The channel is not well defined as it flows onto the alluvial fan. As a result, it is difficult to
locate a bridge to take into account both significant lateral channel migration and bed elevation
changes. A possible solution to the problem would be to build a debris basin upstream of the
bridge to trap the large-sized bed material. This would provide a source of gravel which is in
demand in that area. This might be a site where a bedload transport model may be helpful in
determining the solution to the aggradation problem.
9.5.11 Outlet Creek at US-101 Near Longvale, California (Example 11)
Examples 11 through 13 were taken from Federal Highway Administration Report No.
FHWA/RD-80/158. These examples illustrate situations where the river channel was
relocated to accommodate highway encroachments and crossings. These examples are
reprinted from the original publication except for minor editing to conform with the format of
this text.
In Example 11, the Outlet Creek channel was shortened from 435 m (1,427 ft) in length to 335
m (1,100 ft) and relocated to avoid two crossings on the realigned highway curve (Figure
9.12). The stream is semi-alluvial, and resistant bedrock crops out in the bottom of the
relocated channel. The highway embankment, which forms the right bank of the relocated
channel, is heavily riprapped, and the main potential for instability is at the left bank, which is a
steep (3/4:1) slope cut into colluvial material. However, no erosion or slumping was
observed.
Figure 9.12.
Plan sketch of channel relocation, Outlet Creek (Example 11).
The site location is on US-101, 3.2 km (2 mi) south of Longvale, California. Outlet Creek is
discharge of 12 m3/s (424 ft3/s). The channel width is 9-15 m (30-50 ft), with a channel slope
of 0.0036. The bed material is gravel and cobble, bank material is gravel and sand where
alluvial.
The length of a bend in the natural channel was shortened by a factor of 0.77, and room for
the relocated channel was made by grading back a steep valley side-slope. The relocated
channel lies between this graded slope and the riprapped highway embankment. Riprap on
the embankment includes rocks weighing several tons, and erosion of the embankment is
unlikely.
9.33
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