How to Meet SWMM Requirements for Projects in the Right-of-way

Option 1. Discharge to Existing Facilities in the Right-of-way.   

Upgrades or modifications may be required.

• Identify any existing stormwater facilities within the vicinity of the project site.
• If existing sumps or other facilities are present, coordinate with Environmental Services' staff to determine if an existing facility is adequate to meet SWMM requirements for the triggered impervious area. Adequacy will depend on conveyance to the facility, capacity, condition and configuration. For sumps, an existing facility will not be sufficient as-is if a sedimentation maintenance hole is not present.
• If modifications are necessary, they will be reviewed through the Public Works Permit application.

Option 2. Construct New Stormwater Facilities.

The type of new stormwater facility will depend on the site evaluation criteria described in SWMM Section 2.2. Refer to SWMM Chapter 4 for public stormwater facilities design criteria and submittal requirements.

• Stormwater Facilities and Right-of-Way Dedication: The type, size and placement of required stormwater facilities can impact necessary right-of-way dedication widths. Additional right-of-way dedication for stormwater facilities is often necessary when there is no existing stormwater treatment system and a sedimentation maintenance hole and sump system is not feasible.
• Calculating Facility Basin Areas for Sizing: The drainage basin area for sizing a facility will vary depending on the specific type of stormwater facility proposed.
  • Sumps: Sumps must be sized for the full contributing drainage area, not just the new impervious surface area.
  • Vegetated Stormwater Facilities: These must be sized based on the total triggering area. For most projects, this will consist of the new or redeveloped impervious area alone, but some projects will also trigger the SWMM because of a New Connection or Route of Conveyance (Section 1.2.2). For the latter, the relevant area is the existing impervious area that sheet flows directly to the new curb subject to the new connection criteria described above (for public works permitted projects, upstream gutter flow from outside of the project frontage can be excluded).
  • For other facilities types, discuss drainage basin delineation with the assigned BES reviewer.
• Lined Vegetated Facilities Discharging to the Combined Sewer: As stated in Section 1.3.6, vegetated stormwater facilities that are lined or cannot fully infiltrate and will discharge to a combined sewer system should only be used where there are local or regional capacity problems, as identified by BES staff. If there are no local or regional capacity problems, the applicant must maximize the use of tree credits and then request to pay an Offsite Stormwater Management Fee through the Special Circumstances process (Section 1.8).

Option 3. Apply Tree Credits.

• Trees in the right-of-way can be used to provide stormwater management for some impervious surfaces, subject to the design criteria in SWMM Section 4.2.5.1.
• Tree credits can only be applied to triggered impervious areas located within the same block face and half-street as the tree itself.
• Refer to Section 4.2.5 for additional information. Submit the Tree Credit Worksheet with the Public Works Permit.

Option 4. Pay an Offsite Stormwater Management Fee through Special Circumstances.

A project designer can request to pay an Offsite Stormwater Management Fee instead of building a stormwater management facility for some or all of the stormwater management requirements for a project. Under the 2020 SWMM, there are two types of Special Circumstances that each apply under different scenarios.

Staff Review Special Circumstances

For projects in the right-of-way that meet the criteria for Staff Review Special Circumstances (see SWMM Section 1.8.1), the Offsite Stormwater Management Fee will be allowed for the qualifying impervious areas. To determine if all or a portion of your project qualifies, answer the following questions:

Does the project add impervious area behind an existing curb, propose improvements to meet ADA requirements, or propose a new pedestrian path?

• Yes. What is the total square footage for all frontages?
  • Square footage for all frontages is less than 500 square feet. A Staff Review Special Circumstances Offsite Stormwater Management Fee is not required. You are done with this aspect of the project. Follow the SWMM process for all other triggered impervious areas, as required.
  • Square footage for all frontages is greater than 500 square feet. These areas qualify for the Staff Review Special Circumstances Offsite Stormwater Management Fee. If you would like to pay the fee instead of constructing facilities, submit the Staff Review Special Circumstances application form and follow the SWMM process for all other triggered impervious areas, as required. 

• No. Improvements do not qualify for Staff Review Special Circumstances fee payment. Follow normal SWMM process for all triggered impervious area or apply for a Committee Review Special Circumstances.

Additional notes on the Staff Review Special Circumstances

• For sites with existing facilities, payment of the Offsite Stormwater Management Fee is not necessary if the existing facility is adequate per BES staff (this is typically true in the sumped areas of Portland).
• For sites with public improvement requirements on multiple frontages, the impervious areas eligible for the Staff Review Special Circumstances should be calculated together to determine if they exceed 500 square feet for the purpose of determining Offsite Stormwater Management Fee payment. If the total area eligible for a Staff Review Special Circumstances is less than 500 square feet, fee payment is not required.

Committee Review Special Circumstances

Committee Review Special Circumstances request must demonstrate why it is technically infeasible to manage stormwater runoff for the area not being managed. Typically, site constraints related to grading, slopes, or topography are most likely to make construction of a stormwater facility infeasible. Refer to SWMM Section 1.8.2 for additional information.

To determine if this option is appropriate for your project, ask yourself this question: Is it technically infeasible to meet SWMM requirements for all or a portion of improvements in the right-of-way (consult with BES reviewer as applicable)?

• No. Follow normal SWMM process for all triggered impervious area.
• Yes. Apply for Committee Review Special Circumstances. 

• If the application is denied, follow normal SWMM process for all triggered impervious area.
• If the application is approved, pay offsite stormwater management fee for approved areas.

Always consult the current SWMM for complete requirements.