River and watershed information consists of upstream sediment loading information and the
discretized hydrographs for the mainstem and tributaries. The mainstem hydrograph was
discretized into 12 time steps with 5 time steps on the rising limb of the hydrograph. Time
steps vary in length from 0.5 hours at the peak to 4.0 hours for the end of the recession limb.
Time steps average typically 1.5 hours. The sediment output from the Pantano Wash was
used as input to Tanque Verde at the Craycroft Road section. Two cases of sediment supply
from Pantano were considered. The first case assumed stable conditions existed in the
Pantano reach. This case corresponds to a no-fail grade control condition. The second case
assumed the roadway grade control would fail during the 100-year flood. The sediment load
resulting from the failure of the roadway was calculated independently of the routing procedure.
Four Tanque Verde sediment routing analyses were conducted for the as-is condition. The
cases analyzed are:
Case I. Tanque Verde floods with overflow to the Pantano Wash and no grade control failure.
Case II. Tanque Verde floods with overflow to the Pantano Wash and the grade control fails.
Case III. Pantano Wash floods with overflow to the Tanque Verde and no grade control failure.
Case IV. Pantano Wash floods with overflow to the Tanque Verde and the grade control fails.
Figures 10.22 and 10.23 show the bed level changes over the duration of the 100-year flood
for both Tanque Verde flooding and Pantano flooding. The bridge sites show only slight
aggradation/degradation for the 100-year flood if the Pantano grade control structure does not
fail. This agrees with the conclusion drawn from the geomorphic analysis. The major potential
cause of aggradation is the failure of the existing Pantano roadway grade control. If the
existing Pantano Wash roadway grade control structure fails during Pantano flooding, there is
a maximum aggradation downstream of Craycroft Road of 4.6 ft (1.4 m). The erosion and/or
headcut that may occur during Pantano flooding can supply significant amounts of sediment in
a short period of time (Figure 10.23).
10.2.5 Results of Analysis
Each of the sites evaluated has its own unique problems that must be considered in the
formulation of alternative designs. This section presents several conceptual alternatives based
on the analysis of the as-is conditions. Analysis of the design alternative is broken into three
areas, (1) low chord criteria, (2) total scour criteria and (3) other additional considerations.
The criteria for an acceptable low chord elevation for this alternative is that the bridge can pass
the 100-year flood peak with sufficient freeboard between the water surface elevation from a
rigid-bed hydraulic analysis, plus additional components resulting from sand-wave movement,
general aggradation, and superelevation caused by flow curvature. In addition, an increment
of height is added to provide freeboard for debris passage.
When designing a bridge foundation, proper consideration of scour must be made to determine
the required safe depth of piles or other supports. A design which gives adequate support for
the structure when the channel bed is at its initial elevation may be inadequate after scour
occurs and lowers the channel bed. The physical processes that must be considered are
long-term changes in bed elevation, local scour, contraction (or general) scour, and passage of
sand waves. The total scour is the sum of these, and must be subtracted from the initial design
elevation to establish the design depth for all supports. The supports must have a depth of
burial below this elevation sufficient to support the structure.