Subsurface bed material samples and bank material samples were also taken. The
subsurface bed material is slightly coarser in most cases, but still lacked sizes in the
nontransporting range. Bed material samples had more fine material and these distributions
varied substantially from one location to the next.
Riparian Vegetation. The yield of debris to the bridge sites was determined by visual
inspection of aerial photographs for the riparian zone of the system. A review of these photos
indicated that large trees along the bank present the greatest problem. Trees along the banks
were counted and the root zone size estimated. The root zone is the area of the tree capable
of supporting the weight of the tree. This diameter is estimated as 5 to 6 ft (1.5 - 1.8 m) for
trees on the Tanque Verde.
Tree yield will be from the banks of the river for large floods. The accumulation of smaller
debris at a bridge is assumed to occur only in conjunction with the trapping of larger debris.
Actual debris yield and trapping at a bridge were analyzed by a qualitative approach. For
additional information on estimating debris accumulation, see HEC-20, Chapter 4 (Lagasse et
al. 2001).
Resistance to Flow. During the December 1965 flood, the Rillito River was in upper regime,
having antidunes with breaking waves. Similarly, the geomorphic analysis shows that the bed
forms of the channels in the study system will be antidunes or standing waves during floods.
Resistance to flow associated with antidunes depends on how often the antidunes form, the
area of the reach they occupy, and the violence and frequency of their breaking. If many
antidunes break, resistance to flow can be large because breaking waves dissipate a
considerable amount of energy. With breaking waves, C / g may range from 10 to 20, and
Manning's coefficient n ranges from about 0.019 to 0.038 for the flow depths being considered.
The existing channels will not contain all of the 100-year flood flows. Some overbank flow will
occur. Sparse vegetation, brush, trees and houses are in the floodplain. These elements
increase the resistance to flow. For a conservative erosion and sedimentation analysis (high
channel velocity), a Manning's roughness of 0.025 for the main channels was assumed for this
study. For overbank flows, a higher Manning's n value of 0.05 was used from Dodge
Boulevard to Sabino Creek and an n value of 0.06 was used from Sabino Creek to Agua
Caliente Wash.
Sediment Transport Rates. The rate of sediment transport is the most important factor in
conducting a quantitative determination of aggradation and degradation in the channel. Since
very little actual data were available to calibrate the sediment transport rate determinations,
there is some uncertainty inherent in the procedure used to compute sediment supply rates.
Fortunately, the uncertainty in the results was reduced by several factors. An indirect check of
the sediment transport rate determinations was available on Tanque Verde Creek. This area
has undergone the least change in river form of all the locations in the study area. The 1941
and present aerial photographs show this portion of the system to have remained nearly
unchanged. Therefore, it is expected that these reaches must have sediment transporting
capacities near equilibrium. The engineering geomorphic analysis is in agreement with this
conclusion.
Another factor that helped provide a reliable determination of sediment supply was the grade
control structure on Pantano Wash at the Craycroft Road bridge site. A channel will quickly
come to equilibrium behind such a structure since the results of an excess or imbalance in
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