6.0 ICE CONTROL
It is sometimes desirable to control the ice regime
in order to eliminate or lessen the impacts of ice related problems. These
problems include ice jam flooding, the loss of hydroelectric power production,
the blockage of navigable channels, and damage to hydraulic structures.
Examples of various control measures are provided in this section.
6.1 ICE JAM MITIGATION
Ice jam mitigation includes those measures done
to eliminate or lessen the problems that may occur due to the formation
or release of an ice jam. Considerable understanding of the ice regime,
is required in the implementation of any of the following mitigation techniques.
6.1.1 Fixed Structures
Fixed structures are used to stabilize an ice cover
or prevent the downstream movement of broken ice. Stabilization of an
ice cover prevents its premature breakup thus increasing the chances of
the ice melting in place, while at the same time the retention of moving
ice floes reduces the supply of ice to potential downstream ice jam locations.
Fixed structures must be designed to withstand applied
forces. The forces include the impact of moving ice floes and the thermal
expansion of ice sheets. Physical hydraulic modelling may be required
during the design and installation of fixed structures to control ice
(Perham, 1984).
Groins and jetties can be used to constrict the channel
width (Cumming-Cockburn & Associates Limited, 1986) in order to promote
"bridging" of ice floes during freeze-up and to raise upstream
water levels thereby improving the upstream hydraulic conditions for ice
cover stability. Such structures also create a storage place for ice floes
at breakup, thus reducing the volume of ice moving downstream.
An ice control dam obstructs the passage of ice in order
to prevent ice jam formation downstream (Michel, 1971). Depending on the
available reservoir storage in relation to the spring flood magnitude,
it may be possible for the flood control structure to retain all contributing
upstream ice until it eventually melts. More commonly, however, the available
storage is only sufficient to delay the passage of weaker ice. Ice control
structures may, however, initiate ice jams near the upstream limit of
the headpond since the momentum of moving ice is arrested considerably
by stronger headpond ice and a significant reduction in the hydraulic
gradient. An ice control dam does provide high reliability with minimal
monitoring and operational requirements; however, other environmental
or economic realities may preclude its use.
Weirs are low-head overflow dams built across a
stream to raise upstream water levels to create suitable conditions for
formation of a stable ice cover or for the accumulation of frazil ice
or ice pans. Properly designed weirs can provide reliable control of ice
at suitable sites on small rivers during both the freeze-up and breakup
periods.
6.1.2 Ice Booms
Ice booms are installed across a watercourse to
control the movement of ice. They can be used to reduce the supply of
ice to downstream jamming sites by restricting the movement of upstream
ice and prolonging the period of ice discharge. Ice booms are most commonly
used, however, to stabilize or retain an upstream ice cover where velocities
are less than 0.6 m/s or Froude numbers are less than 0.08. Ice booms
usually consist of timbers shackled together by chains and held together
by cables anchored to the river bed. Pier-mounted booms can also be constructed.
For example, the Montreal ice control structure uses floating steel booms
or stop logs placed between fixed concrete piers to collect ice floes
and to help stabilize the ice cover earlier in winter than otherwise would
be the case.
6.1.3 Channel Modification
Research and observations (Calkins et al., 1976) reveal that ice jams tend to form at sites where there are: surface obstructions, river bends, river channel slope changes, constrictions, low velocity pools and shallow river sections.
Channel modifications can be carried out in order to reduce
the likelihood of ice jams. Such modifications include the removal of
constrictions and surface obstructions such as piles, old bridge piers,
and natural islands as well as the removal of sand and gravel bars. Channel
diversions can also be constructed to by-pass an obstruction to ice and
water flow. However, channel modifications should be evaluated on a site-specific
basis.
In New Brunswick, a permit must be obtained before
an alteration is made to the bed or banks of a watercourse. Projects are
reviewed in order to protect individuals or structures, as well as fish
habitat and the environment, which may be affected by the alteration.
6.1.4 Dusting
Dusting is the spreading of a thin layer of a dark
substance over an ice cover in order to weaken it because of increased
heat absorption. The dusting material could be sand, fly ash, or another
dark substance that is environmentally safe.
Dusting can be done for the purpose of preventing the
occurence of an ice jam at sites of known ice jamming or near highly developed
flood prone areas. Dusting is not effective on rough ice surfaces such
as ice jams since the heat absorption property of such surfaces is very
low.
The timing of dusting operations is very important.
If it is done too early, it could be covered by a late snowfall which
would reduce its effectiveness. If attempted too late, the ice could become
too weak and unsafe to allow the spreading of the dusting material, or
the ice could breakup and jam before the dusting material takes effect.
6.1.5 Blasting
Blasting may be used to break an ice cover into floes which can be transported by the water downstream or to weaken a solid ice cover prior to the arrival of upstream ice. The explosive charge is usually placed in the water underneath the ice. A much greater charge would be required if it is placed on or within the ice.
Properly placed explosives may be used for removing ice
jams by blasting the ice sheet holding the jam in place. The ideal time
to release a jam is just after it has formed. If the flow has dropped,
blasting the jam will be ineffective due to the lack of sufficient water
to carry the loosened ice downstream.
Blasting ice jams is rarely effective and is dangerous
to the blasting crew and neighbouring property. The placement of an explosive
charge is dangerous work that must only be performed by trained personnel.
Adequate safety, rescue and first aid measures should be in place before
the commencement of work.
6.1.6 Ice Cutting
Ice cutting refers to the cutting, sawing, or splitting
of an ice cover to cause it to melt faster or to break it into smaller
pieces that will be transported more readily when water levels rise. Ice
cutting can only be done if the ice cover is strong enough to support
cutting machinery and crews.
The City of Ottawa has a program of cutting and blasting
ice in the Rideau River to prevent the formation of ice jams. Strips of
about 15 - 20 m wide are cut where the ice has a tendency to remain solid.
Then crews drill and blast the ice at the downstream end prior to the
arrival of spring flows.
Ice cutting has also been done to prevent ice jams
in the Beaurivage River near Quebec City. Ice cutting has many advantages
over blasting techniques; it breaks more ice than blasting, it is only
one-third the cost, has no environmental impacts and is generally safer.
6.1.7 Mechanical Ice Removal
For small streams accessible from the banks, it
may be possible to remove ice using construction equipment, such as back
hoes and drag lines, before ice jams form. The efficient and safe removal
of ice depends, in part, upon the equipment operator whose judgement must
be exercised as ice removal operations progress.
6.2 ICE ADHESION
The adhesion of supercooled frazil ice to structures
(trash racks, hydraulic gates, etc.) is another major problem. Several
solutions to this problem exist.
One approach is to heat the surface to which ice adheres.
This can be accomplished by applying steam or warm water to the affected
area, or by incorporating electrical resistance or steam/hot water heating
within the structure. Usually the objective is to keep the surface temperature
above the freezing point so that ice will not adhere to it.
Another approach is to use construction materials
and coatings to which ice bonds weakly so that it can be easily removed.
One coating that has been effective is a plastic type liner made from
a copolymer solution of toulene and silicone oil (Ashton 1986).
6.3 WINTER NAVIGATION
There is little or no inland navigation on New Brunswick
rivers during the ice season, with the exception of ferry operations across
rivers and limited penetration of ocean-going vessels into river estuaries.
Icebreakers are commonly used to control river ice for the purpose of
navigation.
Air bubbler systems can also be effective in preventing
the growth of a solid ice cover at navigation locks, port facilities and
ferry crossings, but are impractical for long river reaches. In a bubbler
system, an air-driven warm water jet is directed upward to melt or suppress
the growth of river ice. The plume of warm flowing water created by the
rising air bubbles spreads as it rises and melts the ice cover (Ashton,
1986). The most important requirement for successful performance of an
air bubbler system is a supply of warmer water near the channel bed (U.S.
Department of the Army, 1982).
Homes surrounded by ice and rising water.
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updated September 30, 1996
Please direct
inquiries regarding the information presented
in this Manual and requests for additional copies
to
Environmental Planning and Sciences Branch
New Brunswick Department of Environment
P.O. Box 6000
Fredericton, N.B.
E3B 5H1
(506) 457-4844