Case (12) illustrates a meandering channel with several tributaries and a major storage
reservoir constructed on the main channel. Two crossings are shown on the main channel
upstream of the reservoir. It is assumed that complete channelizing of the meandering river
has been authorized. This shortens the path of travel of the water by an appreciable distance.
Considering local effects at the bridges, bridge site A is first subjected to possibly severe
degradation and then aggradation. Bridge site B is primarily subjected to degradation. The
magnitude of this degradation can be large. With the degree of straightening indicated in the
sketch, severe head cutting may be initiated up the main channel as well as the tributaries.
The whole system may be subjected to passage of sediment waves and the river form can
dramatically change over time. The flood level in the system and the local and general scour
in the vicinity of the bridges is greatly affected by the channelization.
As a result of channelization, the river reach at bridge site B could become braided. Also, in
this reach the rate of sediment transport is increased, head cutting is induced and flood
stages are reduced. The tributaries in the upper reach are subjected to severe degradation.
For the bridge at position A, the channel would probably degrade and then significantly
aggrade. Significant reactions are possible when channelization is undertaken in a river
system. A detailed analysis of all of the responses is necessary before it is possible to safely
design crossings such as those at location A and B.
Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Bridge A is first
1 - A change of river
1 - For Bridge B see
subjected to
form from mean-
upstream effects
degradation and
dering to braiding
2 - For Bridge A the
then aggradation.
is possible
channel first
Action can be very
2 - Rate of sediment
degrades and
severe
transport is
then significantly
2 - Bridge B is primarily
increased
aggrades
subjected to
3 - Headcutting is
3 - Large quantities
degradation. The
induced in the
of bed material
magnitude can be
whole system
and wash load
large
4 - Upstream of B
are carried to the
3 - The whole system is
flood stage is
reservoir
subjected to pass-
reduced
4 - Delta forms in the
age of sediment
5 - Velocity increases
reservoir
waves
6 - Tributaries
5 - Wash load may
4 - River form could
respond to main
affect water
change to braided
channel changes
quality in the
5 - Flood levels are
entire reservoir
reduced at B and
6 - Tributaries
increased at A
respond to main
6 - Local and general
channel changes
scour is significantly
affected
(12) Human-induced reduction of
channel length
9.16
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