site, Tanque Verde Creek has also formed a sharp bend which will be unstable at high flow.
The last bridge in the study area is at Tanque Verde Road. This bridge is on an undisturbed
portion of the river system and crosses only the defined portion of the channel. Because the
capacity of this channel is much less than the 100-year flood, the bridge creates a significant
backwater for the 100-year flood. The roadway approaches would sustain heavy damage
under these conditions.
A three-level analysis approach was applied to the river system to identify potential erosion and
sedimentation problems associated with the bridge sites (see Chapter 9). First, a qualitative
geomorphic analysis was performed documenting the history of the river system, the type of
river form, the qualitative response of the system, and potential local problems at the bridge
sites. The second level was an engineering geomorphic analysis which assesses the general
quantitative response of the system, including determination of sediment supply, sediment
transport rates, equilibrium slopes in the system for selected conditions, and lateral migration
tendencies. The third level applied a detailed water and sediment routing procedure to
evaluate the as-is conditions and various design alternatives. This three-level analysis
provides the necessary information to:
1.
Evaluate the stability of existing and proposed bridge structures
2.
Determine the lateral migration tendencies of the channel
3.
Estimate the extent of expected general channel scour
4.
Determine the potential local scour around bridge piers and abutments
5.
Estimate the long-term effects of sediment degradation or aggradation on the bed and
water surface profiles
6. Determine the effects of debris on scour and water depth at the bridge sites
Applications of the principles of qualitative geomorphic analysis (plus basic engineering
relations), and quantitative analysis (sediment routing) are demonstrated for the Craycroft site
and the Sabino Canyon road bridge. Use of this three-level analysis gives a realistic bridge
design for moveable bed and bank conditions resulting from the 100-year flood.
10.2.2 Level 1 - Qualitative Geomorphic Analysis
General System Response. The purpose of qualitative geomorphic analysis is to identify the
important physical processes which have been acting on the river system. General
geomorphic relationships are used to classify the river system. Aerial photographs are
compiled over a series of years as a means of constructing the recent history of the river
system. Human activities including gravel mining and river training are documented and the
river's response noted. A qualitative prediction of river response is developed, based on
general geomorphic relationships such as the Lane relationship (see Section 5.5). The
information gained by this level of analysis greatly aids in applying more rigorous methods of
analysis.
The data base necessary for this type of analysis includes aerial photographs, topographic
maps (1" = 100', 2' contour intervals), and site observations. With this approach, the basic
characteristics of the river can be understood quickly with limited information.
Much of the system has been significantly disturbed by human activities. Observed activities
include channelization, sand and gravel mining, construction of bridges, construction of grade
controls, road crossings, and encroachment by urbanization. Much of the system's shape and
form, then, is dictated by human activities rather than natural processes. This is especially true
for the portions of the Rillito River and Pantano Wash within the study area. The Rillito River
has been subjected to major channelization up- and downstream of Swan Road (Figure 10.12).
10.19
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