Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Rivers are dynamic
1 - The river could
1 - See upstream
(ever changing) and
abandon its present
effects
the rate of change
channel. Changing
2 - Shifts in the
with time should be
position of the main
position of th e
evaluated as part of
channel may
main chan-
the geomorphic and
require realignment
nel relative to
hydraulic analysis
with training works.
the position of
2 - Alignment of main
the confluence
channel continually
with the tributary
changes affecting
alternatively flat-
alignment of flow
tens or steep-
with respect to
ens the gradi-
Bridge A.
ent of the tri-
3 - If the main channel
butary causing
shifts to the alter-
corresponding
nate position, the
aggradation
confluence shifts
and degrada-
and the tributary
tion
gradient is signi-
3 - Shifts in the
ficantly increased
position of the
causing degrada-
main channel
tion in the tributary.
causes ag-
Local effects on
gradation, de-
Bridge B same as
gradation and
(11) Naturally shifting river channel
1, 2, 3, and 4 in
instabilities de-
Case (8)
pending upon
4 - Excess sediment
direction and
from the tributary,
magnitude of
assuming (3)
channel change
causes aggrada-
tion in the main
channel and
possible significant
changes in channel
alignment
Considering the possible changes in the position of the main channel, training works may be
required at and upstream of Bridge A to assure a satisfactory approach of the flow to the
bridge crossing. Otherwise, the river could abandon its present channel. A shift in the
position of the main channel relative to the position of the confluence with the tributary also
alternately flattens or steepens the gradient of the tributary causing corresponding
aggradation or degradation in the tributary. This type of problem is difficult because of the
continuously changing characteristics of such river systems. Rivers of this type are usually
stable for several years at a time or at least between major flows. Consequently, if crossing
locations are properly selected and appropriate stabilization techniques are taken, it may be
possible to maintain the usefulness of the crossings for the life of the structures. However,
the disadvantages associated with such locations will often require expensive solutions and
these locations should be avoided if possible.
9.15
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