Potential Local Problems of the Sabino Canyon Road Bridge Site. The Sabino Canyon Road
bridge site (Figure 10.15) has several potential erosion and sedimentation problems that
should be considered in the bridge design. The existing bridge has already experienced
several such problems. The flow area of the bridge appears to be inadequate for the 100-year
flood event. Over 4 ft (1.2 m) of scour has occurred around the bridge piers and abutments. In
addition, the channel is located on a reach that is migrating to the left (looking downstream).
This is causing the left abutment to be attacked. The migration tendency of Tanque Verde
Creek is largely due to its braided nature and its lack of confinement by bank stabilization or
channelization works. The lateral migration tendency is studied in more detail in the Level 2
quantitative engineering geomorphic analysis.
Considerable scour is occurring on the left side of the channel under the bridge since it is
located on the outside of a bend. This is to be expected since high-velocity flow and
secondary currents can scour sediment from the outside of a bend.
The final consideration is the gravel mining from the river. Currently, there is a mine
approximately 3,000 ft (900 m) upstream of the bridge. The pit could act as a sediment trap
and cause scour downstream of the pit near the bridge site as the water removes sediment
from the bed to regain an equilibrium sediment transport rate. Because of the distance, the
threat is not large from the present activity, considering the passage of the 100-year flood;
however, gravel mining operations located closer to the bridge site could cause problems if not
properly managed. In addition, over a long period of time the overextraction of sand and gravel
can cause significant degradation for the entire reach downstream of the operating site, and
possible headcuts upstream of the mining.
10.2.3 Level 2 - Engineering Geomorphic Analysis
Hydrology. The Rillito River is formed by the confluence of Pantano Wash and Tanque Verde
Creek northeast of Tucson and flows west-northwest about 12 miles (19.3 km) to its confluence
with the Santa Cruz River. Precipitation in the Rillito River watershed is produced by three
types of storms: general winter storms, general summer storms, and local thunderstorms. The
general winter storms usually last for several days and result in widespread precipitation.
General summer storms are often accompanied by relatively heavy precipitation over large
areas for periods of up to 24 hours. Local thunderstorms can occur at any time of the year;
however, they cover comparatively small areas and cause high-intensity precipitation for a few
hours.
The flow in the Rillito River is intermittent and Tanque Verde Creek is almost always dry, other
than during or immediately after rain. The USGS gaging station on the Rillito River near
Tucson kept daily discharge records from October, 1908 to September, 1975, after which it
was converted to a crest-stage partial-record station. The gage is located 4.75 mi (7.2 km)
upstream of the confluence of Rillito River with the Santa Cruz River and about 4 mi (6.4 km)
downstream of Dodge Boulevard (Figure 10.12).
Utilizing the USGS records at Rillito Station, all of the extreme events since 1915 are plotted in
Figure 10.16. Based on these flood data the flood frequency curves are plotted on log-normal
paper (Figure 10.17). The USGS log-Pearson Type III analysis is shown in Table 10.8. The
hydrograph of the 1965 flood observed at the Rillito River gage near Tucson (Figure 10.18)
was used to establish the 100-year flood hydrographs for Tanque Verde Creek and Pantano
Wash. The design hydrographs for the 100-year flood for Tanque Verde Creek, Sabino Creek,
Pantano Wash, Ventana Wash and Alamo Wash are given in Figure 10.19.
10.23
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