lowering of the channel bed in the vicinity of the diversion structure because of the deposition
upstream of the diversion dam and the release of essentially clear water for a relatively short
period of time until the sediment storage capacity of the reservoir is satisfied. Bridge site B
is subjected to degradation due to the increased discharge and an essentially clear water
release. However, the degradation of the channel could induce degradation in the tributaries
causing them to provide additional sediment to the main channel, see Case (8). This
response would to some degree counteract the degrading situation in this reach of river.
Such changes in river systems are not uncommon and introduce complex responses
throughout the system. Complete analysis must consider the individual effects and sum
them over time to determine a safe design for the crossings.
Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge Location
(10) Change in water discharge, no change in sediment load
Local Effects
Upstream Effects
Downstream Effects
1 - Bridge A may be subjected to
1 - Upstream of Bridge A -
1 - See upstream effects
aggradation due to excess
aggradation and possible
2 - Construction of reservoir C could
sediment left in the channel by
change of river form
induce aggradation in the main
diversion of clear water
2 - Upstream of Bridge B -
channel and in the tributaries.
2 - Bridge B may be subjected to
degradation and change of
Effects s ame as in Case 7
degradation due to increased
river form
discharge in the channel
3 - Channel instabilities
3 - If a storage reservoir was
4 - Significant effects on flood
constructed at C, it could induce
stage
aggradation in both main
tributaries
Case (11) shows a highway that crosses the main channel at Bridge A and its tributary at
Bridge B. The confluence of the main channel and its tributary is downstream of both
bridges. The alignment of the main channel is continually changing. The rate of change in the
river system should be evaluated as part of the geomorphic and hydraulic analysis of the site.
If the main channel shifts to the alternate position shown and moves the confluence closer to
Bridge B, the gradient in the tributary is significantly increased causing degradation as well as
channel instabilities and possible changes in river form. Excess sediment from the tributary
causes aggradation in the main channel and possibly significant changes in channel
alignment.
9.14
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