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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
storage volume and channel capacity without overtopping and provide the required one foot of
freeboard (or from equation 6.25 from the DDMMC if greater than one foot).
The total opinion of probable construction costs ranged from $185,000,000 to $218,000,000. 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 Converting
Rittenhouse FRS to a Levee (Alternative 6A)
The concept for Alternative 6A 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 converted to a levee and
outlet into the Vineyard Road FRS flood pool. The southern 2/3 of the Vineyard Road FRS would
be raised sufficient to account for estimated future subsidence and sediment storage.
The levee is designed to convey the 100-year, 24-hour flood with three feet of freeboard per
Federal Emergency Management Agency (FEMA) requirements. The crest elevation varies with an
overall levee crest slope of 0.0005 ft/ft. The levee has a 3:1 (H:V) upstream slope and a 2:1 (H:V)
downstream slope and a top of crest width of 14 feet. The upstream slope would be treated with
erosion protection which consists of rock riprap overlaying a geotextile. The converted
Rittenhouse levee was evaluated both in HEC-1 and FLO-2D modeling. The required levee height
was determined from the maximum water surface elevation against the FRS embankment along the
structure. FLO-2D produced the most conservative results and was used to determine the required
crest elevations.
The total opinion of probable construction costs for Alternative 6A ranged from $187,000,000 to
$208,000,000. 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 Converting
Rittenhouse FRS to a Levee (Alternative 7)
The concept for Alternative 7 includes replacing Powerline FRS with a 100-year channel draining
to Vineyard Road FRS. Vineyard Road FRS would be segmented to raise and rehabilitate the
northern ½ of the FRS and convert the southern ½ of the FRS to a levee/ floodway that drains south
to Rittenhouse FRS. Rittenhouse FRS would be converted to a 100-year basin that drains to Queen
Creek.
The 6-hour planning PMF future conditions HEC-1 model was used to determine the flood volume
required for the northern ½ of Vineyard Road. The existing Vineyard Road HEC-1 model was
modified to exclude basins no longer contributing to the FRS. The HEC-1 model was updated with
a new 6-hour PMP calculation based on the new contributing drainage area.
The 100-year, 24-hour storage volume required for the basin located at the southern ½ of Vineyard
Road and Rittenhouse was determined from the existing conditions 100-year HEC-1 models. The
USDA- NRCS Page 5-7 January 2013
Kimley-Horn and Associates, Inc.