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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
results in auxiliary spillway flow only during the 500-year, 24-hour storm event at a depth of 0.5
ft.
Vineyard Road FRS experiences flow in the auxiliary spillway for the 500-year, 24-hour existing
conditions storm event at a depth of 0.5 ft in the south spillway. The 500-year, 24-hour event is
the only existing conditions auxiliary flow for Vineyard Road FRS. The future conditions land
use 100-year, 24-hour event for Vineyard Road results in a flow depth of 0.6 ft in the south
auxiliary spillway. The future conditions land use 500-year, 24-hour event results in a flow depth
of 1.8 ft in the south auxiliary spillway and 1.2 ft in the north auxiliary spillway.
Rittenhouse FRS experiences flow in the auxiliary spillway for the 500-year, 24-hour existing
conditions storm event at a depth of 1.0 ft. The 500-year, 24-hour event is the only existing
conditions auxiliary flow for Rittenhouse FRS. The future conditions land use 100-year, 24-hour
event for Rittenhouse FRS results in a flow depth of 0.6 ft in the auxiliary spillway. The future
conditions 500-year, 24-hour event results in a flow depth of 1.9 ft in the auxiliary spillway. The
future conditions land use “with retention” results in no flow for the 100-year 6-hour or 100-year
24-hour events.
For all three FRSs, the future conditions multi-frequency peak flow rates, peak storage volumes
and peak stages increase relative to existing conditions. The percent increase is highest during
more frequent return intervals (i.e. the 2-year storm event) and lowest during less frequent return
intervals (i.e. the 500-year event). Both the 100-year 6-hour and 100-year 24-hour future
conditions with retention models for all three FRSs result in a decrease in peak storage volume
and peak stage and no outflow in the auxiliary spillways.
The 6-hour reduced PMP overtops the Powerline FRS crest by 0.6 ft in both the existing and
future conditions. The structure is not overtopped in the existing or future conditions 24- and 72-
hour reduced PMP events.
The existing conditions 6- and 72-hour reduced PMP events overtop the Vineyard Road FRS
crest by 1.2 and 0.8 ft, respectively. The future conditions 6-, 24- and 72-hour reduced PMP
events all overtop the Vineyard Road FRS crest by 1.3, 0.4 and 0.7 ft, respectively. Vineyard
Road FRS is not overtopped in the 24-hour existing conditions reduced PMP event.
Rittenhouse FRS is overtopped in all existing and future conditions reduced PMP events. The
existing conditions 6-, 24- and 72-hour reduced PMP events overtop the Rittenhouse FRS crest
by 1.0, 0.1 and 0.3 ft, respectively. The future conditions 6-, 24- and 72-hour reduced PMP
events overtop the Rittenhouse FRS crest by 1.0, 0.1 and 0.2 ft, respectively.
2.6. Dynamic Reservoir Routing Results
HEC-1 existing conditions inflow hydrographs developed for the 100-year, 6-hour and 100-year,
24-hour storms and the 6-, 24- and 72-hour reduced PMP storms were used to update the
dynamic reservoir routing FLO-2D models prepared as part of the “DDA Inundation and
Sedimentation” (ASLD, 2007). The FLO-2D models were used to generate auxiliary spillway
outflow hydrographs and freeboard estimates for each FRS.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 4