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Home My WebLink About20160692 Ver 1_Permit_Cond_Memo_20161213 December 13, 2016 Technical Memorandum: To: Mr. Zan Price, N.C. Division of Water Quality Ms. Andrea Leslie, N.C. Wildlife Resources Commission Mr. Chris Goudreau, N.C. Wildlife Resources Commission Mr. Steve Kichefsky, U.S. Army Corps of Engineers From: James C. Borawa, Senior Environmental Scientist Subject: Permit Conditions, North Fork Spillway and Embankment Improvements Project This purpose of this correspondence is to provide the resource agencies information related to issues that were discussed as potential special conditions of the Section 404/401 permits being sought after by the City of Asheville for this project. Those conditions include:  Stormwater flows out of the new auxiliary spillway during rehabilitation of the gates on the main spillway  Instream flow requirements to the North Fork Swannanoa River downstream of the main dam. The permit conditions were discussed at various meetings with the agencies while the project was in the design phase and input from those meetings were taken into consideration for these proposals. Design plans are nearing completion and, based on those plans, have been altered to address those issues as well as minimize impacts to streams and wetlands. Stream and wetland impacts have been reduced to a level that the City will be seeking to proceed with construction under a USACE Nationwide Permit 3 - Maintenance. It is our hope that the information on the two issues satisfactorily addresses agency concerns. It is our intent to obtain agency agreement on the issues prior to submission of the permit application package to the USACE. This information is being provided by Equinox Environmental, acting as an agent of the City through its prime contractor Schnabel Engineering South, PC. Stormwater Flow Impacts – The original proposal was to utilize the new auxiliary spillway for stormwater releases during the time the gates on the main dam were being rebuilt. Although the reservoir would be lowered during this time, there was significant risk that a 2-10 year storm event would require water to be released out of the new spillway. The extent of the damage to Walker Branch, an unnamed tributary, and surrounding wetlands would be dependent on the magnitude of the event. In addition, it was our understanding the City would be liable for restoring the stream channels regardless of the magnitude of the storm event. Based on design considerations and costs related to stream and wetland restorations, this concept has been abandoned. Schnabel intends to complete construction of the new auxiliary spillway and make it fully functional before beginning work to upgrade the gates on the main spillway. Under this scenario, flood waters from normal storm events will be released only through the main spillway. It is believed that reservoir level management during construction on the main gates will minimize the likelihood of a flood release during construction on EQUINOX 37 Haywood Street, Suite 100 Asheville NC 28801 828.253.6856 www.equinoxenvironmental.com the main gates. A flood event that activates the gates on the auxiliary spillway would be a flood of record event for which the City will not be liable to damage of the downstream channels, a condition for which the City will want included in their permit. Instream Flow Requirements – The City of Asheville understands that the resource agencies seek to obtain additional stream flows for the portion of the North Fork Swannanoa River downstream of the main dam to improve aquatic habitat conditions. The City proposes to provide that flow using a passive system that will not require any action by City personnel. To accomplish this, a 3 inch diameter pipe will be installed at the base of one of the gates. The invert of the pipe inlet will be at an elevation of 2584.0 feet; the invert of the outlet will be at an elevation of 2583.5 feet. Normal reservoir full pool operating elevation is 2601.5 feet. A valve will be installed at the upstream end of the pipe as a safety precaution and in case maintenance on spillway is necessary. The valve will remain open at all other times. To release water at elevations lower than 2584.0 feet would require a significant investment by the City. There is currently no practical way to continuously and safely release water below this elevation. The only other existing infrastructure to release water from below this elevation into the North Fork Swannanoa River is the low level outlet (elevation ≈2493.34 feet); the valves on this outlet are not designed for controlled releases. They are intended only to be used in emergency and maintenance related activities. Based on the attached table and graph, the proposed passive release system will provide flows varying from 0.21 cfs at a lake level of 2584.5 feet to 1.02 cfs at a full pool lake level of 2601.5 feet. This self- regulating system will eliminate the need for active manipulation by City staff and will reflect, to some extent, the hydrologic cycle as the reservoir levels change with rainfall events. The flows from this system will not be monitored other than to ensure the inlet is operating properly (not clogged). The flow from the main gate will be supplemented with 0.5 cfs of seepage water that will be piped to the stream channel from a collection box at the base of the dam that is outfitted with flow monitoring instrumentation. We appreciate your review and comment on the City of Asheville’s proposal to address resource agency concerns. We thank you in advance for your timely response and look forward to obtaining agreement on these issues. Please feel free to contact me at 828-231-9130/828-253-6856 or via email at jborawa@equinoxenvironmental.com with any questions that you may have concerning the information provided. EQUINOX 37 Haywood Street, Suite 100 Asheville NC 28801 828.253.6856 www.equinoxenvironmental.com Proposed Discharges for Aquatic Resource Enhancement Lake Level Outflow Seepage Total Outflow (feet) (cfs) (cfs) (cfs) 2584.5 0.21 0.5 0.71 2585.0 0.27 0.5 0.77 2585.5 0.31 0.5 0.81 2586.0 0.36 0.5 0.86 2586.5 0.39 0.5 0.89 2587.0 0.43 0.5 0.93 2587.5 0.46 0.5 0.96 2588.0 0.49 0.5 0.99 2588.5 0.52 0.5 1.02 2589.0 0.54 0.5 1.04 2589.5 0.57 0.5 1.07 2590.0 0.60 0.5 1.10 2590.5 0.62 0.5 1.12 2591.0 0.64 0.5 1.14 2591.5 0.67 0.5 1.17 2592.0 0.69 0.5 1.19 2592.5 0.71 0.5 1.21 2593.0 0.73 0.5 1.23 2593.5 0.75 0.5 1.25 2594.0 0.77 0.5 1.27 2594.5 0.79 0.5 1.29 2595.0 0.81 0.5 1.31 2595.5 0.82 0.5 1.32 2596.0 0.84 0.5 1.34 2596.5 0.86 0.5 1.36 2597.0 0.88 0.5 1.38 2597.5 0.89 0.5 1.39 2598.0 0.91 0.5 1.41 2598.5 0.93 0.5 1.43 2599.0 0.94 0.5 1.44 2599.5 0.96 0.5 1.46 2600.0 0.97 0.5 1.47 2600.5 0.99 0.5 1.49 2601.0 1.01 0.5 1.51 2601.5 1.02 0.5 1.52 EQUINOX 37 Haywood Street, Suite 100 Asheville NC 28801 828.253.6856 www.equinoxenvironmental.com Flow Release vs. Reservoir Elevation for a 3-Inch Diameter Pipe 1.20 1.00 0.80 Pipe Discharge (cfs) 0.60 0.40 Full Pool 0.20 0.00 2582.02584.02586.02588.02590.02592.02594.02596.02598.02600.02602.02604.0 Reservoir Elevation (feet) EQUINOX 37 Haywood Street, Suite 100 Asheville NC 28801 828.253.6856 www.equinoxenvironmental.com