Ark Dam 2 Aerial looking west

Ark Dam 2 Aerial looking west

Arkansas River Dam No. 2 Project 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.

Project Name Arkansas River Dam No. 2
LIHI Certificate Number 51
LIHI Effective and
Expiration Dates
January 19, 2015
January 19, 2020
Owner Arkansas Electric Cooperative Corporation
State Arkansas
Location Located on the Arkansas River about 15 miles southwest of Dumas, in Desha and Arkansas Counties, Arkansas.
Installed Capacity 108,000 kW at unity power factor (102,600 kW at 95% power factor)
Average Annual Generation 351 GWh
Facility Type Run-of-river
FERC No. 3033

The project is located at the Corps navigation Dam No. 2 on the Arkansas River, two miles southeast of river mile 19 on the McClellan-Kerr Arkansas River Navigation System. The Corps constructed the dam to impound and maintain a minimum nine-foot 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 1130 feet and 54 feet at the maximum height. Sixteen conventional tainter gates in the spillway control the upstream pool. The Corps 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 3150 feet in length. The embankment has a crest elevation of 180 feet M.S.L. In the mid 1990’s, AECC constructed the hydropower project as an integral part of this embankment. They hydropower project was completed and began operation in 1999.

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 180 feet wide by 225 feet long. The upstream face of the powerhouse is located about 500 feet 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 powerhouse contains three horizontal-shaft, bulb-type, axial-flow hydraulic turbine- generator units.  Each of the turbines is of the Kaplan propeller type with four adjustable blades, and adjustable wicket gates, complete with an electronically controlled hydraulically operated governor.  The maximum net power head available to the turbines is approximately 44.5 feet.  The turbines operate under a net head which normally ranges from 5 feet to 37 feet.  The turbines are designed to rotate at 72 rpm. Each turbine is directly coupled to a generator by a horizontal shaft. Thus each of the generators also rotate about a horizontal axis at 72 rpm.

The electrical power and energy from the project is conveyed to the electrical network by a single span of 115 Kv transmission line to a new 115 Kv switching station.  The 115 Kv switching station is connected to a new 115 Kv line which was constructed by the Arkansas Power and Light Company (now referred to as Entergy).  This new section of line is outside of the FERC project boundary but was reviewed by FERC, the Corps of Engineers, and other resources agencies throughout the licensing and permitting process since it was specifically required for the hydropower project.  All applications and studies, including the 404 Permit Application, included this new section of 115 Kv line.

The project is run-of-river, with no ability to impound water and no discretion over flow releases, which are dictated by the Corps. The February 25, 1999 Memorandum of Agreement Establishing Procedures for Hydropower Operation at Wilbur D. Mills Dam between the Corps and AECC stipulates:

“During normal operation the Hydropower Project will generate with one, two, or three units up to 24 hours per day when available river flows exist. All of the following flows are approximate. The flow range during which generation is possible is from 4,000 cfs (minimum turbine discharge) to 200,000 (flow at which units are shut down due to insufficient operating head). River flows from 4,000 cfs to 53,400 cfs (Hydropower Project maximum discharge capacity) normally will be passed through the powerhouse but may be passed through either, neither, or both the powerhouse and Wilbur D. Mills Dam in a coordinated effort, as the Corps deems appropriate. It shall be the responsibility of the Licensee to assure release rates are made as set by the Corps. The Licensee shall schedule all hydropower releases in compliance with the Lockmaster’s needs and instructions. For river flows between 53,400 and 200, 000 cfs, discharges will be made through both the powerhouse and Wilbur D. Mills Dam. At flows greater than 200,000 cfs, discharges will be made through Wilbur D. Mills Dam only. The Licensee agrees to fully cooperate with the Corps in the establishment of release schedules and in case of disagreement the Corps’ decision will be final.”

In practice, each project turbine typically operates over a flow range of 4,100 to 16, 400 cubic feet per second (cfs).  The maximum flow rate though the powerhouse is in the range of 51,000 cfs.  At this flow range or below, all flows are normally only through the powerhouse and no flow is through the Corps spillway gates.

At total river flow above 51,000 cfs up to about 210,000 cfs, the Corps’ spillway gates are opened to handle the flows beyond the 51,000 capacity of the powerhouse.  Any river flow above 210,000 cfs usually has insufficient head for turbine operation, in which case all flows are released through the spillway gates.

Ark Dam 2 Powerhouse

Ark Dam 2 Powerhouse

Certification History

April 22, 2015: Executive Director, Mike Sale, using authority delegated by the LIHI Governing Board, has determined that the Arkansas Dam No. 2 Hydroelectric project continues to meet the Low Impact Certification Criteria. The decision to certify the Arkansas Dam No. 2 Hydroelectric Project is for a 5-year term, effective January 19, 2015 and expiring January 19, 2020, with the following condition:

Condition 1.  The facility owner shall provide a status update on any agency studies pertaining to upstream or downstream fish passage issues as part of its annual filing requirement to LIHI. This status update will contain copies of any pertinent correspondence and documents or a statement that no changes have occurred in the past year. The owner shall cooperate fully with agency studies and notify LIHI immediately if any of those studies lead to new fish passage mitigation requirements at this facility. Based on LIHI’s review of this information, and at LIHI’s sole discretion, certification may be modified.

The 2015 reviewer’s report can be found in the files section below.

June 16, 2014: The Low Impact Hydropower Institute has received an application for a second term of certification of the Arkansas River Dam No. 2 Hydroelectric Project. A public comment period will be open for 60 days, ending on August 1, 2014.

April 21, 2010: The Arkansas River Project has been certified as low impact for a five year term, effective January 19, 2010 and expiring January 15, 2010.

March 19, 2010: The public comment period has been closed.

January 19, 2010: Arkansas Electric Cooperative Corporation has submitted an application for certification of the Arkansas River Dam No. 2 Project. A public comment period will remain open for 60 days.