Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Contraction of the
1 - Aggradation
1 - Deposition of
river
2 - Backwater at flood
excess sediment
2 - Increased velocity
stage
eroded at down-
3 - General and local
3 - Changed response
stream of the
scour
of tributaries
bridge
4 - Bank instability
2 - More severe attack
at first bend
downstream
3 - Possible develop-
(5) Excess of sediment at bridge
ment of a chute
site due to upstream tributary
channel across
the second point
bar downstream of
the bridge
1 - None if straight
1 - Similar to local
1 - Similar to local
section is designed
effects
effects
to transport the
sediment load of
the river and if it is
designed to be
stable when sub-
jected to anticipated
flow. Otherwise
same as in Case
(6) River channel relocation at
(4)
crossing site
Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Aggradation of bed
1 - See local effects
1 - See upstream
2 - Loss of waterway
2 - Change in base
effects
3 - Change in river
level for tributaries
3 - Deposition in
4 - Increased flood
tributaries near
stage
confluences
5 - Late ral channel
4 - Aggradation caus -
instability
ing a perched river
(7) Raising of river base level
channel to develop
or changing align-
ment of main
channel
1 - Channel
1 - Degradation
1 - Degradation
degradation
2 - Reduced flood
2 - Increased
2 - Possible change in
stage
velocity
river form
3 - Reduced base level
and transport in
3 - Local scour
for tributaries,
tributaries
4 - Possible bank
increased velocity
instability
and reduced
5 - Possible destruction
channel stability
(8) Reduction of sediment load
of structure due to
causing increased
upstream
dam failure
sediment transport
to main channel
9.12
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