sediment transport rate to the structure are corrected by removal or storage of material behind
the structure. This process allows the channel to quickly reach equilibrium behind the structure
by producing a channel bed slope that will result in a sediment transport rate equal to the
incoming supply.
The method used to compute the sediment transport rate was the Meyer-Peter, Muller
bed-load equation and the Einstein method for suspended bed material discharge (see
Chapter 4). The shear stress on the bed of the channel was calibrated based on the grain
resistance of the bed material. The method produced a total bed-material concentration which
matched available data on the Rillito River and was consistent with similar sand-bed arid
region rivers.
Engineering Geomorphology. To determine equilibrium channel slope and possible changes in
channel alignment quantitatively, an engineering geomorphic analysis was performed. The
analysis considered various river sections in the study area for a series of water discharges
ranging from 5,000 cfs (mean annual flow) to 34,000 cfs (100-year flood). A multiple
regression was developed for sediment transport as a function of velocity and depth. This
equation was combined with Manning's equation and the water and sediment continuity
equations to form a computational procedure for determining the equilibrium slope (for details
see HEC-20, Chapter 6). Computation of equilibrium slopes was performed for the present
sediment supply and reductions of 25 to 50 percent. Computations with the reduced sediment
supplies were carried out to determine the effects of increased urbanization on the stability of
the present channel system.
The calculation of equilibrium slopes was accomplished by trial and error. The results show
(Table 10.9) that for the present supply condition most of the study area is close to equilibrium.
The exceptions occur at the bridge sites, all of which should degrade according to the
equilibrium analysis. The accuracy of the calculations at the bridge sites is less than the other
locations, since the normal depth assumption may not be accurate because of local hydraulic
effects caused by the bridges. In all, the calculations reflect the fact that the system is near
equilibrium and should not experience significant bed elevation changes if no further
disturbances are introduced. This agrees with the results of the Level 1 channel morphology
analysis.
The sediment supply reduction cases result in degradation at all locations along the system.
This is a crucial fact. It illustrates the severe consequences which would arise from a reduction
in sediment supply by urbanization, sand and gravel mining, or other activities. The equilibrium
conditions which presently exist would be destroyed and significant bed elevation changes
would result. For example, a 0.0005 decrease in equilibrium slope resulting from a 25 percent
reduction in supply would cause a degradation of 2.6 ft/mi (.492 m/km).
The possible extent of lateral migration was determined for Tanque Verde Creek by studying
the channel alignment characteristics for 13 miles (21 km) of Tanque Verde Wash above
Craycroft Road. A potentially severe lateral migration potential was identified for Tanque
Verde Wash at the Sabino Canyon Road Bridge during the Level 1 analysis. In order to
understand the migration process more thoroughly, a 13-mile (21 km) reach of Tanque Verde
Wash above the confluence with Pantano Wash was studied closely to determine the range of
measured.
10.30
Previous Page
