Case (14), (15), and (16) illustrate three examples of longitudinal encroachment. In Case
(14), a few bends of a meandering stream have been realigned to accommodate a highway
(see Case 6). There are two problems involved in channel realignment. First, the length of
realigned channel is generally shorter than the original channel which results in a steeper
energy gradient in the reach (Case 4). Second, the new channel bank material in the
realigned reaches may have a smaller resistance to erosion. As a result of these two
problems, the channel may suffer instability by the formation of a headcut from the
downstream end and increased bank erosion. The realigned channel may also exhibit a
tendency to regain the lost sinuosity and may approach and scour the highway embankment.
To counter these local effects one could design the realignment to maintain the original
channel characteristics (length, sinuosity). Another way would be to control the slope by a
series of low check dams. In any case, bank protection by riprap, jacks or spurs will be
needed. The up- and downstream effects of the channel realignment will be the same as
discussed for channel length reduction in Case (12). For example, as the degradation travels
through the realigned reach, sediment load generation in the river by bed and bank erosion will
cause aggradation downstream.
Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Increased energy
1 - Energy gradient
1 - Channel will
gradient and
also increased in
aggrade as the
potential bank and
the reach upstream
sediment load
bed scour
and may cause
coming from
2 - Highway fill is
change of river form
bed and bank
subject to scour as
from meandering to
erosion
channel tends to
braided
is received
shift to old
2 - Rate of sediment
2 - Channel may
alignment
transport is
deteriorate from
3 - Reach is subject to
increased. As the
meandering to
bed degradation
headcut travels
braided
as headcut
upstream severe
develops at the
bank and bed
downstream
erosion is possible
end and travels
3 - If tributaries in the
upstream
zone of influence
(14) Longitudinal encroachment
4 - Lateral drainage
exist they will
into the river is
respond to lowering
interrupted and
of base level
may cause
flooding and
erosion
Case (15) illustrates encroachment on the waterway of an incised stream flowing through a
narrow gorge. Locally, the effect is to reduce the waterway and to increase the velocities and
bank and bed erosion potential. The erosion protection of the highway slope exposed to the
flow, and possibly, the opposite bank are important problems. The backwater induced by this
obstruction may cause upstream aggradation and higher flood levels. On the downstream
side, channel aggradation may be experienced if bed erosion occurs locally in the encroached
reach.
9.18
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