Cold Regions Technical Digest

No. 91-1, March 1991

Frazil Ice Blockage of

Intake Trash Racks

Steven F. Daly

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
CRREL’s Cold Regions Technical Digests are

aimed at communicating essential technical

information in condensed form to researchers,

engineers, technicians, public officials and others. They convey up-to-date knowledge concerning technical problems unique to cold regions. Attention is paid to the degree of detail

necessary to meet the needs of the intended

audience. References to background information are included for the specialist.
COLD REGIONS TECHNICAL DIGEST No. 91-1, March 1991

USA Cold Regions Research and Engineering Laboratory

Hanover, New Hampshire 03755-1290

Frazil Ice Blockage of

Intake Trash Racks

Steven F. Daly

Introduction Operating water intakes in lakes and rivers in northern regions is

complicated by the presence of ice. One of the most difficult

problems is the accumulation of frazil ice on the intake trash rack,

which can completely block the trash rack and rapidly and unexpectedly shut down the intake facility. In extreme cases the

blockage can cause the intake trash rack to collapse. Blockages of

intake trash racks occur sporadically, most often at night, in cold

weather and always underwater. As a result, opportunities for

observation and measurement are limited. This has lent frazil

blockage an air of mystery that it does not deserve.

Contributing to the air of mystery is the lack of any universal cure

for the problem of frazil ice accumulation on intake trash racks.

While at many intakes there are no obvious techniques for preventing blockage, some successful techniques have been instituted at

specific locations. Unfortunately, given the many uses and designs

for intakes, and the wide range of hydraulic conditions at intakes, it

has proven to be quite difficult to generalize these successful

techniques.

Descriptions of frazil blockage can be found throughout the

literature of cold regions engineering. People became aware of the

problem as soon as they began to demand the uninterrupted use of

rivers and lakes throughout the winter. The complete blockage of

the water supply of the city of St. Petersburg, Russia, in 1914 by

frazil ice is one early example. The writings of Murphy (1909),

The author, a

research hydraulic

engineer, is a member

of CRREL’s Ice

Engineering Research

Branch.
2 COLD REGIONS TECHNICAL DIGEST NO. 91-1

Barnes (1928) and others provide very good overall descriptions of

frazil ice blockage.

This digest presents an overview of frazil ice blockage of intake

trash racks for intake operators and designers. It describes the frazil

ice blockage process, techniques (both successful and less so) for

coping with frazil blockage, and guidelines for operating under

frazil ice conditions. It synthesizes information from the literature,

laboratory experiments, and the experience of the author. Broader

and more general information on designing intakes for cold regions

is available elsewhere (Logan 1974, Ashton 1988) as is information

on frazil ice (Williams 1959, Michel 1971, Osterkamp 1978, Martin

1981, Daly 1984, Ashton 1986).

Frazil ice accumulates on intake trash racks under conditions that

make direct observation very difficult. As yet, there have been no

quantitative measurements made while frazil ice was accumulating

on an intake trash rack outside of a laboratory. Our understanding

of the accumulation process comes from photos of river intake trash

racks raised after becoming completely blocked, descriptions from

intake operators, and experiments conducted at CRREL.

The accumulation process starts when the water entering the

intake becomes supercooled, that is, when the water is below its

freezing temperature. The degree of supercooling may be very

small (less than 0.01°C) and nearly impossible to detect without

laboratory-grade equipment. Supercooling requires a large heat

loss rate from the river that is always associated with low air

temperatures (about –6°C or lower), open water and most often,

clear nights. The association of frazil production and open water is

so strong that it can be said quite confidently that trash racks will be

blocked by frazil only before a stable ice cover has formed, after the

ice cover has broken up, or where a stable ice cover cannot form.

The water will supercool first at the surface, and in rivers the

highly turbulent flow will mix the supercooled water quickly

through the entire depth. Carried along with the supercooled water

will be small crystals of ice—frazil ice—which, because of their

small size, will have little effective buoyancy and will be easily

carried to the bottom of the river. The first few crystals enter at the

water’s surface, and through a process known as secondary nucleation, the number of crystals quickly increases. The concentration

of crystals in rivers has not been measured, but estimates are in the

range of 1 million per cubic meter (approximately 30,000 per cubic

foot). Because they are in supercooled water, the crystals will be

growing in size and will stick to any object they contact—including

The blockage

process

Rivers
FRAZIL ICE BLOCKAGE OF INTAKE TRASH RACKS 3

Trash rack bars

(representative

horizontal section)

Flow

direction

b. Frazil deposition on leading edge

c. Frazil bridging and blockage of flow

d. Extrusion of frazil between

bars due to head difference

across trash rack

1. Pattern of frazil ice

accumulation on intake

trash rack bars.

Deposited frazil

e. Massive extrusion beyond

bars due to head difference

across trash rack and continued deposit of frazil

trash racks—as long as these objects are at a

temperature below freezing. Given the effective heat transfer rates provided by flowing

water, any object in the water that is not heated

will quickly be at the temperature of the supercooled water and will accumulate frazil.

The frazil will collect first on the upstream face of the intake trash

bars, regardless of the shape of the bars—pointed, rectangular,

rounded, streamlined, etc.—or the material—steel, aluminum, wood,

plastic, etc. The accumulation will grow into the flow by continual

deposition, extending upstream and increasing in width until the

space between adjacent bars is “bridged” (Fig. 1). At this point the

trash rack will be effectively blocked. Almost all of the ice will have

accumulated on the upstream side of the trash rack (Fig. 2). In

laboratory experiments, and in some field reports, the bridging

process is observed to occur first at the water line of the trash rack

and then extend downward.

Calculations indicate that the quantity of ice deposited on the

trash rack will far exceed the quantity of ice that is grown by the

transfer of latent heat through the trash rack bars to the cold air or

to the supercooled water flowing through the rack.

As the frazil accumulates on the trash rack, the head losses will

increase. That is, a greater and greater differential water level will

be required across the rack to drive the same flow rate through the

trash rack. Because trash racks are designed to cause very little head

loss under normal flow conditions, a considerable amount of frazil

can accumulate before this head loss becomes noticeable. Head

losses measured in laboratory experiments are shown in Figure 3.

Here the nondimensional head loss (the measured head loss divided

by the head loss of the rack without ice) is plotted against

nondimensional time (time divided by the time at which total

blockage occurs). The head loss is relatively small for a substantial

period of time but increases quite rapidly near the time of total

blockage. As the head loss mounts, the differential pressure across

the rack will tend to push the accumulated frazil through the bars

and may even extrude the accumulated frazil completely through

the trash rack (Fig. 1 and 4). Once the frazil has been extruded

between the bars, it can adhere to a much larger area of the bars. As

a result the accumulated frazil can withstand quite high differential

pressures, equal to many feet of water.

The process of blockage of intake trash racks in lakes has never

been directly observed because of the racks’ remote and often deep

locations. As with river intakes, blockages of lake intake trash racks

a. Initial frazil ice adhesion

Lakes
4 COLD REGIONS TECHNICAL DIGEST NO. 91-1

2. Accumulated frazil

on trash racks raised

from the water. (Photo

courtesy of Pennsylvania Power & Light

Company.)

25

20

15

10

5

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Nondimensional Time

Nondimensional Head Loss

3. Head loss through a

trash rack during frazil

ice accumulation in

laboratory tests.

are associated with open water, low temperatures and clear nights.

They are often also associated with strong winds, which increase the

rate of heat loss at the water surface and may provide the turbulence

that can mix the supercooled water to the depth of the intake. The

intake flow can also entrain the supercooled water if it is high

enough. The depth at which a lake intake will be safe from frazil

blockage cannot be estimated simply, and it probably depends on
FRAZIL ICE BLOCKAGE OF INTAKE TRASH RACKS 5

4. Back side of an intake trash rack raised

from the water, showing massive extrusions

of frazil ice through the

rack bars. (Photo courtesy of Pennsylvania

Power & Light Company.)

many factors, such as the fetch length, bottom topography, intake

depth and intake flow rate. Blockages have been reported at intakes

20 m deep and much deeper.

The blockage of lake intake trash racks probably occurs in much

the same way as river intakes. However, inspections of blocked lake

intakes by divers suggest that the individual crystal size is larger at

lake intakes than at river intakes. The reasons for this are not entirely

clear, but it may result from a lower concentration of crystals in

suspension because of the lower levels of turbulence in lakes. This

means that the accumulated crystals may actually grow due to the

release of latent heat allowed by the constant intake of supercooled

water. This is in contrast to the river intakes, where the deposition

of frazil crystals is by far the most significant source of ice.

Lake intakes are most often connected with a wet well from

which water is pumped. As the intake trash rack becomes blocked,

the wet well may be drawn down, and the differential pressure

across the trash rack will increase. This differential pressure has

been known to cause intakes to collapse. Close monitoring of the

water level in the wet well may help in detecting blockage of the

trash rack by frazil ice.

Many techniques have been tried to keep intake trash racks free

from frazil ice. In the next few pages are listed some of the basic

techniques that have been tried (not always successfully) or are

frequently suggested.

Coping with

frazil blockage
6 COLD REGIONS TECHNICAL DIGEST NO. 91-1

An intact, stable ice cover will always prevent the production of

the frazil ice by “insulating” the water surface and preventing large

heat loss rates responsible for supercooled water. If an ice cover can

be successfully created and kept in place over a sufficient area of the

river or lake in which the intake is located, frazil blockage probably

can be completely avoided. There is no ready “rule of thumb” as to

what is a sufficient area of the water surface that must be covered

by ice. Supercooled water traveling beneath an ice cover will

gradually be warmed to the freezing temperature by the latent heat

released by the growing frazil crystals and the surface ice cover and

by viscous dissipation in the flow. While Ashton (1988) suggested

that a cover length equal to 10 minutes of travel time may be

adequate to warm the water sufficiently, supercooled water has

been measured several kilometers downstream from the head of an

ice cover and with travel times as long as 11/2 to 3 hours on large

rivers. The actual length required will vary widely, depending on

the hydraulic conditions in the river. The techniques for creating

and maintaining stable ice covers are described well elsewhere

(U.S. Army Corps of Engineers 1982, Ashton 1986).

The use of heat can be a very successful way to prevent frazil

from adhering to intake trash racks. It is not effective or efficient to

use heat to melt ice once it has accumulated on the trash rack.

Rather, the heat should be used to prevent adhesion to the trash rack

by warming the incoming water so that it is no longer supercooled

or by warming the trash rack bars so that they are above the freezing

temperature. The most attractive source of heat is the warm water

that is often a byproduct at facilities that use intakes, such as the

cooling water at power generation plants. The warm water is often

dumped back into the river downstream of the intake and is

essentially thrown away! By redirecting a portion of this waste heat

to the intake, frazil blockage can be avoided. At small intakes,

ground water may also be an adequate heat source. Enough warm

water should be redirected to the intake to raise the entire intake

flow temperature 0.1–0.2°C. If it is assumed that all the warm water

will be drawn into the intake, the amount of water QD required is

QD = QT

TR

TD

where QT = total intake flow rate

TR = temperature rise above freezing required

TD = temperature above freezing of the warm water to be

delivered to the intake.

Suppressing

frazil production

Using heat
FRAZIL ICE BLOCKAGE OF INTAKE TRASH RACKS

The warm water should be introduced immediately upstream of the

intake trash rack, and it should be well mixed in the intake flow.

Mixing the warm water with the intake flow can require the use of

“spragers” or diffusers, but complicated designs for these devices

are not usually warranted.

Another means of using heat to prevent frazil blockage is to heat

the trash rack bars directly. If the bar temperatures can be kept above

the freezing temperature, frazil will not adhere. The amount of heat

supplied must be sufficient to offset the heat transfer to the flowing,

supercooled water. Standard heat transfer relationships that relate

the heat transfer to the bar shape and water velocity are available to

guide designers (see, for example, Logan 1974); the degree of safety

provided in the design is the temperature difference maintained

between the trash rack bars and the supercooled water temperature.

As a minimum the bars should be maintained at 0.1°C, but it is

advisable that bar designs include the capability to maintain the bars

at 1.0–1.5°C during high frazil concentration events.

There is no simple means of supplying heat to the trash rack bars.

Hollow bars, through which steam or warm water is pumped, have

been used successfully. However, these systems seem to suffer

from many operational problems. Incorporating resistance heaters

in the trash rack bars has also been used successfully. In Sweden and

other locations, energy is delivered to the trash rack bars by direct

electrical connection.This often requires that the trash racks be

redesigned, and safety is then a consideration.

Constructing heated enclosures over intakes is sometimes suggested, especially where large portions of the trash rack are exposed

to air. However, heated enclosures by themselves cannot prevent

frazil ice accumulation. The heat gain to the trash rack from the

warm air developed within the enclosure will be balanced by the

heat loss to the flowing water at a very small depth. The depth will

depend largely on the bar size and the flow velocity but will

probably be no more than several bar diameters below the surface.

Below this depth, frazil will accumulate as described above. While

heated enclosures will not prevent frazil accumulation, people

working near the rack (raking, for instance) will appreciate the

warmth.

Probably the most widespread method of dealing with frazil

accumulation of intake trash racks is to use rakes to remove the ice.

As seen earlier, before the development of large differential pressures across a trash rack, the accumulation of frazil ice will be

largely on the upstream side of the racks. During this time it is

relatively easy to remove by raking. Manual raking is undoubtedly

Mechanical

removal

7
COLD REGIONS TECHNICAL DIGEST NO. 91-1

the single most widely used technique for dealing with frazil—“a

strong back in a warm coat” can effectively clear large areas of racks

covered with frazil ice. The cost of labor, the cold and wet working

conditions, and the late night hours, however, combine to discourage this approach.

An alternative to manual raking is to use mechanical rakes. There

are a number of manufacturers of mechanical rakes with a corresponding range of sophistication, from relatively simple, manually

operated power rakes, to rakes using the best of robotics. Problems

that can arise in operating mechanical rakes in very cold weather

should not be ignored. The rake mechanisms inevitably move from

completely submerged conditions to the cold air, leading to the

formation of hard ice and resultant mechanical problems. Therefore, the usefulness of a mechanical rake for removing frazil in cold

weather must be carefully evaluated.

The use of high-flow air bubblers and water jets is sometimes

suggested as a means of removing frazil accumulations on intake

trash racks. At present the use of high-flow air bubblers remains

strictly experimental and should be approached as such. Water jets

have been used successfully to remove frazil ice accumulations

inside large conduits and may be effective for use with trash racks.

Back flushing is a technique to reverse the differential pressure

across the trash rack. During back flushing, a higher pressure is

created on the back side of the racks, and the flow through the rack

is reversed. If the accumulation of frazil is largely on the upstream

side of the rack (as in Fig. 1c), then it is likely that back flushing will

be successful. This means that back flushing must be done as soon

as possible during the accumulation process, before large differential pressures occur and the accumulated ice is extruded between the

bars. If the accumulation has been extruded between the trash rack

bars (as in Fig. 1d and 1c), then it is much less likely that the back

flushing process will be successful.

Back flushing is accomplished by any means that the intake

operators may have for increasing the pressure on the back side of

the trash rack. Most often, back flushing is tried at intakes connected

to wet wells, where water is allowed to flow from elevated tanks or

stand pipes or is pumped into the wet well. Back flushing should be

repeated frequently as long as frazil ice conditions persist.

Almost all trash racks are made of metal—the vast majority of

steel, a very few of aluminum. Steel in water quickly rusts, and the

adhesion strength between ice and rusted steel is extremely high.

Trash rack coatings will reduce the ice adhesion strength and can

8

Coatings and

alternative trash

rack materials

Back flushing
FRAZIL ICE BLOCKAGE OF INTAKE TRASH RACKS

extend the time to total blockage, but coatings by themselves will

not prevent frazil blockage. Examples of coatings are two-part

epoxies or heavy-duty marine-type paint. Coatings must be applied

only after careful preparation of the racks, which at a minimum

involves a thorough cleaning, usually by sand blasting. While

coatings cannot prevent blockage, they will increase the ease of

removal of frazil ice (such as by raking, for example).

Alternative trash rack materials—that is, nonmetals such as

plastics, fiberglass, graphite or other material—must be carefully

selected to provide the strength and durability required. Materials

such as these will, like coatings, have a lower ice adhesion strength

than steel, but “no material is known to which ice will not adhere”

(Ashton 1986). As a result, using alternative materials for trash rack

construction is not in itself a guarantee against ice blockage, but

again, like coatings, they will allow frazil ice to be removed more

easily.

Vibration of trash racks has been shown to be effective in

removing frazil ice accumulations in laboratory tests (Mussalli et al.

1987). In these tests a minimum vibration acceleration of 15 g was

needed to shed accumulated frazil ice from the model trash racks.

Under prototype conditions, enough energy must be supplied to

remove ice but not to shake the trash rack apart. The natural

flexibility of trash racks may make effective vibration difficult, and

it also may make it difficult to vibrate an entire rack uniformly from

one application point. Electric- or air-powered vibrators are used

for many applications and are commercially available, but they will

probably need to be modified for underwater use. At present the use

of vibrators must be considered an experimental technique.

Dynamite is sometimes used to remove accumulations of frazil

from intake trash racks under extreme conditions. Blasting is

effective, probably because of the vibrations set up in the trash rack

combined with the pressure wave of the explosion. The first

problem with blasting is to determine the correct size of the charge.

It must be large enough to remove the ice but not so large as to

damage the rack. Additional problems arise from environmental

concerns, safety considerations and the complaints of nearby residents. Because of these problems the use of dynamite cannot be

recommended.

Removing the trash racks is an obvious solution and one that is

sometimes used. The trash racks should be removed only after

carefully weighing the problems caused by the frazil accumulation

9

Vibration

Blasting

Removal of

trash racks
COLD REGIONS TECHNICAL DIGEST NO. 91-1

against the damage that may be caused by not having the trash racks

in place. This type of evaluation can only be done after many years

of experience at a particular site and is not a step to be taken lightly.

Maintaining adequate intake flow under frazil ice conditions can

be very difficult. Given below are guidelines for intake operators

who must maintain an uninterrupted supply of water and who face

frazil ice conditions and frazil accumulation on the intake trash

racks.

1. Monitor the water temperature, air temperature and surface ice

cover extent to develop associations between these conditions and

the formation of frazil ice. Almost all intake operators monitor these

conditions as a routine matter and learn to anticipate the conditions

under which frazil ice occurs. When the water temperature is close

to freezing and no surface ice cover exists, forecasts of low air

temperatures and clear nights should alert operators to the possibility of frazil ice.

2. Continuously monitor the degree of blockage on the intake

trash rack. This can be done by measuring the differential pressure

or water level across the trash rack or by visual inspection. At

intakes that pump from a wet well, the water level difference

between the wet well and the water level in the body of water in

which the intake is located should be monitored. The degree of

blockage will be indicated by the extent that the water level

differential exceeds that found under normal operating conditions

(caused by head losses in the intake structure and the intake piping

systems). Continuous readouts of the instruments used to monitor

these conditions should be available to the facility operators, and

alarm conditions should be established. The chief benefit of this

monitoring is to allow the operators to determine quickly and

unambiguously the degree to which the trash racks are blocked, so

that the appropriate measures can be taken.

3. Frazil ice accumulation can be minimized to some extent,

although not eliminated altogether, through proper design of the

intake trash racks. By having the maximum space between the trash

rack bars and by using the thinnest bars possible with the minimum

of supports consistent with strength and vibration considerations,

the length of time until the intake is totally blocked can be extended.

The ability of the rack to withstand the differential pressure under

blocked conditions should also be carefully considered. A conservative approach is to design the rack to withstand the entire head of

water possible on the upstream side of the rack. Some river intake

trash racks are designed to this standard. This standard may not be

practical for all intakes. The design should then reflect the maxi10

Operating

under frazil

conditions
FRAZIL ICE BLOCKAGE OF INTAKE TRASH RACKS

mum differential pressure the intake trash rack may experience

given the monitoring program established as described above.

4. Develop plans for an orderly shutdown of the facility should

the intake trash rack become blocked by frazil.

5. Take an active approach to dealing with frazil ice by using one

of the techniques described in the previous section. If possible,

actively manage the ice cover in the area of the intake to establish

a stable ice cover as soon as possible in the winter season and to

maintain it as long as possible. If there is a source of heat available,

use it! Coat a section of your trash rack and then observe the

results—is the ability to rake improved? If coatings are an improvement, consider coating the entire trash rack at your site.

6. Keep detailed logs of the water temperature, weather conditions and ice conditions throughout the winter to determine the

specific conditions at which frazil ice blockage occurs. Determine

simple alarm conditions based on these observations and make sure

that the operating personnel are aware of them. Operating personnel

have often developed keen insights into frazil problems from hard

experience. Gather information from these people and make sure

that it is not lost due to retirement or personnel changes.

Frazil ice blockage is a difficult and serious problem. By taking

active measures, by compiling information about when frazil blockage occurs, and by anticipating its occurrence, the problems caused

by frazil blockage can be minimized.

This technical digest was reviewed by Fred Parkinson, LaSalle

Hydraulics Laboratory, Montreal, Quebec, Canada, and Dr. George

Ashton, CRREL, Hanover, N.H.

Ashton, G.D., Ed. (1986) River and Lake Ice Engineering. Littleton,

Colorado: Water Resources Publications.

Ashton, G.D. (1988) Intake design for ice conditions. In Developments in Hydraulic Engineering-5 (P. Novak, Ed). New York:

Elsevier Applied Science.

Barnes, H.T. (1928) Ice Engineering. Montreal: Renouf Publishing Company.

Daly, S.F. (1984) Frazil ice dynamics. USA Cold Regions Research

and Engineering Laboratory, Monograph 84-1.

Daly, S.F. (1987) Modeling trash rack freezeup by frazil ice. In

Proceedings of the First International Symposium on Cold Regions

Heat Transfer, 4–7 June, Edmonton, Alberta, p. 101–106.

Logan, T.H. (1974) Prevention of frazil ice clogging of water

intakes by application of heat. U.S. Bureau of Reclamation, Denver,

Colorado, Report REC-ERC-74-15.

11

Acknowledgments

References
COLD REGIONS TECHNICAL DIGEST NO. 91-1

Martin, S. (1981) Frazil ice in rivers and oceans. Annual Review of

Fluid Mechanics, 13: 379–397.

Michel, B. (1971) Winter regime of rivers and lakes. USA Cold

Regions Research and Engineering Laboratory, Cold Regions

Science and Engineering Monograph III-B1a. AD724121.

Murphy, J. (1909) The ice question—as it effects Canadian water

power—with special reference to frazil and anchor ice. Transactions of the Royal Society of Canada, Section III, p. 143–177.

Mussalli, Y.G., L.S. Gordon and S.F. Daly (1987) Frazil ice

control using electromechanical vibrators and ice-resistant coatings. In Proceedings of ASCE Water Power ‘87, 19–21 August,

Portland, Oregon, American Society of Civil Engineers.

Osterkamp, T.E. (1978) Frazil ice formation: A review. Journal of

the Hydraulic Division, ASCE, 104(H49): 1239–1255.

Parkinson, F.E. (1986) Frazil ice problems in intakes at Montreal.

In Proceedings of 4th International Conference on Cold Regions

Engineering, Anchorage, Alaska, American Society of Civil Engineers, p. 609–618.

U.S. Army Corps of Engineers (1982) Ice engineering manual.

EM 1110-2-1612. U.S. Army Corps of Engineers, Washington,

D.C.

Williams, G.P. (1959) Frazil ice: A review of its properties with a

selected bibliography. The Engineering Journal, 42(11): 55–60.

12