Stone Bridge Replacement & Channel Restoration – Nettle Creek

Stone Bridge Replacement and Channel Restoration of Nettle Creek
BEFORE
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AFTER

Stone Bridge served as a culverted pedestrian crossing of Nettle Creek within the Tryon Creek State Natural Area. Nettle Creek is a tributary to Tryon Creek, a priority waterway for restoration of fish passage into the 645-acre State Natural Area. Nettle Creek flows were constrained through an existing undersized concrete culvert, creating both a velocity and passage barrier for resident and migratory fish. Historically, several species of salmonids and Pacific lamprey accessed the Tryon Creek watershed from the Willamette River. Extension of fish passage from Tryon Creek into the Nettle Creek sub-watershed has restored access to approximately 2000-lineal feet of state-protected natural stream habitat.

Downstream of the Stone Bridge culvert, high flows scoured and eroded native streambed grade, creating a deep plunge pool. With erosional displacement of native streambed materials at and below this plunge pool, Nettle Creek’s channel had subsequently incised and became disconnected from its natural floodplain. Hydraulic scour at the plunge pool had also begun to undermine the Stone Bridge, increasingly compromising the integrity and public safety of the pedestrian crossing. Measured jump height from water surface into the culvert exceeded 3 feet on average flows. Upstream of the bridge structure, streambed materials had aggraded due to flow and resulting sediment transport constraints through the undersized culvert. Upstream of the project reach, a more natural channel condition exists with good floodplain connectivity and healthy native riparian habitat.

HENDERSON environmental design-build professionals committed in-kind professional services on a pro-bono basis to assist the Tryon Creek Watershed Council in reaching 30% Design stage for restoration of Nettle Creek. HENDERSON was then awarded a contract to complete the project’s design, permitting, and construction. HENDERSON’S restoration efforts help the Council succeed in replacing the existing structure in its entirety and restoring Nettle Creek’s channel grade to a stable condition similar to upstream creek reaches. Stream grades were restored through construction of a ‘roughened channel’, incorporating boulder grade-control structures and large woody materials. A combination of salvaged bridge material (stone/boulders) along with imported and native bed materials were used to fill the scour hole and downstream incised channel, improving stream grades, reconnecting channel flows with the historic floodplain, and restoring resident and migratory fish passage. Large woody material was installed to stabilize a vulnerable embankment and to enhance in-stream and riparian habitat diversity. Finally, creek banks were stabilized with biodegradable erosion control fabric, and all disturbed areas restored with native herbs, shrubs, and tree species.

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