The Level 1 and 2 analysis results and total scour computations are summarized briefly in
the following sections. Then the BRI-STARS model is applied to refine the initial qualitative
analysis of long-term bed elevation changes.
10.3.2 Level 1 - Reconnaissance and Geomorphic Analysis
A 650-foot (198 m) long bridge (Figure 10.28) is to be constructed over a channel with
m) back from the channel bank. The right abutment is set at the channel bank. The bridge
deck is set at elevation 22 ft (6.71 m) and has a girder depth of 4 ft (1.22 m). Six round-nose
piers are evenly spaced in the bridge opening. The piers are 5 ft (1.52 m) thick, 40 ft (12.19
m) long, and are aligned with the flow.
Figure 10.28. Cross section of proposed bridge.
A site investigation of the crossing was conducted to identify potential stream stability
problems at this crossing. Evaluation of the site indicates that the river has a relatively wide
floodplain. The floodplain is well vegetated with grass and trees; however, the presence of
remnant channels indicates that there is a potential for lateral shifting of the channel. The
river and crossing are located in a rural area with the primary land use consisting of
agriculture and forest.
The bridge crossing is located on a relatively straight reach of channel. The channel
geometry is essentially the same for approximately 1,000 ft (300 m) upstream and
downstream of the bridge crossing. The D50 of the bed material and overbank material is
approximately 2 mm. The maximum grain size of the bed material is approximately 8 mm.
The specific gravity of the bed material was determined to be equal to 2.65. Since this is a
sand-bed channel, no armoring potential is expected. Furthermore, the bed for this channel
at low flow consists of dunes which are approximately 1 to 1.5 ft (0.3 to 0.5 m) high. At
higher flows, above the Q5, the bed will be either plane bed or antidunes.
10.40
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