Neskowin Creek Channel Modification Design, Permitting, & Construction

Overview

A wastewater effluent pipeline crosses Neskowin Creek in a reach that has a channel migration zone of approximately 225 feet. Until recently, the creek channel was situated along the edge of the channel migration zone and ran parallel to the wastewater storage lagoon at the base of the slope that supports the lagoon. Within the last several years the channel has shifted eastward to approximately the center of the channel migration zone. This shift in channel location occurred approximately 300 feet upstream of the pipe crossing, and returns to its previous alignment just downstream of the pipe crossing. The transition of the new channel alignment to the older alignment has resulted in stream flow that is now eroding the streambank containing the buried pipe and is directed perpendicular to the toe of the slope below the lagoon. Stream energy is now dissipated against the slope supporting the lagoon causing scour of the streambank adjacent to the pipe crossing. Scour has progressed to the point that the pvc pipe crossing is exposed beyond the protection of its steel casing and is at some risk of failure.

water-levelModifications to the channel and/or the pipe crossing are necessary to reduce the risk of pipe damage and failure from stream and debris flows. HENDERSON’s general approach includes two alternatives. The first alternative would reroute Neskowin Creek back to its previous channel that parallels the toe of the lagoon slope. The second alternative is to reroute the flow to a newly constructed channel across the right bank floodplain to decrease the impact angle of the channel with the lagoon slope.

HENDERSON intends to support Westech Engineering in completion of design and permitting of the proposed modifications in time to take advantage of the 2015 in-water work window for Neskowin Creek.

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