In another example, the volcanic eruption from the magmatic blast of Mount St. Helens
triggered a major slope failure on the north flank of the mountain. Mudflows and debris flows
were generated and swept down the Toutle and Cowlitz Rivers destroying bridges, inundating
buildings, and eventually blocking the navigation channel of the Columbia River. Bradley
(1984) reported the Cowlitz has aggraded markedly as a result of the post eruption
hyperconcentrated flows. The upper Cowlitz and the lower Toutle have shifted from
meandering to braided streams, thereby causing difficulties in preventing the failure of some
Examination of typical watershed behavior and response provides information on the impact of
changes on the fluvial system. Channel stability assessments and possible gradation changes
are indicated in Table 5.8. Those findings reflect the observations of Keefer et al. (1980). An
example relating to the use of this table is presented in Section 5.9, Problem 3.
5.7.3 Resulting Problems at Highway Crossings
Brown et al. (1980) reviewed design practices to evaluate crossing design procedures and the
effect of grade changes on these procedures. The parameters most influenced by grade
changes are those used as input to the hydraulic design procedures currently in use. These
input parameters include: design discharge; channel roughness; energy slope; bed slope;
velocities; shear stresses; cross-sectional geometry; base level; flow depth; and flow alignment.
Other components of crossing design affected by grade changes include foundation depth,
bridge deck clearance, and flow opening size.
Problems encountered at bridge crossings include bridge capacity, backwater, pier and
flow-control and debris-control structures. With respect to bridge capacity and backwater,
aggradation produces the most severe problems. However, debris problems associated with
degradation can also have a significant impact on flow capacity and scour. Foundation depths
for piers, abutments, and flow-control structures can be influenced in two ways by grade
changes: the normal streambed base level will be altered; and the "normal" hydraulic
conditions at a site used as input to local scour computations will be changed. The important
components of bank protection design adversely affected by grade changes are key depths
and the vertical extent of bank protection above and below the streambed.
Problems encountered at culvert crossings can be the result of general grade changes
produced by long-term changes in stream morphology or inadequate design and/or
construction of culvert systems. The design components most often influenced are culvert
capacity and structural stability. The greatest danger produced by aggradation is partial
plugging of the culvert opening resulting in a damming effect and increasing the magnitude
and frequency of flooding upstream of the structure. Degrading stream reaches affect culvert
systems by reducing their structural stability. General streambed degradation has undermined
the foundations of culverts resulting in their complete failure.