Case (13) is a series of situations, unrelated in some instances and combined in others,
which can affect bridge crossings. Tidal flows, seiches and bores can have significant
effects on scour and depth in the channel system. The tidal flows, seiches and bores, as well
as wind waves, can rapidly and violently destroy existing bank lines.
When considering earthquakes, it is of importance to examine a seismic probability map of
the United States. Large portions of the United States are subjected to at least infrequent
earthquakes. Associated with earthquake activity are severe landslides, mud flows, uplifts in
the terrain, and liquefaction of otherwise semi-stable materials, all of which can have a
profound effect upon channels and structures located within the earthquake area. Historically,
several rivers have completely changed their course as a consequence of earthquakes. For
example, the Brahmaputra River in Bangladesh and India shifted its course laterally a
distance of some 320 km (200 miles) as a result of earthquakes that occurred approximately
200 years ago. Although it may not be possible to design for this type of natural disaster,
knowledge of the probability of its occurrence is important so that certain aspects of the
induced effects from earthquakes can be taken into consideration when designing the
crossings and affiliated structures.
Table 9.1. River Response to Highway Encroachments and to River Development (continued).
Bridge
Local
Upstream
Downstream
Location
Effects
Effects
Effects
1 - Scour or
1 - See local effects
1 - See local effects
aggradation
2 - Channel erosion
2 - Beach erosion
2 - Bank erosion
3 - Changes in
3 - Channel change
channel slope
4 - Bed form
change
1 - Bank erosion
1 - See local effects
1 - See local effects
2 - Inundated
highway
3 - Increase in
velocity
4 - Wave action
(13) Tectonics and other natural causes
1 - Channel
1 - See local effects
1 - See local effects
changes
2 - Slide lakes
2 - Slide lakes
2 - Scour or
deposition
3 - Decrease in
bank stability
4 - Landslides
5 - Rockslides
6 - Mudflows
9.17
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