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 8.0 THE PREFERRED ALTERNATIVE 8.1 RATIONALE FOR ALTERNATIVE PREFERENCE The Preferred Alternative is Alternative 2 – Decommissioning. The decommissioning will be accomplished by replacing the Powerline FRS and Powerline IDSM with a 100-year channel to discharge to the Vineyard Road FRS. The decommissioning alternative will meet the plan purpose of flood prevention and the Sponsor’s formulation goal of providing continued flood protection and eliminating the existing safety deficiencies associated with the Powerline FRS. The existing FRS would no longer be defined as a jurisdictional dam by ADWR. A plan schematic/exhibit for Alternative 2 is included in Appendix C. The NRCS has identified in their memorandum with the subject of “NED Alternative and Preferred Alternative for the PVR FRS Rehabilitation Project” (NRCS, Jan. 2013) that the NED alternative (least-cost technically-feasible) is identified as consisting of the following major components and filter options for the Powerline FRS, Vineyard Road FRS and Rittenhouse FRS: • Replace Powerline FRS with 100-year channels with hardened segments; • Raise and rehabilitate Vineyard Road FRS with the ‘hybrid” filter option; • Convert Rittenhouse FRS to a levee. The approximate total project cost for this alternative/filter options is $83M as noted in the Level III report and is reported as the least cost alternative. Consistent with P&G, the alternative that has the highest positive net benefits would be identified as the NED Plan. However, per Section 502.2 of the NRCS National Watershed Program Manual, “In cases where human life is at risk in the event of catastrophic failure of an existing dam that does not meet current safety and performance standards, an overriding reason for an exception to the NED plan requirement exists. To avoid seeking individual exceptions in such cases, the NED plan is defined as the federally assisted alternative with the greatest net economic benefits.” Section 5.2 presented a discussion of Alternatives 8B and 8A in relation to the Preferred Alternative and the NED Plan. As noted Alternatives 8A and 8B were carried forward into a Level III analysis (Level III Final Alternatives Summary Report – KHA January 2013). The Level III analysis conducted further evaluations of Alternatives 8A and 8B. Both alternatives are identical in the replacement of Powerline FRS with a new 100-year flood channel. The alternatives differ in the treatment of Rittenhouse FRS and in the filter treatments for Vineyard Road FRS and Rittenhouse FRS in Alternative 8B. The Level III report identified the Preferred Alternative as Alternative 8A with the option of a ‘hybrid’ filter for the dam rehabilitation of Vineyard Road FRS, converting the Rittenhouse FRS to a levee, and replacing Powerline FRS with a 100-year channel. The Preferred Alternative and the NED Plan for the Powerline Channel is one in the same for this Plan/EA. The Preferred Alternative and NED Plan for the Vineyard Road FRS and Rittenhouse FRS are provided in the “Draft Supplemental Watershed Plan and Environmental Assessment for the Vineyard Road FRS and Rittenhouse FRS” (Kimley-Horn January 2013). USDA- NRCS Page 8-1 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment 8.2 MEASURES TO BE INSTALLED The features of the Preferred Alternative are summarized below which are based on the results of the technical investigations and analyses, and in accordance with the Sponsor and NRCS requirements. 8.2.1 POWERLINE CHANNEL The concept for the Preferred Alternative includes constructing a new 100-year channel upstream of Powerline FRS and removing both the existing FRS and IDSM embankments. The proposed Powerline channel was sized using the existing conditions 100-year discharge. Typical cross sections are provided in Appendix C as Figure C-6. The Powerline channel is comprised of two channels. The first is a collector channel running along the existing FRS alignment to capture inflow that would otherwise reach the structure. The second channel is specifically for Siphon Draw, the largest wash within the watershed. The two channels combine before discharging into the Vineyard Road FRS flood pool. The channels were initially sized using normal depth routing within HEC-1. The preliminary channel sizes were then input into the FLO-2D model. Where breakouts occurred from potential channel overtopping, the channel sizes were increased to provide additional capacity. The resulting bottom widths and geometries of the two proposed channels are summarized in Table 8-1. Table 8-1 Summary of Powerline Channel Geometry and Results. Powerline Channel Design Parameters Analysis Results Hydrologic Segment Start Station End Station Lining Channel Bottom Width [ft] Total Depth 1 [ft] Side Slopes [ft:ft] [H:1] Peak Discharge 2 [cfs] Depth of Flow 3 [ft] Velocity 3 [ft/s] CP25 10+00 27+50 Earthen 60 5.1 6 984 4.0 2.9 CP25 3.0 2 939 2.0 7.2 Hardened 27+50 46+50 Hardened 60 CP9 46+50 61+50 Earthen 150 6.2 6 3,153 4.9 3.6 CP5R 5.8 2 7,746 4.2 11.7 Hardened 61+50 77+50 Hardened 150 Vineyard 5.6 6 8,129 4.4 3.5 CP5R 77+50 Flood Pool Earthen 500 CP4 (Siphon 4.1 6 13,444 3.1 4.2 Draw) Earthen 1000 1 Total depth including freeboard 2 Peak discharge reported from FLO-2D results 3 Depth of flow and velocity reported from FlowMaster normal depth calculations Segments of the proposed collector channel will be hardened with concrete as shown in the typical section in Appendix C. The available earth fissure risk hazard mapping at this level of analysis does not fully cover the proposed channel limits so existing mapping zones were projected upstream. A map of Fissure Risk Hazard Zones (FRZs) is provided in Appendix C. The segments USDA- NRCS Page 8-2 January 2013 Kimley-Horn and Associates, Inc.
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Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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