Table 9.1. River Response to Highway Encroachments and to River Development.
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Fan reduces
1 - Erosion of banks
1 - Aggradation
waterway
2 - Unstable channel
2 - Flooding
2 - Direction of flow
3 - Large transport
3 - Development of
at bridge site is
rate
tributary bar in
uncertain
the main
3 - Channel location
channel
is uncertain
(1) Crossing downstream of an
alluvial fan
1 - Headcutting
1 - Increased velocity
1 - Increased
2 - General scour
2 - Increased bed
transport to main
3 - Local scour
material transport
channel
4 - Bank instability
3 - Unstable channel
2 - Aggradation
5 - High velocities
4 - Possible change
3 - Increased flood
of form of river
stage
(2) Lowering of base level for the
channel
Case (4) illustrates a situation where artificial cutoffs have straightened the channel
downstream of a particular crossing. Straightening the channel downstream of the crossing
significantly increases the channel slope. This causes higher velocities, increased bed
material transport, degradation and possible head cutting in the vicinity of the structure. This
can result in unstable river banks and a braided streamform. The straightening of the main
channel can drop the base level, adversely affecting tributary streams flowing into the
straightened reach of the main channel, which was discussed in Case (2).
Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - At low flow a low
water channel
develops in river
bed
2 - Increased danger to
-----
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piers due to
channelization and
(3) Channel characterized by
local scour
prolonged low flows
3 - Bank caving
1 - Steeper slope
See local effects
1 - Deposition down-
2 - Higher velocity
stream of
3 - Increased transport
straightened
4 - Degradation and
channel
pos sible headcutting
2 - Increase in flood
5 - Banks unstable
stage
6 - River may braid
3 - Loss of channel
7 - Danger to bridge
capacity
foundation from
4 - Degradation in
(4) Cutoffs downstream of
degradation and
tributary
crossing
local scour
9.10
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