Nebraska Highway 12 Niobrara East and West Supplemental Draft Environmental Impact Statement
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APPENDIX F Flood Storage Technical Memorandum 1-)~ Technical Memorandum Date: Monday, June 17, 2019 Project: N-12 Niobrara East and West Supplemental Draft EIS To: USACE From: HDR Reassessment of September 2015 Missouri River Storage Assessment Technical Subject: Memorandum During development of the 2015 Draft Environmental Impact Statement (EIS), a technical memorandum was prepared to assess the Action Alternatives’ impact on the Missouri River floodplain and associated Lewis and Clark Lake storage. That memorandum reviewed the No-Action Alternative and the following four build alternatives: • Alternative A1 – Existing Alignment • Alternative A2 – Parallel Alignment • Alternative A3 – Base of Bluffs Alignment • Alternative A7 – Base of Bluffs Elevated Alignment (2015 Applied-for Project) In the Supplemental Draft EIS, the proposed N-12 Action Alternatives are the following, referred to as the S Alternatives in this memorandum: • Alternative S-1 – Elevation Raise on Existing Alignment with Temporary Road Offset to the Right • Alternative S-2 – Elevation Raise on Parallel Alignment with Temporary Road Offset to the Right • Alternative S-3 – Elevation Raise on Parallel Alignment with Temporary Road Offset to the Left The roadway horizontal alignment of the S Alternatives would be identical to previously evaluated Alternative A1, but the vertical alignment would be at a lower elevation throughout the length of the project. The vertical profile would still be above the 1 percent annual chance flood. Missouri River Conveyance Similar to the 2015 Action Alternatives, the S Alternatives would be along the existing N-12 alignment. Each of the S Alternatives would have less than 0.2 foot of stage increase on the Missouri River for the 1 percent annual chance flood. Effects on Missouri River conveyance would be identical to the anticipated effects of Alternative A1, which were determined to be negligible. Missouri River Floodplain Storage The bridges and culverts for the S Alternatives are assumed to be similar to the bridges and culverts included as part of Alternative A1. The lower vertical profile of the S Alternatives would have the same effect on 1 percent annual chance flood storage equalization, which was determined to be major negative, because there would still be no overtopping flow during the 1 percent annual chance flood and all equalization would occur via backflow through the bridges and culverts. hdrinc.com 1917 S 67th Street, Omaha, NE 68106-2973 (402) 399-1000 1 1-)~ Lewis and Clark Lake Storage The lower vertical alignment of the S Alternatives would have a smaller cut/fill volume than Alternative A1. The Corps published a regulation titled, “Land Development Guidance at Corps Reservoir Projects” that provides guidance when evaluating effects associated with cut and fill volumes (Corps 2004).1 The lower profile of the S Alternatives compared to Alternative A1 would increase weir flow depths, an incremental improvement to the equalization process. However, to reduce the impact from a major negative to a moderate negative, the change in the equalization would have to be reduced to under 25 percent of the 1,975,000 acre-feet per day for the existing profile (448,750 acre-feet). Alternative A1 had a cumulative equalization capacity of 30,200 acre-feet per day. While the S Alternatives would have an improvement, the improvement would still be significantly less than the 448,740 acre-feet per day needed to be under 25 percent of the change from existing conditions. Conclusions The No-Action Alternative has not changed since the analysis in the September 2015 Technical Memorandum. The No-Action Alternative would have a negligible effect on Missouri River conveyance, Missouri River floodplain storage, and Lewis and Clark Lake storage. Because the S Alternatives are similar to previously evaluated Alternative A1, the S Alternatives would likely have the same, or minimally reduced, effects on Missouri River storage. If all of the bridges and culverts under Alternative A1 are similar under the S Alternatives, a slight improvement to the equalization volume per day, as represented in the 2015 technical memorandum in Tables 2-10 and 2-11, may be realized. While these values would increase, the increase in the values would not be enough to change the impact assessment as determined for Alternative A1. Therefore, the following summary of effects for the S Alternatives would still be applicable: Assessment Summary of Alternatives’ Effects Lewis and Clark Missouri River Missouri River Alternative Lake Storage Conveyance Floodplain Storage Zone 3 No-Action Alternative Negligible Negligible Negligible S1, S2, and S3 Negligible Major Negative Major Negative 1 Corps. 2004. Land Development Guidance at Corps Reservoir Projects. CENWD-MT-E Regulation No. 1110-2-5. April 30. Online at: https://usace.contentdm.oclc.org/digital/collection/p16021coll7/id/1060/. hdrinc.com 1917 S 67th Street, Omaha, NE 68106-2973 (402) 399-1000 2 L ~R I O E COMPANY .i:-J..A.. Many Solutions'" Technical Memorandum To: Project File From: HDR Engineering, Inc. Project: N-12 Niobrara East & West EIS CC: Date: September 2015 Job No: 84534 Missouri River Storage Assessment Technical Memorandum 1.0 Introduction The U.S. Army Corps of Engineers (Corps) is preparing an environmental impact statement (EIS) for the proposed reconstruction of the Nebraska Highway 12 (N-12) roadway east and west of the Village of Niobrara (Niobrara), Nebraska (Project). Because the Project would have impacts on regulated waters of the U.S. and would require a Clean Water Act Section 404 permit, and no other federal action is required, the Corps is the lead federal agency for compliance with the National Environmental Policy Act (NEPA). The purpose of this hydraulic assessment is to estimate the potential effect each Project alternative may have on the ability of the valley to convey the 1-percent-annual-chance (also known as the 100-year) Missouri River flood (1 Percent Flood) and the potential effect each alternative may have on the storage capacity of the Missouri River 1 Percent Flood floodplain. To provide a national standard without regional discrimination, the 1 Percent Flood has been adopted as a standard threshold for both the NFIP and EO11988 Compliance. This assessment was also performed to estimate the potential effect each alternative may have on the reservoir storage capacity of the Lewis and Clark Lake formed behind Gavins Point Dam using conceptual-level evaluation based on available existing information and assumptions to determine whether additional study is necessary. Groundwater flow was not considered in this analysis; however the contribution of groundwater to floodplain equalization flows would be directly proportional to the duration of the flood event. Only the potential effects on surface water in the Missouri River floodplain are included in this assessment. An initial analysis of the Missouri River Storage Assessment was completed in February 2012. The best available hydraulic data for this area was updated 20 May 2015 by the Corps and comes from the Hydraulic Modeling and Mapping Summary Steady RAS Confluence of Missouri and Niobrara River near Niobrara, Nebraska May 2015 Study), the following analysis and documentation has been updated based on the May 2105 Study. The q100 increases from 100,000 cfs at the upper end of the project reach to 101.700 cfs at the lower end. Nebraska Highway 14 (N-14) would also be raised to meet the future design considerations. However, it was determined that the 1 Percent Flood does not overtop N-14 during future conditions except for a small portion that has very minor overtopping. Therefore, no additional analysis was needed for N-14 to assess the additional road raise required to meet future design considerations for N-12. A review of the influence of climate change was performed by the Corps per their policy on civil works studies, designs, and projects (Corps 2014a). The area of influence for this Project is the contributing drainage area of the Missouri River watershed upstream of Gavins Pont Dam. This includes the majority of the states of Montana, North Dakota, Wyoming, and South Dakota (Corps 2015b). This study concluded that the potential increases in flood magnitudes and stages are likely in the uncertainty range for the existing hydrology used to compute flood stages and also the stage effects caused by projected sediment deposition. See study in Appendix A. For these reasons, it was not recommended to change the flood frequency values to anticipated climate trends. For purposes of discussion in this memorandum, floodplain is synonymous with the 100-year floodplain. HDR Engineering, Inc. 8404 Indian Hills Drive Phone (402) 399-1000 Page 1 of 22 Omaha, NE 68114-4098 Fax (402) 399-1238 www.hdrinc.com Alternative A7 was added as a practicable alternative following the February 2012 analysis. However, Alternative A7 is on the same alignment and has the same vertical profile as Alternative A3. Alternative A7 contains approximately 7,000 feet more total bridge length than Alternative A3. Due to these similarities, the analysis of Alter naive A3 would yield similar results to what would be seen for Alternative A7. Instances of comparison of analysis between Alternative A3 and A7 are provided in sections below when appropriate. For the purpose of this assessment, no additional modeling was performed on Alternative A7. In addition, Alternative B1 is included in this analysis as it was initially performed prior to completion of practicability screening. The analysis of Alternative B1 is retained in this memorandum but is not a practicable alternative as determined by the Corps. 1.1 Alignment Descriptions Refer to Figures I-1 and I-2 for a plan view of the alternatives’ alignments. A complete description of each alternative can be found in Chapter 2 of the N-12 Draft EIS. Alternatives A1, A2, and A3 alignments are in the area inundated by the 1 Percent Flood, see Figures I-1 and I-2.