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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
The NRCS criteria for sediment storage for dams may be found in Technical Release 60, Earth
Dams and Reservoirs (TR-60). TR-60 refers to the NRCS publication National Engineering
Handbook (NEH) No 3 – Sedimentation (NEH-3) for criteria and general guidelines for
estimating sediment yield. The NRCS National Watershed Program Manual (NWM) (Dec,
2009) provides guidelines for the rehabilitation of existing dams and sediment storage volume
requirements. On page 78, Section 505.35, B, paragraph iii states that for the rehabilitation of an
existing dam: “Sediment storage life will be for the longest reasonable period practical (100
years maximum) but in no case will the evaluation life be less than 50 years”.
2.10. Freeboard
Freeboard was estimated as the sum of wave runup plus wave setup in accordance with
procedures described in Freeboard Criteria and Guidelines for Computing Freeboard Allowances
for Storage Dams, ACER Technical Memorandum No. 2 by the United States Bureau of
Reclamation (USBR), revised in 1981 (ACER-TM2). Wave runup is the movement of water up
a structure on the breaking of a wave and the amount of runup is the vertical height above still
water level. Wave setup is the vertical rise in the still water on the leeward side of a body of
water caused by the wind stresses on the surface of the water. The two are summed for wave
action.
The existing embankments for the PVR structures have been in place since 1968-1969 and are
assumed fully settled. For the purposes of this planning level study, it is assumed that additional
freeboard to compensate for future settlement is not required. Freeboard also accounts for
anticipated future land subsidence at each structure.
2.11. Level II Hydrology and Hydraulics Technical Memorandum
The purpose of this Hydrology and Hydraulics Technical Memorandum was to document the
hydrologic and hydraulic approach, methodology and results for evaluation of the Level II
planning alternatives for the PVR project. This hydrologic and hydraulic study was used to
conduct preliminary evaluations of the identified alternatives.
The scope of this study was to conduct updated hydrologic/hydraulic analyses to evaluate future
project alternative conditions for the PVR Level II analysis planning alternatives. Previous
hydrologic and hydraulics studies conducted by Kimley-Horn for existing PVR conditions are
used as a baseline and updated for project alternatives. The tasks performed in developing this
memorandum are listed below.
• Update previously prepared HEC-1 hydrologic models for five planning alternatives.
• Conduct FLO-2D modeling for the five planning alternatives. Model alternatives
including dam rehabilitation for the inflow design flood and alternatives including basins
and channels for the 100-year, 24-hour storm.
2.12. Final Level III Alternatives Summary Report
This report documents the evaluation of Level III alternatives for the PVR structures and
presents the results of the analysis of those alternatives. The analysis includes hydrology and
hydraulics for each of the Level III alternatives, geotechnical and geohazard evaluation,
identification of existing and planned utility corridors and transportation infrastructure,
development of conceptual landscape themes and recreational opportunities, and development of
preliminary construction cost estimates.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 7