice were pressed together they froze together, and that this
took place under water even when above the freezing point. Professor
James D. Forbes found that the same thing occurred by mere contact
without pressure, and that ice would become attached to other substances
in a similar manner. Regelation was observed by these philosophers in
carefully arranged experiments with prepared surfaces fitting together
accurately, and kept in contact sufficiently long to allow the freezing
together to take place. In nature these favorable conditions would
seldom occur in the masses of ice commonly observed, but we must admit,
on the evidence of the recorded experiments, that, under particular
circumstances, pieces of ice will freeze together or adhere to other
substances in situations where there can be no abstraction of heat.
[Footnote 1: Paper clx., in the Transactions of the Society, 1878, vol.
vii., pages 109-168.]
When a piece of ice of considerable size comes in contact under water
with ice or other substance, it would usually touch in an area very
small in proportion to its mass, and other forces acting upon it,
and tending to move it, would usually exceed the freezing force, and
regelation would not take place. In the minute needles formed at the
surface of the water the tendency to adhere would be much the same as in
larger masses touching at points only, while the external forces acting
upon them would be extremely small in proportion, and regelation would
often occur, and of the immense number of the needles of ice formed at
the surface enough would adhere to produce the effect which we observe
and call anchor ice. The adherence of the ice to the bed of the stream
or other objects is always downstream from the place where they are
formed; in large streams it is frequently many miles below; a large
part of them do not become fixed, but as they come in contact with each
other, regelate and form spongy masses, often of considerable size,
which drift along with the current, and are often troublesome
impediments to the use of water power.
Water powers supplied directly from ponds or rivers, or canals frozen
over for along distance immediately above the places from which the
water is drawn, are not usually troubled with anchor ice, which, as I
have stated, requires open water, upstream, for its formation.
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