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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
2.7. Principal Spillway Hydrograph, Stability Design Hydrograph and Freeboard
Hydrograph
Kimley-Horn prepared the “Principal Spillway Hydrograph, Stability Design Hydrograph and
Freeboard Hydrograph Technical Memorandum” (PSH, SDH and FBH Technical Memorandum,
Kimley-Horn 2010) to document the future land use conditions principal spillway hydrograph
(PSH) analysis and the existing and future conditions stability and integrity analysis for the
existing PVR auxiliary spillways.
Kimley-Horn used the PVR FRSs Existing Conditions PSH Analysis single basin models as the
basis for the future land use PSH analysis. The NRCS SITES Analysis Computer Program was
used to complete the analysis. Future conditions land use was taken from the “Existing and
Future Hydrology and Hydraulics Update” (Kimley-Horn, 2010). The time for PSH drawdown
increased for Powerline and Vineyard Road FRSs during future conditions. The PSH drawdown
did not change for Rittenhouse FRS because the composite CN value did not change for future
land use conditions. None of the FRSs experience auxiliary spillway flows during the 100-year,
10-day event for existing or future land use conditions.
An existing and future conditions stability and integrity analysis for the existing PVR auxiliary
spillways was completed. The 6-hour reduced PMP storm event was determined to be the
critical storm duration for each FRS in the “Final PMP Tech Memo” (Kimley-Horn, 2010) and
was used to develop the SDH and FHB.
Precipitation depths used for the FBH and SDH equation were taken from the “Existing and
Future Conditions Hydrology and Hydraulics Update Technical Report” (Kimley-Horn, 2010).
The reduced PMP value was used for the SDH and FBH. SDH and FHB were calculated using
the 6-hour HMR-49 distribution.
Precipitation depths were input in HEC-1 models from the “Existing and Future Conditions
Hydrology and Hydraulics Update Technical Report” (Kimley-Horn, 2010h) to determine the
flood hydrographs at each structure. HEC-1 inflow hydrographs were imported into FLO-2D
and dynamically routed through the reservoir to obtain the spillway hydrographs. The
hydrographs were then input into SITES to evaluate the stability and integrity of each auxiliary
spillway.
The allowable stress for each spillway was calculated using a spreadsheet developed by the
NRCS that follows Agricultural Handbook (AH) 667 and TR-60 methodology. The soil
effective stress was obtained from the SITES output file for each spillway and compared to the
calculated allowable stress. The soil is stable for Vineyard Road and Rittenhouse auxiliary
spillways under existing conditions with the SITES soil effective stress being less than the
calculated allowable stress. The soil is erodible for Powerline auxiliary spillway under existing
conditions where the effective stress is greater than the calculated allowable stress. Under future
conditions, the soil is stable for the Vineyard Road north and Rittenhouse auxiliary spillways.
The soil is erodible for the Powerline and Vineyard Road south auxiliary spillways.
An integrity analysis was completed for each auxiliary spillway using the SITES model. There
are four spillways, one at Powerline, two at Vineyard Road, and one at Rittenhouse. Soil borings
were taken at each spillway and documented in detail in the “Final Geotechnical Summary and
Analysis Report” (AMEC, 2010).
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 5