Figure 10.13. Slope-discharge relationship for Rillito River.
Each proposed or existing bridge site has possible problems associated with local erosion and
sedimentation processes. These problems are identified below.
Potential Local Problems at the Craycroft Road Site. Location of a bridge at the confluence of
Tanque Verde and Pantano Wash (Figure 10.14) could cause several problems. First, the
confluence of two sand-bed rivers is usually very dynamic and can shift upstream or
downstream and laterally quite quickly. This is especially true when an abnormal sequence of
events results in a shift in the relative balance of flows between the two rivers. To compound
this problem, the grade control structure (dip crossing) has created a situation in which
Pantano Wash has a bed elevation several feet higher than Tanque Verde Creek at the same
location. This provides an additional tendency for flows from Pantano Wash to migrate toward
Tanque Verde Creek, where the flow could attack bridge piers and abutments at angles other
than designed. As a result, local scour around piers and abutments of a bridge at this site
could be significantly increased.
Neither Pantano Wash nor Tanque Verde Creek can contain a 100-year flood within its own
channel. Since the two usually do not reach peak flows at the same time, the flow spills out of
the flooding channel across the floodplain area between the two channels and into the opposite
channel. In the process, the overflow deposits most of its sediment in the floodplain and the
clear water entering the opposite channel causes degradation. There is also the problem of
poor flow alignment past piers and abutments under these conditions. These problems are
analyzed further when engineering alternatives to eliminate the problems are presented.