Clackamas County Group

Clackamas county group stormwater laws & regulations

The Clackamas County Group is a combined effort by Clackamas County and municipalities within the county in maintaining their joint NPDES MS4 Discharge Permit. Each municipality has authority of the MS4 within their jurisdictional area, and thus have individual municipal regulations regarding the permitting and compliance of stormwater discharge.

The Clackamas County Group includes 13 authorized co-permittees:

  • Clackamas County
  • City of Gladstone
  • City of Happy Valley
  • City of Johnson City
  • City of Lake Oswego
  • City of Milwaukie
  • City of Oregon City
  • City of Rivergrove
  • City of West Linn
  • City of Wilsonville
  • Oak Lodge Sanitary District
  • Clackamas County Service District No. 1 (CCSD #1)
  • Surface Water Management Agency of Clackamus County (SWMACC)

EXCERPT- NPDES MS4 Discharge Permit, August 2008

  1. Post-Construction Site Runoff: Co-permittees must continue to implement their post­ construction stormwater pollutant and runoff control program.

I. By November 1, 2014, the post-construction stormwater pollutant and runoff control program applicable to new development and redevelopment projects that create or replace impervious surfaces must meet the conditions described in this subsection.  The minimum project threshold applicable to each co-permittee post-construction stormwater pollutant and runoff control program is identified in Table A-1.  The post-construction stormwater site runoff permit conditions are as follows:

  1. Incorporate site-specific management practices that target natural surface or predevelopment hydrologic functions as much as practicable.  The site-specific management practices should optimize on-site retention based on the site conditions;
  2. Reduce site specific post-development stormwater runoff volume, duration and rates of discharges to the municipal separate storm sewer system (MS4) to minimize hydrological and water quality impacts from impervious surfaces;
  3. Prioritize and include implementation of Low-Impact Development (LID), Green Infrastructure (GI) or equivalent design and construction approaches; and,
  4. Capture and treat 80% of the annual average runoff volume, based on a documented local or regional rainfall frequency and intensity. ·



Post-Construction Minimum Thresholds - Impervious Surface Area


Minimum Project Threshold (ft)

Clackamas County*


City of Gladstone


City of Happy Valley


City of Johnson City


City of Lake Oswego


City of Milwaukie


City of Oregon City


City of Rivergrove


City of West Linn


City of Wilsonville


Oak Lodge Sanitary District


*Includes jurisdictional areas within CCSD# 1, SWMACC, and jurisdictional areas with post-construction program oversight by the Clackamas County Department of Transportation and Development.


ii. The co-permittee must identify, and where practicable, minimize or eliminate ordinance, code and development standard barriers within their legal authority that inhibit design and implementation techniques intended to minimize impervious surfaces and reduce stormwater runoff (e.g., Low Impact Development, Green Infrastructure).   Such modifications to ordinance, code and development standards are only required to the extent they are permitted under federal and state laws. The co-permittee must review ordinance, code and development standards for modification, minimization or elimination, and appropriately modify ordinance, code or development standard barriers by November 1, 2014. If an ordinance, code or development standard barrier is identified at any time subsequent to November 1, 2014, the applicable ordinance, code or development standard must be modified within three years.

  1. To reduce pollutants and mitigate the volume, duration, time of concentration and rate of stormwater runoff, the co-permittee must develop or reference an enforceable post­ construction stormwater quality management manual or equivalent document by November 1, 2014 that, at a minimum, includes the following:
    1. A minimum threshold for triggering the requirement for post-construction stormwater management control and the rationale for the threshold.
  1. A defined design storm or an acceptable continuous simulation method to address the capture and treatment of 80% of the annual average runoff volume.
  2. Applicable LID, GI or similar stormwater runoff reduction approaches, including the practical use of these approaches.
  3. Conditions where the implementation of LID, GI or equivalent approaches may be impracticable.
  4. BMPs, including a description of the following:
    1. Site-specific design requirements;
    2. Design requirements that do not inhibit maintenance; and,
    3. Conditions where the BMP applies.
  5. Pollutant removal efficiency performance goals that maximize the reduction in discharge of pollutants.

iv. The co-permittee must review, approve and verify proper implementation of post­ construction site plans for new development and redevelopment projects applicable to this section.

  1. Where a new development or redevelopment project site is characterized by factors limiting use of on-site stormwater management methods to achieve the post-construction site runoff performance standards, such as high water table, shallow bedrock, poorly­ drained or low permeable soils, contaminated soils, steep slopes or other constraints, the Post-Construction  Stormwater Management program must require equivalent pollutant reduction measures, such as off-site stormwater quality management.   Off-site stormwater quality management may include off-site mitigation, such as using low impact development principles in the construction of a structural stormwater facility within the sub-watershed, a stormwater quality structural facility mitigation bank or a payment-in-lieu program.
  2. A description of the inspection and enforcement response procedures the co-permittee will follow when addressing project compliance issues with the enforceable post­ construction stormwater management performance standards.


State of Oregon documents:

Stormwater Permit Application Forms and Fees
Stormwater Contacts