Benton County, OR Environmental Rules: Stormwater Management (2026)

BCC § 99.670 is Benton County's post-construction stormwater rule, adopted to protect water quality and prevent flooding and sedimentation from runoff created by new development and redevelopment. It applies to all land development within unincorporated Benton County. A property owner must obtain a Post-Construction Stormwater Permit before any ground-disturbing activity if the project needs another county permit or review and the proposed new or replaced impervious surface, combined with the cumulative total of impervious surface established on the property since March 1, 2023, reaches 0.25 acre (10,890 square feet) or more; a project below that threshold on its own still counts toward the running cumulative total. Roads, driveways, structures, and site work such as a septic drainfield or landscaping all count as triggering activity.

The application must include a stormwater site plan stamped by an Oregon-licensed engineer or engineering geologist, designed to the county's Stormwater Support Documents, plus the fee set by the Board of Commissioners, and the property owner must sign a Stormwater Management Facilities Long-Term Maintenance Agreement before the County Engineer will issue the permit under 99.670(4)(c) and (6). For a subdivision or partition, the stormwater plan must address all impervious surface the whole project will eventually create, with maximum impervious area recorded as a deed restriction or on the plat, and detention and treatment for full buildout designed before final plat approval. Some activities are exempt, including accepted farm practices, agricultural buildings outside the county's MS4 area, and Oregon Forest Practices Rules activities, though forestry land reverts to permit-required status if it is not replanted to state stocking standards.