Freeze-up conditions

DATA COLLECTION PROGRAM DESIGN

Figure 17. Nebraska ice report data collection form

SR96_01
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concerns limit ground-based observations, aerial

Table 15. Data to be collected in ice data collection

program.

reconnaissance would be desired. A particularly

useful application of aerial reconnaissance would

Historical data

be to monitor the location of the main flow chan-

past flood-fighting efforts

nel in the rivers at freeze-up, since effective ap-

stages

plication of some mitigation measures requires

discharges

meteorological conditions

this information. Ideally the channel should be

ice records

videotaped to record the location of the main chan-

nel, and the main channel location should be trans-

Physical (field) data

ferred to maps. In the future, as technology fur-

Freeze-up conditions

ther develops, it may be possible to "mark" the

general ice conditions in river

predominant ice type

main channel directly on the videotape. Ideally

location of main channel at freeze-up

observations of freeze-up conditions should be

Ice cover characteristics

made in a consistent manner by the various ice

general appearance

observers. It should be emphasized that the ob-

location of upstream and downstream boundaries

servers should include as many additional obser-

ice cover roughness

presence of leads or cracks

vations and comments as they desire when using

changes in water level affecting ice cover

data collection forms. It is often the unexpected

Breakup details

observation that provides the most information.

thermal effects on ice cover

presence and growth of leads or cracks

Ice cover characteristics

changes in water level leading to breakup

breakup process

As the winter continues, periodic observations

Jam formation and evolution

of general ice conditions should be made at se-

type of jam

lected locations. Observations of general ice con-

location of jam toe

ditions include monitoring the locations of the

jam extent

downstream and upstream ends of the ice cover,

ice characteristics

upstream ice supply

if possible, and any changes over time. Aerial re-

jam progression or regression with time

connaissance is the easiest way to obtain this in-

river geometry and hydraulics affecting jam

formation, but with an adequate network of ob-

water stage

servation stations, ground-based observations

ice thickness

may be suitable. This is the case at several of the

actual and potential ice jam flood areas

seven recurring ice-jam locations, such as the Loup

River at Columbus and along the Union Dike,

where levees and dikes provide good access and

Freeze-up conditions

visibility along the jam reaches.

Visual observations of the freeze-up process

The relative roughness of the ice-cover surface

are quite valuable. In particular, the date of initial

should be monitored over time, along with the

ice formation and when and where the ice freezes

locations and sizes of open-water areas (leads)

across the channel to form an ice cover are impor-

within the ice cover. Changing water levels may

tant. Border ice growth, as well as the types and

affect the ice cover in various ways. If the water

relative amounts of moving ice observed, should

level drops after freeze-up, the ice cover may also

be documented. The manner in which the ice cov-

drop, forming a hinge crack along the shoreline.

er actually forms in a reach where jams are known

It is important to note the presence of such cracks

to form would be of special value. It may also be

parallel to the shore since they have a direct im-

possible to determine whether the ice cover is

pact on breakup processes. The appearance and

made up primarily of sheet ice or frazil ice through

location of cracks perpendicular to the shore

observations of ice-cover formation and freeze-

should also be noted. Since snow insulates an ice

up conditions. Identifying the type and the rela-

cover, the dates of significant snowfall and the

tive quantity of each type of ice that constitutes

change in estimated snow depths with time should

the ice cover or jam can help both in estimating its

be reported.

possible effects and in evaluating suitable mitiga-

The ice cover will begin to deteriorate as the

tion measures.

air temperature, length of day and sun angle in-

Ground-based observations of freeze-up con-

crease during late winter and spring. The surface

ditions at selected locations are usually adequate,

of a formerly smooth ice cover may become pit-

but if conditions are rapidly changing or if safety

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