Pending Application - Lower Great Falls Project, New Hampshire and Maine
|Lower Great Falls
|LIHI Certificate No.
|LIHI Certificate Term
|Green Mountain Power
|New Hampshire and Maine
|River Mile 3.1 – Salmon Falls River.
|Average Annual Generation
|P-4451. License issued January 20, 2023, expiring December 31, 2062.
The Lower Great Falls Project is located on the Salmon Falls River in the states of New Hampshire and Maine. Most of the infrastructure associated with the Project, including the intake, penstock, and powerhouse, is located within the City of Somersworth, NH. The left abutment of the dam is in the Town of Berwick, ME.
The project dam is located at approximately river mile 3.1 and is the third dam on the mainstem of the Salmon Falls River. The total drainage area is approximately 220 square miles, which is about 93.2% of the Salmon Falls drainage area (236 mi2).
The project dam has been in place since 1825 and has a total length of 297 feet, which consists of a 50-foot-long left abutment, a 176-foot-long spillway section, and a 71-foot-long right abutment. The dam is a stone masonry and concrete gravity dam that is approximately 32 feet high and topped with 4-foot-high flashboards. The dam was rehabilitated in 1984 followed by rehabilitation of the spillway in 1991.
There is one low level outlet gate located in the left abutment which controls flow into a seven-foot-diameter, 40-foot-long bypass pipe. The gate is 8 feet wide by 8 feet high. A second inoperable outlet pipe that had previously been partially filled with concrete is present. In the Fall of 2022, the owner completed several FERC mandated dam safety repairs at the Project that included filling the remaining portion of the inoperable outlet pipe with concrete. The gate that is operable is opened when the impoundment level rises approximately 10 inches above the flashboards to avoid failure of the flashboards during high flow periods. The gate is operated manually by a hydraulic unit mounted to the gate structure that is run by a dedicated portable generator.
A small trash gate is located adjacent to the intake structure with a manually operated screw stem operator. The gate is typically used to sluice debris and is also opened when the impoundment level rises approximately 10 inches above the flashboards during high flow periods. The gate is inspected during flashboard repair activities but requires no routine maintenance other than regular lubrication of the gate operator mechanism.
The impoundment has a gross volume of 584 acre-feet with an approximate surface area of 40 acres at the normal pond elevation of 106.4 feet, NGVD 1929. Since the Project operates as a run-of-river facility, the useable storage volume is negligible.
The 40.5-foot-wide by 20-foot-high intake is a concrete structure with a wooden deck that includes four steel frame gates with a sloping steel trashrack at the upstream side of the wooden deck. Two pairs of gates control flow to each penstock. Each gate is 5 feet wide by 10.5 feet high and has a sill elevation of 92.0 feet.
The gates are hand operated with a chain hoist suspended from a monorail. These gates provide closure to two 8.5-foot penstocks and in turn each penstock bifurcates into two penstocks that convey water to the powerhouse. All the penstocks are buried and extend approximately 200-225 feet from the intake structure to the powerhouse.
The powerhouse is located approximately 250 feet downstream of the project dam. The powerhouse is a 30-foot by 46-foot concrete and brick building with a wood frame superstructure. The powerhouse includes the 4 turbine/generators, controls, and station switchgear. Power is delivered directly to a 4.16 kV distribution line via a 260-foot-long underground transmission line that stretches to an Eversource Energy pole-mounted air break.
The Project tailrace is approximately 55 feet wide and 30 feet long. A 250-foot-long bypass reach extends between the Project dam and the tailrace. The bypass varies in width between approximately 90 feet at the downstream end to approximately 160 feet at the upstream end.
February 12, 2024: The Low Impact Hydropower Institute has received a complete application for Low Impact Certification of the Lower Great Falls Hydroelectric Project. LIHI is seeking public comment on this application. Specifically, we are interested in knowing whether you think the Project meets the LIHI Low Impact Certification Criteria, as revised in the 2nd Edition Handbook. Please review the program and criteria in LIHI’s revised Handbook and then review the Project’s application materials below. Comments that are directly tied to specific LIHI criteria (flows, water quality, fish passage, etc.) will be most helpful, but all comments will be considered. Comments may be submitted to the Institute by e-mail at firstname.lastname@example.org with “Lower Great Falls Project Comments” in the subject line, or by mail addressed to the Low Impact Hydropower Institute, 1167 Massachusetts Avenue, Arlington, MA 02476. Comments must be received on or before 5 pm Eastern time on April 12, 2024 to be considered. All comments will be posted to the web site and the applicant will have an opportunity to respond. Any response will also be posted.