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

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Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment Alternative 1 – No Action: In this alternative the Powerline FRS and IDSM would remain in place and would continue to provide flood protection up to the 100-year event, given assumed future conditions on the upstream watershed to be the same as existing conditions, until year 2017 (end of design life). The Powerline FRS would not meet all of the standards set by ADWR and the NRCS for public safety and reliability. The previously described inadequacies in dam foundation, central filter, auxiliary spillway capacity, and embankment cracking would remain. At the end of the design life period, the Sponsors would continue to operate and maintain the dam until such time that available construction funding becomes available to replace the dam. Therefore, Alternative 1 is not acceptable to the Sponsor and local community as a viable solution since it does not meet the purpose and need to reduce the risk of loss of life due to catastrophic dam failure. The structures would continue to have safety and performance deficiencies and continue to pose a significant risk of catastrophic failure until eventual rehabilitation or replacement of the structures. Alternative 2 – Decommissioning: The alternative eliminates impoundment by the Powerline FRS so that it will no longer need to conform to dam safety criteria or performance standards. Alternative 2 provides a new channel that replaces the Powerline FRS. The channel will convey floods up to the 100-year event to the Vineyard Road FRS. Therefore, Alternative 2 meets the project purpose and the Sponsor’s formulation goals. 6.2.1 RISK AND UNCERTAINTY A variety of factors contribute to the potential for flood control structure failure, including storm event intensity, control structure construction materials and techniques, and operation and maintenance activities. These uncertainties described below will be addressed during the design phase of the project. Under Alternative 1 – No Action, the Sponsor (District) would continue to operate and maintain the dam in its present condition and in accordance with the current Emergency Action Plan. The Powerline IDSM is an interim measure (design life of 15 years) to mitigate the potential for an earth fissure near or under the Powerline FRS. The alignment/location of the IDSM is based on current investigations and analyses for projected future earth fissure risk zones. The District will continue the on-going monitoring program at Powerline FRS for indications of further subsidence and potential of earth fissure development. The dam would continue to have structural inadequacies in auxiliary spillway capacity. As a result, there is a risk of dam break until the FRS is decommissioned. Current hydrologic analytical results indicates the FRS and auxiliary spillway is not capable of passing the planning 6-hr PMF (for both existing and future land use conditions), which overtops the dam by 0.6 foot. The dam experiences no flow in the auxiliary spillway for any of the existing conditions multi-frequency (2-year through 500-year) events. The future conditions land use for Powerline FRS results in auxiliary spillway flow only during the 500-year, 24-hour storm event at a depth of 0.5 ft. The erosion analysis using the SITES model indicates that the auxiliary spillway erodes during the stability design hydrograph and breaches during the existing and future conditions freeboard design hydrographs. As a result there remains a risk of breach of the auxiliary spillway. USDA- NRCS Page 6-6 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment Under Alternative 2 – Decommissioning, the Powerline FRS would be replaced by a 100-year channel. This alternative is subject to uncertainty because of missing or approximate information utilized during the planning study. Uncertainties and the effects of these uncertainties, and suggestions for future studies are listed below. Geohazard Investigations and Analyses The proposed Powerline Channel includes channel segments that provide mitigation measures within the earth fissure risk zones. The location of the “hardened” channel segments are based on delineation of these zones conducted with current data at the time of this Plan/EA and projected in a linearwise fashion for planning purposes. The limits of the channel hardening (if required) will be determined in final design. The future projection of the delineation of the risk zones and level of risk zone may change during the design life (100-years) of the proposed project. Continued monitoring of land subsidence and geohazard investigations are recommended for the design life of the project. Construction Sequence The construction of the Powerline Channel needs to occur after the dam rehabilitation is completed for the Vineyard Road FRS (the rehabilitation of the Vineyard Road FRS is described in the separate document “Draft Supplemental Watershed Plan and Environmental Assessment for the Vineyard Road and Rittenhouse FRS” by Kimley-Horn, January 2013). The dam rehabilitation improvements for Vineyard Road FRS include/provide for flood storage capacity from the Powerline watershed for the 100-year flood event. Powerline FRS and IDSM Embankment The construction cost estimate assumed that the embankment for the Powerline FRS and IDSM would be removed, the sites graded out, and no special handling of the excess spoils were considered. The embankment soils are owned by the Arizona State Land Department. The embankment for the Powerline FRS may be opted by the District to be breached in one or more locations instead of being removed. This will be determined in final design with considerations from ASLD and ADWR. Hydrology and Hydraulics The SITES modeling for the principal spillway hydrograph (PSH), stability design hydrograph (SDH), and integrity or freeboard hydrograph (FBH) was completed for the existing dam conditions as part of this planning study. SITES modeling was completed for the alternatives (8A and 8B) to ascertain the volume and peak discharge contributing from the Powerline watershed to the Vineyard Road FRS watershed via the Powerline Channel. PMF modeling was conducted using HEC-1 and FLO-2D to estimate the contribution from the Powerline watershed to the Vineyard Road FRS from the Powerline Channel. However, SITES hydrologic and spillway erosion modeling was not completed for the alternatives for the SDH or the FBH. These need to be completed as part of final design to determine the final Vineyard Road FRS flood volume storage and assist with design of the auxiliary spillways for Vineyard Road FRS. The PVR planning study prepared a planning level Probable Maximum Precipitation (PMP) that is reduced over using the results from Hydrometerological Report 49 (HMR 49) methodology. The planning level PMP was transformed into the planning Probable Maximum Flood (PMF) using the USDA- NRCS Page 6-7 January 2013 Kimley-Horn and Associates, Inc.
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DRAFT SUPPLEMENTAL WATERSHED PLAN A
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