Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Reduced
1 - Backwater
1 - Large sediment
waterway
generated by the
load may cause
causes a local
obstruction
aggradation
obstruction to flow
increases flood
2 - Local scour at
and higher
stage
end of con-
velocities
2 - Deposition induced
tracted section
2 - Significant erosion
by the backwater
problem on the
highway fill and
induced bed
degradation
(15) Longitudinal encroachment
3 - Lateral drainage
into the river is
interrupted and
may cause
flooding and
erosion
Case (16) is a case of floodplain encroachment. It is assumed that during bankfull and lower
stages the highway does not interact with the flow. However, during high stages, the total
flow area is decreased by the encroachment. Locally, the highway must be protected against
inundation and erosion during a flood. The effect on the river channel depends on the extent
of encroachment on the waterway. If the highway significantly reduces the floodplain, it may
increase river stages for a given flood. If the river channel tends to shift, the highway
encroachment may alter the interaction of floodplain flow and channel flow, affecting the
channel floodplain flow pattern. Very often this type of encroachment has little or no effect on
flood stages or on the stream upstream or downstream, however, possible adverse effects
should be investigated.
In all cases of longitudinal encroachment, the lateral drainage into the river will be intercepted.
An important consideration in the design of encroachments will be to provide for this drainage.
Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Erosion of highway
1 - If significant
1 - If the river
fill and submer-
encroachment on
channel is highly
gence possible
the floodplain
shifting, the
during floods
waterway, back-
channel align-
2 - Pattern of overbank
water may be
ment may
spill are affected
induced
change
by the encroach-
2 - If significant
ment and in highly
erosion
shifting channels
experienced
may change river
upstream,
course downstream
aggradation will
3 - Lateral drainage into
occur
the river is inter-
rupted and may
cause flooding and
(16) Longitudinal encroachment
erosion
9.19
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