Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
for the combined structure. A single HEC-1 model was created combining the full Rittenhouse
watershed and the contributing Vineyard Road subbasins. The model was updated with the new
PMP and a new stage-storage-discharge curve for the combined structure. The existing 100-year,
24-hour HEC-1 models were used to size the channels and basins for Powerline and the northern
1/3 of Vineyard Road.
There are two primary constraints for Alternative 5. First, the combined Vineyard Road and
Rittenhouse FRS would include a new principal spillway that would drain to Queen Creek Wash
via a new constructed channel from the southern end of the combined FRS to Queen Creek Wash.
The slope of the channel is limited by the invert elevation of the tie-in with Queen Creek Wash.
Second, the alternative is also limited by the existing capacity of Queen Creek. The Sonoqui
Detention Dike, owned and operated by the CAP, discharges to the existing CAP overchutes at the
CAP canal. The major design consideration for a new overchute at Queen Creek Wash is to not
adversely impact the capacity and stability of the existing Queen Creek Wash downstream of the
CAP canal.
The opinion of probable cost for replacing Powerline FRS and the northern 1/3 of the Vineyard
Road FRS with basins and channels, raising and rehabilitating the southern 2/3 of the Vineyard
Road FRS and combining the southern 2/3 of the Vineyard Road FRS with a new Rittenhouse FRS
as considered in this Level I analysis is $183,952,000. This alternative was eliminated from further
detailed study in the workshop conducted on April 26, 2011 due to high costs.
Combination of Decommissioning and Rehabilitation of Vineyard Road and Rittenhouse FRS
(Alternative 6)
The concept for Alternative 6 includes replacing the Powerline FRS, the Powerline IDSM, and the
northern 1/3 of the Vineyard Road FRS with detention basins in low-moderate fissure risk hazard
zones and channels in moderate fissure risk hazard zones. The basins and channels would drain to
a 100-year detention basin near the existing shared Powerline FRS/Vineyard Road FRS north
auxiliary spillways and outlet to the Powerline Floodway.
The southern 2/3 of the Vineyard Road FRS would be raised and rehabilitated and outlet north
towards the Powerline Floodway. The existing Rittenhouse FRS would be raised and rehabilitated
and outlet into the Vineyard Road FRS flood pool (as under existing conditions). The southern 2/3
of the Vineyard Road FRS and Rittenhouse FRS would be raised sufficiently to account for
estimated future subsidence and sediment storage.
The interconnected basins and channels for Alternative 6 were preliminarily sized using HEC-1 and
approximate basin geometry. The trapezoidal channels serve as in-line connections between the
basins and are sized to convey routed discharge from 100-year, 24-hour basins to the large
detention basin near the existing shared Powerline and Vineyard Road FRS auxiliary spillways.
concentration points were inserted at the basin location to determine the required storage volume
and the basins were located on existing topography. Channels were sized sufficiently to route flow
between the basins. The basins and channels were then input into the FLO-2D models for the
alternative. Initially, the dynamic, two-dimensional routing results indicated that the preliminary
basin and channel sizes were not sufficient to pass the 100-year, 24-hour event without
overtopping. Storage volume and channel capacity were added by iterative trial to provide enough
USDA- NRCS Page 5-6 January 2013
Kimley-Horn and Associates, Inc.