Operational Plan for the Restoration of Diadromous Fishes to the ...

exclusion. Information from published literature was not located to document this
assumption.
Based on the approximately 14-foot difference in elevation between the Penobscot River
and the Howland Dam impoundment at normal flow conditions, a 10-foot high hydraulic
barrier integrated into the TSCF would reduce rise of the Proposed Bypass from 14 to 4 feet
at normal flows. Changes to this head differential during periods of high flow would need to
be evaluated to determine whether increased backwater effects would adversely affect the
efficacy of the hydraulic barrier at the weir.
The use of a weir in a TSCF located at the upstream end of the Proposed Bypass would
require setting the invert of the Proposed Bypass at a substantially lower elevation
downstream from the TSCF. This would substantially increase the footprint of the Proposed
Bypass and required excavation, thereby increasing the overall project cost while reducing
the area of adjacent land for other uses.
3.2.4 Debris Management
Debris management may also be easier at the upstream limit of the Proposed Bypass, as a
single system could be used to manage all debris in the vicinity of the upstream end of the
channel. Regardless of the location of the TSCF, consideration should be given to excluding
large debris from entering the upstream end of the Proposed Bypass so as to minimize the
need for debris management in the channel.
A variety of approaches may be appropriate for debris management, including trash racks,
debris booms, and/or piers in the adjacent reach of the Piscataquis River to deflect debris. A
trash rack system with a 1-foot clear spacing located immediately upstream from the
hydraulic entrance to the Proposed Bypass would provide an effective means for debris
management, but might require substantial maintenance. A debris boom or piers to deflect
debris might be less effective than a dedicated trash rack system but would likely require
less maintenance. Existing piers in the Piscataquis River could potentially be used for debris
management.
The evaluation of the suitability of piers for debris management would need to evaluate flow
routing over the Howland Dam during periods of high flow when large debris (e.g., whole
trees) are likely in the adjacent reach of the Piscataquis River.
3.2.5 Design Alternatives
The conceptual TSCF design considers both "series" and "parallel" TSCF systems. A series
TSCF refers herein to a system with a single route for upstream passing fish. A parallel
TSFC system refers herein to a system with multiple routes for upstream passing fish, and
may include routes without trapping facilities. The use of definitions based on upstream fish
passage routes follows from the primary objectives of the Proposed Bypass—upstream fish
passage, the recognized need for hydraulic conveyance in the Proposed Bypass, and the
associated need for debris management.
Both series and parallel TSCF systems would therefore consist of a trapping system
handling a fraction of the total flow in the Proposed Bypass. The balance of the flow would
be routed around the trapping facility through the TSCF conveyance channel with means to
preclude upstream passage of all target species (series TSCF system) or allow upstream
passage for a subset of target passage species (parallel TSCF system).
PRFP Page 275