Ice thickness

Air and water temperature - CR97_020020

Ice movement and velocity - CR97_020022

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above, with the added disadvantage that the wires

Ice thickness

The standard CRREL ice thickness kit contains

can break.

a two-part iron bar used to test the ice for

The USGS, in making discharge measurements

safety, an auger with carbide-tipped bit and bit

through an ice cover, measures the distance from

brace for drilling holes, extension rods to increase

water surface to the bottom of the ice cover, so that

the depth to which holes can be drilled, and a de-

the total ice thickness is not readily known. Some-

vice to measure ice thickness (White and Zufelt

times, the total thickness will be noted, but not

1994). A small diameter auger is preferred because

often. Moreover, the thickness is not normally

holes can be drilled faster, but a minimum diam-

published, but is noted only on the discharge mea-

eter of 2 in. (5 cm) is recommended if velocity mea-

surement notes. If ice thickness measurements at

surements are desired. Thickness is measured us-

a particular USGS gaging site are desired, a search

ing a tape equipped with a hinged weight at the

through the original measurement notes stored at

end (Ueda 1983). The weight and tape is lowered

the State or appropriate field office will be neces-

through the hole, usually until the weight hits

sary. This task can be time consuming, but if con-

bottom so that total depth of flow is known. The

ditions warrant, the average ice thickness can be

tape is then pulled upward until the weight en-

reported. As an example, the State of Nebraska has

counters the ice bottom and catches on the ice. It

set up an ice data collection program (USAED,

is then read so that the thickness is known. The

Omaha 1994) for which the USGS has agreed to

measurement can be complicated if frazil is present

report the average ice thickness measured at each

underneath the ice surface, but with a little prac-

gaging location of interest. The thickness is re-

tice the observer can differentiate between the

ported to the state for inclusion in its database,

frazil and solid ice. If frazil is present, both the

and is also included in computer files that the Dis-

depth to bottom of frazil and bottom of solid ice

trict Water Control personnel can retrieve.

should be recorded. After the tape is read, the

Visual estimates of an in-place ice thickness are

weight is hinged, or folded, and pulled back up

highly subjective and subject to error. An indirect

through the hole. This method is relatively quick

measurement of ice thickness can be made after

and accurate. One disadvantage of this method of

the ice cover has broken up, when pieces of the

measuring ice thickness is that of safety for indi-

broken ice cover that remain on shore can be mea-

viduals going on the ice cover. Another disadvan-

sured. Observation must take place shortly after

tage is that only a solid ice cover strong enough to

breakup, before warmer temperatures or rain can

support the weight of the observers can be mea-

significantly reduce thickness. Ice jam thickness

sured; floating frazil or very thin ice cannot be

is often estimated based on observation of height

measured. The thickness of an ice jam could be

of ice shear walls, if they remain, after an ice jam

measured in this manner, but unless the jam is

releases. While these indirect methods of thickness

grounded and/or frozen in place, it would be

measurement are helpful for future use, they are

highly inadvisable to attempt such a task due to

not applicable for making real-time measurements

safety reasons.

of thickness.

Sherstone et al. (1986) report on the use of "hot-

wire" resistance gages to measure ice thickness in

Water velocity

the MacKenzie Delta. The gages are installed af-

Water velocity is usually measured for the pur-

ter the initial formation of the ice cover. An 18-

pose of determining stream flow, or determining

gauge chrome A resistance wire of known length

the effects of ice roughness on flow. Normally, the

is suspended from a platform above the ice sur-

velocity measurement would be done using a

face through a hole drilled in the ice. The resis-

Price-type current meter. This type of meter must

tance wire is weighted on the bottom. A second,

be used with care, as it often freezes up, either from

insulated, wire is connected to the bottom of the

contact with frazil ice or when removed from the

resistance wire. Once the hole refreezes, ice thick-

water. A few Districts do have electromagnetic

ness can be measured by applying a current to the

velocity probes, which have the advantage over

resistance wire, heating it, and raising the wire

Price-type meters in that there are no moving parts

until the weight hits the bottom of the ice thick-

to freeze and ice accumulations may be easily re-

ness. The ice thickness can then be determined by

moved (White and Zufelt 1994). However,

measuring the amount of resistance wire remain-

Yamaguchi and Hirayama (1990) report the use of

ing above the surface. This method has the same

an electromagnetic probe as being unsuitable for

disadvantages as the drilling method described

velocity measurement in frazil-laden flow due to

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