The construction layout is shown in Figure 10.11. The riprap design and the gravel filter design
at the spur nose are the same as those used in the continuous revetment. The length requiring
nose riprap size can be estimated by considering the flow separation zone. The riprap size in
the shank inside this zone may use a smaller rock size. A reduction of fifty percent in rock size
is determined by considering the decrease in flow velocity. The riprap at the downstream side
of shank may be eliminated if a larger risk is accepted. This is because the downstream side
of the shank is not expected to be subjected to a strong velocity. However, it may be subject to
scour due to overtopping.
The crest width of rock riprap spurs usually ranges from 3 to 20 ft (1 to 6 m) and the side slope
from 1.25:1 to 5:1. Considering the convenience in hauling and placing rock riprap, the crest
width of spurs is determined to be 15 ft (5 m). The side slope was determined to be 2.5:1.
A second set of alternatives would result from replacing the rock riprap with soil-cement
revetment in design alternatives 1, 2, and 3. This alternative may be of interest because large
rocks can be very difficult to obtain and soil-cement revetment may be manufactured at the site
without much difficulty (see Chapter 6 and HEC-23).
10.2 OVERVIEW EXAMPLE 2 - RILLITO RIVER
The Rillito River System in Tucson, Arizona provides an example of the problems encountered
in bridge crossing design. The objective of this example is to illustrate the methodologies used
in the analysis and design of a bridge crossing, including a three-level analysis and the
evaluation of conceptual alternatives. 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.
Two bridge sites are reviewed which provide insight into several outstanding problems
characteristic of the Rillito system. These are the Sabino Canyon Road site with an existing
bridge crossing (constructed 1936) and the Craycroft Road site with a dip crossing (where the
roadway is at the same elevation as the channel bed). The study reach of Rillito River includes
approximately 11.5 miles (18.5 km) of channel extending from Dodge Boulevard to Agua
Caliente Wash (Figure 10.12). This includes 2 miles (3.2 km) on the Rillito River below
Craycroft Road, 6-1/2 miles (10.5 km) upstream of Craycroft road on Tanque Verde Creek, 2
miles (3.2 km) upstream of Craycroft Road on Pantano Wash and 1 mile (1.7 km) on Sabino
Creek upstream of the confluence with Tanque Verde Creek.
The history of flood events and the recent geomorphology of the Rillito system has shown that
it is very dynamic and illustrates the characteristics of a braided river. The channel is steep,
dropping at the rate of 21 ft per mile (3.98 m per kilometer). The bed material is predominantly
in the medium to coarse sand sizes. The natural sinuosity of the river is low. Additionally, the
river is generally unstable, changes alignment rapidly, and carries large quantities of sediment.
A large portion of the river system is in the metropolitan area of Tucson where human activities
in, and adjacent to, the river environment have induced a number of changes in the system.
Primary impacts on the system have occurred due to encroachment by urban development
and channelization of segments of the river. Uncontrolled sand and gravel extraction has also
led to even more rapid and significant changes in the river system. A secondary effect of
urbanization is a reduction in sediment supply from tributaries draining urban areas.