(A) Bank protection requirements.
(1) Bank protection measures to be designed along alluvial watercourses will require scour analyses to determine bank protection toe-downs, unless the entire channel bottom will be lined. Toe- downs refer to the vertical distance that the bank protection extends below the invert of the channel. The design scour depth, to which the bank protection toe-downs must extend, are determined by adding various scour components, as appropriate, and applying a safety factor to the result. The scour components that must be addressed include the following:
(a) General scour.
(b) Anti-dune trough depth.
(c) Local scour.
(d) Bend scour.
(e) Low-flow thalweg.
(2) Each of these scour components must be determined and summed in order to determine the Total Scour. A safety factor of 1.3 is typically applied to the Total Scour to determine the depth of toe- down for the bank protection. Detailed procedures and equations for estimating each of the above scour components is provided within the following publications:
(a) Design Manual for Engineering Analysis of Fluvial Systems, Arizona Department of Water Resources (1985).
(b) Standards Manual for Drainage Design and Floodplain Management in Tucson, Arizona, (1989).
(c) Watercourse Bank Stabilization SSA 7-98, Arizona Department of Water Resources (1998).
(3) Engineers engaged in the analysis and design of flood-control and erosion-control facilities on sand and gravel bed watercourses within the town shall become familiar with these references and utilize the procedures contained therein. The procedures presented within these manuals are acceptable for designing bank protection toe-downs within the town.
(B) Grade-control structures.
(1) When channelization of a natural watercourse occurs, the top width is usually narrowed, and channel roughness is normally decreased. This usually results in an increase in flow velocity and possibly depth of flow, with a corresponding increase in sediment-transport capacity. Sediment transport capacity then exceeds sediment supply; then if the channel is composed of transportable sediments, the channel bed will begin to degrade. Another factor that contributes to this degradation is the upstream urbanization of the watershed. Urbanization typically increases runoff and decreases sediment yield, which further increases sediment transport. The result of all these factors is that channel bed degradation will continue until the channel slope flattens enough that the sediment transport rate equals the incoming sediment supply. Streambed degradation can become extreme over time and can threaten underground utilities, bank protection, culverts and other structures that are within or near the watercourse. Under these conditions, grade-control structures or lining of the channel bed are usually required in order to prevent damage by streambed degradation.
(2) An analysis to assess the potential need for grade-control structures will be required for all proposed channelization projects on alluvial or sand and gravel bed watercourses within the town. Analysis procedures to perform this assessment are provided within the two previously referenced manuals (i.e. Arizona Department of Water Resources and City of Tucson).
(C) Bridge scour. In addition to standard hydraulic analyses associated with bridge design, a scour analysis is also required for proposed bridges located within alluvial or sand and gravel bed watercourses. These analyses require assessment of the scour components listed previously within this chapter, with special attention given to local scour at bridge piers and abutments. In addition to the two previously cited references relating to scour analysis, the following publication provides additional analysis procedures specific to scour analyses of bridge structures: Highways in the River Environment (1990), U.S. Department of Transportation, Federal Highway Administration.
(D) Building setbacks.
(1) Prudent building setbacks shall be established for all proposed development adjacent to alluvial watercourses within the town. Channel bark erodibility may be assessed, and erosion setbacks may be established by using procedures developed by the Natural Resource Conservation Service. These procedures are presented in detail in the previously referenced City of Tucson Standards Manual for Drainage Design and Floodplain Management (1989) and consist of the Allowable-Velocity Approach, Tractive-Stress Approach, and the Tractive-Power Approach. These procedures are also presented in State Standard for Watercourse System Sediment Balance, Guideline 1 Lateral Migration Setback Allowance for Riverine Floodplains in Arizona SSA 5-96, Arizona Department of Water Resources (1996). In lieu of performing a detailed erosion study, the following equations (8.1a, 8.1b and 8.1c) may be used to establish setbacks to guard against lateral migration of unstabilized channel banks associated with alluvial watercourses which have drainage areas of less than 30 square miles (City of Tucson, 1989):
SB
$
1.0 (Q
p100
)
0.5
, for r
c
/T
$
10 (Eqn. 8.1a)
SB
$
1.7 (Q
p100
)
0.5
, for 5 < r
c
/T < 10 (Eqn. 8.1b)
SB
$
2.5 (Q
p100
)
0.5
, for r
c
#
5 (Eqn. 8.1c)
Where:
SB = Minimum setback, in feet, measured from the top edge of the highest channel bank or from the edge of the 100-year floodplain limit, whichever is closer to the channel centerline.
Q
p100
= Peak discharge of 100-year flood, in cubic feet per second.
r
c
= Radius of curvature of channel centerline, in feet.
T = Top width of channel, in feet.
Notes: 1. Equations 8.1b and 8.1c apply only to setbacks on the outside of the channel bend. Equation 8.1a may be used for setbacks on the inside of a channel bend.
2. See Figure 6.2 of this chapter for determining r
c
and T
w
(2) Lesser setbacks than those determined from Equations 8.1 may be allowed provided they can be justified by use of the following methods, which would indicate that a lesser setback is appropriate. The Public Works Engineer must approve the use of lesser setbacks in writing.
(3) A detailed sediment transport analysis, prepared by an Arizona Registered Professional Civil Engineer.
(4) The Allowable-Velocity Approach, Tractive-Stress Approach, or Tractive-Power Approach (City of Tucson, 1989), any or all of which must indicate that the channel banks are not erosive for the flow conditions associated with runoff events up to and including a 100-year flood on the affected watercourse.
(Res. 1637, passed 2-28-02)