Factors Influencing Ice Conveyance at River Confluences
ROBERT ETTEMA, MARIAN MUSTE, ANTON KRUGER, AND JON ZUFELT
ing ice movement in two initially separate chan-
INTRODUCTION
nels that merge as a single channel through the
River confluences, the intersection and merg-
usually complicated bathymetry of a confluence.
ing of two sets of water, sediment, and, in cold
To make the analysis feasible, the following two
regions, ice discharge conditions, are prime loca-
major approximations are made:
tions for ice jam development. The potential con-
Confluence geometry is treated as the inter-
gestion of ice forced from two channels into a
section of two prismatic (rectangular) chan-
single channel, together with the complex bathym-
nels, whose widths greatly exceed their
etry that usually occurs there, hamper ice dis-
depths, thereby simplifying description of
charge through confluences. Fairly numerous
channel geometry.
accounts exist of ice jams at confluences. Tuthill
Ice discharge through confluences is catego-
and Mamone (1997) provide a comprehensive
rized according to whether ice enters the
summary of problematic confluence sites in the
confluence as a single layer of free drifting
U.S. The factors influencing ice jam development
ice moving with a velocity slightly less than
at river confluences, however, have not received
the water surface, or as a contiguous, thick-
rigorous analytical attention.
ened accumulation extending approximately
This report documents the approaches to and
across the full width of the channel, and mov-
preliminary results of a comprehensive study of
ing with a velocity that is much less than the
ice discharge and jamming at river confluences. It
water velocity along the underside of the
identifies the important parameters influencing
accumulation.
the ice discharge capacity of confluences, and, as
a case study, it presents a well illustrated descrip-
These approximations enable the key non-
tion of ice-conveyance and jam-initiation processes
dimensional, gross parameters influencing ice
in a hydraulic model of the confluence of the Mis-
movement through confluences to be identified,
souri and Mississippi rivers. The confluence of
and reduce the number of parameters needed to
those two rivers is well known for its ice jams.
describe the essential processes that occur when
The study entails two investigative approaches.
two flows of ice merge. Other, less significant,
One approach is to identify a group of influential,
simplifications are made subsequently in the
nondimensional parameters for use in generally
analysis.
describing ice discharge and jamming in river
The analysis does not take into account the
confluences. The other approach is to examine in
influences of such channel features as bars, large
detail the features of water flow and ice movement
dunes, rock outcrops, etc. Nor does it include the
through a representative confluence. The first
presence of engineered features such as bridges,
approach required a dimensional analysis of the
wharves, channel-control structures, etc. The
variables most likely to affect ice discharge
hydrometeorological influences of air temperature
through a generic confluence. The second
(as affecting freezing consolidation of drifting ice)
approach required the use of a hydraulic model
and wind also are neglected. These channel, engi-
of a confluence and the development of a sophis-
neered, and hydrometeorological factors likely are
ticated imaging process for mapping vectors of
very important; however, they do not play a role
water and ice movement.
in the essential process of waterborne ice discharge
through the confluence of prismatic channels,
which is considered in the ensuing dimensional
DIMENSIONAL ANALYSIS
analysis.
Dimensional analysis of ice discharge through
a confluence of two channels is complicated by a
Categories of ice discharge through confluences
potentially large number of variables. These vari-
Ice discharge through confluences is classified
ables all may need to be considered when describ-
below using two general categories: free-drifting
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