Roadway Standards Section 420 - Exceptions To WES Standards

The following standards are exceptions to WES stormwater standards.  These standards are directed towards the design and construction of public storm drainage facilities and for the coordination of stormwater runoff from private drainage systems into public systems.  The intent is to ensure a comprehensive engineering review and sufficient design to identify and mitigate existing deficiencies, protect the environmental health of our watersheds, and identify the capacity requirement of new system improvements, resulting in an overall benefit of reduced flooding.

If any conflicts arise between these standards and WES standards, the following shall govern.

420.1 Best Management Practices (BMP) & Low Impact Development Approaches (LIDA)

Engineering acknowledges the need for Best Management Practices (BMP) or Low Impact Development Approaches (LIDA) with development.  In addition to any LIDA standards adopted by WES, Engineering encourages designers to submit LIDA designs for review to meet the water quality and infiltration requirements outlined in WES stormwater standards.  Calculations will be required to illustrate to Engineering how a given LIDA provides water quality benefit. Private improvements in rural areas may work with DTD to provide a simplified approach to stormwater management that utilizes vegetation and infiltration if the site conditions warrant it. City of Portland's Simplified Approach for infiltration testing and typical details for stormwater management may be used in the rural area.

420.2 Acreage as a BMP

Development outside WES service districts is predominated by larger lot sizes.  A reasonable BMP outside the UGB may be the utilization of undeveloped acreage.  The applicant shall demonstrate to Engineering that water quality, detention, and/or infiltration requirements are met using the acreage BMP.

420.3 Surface Water Management Applicability

Surface water management plans in conformance with this chapter will be required for any of following:

1. When 5,000 square feet or more of new or reconstructed impervious surface is proposed within the UGB.
2. When 10,000 square feet or more of new or reconstructed impervious surface is proposed outside the UGB.
3. When grading or any new or reconstructed impervious surface is proposed or replaced within 50 feet of a perennial stream, creek, wetland, or lake, or within 10 feet of a property line.

420.4 Underground Injection Control (UIC) Devices

UICs are only permitted in Clackamas County right-of-way when registered with DEQ and maintained by an established stormwater district, unless otherwise approved by DTD Transportation Maintenance.  When UIC are proposed in County right-of-way, an approved IGA and maintenance agreement with an established stormwater district shall be in place prior to Development Permit issuance.