Culvert Replacement Corps Permitting and Engineering Review

  • Snohomish County proposed to replace 29 culverts, such as this one, that impeded fish passage.
  • Confluence conducted fieldwork to confirm determinations about regulatory compliance, permitting, and jurisdictional status.

Snohomish County Surface Water Management (SWM) proposed 29 culverts and 2 water quality facilities for replacement as part of a program to improve fish passage and habitat conditions. Confluence managed the effort to determine applicable U.S Army Corps of Engineers (Corps) regulatory requirements for the proposed projects. Confluence worked with design engineers to assess field conditions and fish passage requirements to develop appropriate replacement designs and to minimize environmental impacts (both through design and best management practices during construction).


The culvert replacements have been designed to meet the current Washington Department of Fish and Wildlife (WDFW) guidelines for fish passage, and to correct barriers to fish passage. Stream channel, bank modification, and restoration/mitigation are elements of these projects. Confluence’s work involved evaluating culvert and water quality facility designs as well as construction means and methods to determine applicability of Corps Section 404 maintenance exemptions or Nationwide Permits. Confluence completed field investigations of each proposed project to determine potential impacts to waters of the United States. If it was determined that the Corps had jurisdiction over the proposed activity, we developed a permitting approach to address anticipated regulatory requirements. Project work also involved coordination with Snohomish County SWM, WDFW, the Corps, and the Tulalip, Muckleshoot, and Stillaguamish Indian tribes. Several of the culverts have already been replaced over the 2017 and 2018 in-water work windows, with the remaining culverts to be replaced during in-water work windows between 2019 and 2022.


Each of the culvert replacements triggered multiple regulatory approvals. Confluence helped SWM maximize avoidance and/or minimization of project environmental impacts to increase efficiency and decrease costs of permitting and constructing the projects. When the proper replacement solution resulted in unavoidable impacts, Confluence documented the alternatives assessment as well as the reasoning for the selected replacement solution. For some culvert replacements, additional restoration or mitigation was needed for those unavoidable impacts, and Confluence worked with Snohomish County to develop the mitigation design while still providing the necessary natural resource improvements.

Project Location
Snohomish County, Washington

Snohomish County Department of Public Works—Surface Water Management

Project Timeline
Ongoing since 2017