Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone 703-385-6000

fax 703-385-6007

DATE: April 10, 2009

TO: Paul Shriner and Jan Matuszko, EPA

FROM: Tim Havey and Kelly Meadows, Tetra Tech

SUBJECT: Icing at Cooling

In cold climates (where prolonged periods of sub

cooling water intake structures

accumulation on and around critical intake components. 

operations in multiple ways; s

screens and equipment, block

blocking water flow and damaging the screens or pumps

become lodged in the open space of the intake screen

also form below the surface and collect on submerged portions of the intake structure, 

rapidly restricting water flow. 

This memo describes the various types of icing that are typically encountered at CWISs 

as well as the design features and operational measures that facilities use to minimize the 

effect of icing on maintaining normal operations.

to address icing at facilities EPA visited in preparation for the 

Types of Ice

Depending on the meteorological and hydrological conditions at the CWIS, several 

different types of ice may form. Each type behave

on CWIS operations. 

MEMORANDUM

10306 Eaton Place, Suite 340

April 10, 2009

Paul Shriner and Jan Matuszko, EPA

Tim Havey and Kelly Meadows, Tetra Tech

Icing at Cooling Water Intake Structures

where prolonged periods of sub-zero air temperatures are common)

cooling water intake structures (CWISs) must be designed and maintained to handle ice 

accumulation on and around critical intake components. Ice can threaten CWIS 

operations in multiple ways; surface ice may develop in thick sheets and damage intake 

locks of ice may drift and collect in front of an intake screen, 

blocking water flow and damaging the screens or pumps, or small fragments of ice may

become lodged in the open space of the intake screen. Under the right conditions, ice can 

form below the surface and collect on submerged portions of the intake structure, 

rapidly restricting water flow. 

the various types of icing that are typically encountered at CWISs 

as well as the design features and operational measures that facilities use to minimize the 

effect of icing on maintaining normal operations. Lastly, it describes the measures used 

ess icing at facilities EPA visited in preparation for the Phase II rulemaking.

Depending on the meteorological and hydrological conditions at the CWIS, several 

different types of ice may form. Each type behaves differently and has a differ

1

common), 

must be designed and maintained to handle ice 

can threaten CWIS 

urface ice may develop in thick sheets and damage intake 

ice may drift and collect in front of an intake screen, 

l fragments of ice may

Under the right conditions, ice can 

form below the surface and collect on submerged portions of the intake structure, 

the various types of icing that are typically encountered at CWISs 

as well as the design features and operational measures that facilities use to minimize the 

Lastly, it describes the measures used 

Phase II rulemaking.

Depending on the meteorological and hydrological conditions at the CWIS, several 

a different impact 
2

Ice Formation

Ice begins to form when water temperature is reduced to 32°F and the water continues to 

lose heat to the atmosphere. For pure water, supercooling1to temperatures below 

freezing is necessary to start ice formation, but with natural water, the required 

supercooling is much less. 

There are two general types of ice formation: static ice and dynamic ice.

Static ice forms where natural water loses heat to the atmosphere and there is little or no 

turbulence, supercooled water and existing surface ice crystals are not carried to a 

significant depth, and the result is the formation of a layer of surface ice (as opposed to

frazil ice, described below) above the liquid water.

Dynamic ice formation occurs in turbulent water such as areas of greatest flow in rivers 

and lakes mixed by wind and wave action. Under turbulent conditions, uniform cooling 

of a large fraction of the waterbody can occur. If the initial water temperature is slightly 

above freezing and cooling is rapid, a small amount of supercooling occurs, and small, 

disk-shaped crystals form and are distributed throughout the turbulent mass. These small 

crystals are the initial stage of frazil ice production. Other ice forms can develop from 

this initial ice production in sizable quantities. 

Surface Ice

Surface ice is simply blocks of ice floating in the source waterbody. In some locations,

surface ice and ice floes can create a structural hazard to exposed intakes. On lakes, an 

accumulation of wind-driven ice floes near a shoreline intake can produce a deep, nearly 

solid layer of ice restricting or completely blocking intake ports. Under such conditions, 

reliable intake operation becomes extremely difficult.

Ice jams can cause partial or complete blockage of river intakes. Jams below a river 

intake can also cause extremely high river stages, and an upstream jam can produce low 

water levels at the intake location, reducing its capacity.

Surface ice formation reduces heat loss from the water and usually prevents frazil ice 

formation.

Frazil Ice

Frazil ice is composed of small, free-flowing ice crystals that form in supercooled water 

and, without sufficient buoyancy, can be carried throughout the water column by

turbulence. Frazil ice formed under dynamic conditions adversely affects hydraulic 

characteristics of intakes.2

 

1

Supercooled water has a temperature below the freezing point but remains in a liquid state.

2 Note that frazil is not the same as anchor ice, a generic term used to describe ice crystals that form 

underwater and adhere to each other and a submerged object, such as rocks or intake structures.
3

Two kinds of frazil ice have been identified: active and inactive. Freshly formed frazil 

crystals dispersed in supercooled water and growing in size are in an active state. When 

in this condition, they will readily adhere to underwater objects such as intake screens or 

rocks. Frazil ice production and adhesiveness are associated with the degree of 

supercooling, which is related to the cooling rate of the water mass. Frazil ice particles 

remain in an active, adhesive state for only a short time after their formation. With the 

reduction of supercooling and the return of water to 32°F, frazil crystals stop growing and 

change to an inactive, or passive, state. Passive frazil has no adhesive properties and is 

therefore less troublesome. 

Frazil ice has been aptly called the “invisible strangler.” When conditions favor its 

formation, the rate of buildup on underwater objects can be rapid; frazil ice formation can 

reduce an intake’s capacity substantially or clog it completely in a few hours. 

The climatological conditions that encourage frazil ice formation are:

 Clear night sky

 Air temperature of 9.4°F or less

 Daytime water temperature of 32.4°F or less

 Cooling rate greater than 0.01°F per hour

 Wind speed greater than 10 mph at water surface

Frazil ice generally accumulates in the late evening or early morning hours and seldom 

lasts past noon. Conditions favorable to frazil ice formation vary considerably from site 

to site, making it difficult to use weather data alone as a forecasting tool.

Design Features and Operational Measures to Minimize the Effects of Icing

Facilities can use a specific intake design to combat ice formation or they may use 

operational measures to prevent ice buildup.

Design Features

In many cases, locating the intake at a significant depth is a simple method to reduce 

problems with icing. Research and experience on the Great Lakes and elsewhere indicate 

that the location and design features of submerged intakes can reduce intake ice 

problems, but probably not completely eliminate them. Submerging lake intakes in deep 

water and sizing inlet ports for a velocity of 0.3 ft/s or less minimizes the amount of frazil 

ice transported downward into the structure. However, during winter storms, strong wind 

and wave action carry ice crystals and supercooled water to considerable depths, making 

accumulation of ice on and around the intake likely. 

Alternatively, a facility can promote the formation of surface ice to prevent frazil ice. At 

the Billings, Montana waterworks, frazil ice was a severe winter problem in the turbulent 

Yellowstone River. In this case, the solution was to enlarge an off-river intake channel 
4

into an earthen forebay with a retention time of approximately one hour. Surface ice 

formed on the newly quiescent forebay. This insulating ice cover prevented the 

formation of additional ice and provided the opportunity for river frazil carried into the 

forebay to combine with the surface ice and revert to a passive condition.

To prevent (or at least minimize) ice clogging, the intake structure can be constructed 

with materials with a low heat transfer rate and smooth surfaces to prevent the 

accumulation of ice crystals. Metals such as steel are more susceptible to frazil ice 

formation because they have a high heat conductivity and act as a sink for the latent heat 

released with the ice begins to form, which encourages ice buildup. In contrast, ice does 

not readily crystallize or grow rapidly on wood or plastic--screens can be constructed 

using fiberglass-reinforced plastic with low thermal conductivity and a smooth surface. 

Any exposed metal surfaces can be coated with an inert material such as black epoxy

paint to effect better thermal properties and to increase radiation heat gain.

Heated screen elements have also been used successfully at power plant installations (see 

examples below). In these cases, screen elements (such as the bars in a trash rack) are 

wrapped in conductive material, where an electric current is converted to heat. Frazil ice 

does not adhere to objects whose temperature is slightly above the freezing point.

Operational Measures

Facility operators can employ a number of operational measures to address icing. These 

measures generally fall into one of three categories: physical measures, thermal 

measures, and agitation.

Physical measures are most common with surface ice; barges or other structures are 

placed in the waterbody to guide floating debris and ice away from the CWIS. A facility

might also use a curtain wall or floating boom to keep surface ice away. Once ice has 

formed on or collected near the CWIS, operators can also manually remove ice with hand 

tools or operate a mechanical rake or other cleaning mechanism.

Thermal measures are commonly used by power plants. The typical method is to 

construct a pipe from the discharge of the condensers to carry a portion of the heated 

effluent back to the CWIS. The hot water, in combination with the agitation from the 

discharge, acts to prevent ice buildup on the screens. A facility may also backflush its 

cooling water flow, which would serve to route warmer water back to the CWIS.

Agitation is often important in controlling frazil ice at an offshore intake structure. A 

facility can injecting steam or compressed air at the intake opening and backflush the 

intake with settled water.

Site Visit Data

As part of the process in revising the 316(b) regulations, EPA has conducted dozens of 

site visits to power plants across the country, including facilities in northern climates 
5

where icing is a significant concern. The rulemaking record contains the full set of cite 

visit reports, but a summary of the methods used by sites that EPA visited is presented 

below.

Midwest

EPA visited several facilities in Nebraska, Kansas, and Missouri and gathered data on 

several more. Most facilities stated that they experience significant icing; these facilities 

typically have more problems with floes of surface ice, which can collect near the intake 

and block water withdrawals. Intake screens at these facilities can also form ice 

(especially in the open space of the screen panels), making them heavier, more difficult to 

operate, and less able to provide cooling water.

Despite the threat of ice, facilities in the Midwest have adapted their operations and 

rarely experience screen failures or unit shutdowns due to icing.

3

 At least seven of these 

facilities recirculate heated effluent to their CWIS to prevent ice buildup. At least three 

facilities also use an ice deflector or barge to divert floating ice away from the CWIS.

Some facilities also have desilting sprays or other devices designed to expel sediment that 

accumulates in the area of the intake screens; these technologies could also be used to 

agitate the source water or push ice floes away from the CWIS.

Great Lakes

EPA visited three facilities located on the Great Lakes. Frazil ice is the more common 

problem here, as winds on the lakes create a more turbulent environment. Additionally, 

many CWISs in the Great Lakes are located offshore, where surface ice would not be a 

concern.

Two of these facilities are able to recirculate heated effluent to the offshore CWIS to 

inhibit ice buildup; the third uses a heated bar rack.

Northeast

EPA also visited a number of sites in the northeastern U.S. None mentioned any

significant problems with icing, but one did note that it is forced to operate the air sparge 

on its wedgewire screens more often in the winter to prevent ice buildup on the screen.

Other Sites

EPA also visited sites in the Mid-Atlantic and southeast, but these facilities did not 

indicate any problems with icing.

 

3 One facility did note, however, that water levels were abnormally low due to an ice dam upstream of the 

facility, forcing them to shut down one generating unit.
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Conclusions

Many facilities face challenging operating conditions and icing is a significant concern to 

many power plants. However, as evidenced by EPA’s site visits, power plant operators 

have installed design features and operational measures to reduce the impact that icing 

can have on their ability to generate power. These technologies are in common use 

across the affected regions and are known to be effective.