The following potential impacts to water quality may be associated with various construction and operational aspects of the project:
• Suspended sediment loading during in-stream construction
• Release of sediment from surface runoff
• Water contamination from spill
• Water contamination from fill/construction material
• Sediment or contaminant release during maintenance
• Sediment release from bank erosion or surface runoff
• Water contamination from spills
The main water-quality issues related to the bridge project relate to the potential release of sediments or chemicals into the river channel, primarily during construction. If sediment is released during construction the main impact would occur downstream of the bridge. Sediment loading from bridge construction would be short-term. A variety of construction techniques could be used to minimize, or eliminate, the possibility of large and sustained sediment release. Timing of construction to avoid critical periods for fish species and liferequisite activities (spawning) also may be an effective mitigation tool. Sediment release can also occur during the operation period of the bridge as a result of shoreline erosion, from surface runoff along ditches, and from fine sediments washing off bridge deck into the river (e.g., sand, gravel, and dust from road, maintenance activities, and traffic). Other potential water-quality impacts include the release of contaminants into river from fuel or chemical spills. This can occur during construction when heavy equipment is working on near the river as well as during operations from traffic and maintenance activities.
The following potential impacts to water quantity are usually identified for activities associated with a bridge construction (construction of bridge approaches and in-stream pier construction/placement):
• Alternation to channel hydraulics preventing fish movement,
• Changes in channel hydraulic form resulting in downstream erosion or deposition,
• Channel blockage (flow restrictions),
• Channel aggradation/degradation overtime.
Potential impacts of a bridge include changes to channel hydraulics upstream and downstream ofthe structure as well as immediately around the bridge piers. Any narrowing of the channel can restrict flows and increase river velocity at crossing site. This increase of velocity can lead to erosion around the bridge abutments and piers. The channel restriction can also impede the movement ofice at breakup and lead to ice jams ifthe bridge is not designed properly. During construction, depending on construction techniques, the river may be constricted to allow construction to occur away from the flow of the river to reduce sediment release. Overall, these types of changes may lead to erosion of the banks or channel bed, increase sedimentation, changes in river velocity in the vicinity of the bridge, deposition of sediment in new areas of the channel, and restriction of fish movements.
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