Project Description
Introduction
The project is located in southeast Alaska approximately five air miles northeast of the city of Ketchikan. The site generally consists of two lakes, Upper and Lower Mahoney Lake, that are approximately one mile apart, but differ in elevation by more than 1,800 feet. It is this difference in elevation that would be utilized by the proposed project to generate electricity.
The proposed 9.6 mW project would involve construction of a lake tap, which would enter Upper Mahoney Lake about 75 feet below its surface via a vertical shaft, and two tunnels to convey water from Upper Mahoney Lake to the powerhouse. The powerhouse would be located above Lower Mahoney Lake near the base of a large waterfall. Power generated from the project would be carried more than four miles over a combination of underground and overhead transmission lines. The project would not require construction of a dam.
Goals and Objectives
The objective of this project is to secure a FERC license for the construction of the 9.6 mW hydropower project for the Cape Fox Corporation. At the end of this U.S. Department of Energy (DOE) partially funded project, the FERC license was obtained.
Project Actions and Resultant Data
Proposed Project Facilities
Lake Tap of Upper Mahoney Lake
Existing Upper Mahoney Lake would serve as the reservoir for the project. A natural outlet from the lake maintains the lake water surface elevation at about an elevation of 1,959 feet. Surface area of the lake is 74 acres. The proposed project involves construction of a lake tap near the natural outlet and about 75 feet below the normal water surface elevation. A lake tap deeper than this level would provide more drawdown capability, but a deeper lake tap would require a much different, and more costly, tunnel and shaft arrangement with only a marginal increase in energy production. Preliminary surface investigations indicate the general rock quality in the vicinity to be adequate for lake tap construction. Usable storage for power generating purposes is about 4,000 acre-feet based on the proposed lake tap elevation.
Upper Tunnel
Construction of the Lake tap would first require construction of a 1,700-foot-long tunnel to access the tap. A pipe four feet in diameter and 20 feet in length would be encased in concrete about 200 feet downstream of the lake tap. The pipe would be closed off with a valve just prior to blasting out the last portion of the tunnel that would create the lake tap, to contain the immediate in-rush of snow and rock from the blast. The valve would be opened once the upper tunnel pipeline is installed and is ready to be pressurized. Tunnel walls would be left unlined except in areas requiring additional support. Rock traps would be excavated in the tunnel invert to capture and retain rock debris from lake tap blasting operations, and from any fit-loose rock entering the ungated intake area.
A 4-foot diameter steel pipe would be installed from the lake tap pipeline valve to convey water from the upper tunnel to the vertical shaft. A 300-square-foot (approximately) concrete valve house would be constructed at the upper tunnel portal and immediately above the beginning of the vertical shaft. The valve house will contain two 48-inch diameter butterfly valves and a vent pipe. One valve would act as the primary intake shut-off valve and the other as an emergency shut-off. Both valves would be motor-operated and connected by power and lines to the powerhouse. A 12-inch diameter bypass pipeline would be constructed from the valve house back to Upper Mahoney Creek to provide flow continuation under certain plant shutdown conditions. Maximum discharge velocity in the 8-foot horseshoe section of the upper tunnel would be 1.4 feet per second.
Vertical Shaft
A 1,370-foot-long partially lined vertical shaft would be constructed to connect the upper tunnel to the lower tunnel. Preliminary studies indicate the preferred method for excavating the shaft to be the Alimak raise climber method. This mining method would excavate the shaft from the bottom up. A cross section of the shaft would be about 5 feet by 7 feet in unlined sections and four-foot diameter in concrete lined sections. It is assumed that half the shaft length can be left unlined. Ultimately the shaft construction technique used by the construction contractor would dictate the size of the final shaft cross section.
Lower Tunnel
Construction of the vertical shaft would initially require access to the lower end of the shaft. Such access would be provided by constructing an 8-foot horseshoe-shaped tunnel, 3,350 feet long from the powerhouse portal area. The tunnel would be constructed at a 10% grade to shorten the required length of shaft and to provide positive drainage from the tunnel. The tunnel would provide access during construction and post-construction. Portions of the tunnel would be lined with shotcrete and supported by rock bolts and steel sets as required. Roof drains and a floor gutter would be constructed to collect and contain tunnel drainage and/or seepage.
Turbine flow would be conveyed in a 32-inch diameter welded steel pipe supported on concrete saddles inside of the tunnel. A concrete plug would be constructed at the upstream end of the tunnel (bottom of the vertical shaft) in order to pressurize the shaft. A 48-inch diameter steel pipe would be embedded in the plug to convey flow to the 32-inch diameter pipe and to provide permanent access to the bottom of the shaft for future inspection and maintenance. Permanent access to the lower tunnel would be provided from the powerhouse.
Powerhouse
The powerhouse would be a semi-underground concrete structure constructed at the portal entrance to the lower tunnel. It would be essentially an over-excavated tunnel portal providing approximately 1,600 square feet of space for powerhouse equipment. Because the powerhouse would be set back into the surrounding rock formation, it would be protected from any potential avalanche hazards. Its location is near the base of the first waterfall on Upper Mahoney Creek, about 1,100 feet upstream of the lake shore from Lower Mahoney Lake.
Turbine/Generator
The powerhouse would contain a single twin-jet horizontal Pelton turbine. The turbine would be rated at 12,900 horsepower at a discharge of 78 cfs and a rated net head of 1,730 feet. Minimum operating discharge would be 8 cfs Centerline of the turbine shaft would be at an elevation of 150 feet. The turbine would be coupled to a 13.2 kV synchronous generator capable of continuous operation at 9,600 kW.
Tailrace
Discharges from the powerhouse would be conveyed back to Upper Mahoney Creek in a 200-foot-long tailrace channel. The channel would consist of a of pre-cast concrete box culvert or corrugated metal pipe for approximately 70 feet immediately downstream of the powerhouse. The remaining channel length would be a rip-rap lined earthen channel. The discharge would enter Upper Mahoney Creek at a large pool at the base of the waterfall.
Access Road
A new 2.6-mile-long access road would be constructed between the end of the existing access road northeast of Lower Mahoney Lake and the powerhouse. The new access road would be routed to the south and east of Lower Mahoney Lake. The new road would be a single-lane gravel surfaced road with turnouts.
The new access road would require construction of two bridges. An approximately 80-foot-long, single-lane bridge would span Lower Mahoney Creek. A second single-lane bridge would span approximately 30 feet across South Creek, a major drainage on the south side of Lower Mahoney Lake.
Transmission Line/Switchyard
The transmission line route would follow essentially the same route as the new access road between the powerhouse and switchyard. The switchyard would be located approximately 1 mile from the powerhouse in a low avalanche hazard area on the east side of Mahoney Lake adjacent to the access road. The transmission line would be a buried 13.2 kV conductor following the access road from the powerhouse to the switchyard. A power transformer would be located in the switchyard to step the voltage up to 34.5 kV transmission voltage. From the switchyard the 3.6-mile-long, 34.5 kV transmission line would be constructed south as a combination of buried and overhead lines to the proposed intertie point with Ketchikan Public Utilities Beaver Falls Hydroelectric Project (FERC No. 1922) transmission line, located adjacent to that project's powerhouse. This line would be buried for the first 0.5 mile to minimize impacts to an existing eagle nesting site. After the first 0.5 mile, the line would move overhead onto poles. Pole-mounted transmission lines would be designed according to the latest raptor protection guidelines.
Other Mechanical, Electrical, and Transmission Equipment
Electrical accessory equipment would include medium voltage switchgear, station service equipment, d.c. power supply, ventilation equipment, and lighting. Instrumentation would include continuous readout of upper reservoir pool elevation, valve status indicators, drain sump level controls, and ventilation controls.
A battery back-up system with an on-line charger would be provided to supply control power sufficient to shutdown the plant in the event of a power outage. Battery backups would be at both the powerhouse and the valve house. Power and communications cables for instrument signals would be run in conduit from the powerhouse to the valve house through the lower tunnel and shaft. Level signals from Upper Mahoney Lake, and signals to open and close the pipeline shutoff valves would be sent over the communication cable. The power line would provide power to operate the instruments, valve motor operators, and small space heaters in the valve house.
A computer-based plant control panel located in the powerhouse would monitor all plant functions and would shutdown the turbine if any problems arise. A telephone autodialer would then call out to the plant operator to report the problem. The fail-safe operation mode would be to shut down the project in the event of an emergency. Remote monitoring of all plant functions and equipment condition would be performed via a SCADA system over telephone lines.
Lands of the United States
The Mahoney Lake project would occupy approximately 113.97 acres of federal land that is managed by the U.S. Forest Service as follows:
Upper Lake Area - The total upper lake area is 84 acres.
Access Road Area - The total access road area is 1.6 acres Upper Lake Area - The total upper lake area is 84 acres.
Transmission Line Area - The total transmission line area is 28.37 acre Upper Lake Area - The total upper lake area is 84 acres.
Total Project Area (on National Forest Lands) - Total project area on National Forest system lands is 113.97 acres Upper Lake Area - The total upper lake area is 84 acres.
Results, Conclusions, Findings, and Recommendations
Table A-1 summarizes the resultant project and the basic characteristics of the proposed features contained therein:
| Project Location | Sections 24, 25, 26, 27, 34, 35, and 36, Township 74S, Range 91E; Sections 30 and 31, Township 74S, Range 92E; and Sections 5, 6, and 8, Township 75S, Range 92E, Copper River Meridian. |
| Diversion Type | Lake Tap at El. 1880 |
| Reservoir | Upper Mahoney Lake:
|
| Tunnels/Shaft/Pipeline | Upper Tunnel:
|
| Powerhouse |
|
| Transmission Line |
|
| Average Annual Energy | 46,000,000 kWh |
<p><strong>Tribe/Awardee</strong><br />Saxman, Organized Village of/Cape Fox Corporation</p><p><strong>Location</strong><br />Ketchikan, AK</p><p><strong>Project Title</strong><br />FERC Licensing of a Hydro-Electric Power Project on Upper and Lower Mahoney Lakes</p><p><strong>Type of Application</strong><br />Feasibility</p><p><strong>DOE Grant Number</strong><br />DE-FG48-95R810572</p><p><strong>Project Amounts</strong><br />DOE: $125,000<br />Awardee: $110,700<br />Total: $235,700</p><p><strong>Project Status</strong><br />Complete</p><p><strong>Project Period of Performance</strong><br />Start: September 1995<br />End: June 1997</p>