Integrating Interdisciplinary Studies Across a Range of Spatiotemporal Scales for the Design of Effective Flood Mitigation and Habitat Restoration Strategies, Green Valley Creek, California

Abstract:

Green Valley Creek provides some of the most critical habitat for endangered coho salmon in the Russian River Watershed. Extensive changes in land-use over the past century have resulted in a dynamic system characterized by ongoing incision in the upper watershed and deposition and increased flood risk in the lower watershed. Effective management requires a watershed-scale understanding of the underlying controls on sediment erosion and transport as well as site-specific studies to understand local habitat conditions and flood dynamics. Here we combine an evaluation of historical changes in watershed conditions with a regional sediment source assessment and detailed numerical hydraulic and sediment transport models to find a sustainable solution to a chronic flooding problem at the Green Valley Road bridge crossing.

Ongoing bank erosion in the upper watershed has been identified as the primary source of coarse sediment being deposited in the rapidly aggrading flood-prone reach upstream of the bridge. Efforts at bank stabilization are part of the overall strategy, however elevated sediment loads can be expected to continue in the near-term. The cessation of historical vegetation removal and maintenance dredging has resulted in a substantial increase in channel roughness as riparian cover has expanded. A positive feedback loop has been developed whereby increased vegetation roughness reduces sediment transport capacity, inducing additional deposition, and providing fresh sediment for continued vegetation recruitment.

Our analysis revealed that traditional engineering approaches are ineffective. Dredging is not viable owning to the habitat impacts and short timeframes over which the dredged channel would be maintained. Roadway elevation results in a strong backwater effect increasing flood risk upstream. Initial efforts at designing a bypass channel also proved ineffective due to backwater effects below the bridge. The only viable solution involved reducing the downstream backwater effect through removal of an existing partial levee and designing multiple high-flow bypass channels to convey flows past the flood-prone reach. In addition to reducing the flood hazard, the design is relatively resilient to continued deposition and creates new floodplain and side-channel habitat.