There is also a large instream gravel pit below Swan Road. The Pantano Wash system has a
large instream gravel pit below, and bank stabilization works in the vicinity of Tanque Verde
Road. The results of these activities have been to change these systems from their natural
braided forms to defined channels. Pantano Wash still possesses a stretch of over 3,000 ft
(900 m) which is braided. Tanque Verde Creek, however, has experienced less impact from
human activities than the other two systems. The islands, bends and natural channel
alignment observed in 1941 aerial photographs of Tanque Verde Wash are still intact.
The Tanque Verde Creek system should be classified as a braided system (see Section 5.4.5).
The evidence of multiple channels, islands, and shifting alignment support this conclusion. A
braided river can be identified by Equation 5.2.
S Q 0.25 ≥ 0.01
in which S is the average bed slope and Q is the dominate discharge (cfs). The mean annual
flood of 5,000 cfs (142 m3/s) is assumed to represent dominant conditions in the system. The
average slope for 13.1 miles (21 km) of the Tanque Verde Creek and Rillito River is 0.0044.
This slope and discharge give a value of 0.037, which is well within the braided range (Figure
10.13). Pantano Wash has a slightly steeper grade, which would place it even further into the
braided range. Even though much of the river has been channelized, it should be recognized
that the river is in the braided range and, hence, is very dynamic.
Often, the general response of a river system to a flood event can be assessed qualitatively by
studying its profile and plan view. This is especially true of a system which has been altered by
human activity. This type of analysis is based on estimating the relative velocity along the
system. In locations where a channel is constricted or the profile steepens, the velocity would
be expected to increase. Since velocity is the dominant factor in determining sediment
transport rate (when the sediment size does not change greatly), areas with large increases in
velocity should degrade; areas where velocities are slowed considerably should experience
aggradation. This is expressed in Lane's relationship (Equation 5.28), which can be written:
Q s D50 α Q S
In this relationship, Qs is the sediment transport rate, D50 is the median sediment size, Q is the
flow rate of water and S is the slope of the bed.
For example, the instream gravel pit below Swan Road will trap sediment and reduce sediment
supply to the downstream reach. This can be expressed using the Lane relationship as:
Q s D50 α Q S -
-
From this, one would expect an overall response of possible degradation in the reach of the
Rillito River below the gravel pit to Dodge Boulevard. A similar application of the Lane
relationship to various reaches in the study area indicates that over half of the channel reaches
are well balanced for sediment transport. None of the reaches has a great potential for either
aggradation or degradation. In all, the system should not experience large bed elevation
changes except for those related to increased development and localized flow conditions. This
qualitative assessment is confirmed by more detailed Level 2 analyses in the next section.
10.20
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