Necanicum River - Highway 101 Flood Mitigation

A flood mitigation and habitat restoration project led by the North Coast Land Conservancy (NCLC) and funded by the Oregon Department of Transportation (ODOT) removed a levee built in the 1960s and reestablished the Necanicum River floodplain, mitigating flooding of Highway 101.

A one-mile section of Highway 101 south of Seaside, Oregon has for decades experienced chronic seasonal flooding from the Necanicum River, which runs alongside it. The flooding has frequently caused traffic restrictions or road closures, resulting in lengthy detours, limited access to the local hospital, and delayed emergency services. A hydrological study commissioned in 2011 by ODOT, Clatsop County, the cities of Astoria, Cannon Beach, Warrenton, Gearhart, Seaside, and the Port of Astoria, indicated that the flooding was primarily caused by a 1.5 mile levee built in the 1960s along the Necanicum River on the bank opposite the highway. When heavy fall and winter rains combined with high tides, the levee prevented the river from overflowing into the natural floodplain, and instead the excess water flooded the highway.

The NCLC owns 364 acres of land adjacent to the river, but was unable to raise the approximately $1 million necessary to remove the levee. However, in early 2013 ODOT offered to partner with NCLC to implement the wetland restoration project as part of its wetland Mitigation and Conservation Banking Program. The program offsets unavoidable impacts on natural resources from ODOT’s highway construction projects by funding wetland restoration projects in other areas. The levee was removed by a contractor in summer 2013.

During spring and summer 2013, NCLC staff and volunteers reestablished the wetland by adding woody debris complexes throughout a 100-acre area in the floodplain. The debris complexes, made of logs, trees, and rootwads, create habitats for fish and birds when the area is flooded, and help to prevent surges of water from flushing out to the ocean. The debris complexes help hold flood waters in place, allowing for restoration of groundwater, and potentially providing a habitat for juvenile salmon.

Unusually heavy rainstorms in late September 2013 caused the Necanicum River to overflow and inundate 120 acres of the newly-accessible floodplain; however, Highway 101 was not flooded. ODOT attributed the lack of highway flooding to the levee removal and wetland restoration project, and indicated that this first test of the new system was a promising initial result.

 

This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on December 29, 2014.

Publication Date: 2013

Related Organizations:

  • Oregon Department of Transportation (ODOT)
  • North Coast Land Conservancy

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  • Best practice
  • Case study

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