Alternatives Unlimited’s project is located on the Mumford River in the Town of Northbridge, Massachusetts. The project’s dam impounds 2 acres and stores 19,735m3 at a proposed pool elevation of 285.1. The height of the dam from its toe to the top of its abutment is 4.7m. The spillway height is 2.9m and the normal hydraulic head on the structure, with 30-inches of flashboards installed, is 3.7m.

The project consists of a 28m concrete ogee spillway, a waste gate, and two inlet structures located at the north and south ends of the spillway. The ogee crest is fitted with sockets to accommodate 2½ inch pipes, 30-inches tall on three-foot centerlines. The crest is designed to be fitted with 30-inch tall flashboards. The crest elevation is 282.6 (282.0 msl original datum). The deeded water rights allowed the impoundment to be raised to 285.1 (284.5 msl original datum) with the flashboards installed.

The dam was originally constructed to impound and convey the waters of the Mumford River to the shops of the Whitinsville Machine Works constructed on both banks of the river. Both intakes are now abandoned. The turbine generator sets for the northern intake were removed in the 1980s when the shop was converted to an assisted living facility. At that time, the intake canal was sealed off and the hydro power project was abandoned. The Machine Works Shop is now occupied by Cotton Mill Apartments, an assisted living complex on the downstream side of Douglas Road.

The southern intake was abandoned at the turn of the century. It consists of a silted forebay, which leads up to a wooden headgate. This gate is located beneath the lower Alternatives Building. The top of the headgate is visible in the silt and has slid sideways. Under prior conditions, the water was conveyed through a short five-foot diameter stub penstock to a turbine casing located beneath the same building. An existing tailrace conveyed the water back to the Mumford River just upstream of the southerly abutment of the Douglas Street Bridge.

The original timber crib dam was destroyed in a 1938 flood and was replaced in its entirety in 1941 with a concrete gravity structure. It was originally proposed to reconstruct the existing southern forebay, intake gate, discharge pit and tailrace and install the turbine in the approximate location of the former hydro equipment. However, due to the deterioration of the existing hydro-infrastructure, the turbine was installed outside the building, on the south side of the dam, just downstream of the flood-gate opening. The power unit is one (1) 47kW Leroy Somers Tube Turbine. The expected plant factor is 82%. It is anticipated that all generated electricity would be utilized by Alternative’s facility, except during low power demand, when net metering will occur. Massachusetts Electric Company provides transmission at the site. The project is to be operated strictly run-of-river where outflow will equal inflow and no “Peaking” or “Ponding” will occur. All minimum stream flows will be maintained as required by the agencies. The Trashrack provided is a 6m long with clear spacing of 3/4-inch and 45º angle to flow as designated by the US Department of the Interior and MA Department of Fish and Wildlife.

The Arkansas River Dam No. 2 Project (FERC No. 3033) is one of a number of hydroelectric projects constructed by retrofitting existing dams on the Arkansas River. Arkansas Electric Cooperative Corporation (AECC), a rural electric cooperative corporation, developed the projects to reduce long-term costs of producing energy for its 17 distribution cooperatives around the state and, equally important, to displace a portion of AECC’s fossil-fuel derived energy with energy generated from renewable hydropower. No other forms of renewable energy such as wind, solar, and geothermal energy were or are economically or technically competitive with low-head hydroelectric generation in Arkansas.

Arkansas River Dam No. 2 is part of the McClellan-Kerr Arkansas River Navigation System (McClellan-Kerr). The McClellan-Kerr is a 445-mile-long-waterway with 18 locks and dams; for a majority of that distance, it and the Arkansas River are one and the same. The US Army Corps of Engineers (USACE) constructed the waterway to achieve the primary goal of allowing barge traffic to overcome a 128m elevation difference between the Mississippi River and the head of navigation at Catoosa, Oklahoma. USACE constructed the McClellan-Kerr between 1964 and 1970; Arkansas River Dam No. 2, also known as the Wilbur D. Mills Dam, was constructed between 1963-1968.

On August 10, 1983, the Federal Energy Regulatory Commission (FERC) issued to AECC a 50-year license for the Arkansas River Dam No. 2 Project. Due to construction challenges and other factors AECC did not begin building the project until the 1990s. In 1994 AECC obtained a Clean Water Act Section 404 (wetland fill) permit from USACE. The project underwent regulatory review by state and federal natural resource, environmental, and cultural resource agencies in the early 1980s and again in the early 1990s.

The project is located at the USACE navigation Dam No. 2 on the Arkansas River, two miles southeast of river mile 19 on the McClellan-Kerr Arkansas River Navigation System. USACE constructed the dam to impound and maintain a minimum 2.7m deep navigation pool along this section of the Arkansas River. Built in the mid 1960s, the dam is located in Arkansas and Desha Counties approximately 12 miles northeast of Dumas, Arkansas.

The dam consists of a gated spillway and an earthfill overflow and non-overflow embankments. The spillway section is a straight concrete gravity type structure with a crest length of 344m and 16m at the maximum height. Sixteen conventional tainter gates in the spillway control the upstream pool. USACE constructed a non-overflow earth embankment on the west end of the dam to block the old Arkansas River channel. The access road to the dam is located on this non-overflow embankment which measures 960m in length. The embankment has a crest elevation of 55m M.S.L. In the mid 1990’s, AECC constructed the hydro power project as an integral part of this embankment. The hydro power facility included development of a reinforced concrete powerhouse with upstream and downstream channels to divert water from the Arkansas River. The channels pass water from Pool 2 (upstream of the existing USACE dam) through the powerhouse and immediately return it to the river downstream of the dam.

A headrace channel was constructed to divert the river flow and convey it into the powerhouse. Rip-rapped slope banks and reinforced concreted training walls forming the sides of the channel were built upstream from the powerhouse. Beyond the training walls, the stabilized sides of the channel were sloped to meet existing ground level. A log boom across the headrace is intended to trap large debris; however, a trash rack crane on the powerhouse is required to clear the trashrack of the extensive debris the Arkansas River can deliver. Water flow is directed back into the river by the tailrace channel. A portion of the existing navigation revetments were removed to construct the headrace and tailrace channels. These channels provide a transition to the undisturbed revetment areas.

The powerhouse is approximately 55m wide by 68.6m long. The upstream face of the powerhouse is located about 150m downstream from the dam axis. The powerhouse structure is located on the west side of the Arkansas River adjacent to the dam on the existing non-overflow embankment. The powerhouse elevation is at least as high as the non-overflow embankment. For this reason the high roof of the powerhouse will always be above flood level.

The hydro power project was completed and began operation in 1999.