Southport Sacramento River Levee Setback: Adaptive Management of a Floodplain Restoration Project

Year: 2022
Presenter/s: John Stofleth
Symposium Session: 2022 - 02 Big Challenges Need Adaptive Management
Topics covered: fish-salmon, floodplain, hydraulics, lessons learned, modeling, monitoring, outside PNW, riparian, risk, sediment transport, and urban


ABSTRACT

Many flood control levees along rivers in the California Central Valley originated as dredged material removed from the channel following massive sedimentation associated with the 19th century gold rush. Dredge spoils were placed on natural levees and therefore eliminated most of the active floodplain. Despite various improvements, much of the flood control infrastructure remains unsound and riparian and floodplain resources (e.g., salmonid rearing habitat) are degraded. To address these issues, a major levee setback was constructed along a five-mile reach of the Sacramento River in West Sacramento in 2018, creating 140 acres of riparian and floodplain habitat. The new floodplain was about 10 ft lower than the existing floodplain, requiring excavation of about 2.1 million yd3 of material.
The new floodplain design comprised hydrodynamic modeling and geomorphic interpretation to maximize ecological functions. We used the two-dimensional hydrodynamic and sediment transport model, MIKE21C, as an analytical tool for assessing floodplain inundation relative to specific ecological criteria and morphological change (erosion and sedimentation) under existing and design conditions. The resulting restoration design fell within the constraints of the overall flood protection project with special habitat features such as instream wood structures, variation in floodplain topography, and large-scale plantings to enhance salmonid rearing habitat. Inundation modeling allowed delineation of zones for riparian forest, shaded riverine aquatic, and seasonal wetland habitat.

Project plans included monitoring and adaptive management. Vegetation monitoring followed standard protocols for on-the-ground visual inspection to document plant survival, cover, and health to meet regulatory requirements for mitigation. Aerial surveys provided supplemental support when delineating vegetation zones required for the post-project wetland delineation. Erosion and sedimentation were monitored using visual inspection, drone photography, and ground surveys.

Monitoring revealed that the hydrologic regime following construction activated adaptive management plans. An extended period of inundation occurred during the 2018-2019 flood season causing minor localized erosion and significant localized sediment deposition. Adaptive management measures included excavating a low-flow swale that provided connection to the river after it filled with sediment during the first high flow season and adjusting the planting palette following sediment deposition / topographic change. The following two years (2019 – 2021) were very dry, which required supplemental irrigation for the planted vegetation and adjusting the delineation of the designed vegetation zones after native riparian species colonists outcompeted plantings in areas initially intended as wetlands.

In hindsight, project implementation might have been more efficient if final topographic contouring and planting of the new floodplain had been delayed until after the first major high flow. This approach might have eliminated the need to develop predetermined vegetation zones, as natural recruitment could have been leveraged to produce a more sustainable plant community mosaic. Furthermore, the allowance of access routes for continued maintenance and adaptive management of sedimentation in the low-flow swales could reduce the potential for fish stranding in the new floodplain areas.