fleeting activities in the vicinity of the confluence, as well as damaging shoreline
structures.
A further objective of the study is that its findings be of direct use in reducing
the incidence of ice jams at confluences, and generally in numerical modeling of
ice movement through channel confluences. The insights obtained are to be of
use in the design of structural and bathymetric techniques for ice jam mitigation,
notably at the confluence of the Mississippi and Missouri Rivers. Yet a further
objective of the present study is that it formulate ice jam development in river
confluences.
These objectives extend a survey by Tuthill and Mamone (1997) on the incidence
of ice jams at confluences in the United States. The particular objective of their
survey was to identify confluences for which structural mitigative techniques might
be implemented, e.g., ice booms or ice retention barriers. Their study and this
study were conducted as part of an overall CRREL effort focused on ice jams at
river confluences.
The following tasks were conducted for the present study:
Review published accounts of ice jams at confluences.
Identify the main features of confluence flow and bathymetry.
Identify the nondimensional parameters of importance for characterizing
water flow and ice movement through confluences.
Determine the mechanisms whereby ice jams develop at confluences.
Formulate ice jam mechanisms.
Investigate, as a case study, ice jam formation at the confluence of the Missis-
sippi and Missouri Rivers. As part of this investigation, evaluate the likely
effect on ice movement through the confluence of a set of bendway weirs
(submerged, relatively long, broad-crested groins) to be placed opposite a
point bar formed in the confluence.
Bendway weirs are used typically to shift channel thalweg toward the inner
bank of a curved channel. They function to turn flow, increase flow resistance
along one side of a channel, and to shift channel thalweg. The St. Louis District of
the U.S. Army Corps of Engineers is to place a series of bendway weirs in the
confluence in order to reduce the extent of a large bar formed in the confluence.
The bar is evident in Figure 1.
In the broad context of channel connections within watersheds, possible several
categories of channel confluences can be identified. The confluence type of pri-
mary concern for the present study comprises two inflow channels that merge
into a single outflow channel. Another category of confluence comprises the merg-
ing of one channel in series with another channel of greatly different geometry.
This type of channel confluence most notably includes a river flowing into a reser-
voir or lake, or a river issuing from a lake: in essence, a narrow channel connecting
with an extremely wide channel. The present study does not closely examine ice
jam formation in this latter class of confluence other than to review the reasons
why jams commonly form in them.
The findings of the present study are based on two sets of laboratory experi-
ments. One set was conducted to determine the processes whereby jams form at
confluences. That set used a small-scale model built to simulate diverse combina-
tions of confluence geometry and to evaluate the sensitivity of jamming mecha-
nisms to selected parameters, such as relative magnitudes of confluent flows. The
results of these experiments were augmented using information interpreted from
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