insufficient to transport gravel brought in by tributary streams. Channel avulsions, which can
present a serious threat to many engineering structures, are associated with most aggrading
situations. Rapid lowering of river stage may result in severe bank slumping from pore-water
pressures in the banks. However, the more general effect of reservoirs is probably to reduce
hydraulic problems at highway crossing bridges, both by reduction of flood peaks and a
reduction of lateral erosion rates.
Interbasin transfers of flow and diversions result in periods of channel instability and bank
erosion until the new channel regime is established.
5.7.2 Natural Causes
Although problems resulting from natural causes are not as frequent as those resulting from
human activities, it is important to recognize natural causes in both design and maintenance of
highway crossings.
Natural causes and complications from gradation problems include: alluvial fans, natural
armoring, braiding, meandering/migration (natural cutoffs), recurrent flooding, high stream
velocity, channel bed and bank material erodibility, fire, floating debris, mud and debris flows,
earthquakes, tectonic activity, volcanic activity, and landslides.
Floating Debris. Floating debris causes hydraulic problems at highway crossings nationwide.
The problems are the greatest in the Pacific Northwest and the upper and lower Mississippi
River Valley. Debris hazards are generally a local phenomena often associated with large
floods. Most bridge destruction from debris is due to accumulation of debris against bridge
components. Debris may partially or totally block waterways, create adverse hydraulic
conditions that erode pier foundations and bridge abutments or may overtop roadways and
cause structural damage.
Many debris problems exist in forested areas with active logging operations. Highway
crossings on streams where stream slopes are mild or moderate, in contrast to headwater
streams, are more vulnerable to debris related hazards. Debris hazards occur more frequently
in unstable streams where bank erosion is active. Countermeasures presently used by
highway agencies include: (1) sufficient freeboard, (2) proper pier spacing, (3) solid piers, (4)
debris deflectors, (5) special superstructure designs, (6) flood relief structures, and (7) routine
and emergency removal of debris at bridge crossings. Most debris transported in floods does
not travel a great distance and often is observable locally along the streambanks upstream
from the bridge prior to the flood. Debris usually moves as individual logs in a non-random
path concentrating in the thalweg of the stream. Therefore, methods for evaluating its
abundance and for mitigating its hazard are deemed feasible. HEC-20 (Lagasse et al. 2001)
summarizes the results of studies by the U.S. Geological Survey to develop methods to
estimate potential debris accumulations at bridges (Diehl 1997 and Diehl and Bryan 1993).
Examples of debris control structures for culverts are given in Reihsen (1964).
Mud Flows And Debris Flows. Fast melting snowpack and overabundance of soil moisture
on steep slopes throughout the Western United States causes mudflows, debris flows and
landslides, threatening bridges and highway structures. There is considerable evidence of
damages to highway structures in the literature. For example Hungr et al., (1984) documented
a bridge for which a concrete bridge beam was demolished by point impact during a debris flow
event.
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