By approving the Central City 2035 Plan, Council enacted a new ecoroof requirement that applies to most buildings in the Central City Plan District. Here, we provide clarification for how the requirement of CC2035 relates to the Stormwater Management Manual.
- By approving the Central City 2035 Plan (CC2035), Portland City Council enacted a new ecoroof requirement in Portland’s Zoning Code that applies to most buildings in the Central City Plan District (see Portland City Code 33.510.243).
- Ecoroofs are an approved stormwater management facility in the City’s Stormwater Management Manual (SWMM) for water quality and flow control. The City’s stormwater management approach prioritizes vegetation and infiltration stormwater management facilities.
- Given these two overlapping ecoroof regulations, there is a need to clarify how the SWMM and CC2035 relate to each other.
- While the adoption of the CC2035 Plan and its ecoroof requirement are the drivers for this clarification, this policy will apply City-wide.
Directions to Designers
Clarifications of Infiltration Requirements
With the advent of the new zoning requirement, Environmental Services is clarifying our approach to ecoroofs and how they affect the requirement to infiltrate onsite.
- Building projects that satisfy the minimum coverage of 60 percent ecoroof to meet the zoning requirement will not be required to evaluate onsite infiltration for the building area, regardless of the available stormwater disposal location (storm-only versus combined sewer).
- Ground-level impervious areas that can be considered incidental and associated with an adjacent building meeting the 60 percent ecoroof standard, such as areas within a property setback or building articulation, are not expected to meet a different design approach. Therefore, they can also be waived from infiltration requirements.
- Ground-level impervious areas that allow sufficient space to install an infiltration facility, such as surface parking lots, will need to investigate infiltration for those impervious areas.
Stormwater Pollution Reduction Requirements
- MS4 areas: All non-ecoroof impervious areas must be treated for water quality.
- Combined areas: Per the SWMM, pollution reduction is not required for discharge to the combined sewer system.
Stormwater Flow Control Requirements
- Sites are required to meet all flow control requirements for the relevant MS4 or combined sewer system, as defined in the SWMM.
- Partial Ecoroofs: Partial ecoroofs will contribute to meeting flow control requirements for the site. In some cases, a partial ecoroof may be all that is required. To address this, the project's stormwater report must include calculations using the following engineering assumptions:
- Calculation Method: Santa Barbara Urban Hydrograph method
- Time of Concentration: 5 minutes
- Ecoroof Curve Number: 61
- Use the following equation to calculate the weighted curve number for the entire roof area: