Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
• Reconstruction of Mt. Adams Orchard diversion and temporary water supply pipeline
• Construction or upgrading of haul roads
• Preparation of staging areas and disposal sites
• Preparation of work areas
• Potential fuel or oil spills from equipment and storage areas
Potential impacts related to each of these activities are described below. All of these potential
impacts are considered short-term construction effects.
Reinforcement of the Northwestern Lake Bridge
The bridge was designed and constructed in 1958 to replace an existing bridge spanning
Northwestern Lake. The bridge was constructed well after accumulation of sediments in the lake
had begun, and there is concern that the rapid drawdown and accompanying erosion of the
accumulated sediments will destabilize the bridge. To alleviate this problem, seven measures are
proposed to protect the bridge:
• Sheet pile around two central piers
• Concrete wing walls and crib structure to tie existing bridge abutments to new sheet
pile cofferdams
• Temporary bracing to existing piers and sheet pile walls in stages, as required for
temporary construction stability
• Reinforce concrete grade ties from the existing concrete pile caps to new concrete
wingwalls
• Possible construction of new base footing within cofferdam
• Backfill of cofferdam and concrete crib structures
• Armor rock along river revetment slopes on both sides of the bridge near the
abutment approaches
Fine grained sediments would be suspended in the water surrounding the bridge piers, potentially
increasing turbidity during installation of the sheet pile, construction of the wing walls and crib
structure, construction of a new base footing (contingency), backfilling of cofferdam and crib
structures, and placement of armor rock. Coarse-grain sediments disturbed during this process
would not migrate due to this construction activity. In accordance with the Sediment
Assessment and Management Plan (PacifiCorp 2004) silt curtains would be installed to help
minimize migration of suspended fines. With the implementation of the proposed measures, the
amount of fines that would migrate from the construction area for the Northwestern Lake Bridge
is expected to be small and would occur over a short duration, and therefore is considered to be a
less than significant impact.
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