Presenter/s: Vaughn Collins
Symposium Session: 2023 - 07 - Restoration as Risk Reduction: Flooding
Topics covered: climate change, community involvement, flood, floodplain, lessons learned, and risk and resilience
Many restoration projects involve removal or modification of infrastructure such as roads and levees, due to the ecological impacts of these structures. Many of these structures, even when intended for flood control, increase flood risk on adjacent areas due lack of holistic analysis, changes in land use, and ad-hoc construction over many decades. As a result, it is not uncommon for restoration projects to claim tangential flood risk reduction benefits. Less common, however, is when a focused flood risk reduction study driven by damages to infrastructure and developed areas leads to selection of large-scale habitat restoration as the preferred alternative. Additionally, many restoration projects have ecologically based post-construction monitoring studies conducted, but rarely do these studies validate any expected flood risk reduction benefits.
Two projects on the north Oregon Coast initiated to address highway flooding and economic flood damages serve as examples of restoration projects being constructed with flood mitigation as a primary objective, and where post-construction analysis and monitoring validated the expected flood risk reduction. The first project addressed chronic flooding of Highway 101 near Seaside that led to repeated roadway closures every winter. An alternatives analysis showed that structural solutions such as road elevation were far more expensive than implementing floodplain restoration on 300 acres of land across the river from the highway. Hydraulic modeling in the design phase was used to estimate reduction in flood levels and roadway inundation times: post-construction monitoring and modeling validated this analysis.
In the second project, an alternatives analysis showed that large scale restoration of over 500 acres —well beyond that contemplated in prior studies—provided the best flood mitigation benefits for Highway 101 and thousands of acres of adjacent farm and urban lands in Tillamook. A rigorous benefit-cost analysis was used to justify award of $3.5 million in FEMA hazard mitigation funds based purely on flood mitigation benefits. As part of an extensive pre- and post-construction monitoring program, the flood level reduction benefits have been validated through modeling and analysis.
These two projects provide examples of how past floodplain development has exacerbated flood risk and how restoration projects can provide significant and quantifiable risk reduction in addition to ecological benefits.