- Report Published -
|Harvell Dam Fishway Alternatives Assessment - November 30, 2011|
|Department of Game and Inland Fisheries|
|Chapter 215 (Regular Session, 2011)|
|Harvell Dam is located on the Appomattox River between the City of Petersburg and the City of Colonial Heights, Virginia. The original dam was reportedly constructed in 1856 and was used for hydropower generation as early as 1885. The current hydroelectric facilities, located at the right abutment of the dam, consist of a main powerhouse (capable of producing up to 600kW) and two smaller siphon turbines (capable of producing 180 kW). In 1998 a Denil fishway was constructed near the right abutment of the dam to satisfy a Federal Energy Regulatory Commission (FERC) requirement. Ownership of the dam was recently transferred from the Virginia Hydrogeneration and Historical Society to Mr. William Patton. Mr. Patton is the current owner of the dam.|
The Denil fishway at Harvell Dam is a unique and complicated structure that contains two fish entrance locations at different elevations (one of which is influenced by tidal flow), has a dual attraction flow system, and is intended to operate in combination with hydropower generation under a wide range of river flow conditions. The complexity of the fishway requires continuous monitoring and careful adjustments in response to changing river flow conditions to ensure that all aspects of the facility function properly and optimal flow conditions for attracting upstream migrating fish into the fishway are constantly maintained.
Fish passage monitoring results from 1998, 1999 and 2001 (during hydropower generation) reported some fish movement through the Denil fishway, including a few American shad. However, hydropower generation ceased in 2004 at Harvell Dam with no plans for resuming generation. The performance of the Denil fishway is compromised without adequate attraction flow created by hydroelectric generation. Furthermore, the existing internal attraction flow system of the Denil fishway is undersized, supplying adequate attraction flow for only 60 percent of the time (i.e., river flows up to 1,400 cfs) during the spring migration season. Currently, no monitoring results are available to evaluate the performance of the fishway since hydropower generation has ceased at this site.
The complexity of the fishway, continuous monitoring requirements, inadequate attraction flow and the absence of hydropower generation all contribute to the reduced efficiency of the Denil fishway. It is believed that the performance of the Denil fishway can be improved during low and average river flow conditions by altering the fishway structure's design and simplifying the operational procedures required to maintain the facility at its maximum efficiency. The recommended modifications are described in greater detail in Section 10 of the full report. The most significant modifications include the following:
• Eliminate the main (lower) fishway entrance and only use the secondary (upper) fishway entrance.
• Construct a rock weir barrier across the siphon tailrace channel to the main (lower) fishway entrance to prevent migrating fish from entering this channel.
• Reconstruct the secondary (upper) entrance and provide an automated gate system for continuous monitoring and adjustment of the entrance flow conditions.
• Construct an improved approach channel servicing the secondary entrance.
• Extend the spillway barrier on the dam crest and relocate the down migration chute.
• Provide continuous monitoring of the facility during the upstream migration season to document the effectiveness of the facility to pass the desired fish species.
The aforementioned modifications are intended to reduce operation and maintenance costs and enhance the performance and efficiency of the existing Denil fishway during low and average river flow conditions (up to 2,600 cfs). Based on historical river flow records, it is expected that river flows will exceed 2,600 cfs approximately 22 percent of the time during the spring migration season (March 1st through June 30th). As river flows exceed 2,600 cfs and approach the upper design range (7,500 cfs), elevated tailwater conditions reduce the effectiveness of the attraction flow system and the fishway will become less effective in attracting fish into the fishway. Improvements to the attraction flow system to allow it to function during high river flow conditions would require significant alterations to the structure or a complete replacement of the structure with a vertical slot fishway and may be impractical. The estimated construction costs to modify the existing Denil fishway to achieve satisfactory fish passage during low and average flow conditions are estimated to range between $350,000 and $500,000. Accounting for design and permitting, the total project costs are estimated to be between $400,000 and $600,000.
It should be noted that the physical limitations of the current structure, even if modified, will not support the projected ultimate numbers of target fish species. Should the target species populations increase over time to a point where they approach the physical limitation of the existing Denil fishway, additional fishway(s) and/or other types of fishways will be required at Harvell Dam to accommodate the target migration goals for the Appomattox River.
Should additional fish passage capacity be required, it is recommended that nature-like fishways, such as the rock ramp fishway, be considered. These types of facilities have an unlimited capacity, allow a wider variety of aquatic species to pass through the structure, provide aquatic habitat, and have minimal maintenance requirements. Alternatively, if the rock ramp solution is not practicable, a vertical slot fishway could be considered.