Sabino Canyon Road Bridge. Four alternatives for the Sabino Canyon road bridge are
analyzed. Alternative I is the present condition (Figure 10.15). The other three alternatives
consider variations on a similar scheme. The obvious options involve channelization
downstream of the bridge to reduce water surface elevations, widening of the bridge opening,
protection of the south side of the bridge by a guide bank, and bank protection at key locations.
Alternatives II, III and IV consider a channel bottom width of 300, 350, and 400 ft (91, 107, and
122 m), respectively. A sketch of the basic design is presented in Figure 10.27. The drawing
shows a 300-foot (91 m) channel for Alternative II.
A low chord and total scour analysis similar to the Craycroft Road site presented in Table 10.11
could be used for the analysis of the Sabino Canyon bridge. Because of the problems
associated with a bridge located on a bend, Alternatives II, III, and IV include bank protection
and stabilization 200 ft (60 m) up- and downstream of the bridge site. Also, additional bank
protection should by provided along the south bank above Sabino Canyon Road. This latter
protection will prevent the upstream bend from further migration that would cause flow
alignment difficulties at the bridge. A guidebank should be constructed on the south side to
protect the bridge from southward channel migration and to assist in controlling overbank flow
and guiding the flow through the bridge opening (Figure 10.27).
By using the radius of bend curvature as a function of channel width determined earlier,
rc = 1.06 B1.35
the appropriate radius of curvature for each alternative can be determined. The results show
that for the channelization alternatives (II, III, IV), the radius of curvature is within the stable
range. However, for the present condition (Alternative I), the radius of curvature is too small.
This is one cause of the present migration problem on the south bank. A very preliminary
assessment concluded that Alternative II [300 ft (91 m) bottom width channelization] is
probably the most feasible and practical solution for the new bridge. Very little is gained in
terms of low chord and scour reduction by the wider channels; however, they would require a
large amount of additional earthwork. The narrower channel would also cause less conflict
with private property ownership.
10.3 BRI-STARS SEDIMENT TRANSPORT MODELING EXAMPLE
10.3.1 Background
Overview examples 1 and 2 illustrate the application of the three-level analysis procedure.
Example 1 applied the principles and methods introduced in this manual to the design of
countermeasure alternatives on a migrating alluvial channel bendway. Techniques used
included a Level 1 reconnaissance and geomorphic analysis and a Level 2 quantitative
engineering analysis. Example 2 developed conceptual design alternatives for two bridge
crossings on a dynamic sand bed river system in the southwest using, primarily, Level 1 and
2 analysis procedures; however, the results of a Level 3 sediment routing analysis were
demonstrated.
This final overview example illustrates in more detail the application of Level 3 sediment
transport analysis techniques using FHWA's BRI-STARS model. This example problem is
taken from a paper by Arneson et al. (1991) which is presented as the total scour
comprehensive example in HEC-18 (Richardson and Davis 2001). For this problem in HEC-
18, FHWA's WSPRO computer program was used to obtain the hydraulic variables. The
program uses 20 stream tubes to give a quasi 2-dimensional analysis. Each stream tube
has the same discharge (1/20 of the total discharge). The stream tubes provide the velocity
distribution across the flow and the program has excellent bridge routines. Since the data for
this case study are available in English units, the figures and tables retain English unit
notation. SI (metric) units are given parenthetically in the text for reference.
10.38
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