The left and right banks are relatively well vegetated and stable; however, there are isolated
portions of the bank which appear to have been undercut and are eroding. Brush and trees
grow to the edge of the banks. Banks will require riprap protection if disturbed. Riprap will
be required upstream of the bridge and extend downstream of the bridge.
Evaluation of stage discharge relationships and cross sectional data obtained from other
agencies does not indicate progressive aggradation or degradation. Review of bridge
inspection reports for bridges located upstream and downstream of the proposed crossing
indicates no long-term aggradation or degradation in this reach. Based on these
observations, the channel is relatively stable vertically, at present. Furthermore, there are no
plans to change the local land use in the watershed. The forested areas of the watershed
are government-owned and regulated to prevent wide spread fire damage, and instream
gravel mining is prohibited. These observations indicate that future aggradation or
degradation of the channel, due to changes in sediment delivery from the watershed, are
minimal.
Based on these observations, and due to the lack of other possible impacts to the river
reach, it is determined that the channel will be relatively stable vertically at the bridge
crossing and long-term aggradation or degradation potential is considered to be minimal.
However, there is evidence that the channel is unstable laterally.
10.3.3 Level 2 - Hydraulic Engineering Analysis
Hydraulic characteristics at the bridge were determined using FHWA's WSPRO model.
Three cross sections were used for this analysis and are denoted as "EXIT" for the section
downstream of the bridge, "FULLV" for the full-valley section at the bridge, and "APPR" for
the approach section located one bridge length upstream of the bridge. The bridge geometry
was superimposed on the full-valley section and is denoted "BRDG."
The WSPRO HP2 option was used to provide hydraulic characteristics at both the bridge and
approach sections. This option subdivides the cross section into 20 equal conveyance
tubes. Figures 10.29 and 10.30 illustrate the location of these conveyance tubes for the
approach and bridge cross sections. Figure 10.30 illustrates the average velocities in each
conveyance tube and the contraction of the flow from the approach section through the
bridge. Figure 10.30 also identifies the equal conveyance tubes of the approach section
which are cut off by the abutments.
Contraction scour will occur both in the main channel and on the left overbank of the bridge
opening. For the main channel, contraction scour could be either clear-water or live-bed
depending on the magnitude of the channel velocity and the critical velocity for sediment
movement. In the overbank area adjacent to the left abutment, clear-water scour will occur.
This is because the overbank areas upstream of the bridge are vegetated and the velocities
in these areas will be low. Thus, returning overbank flow which will pass under the bridge
adjacent to the left abutment will not be transporting significant amounts of material to
replenish the scour on the left overbank adjacent to the left abutment.
10.41
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