Appropriate and effective stormwater management is critical to maintaining and enhancing the City’s livability and improving watershed health. Stormwater management standards and requirements are found in the Stormwater Management Manual.
These requirements allow the City of Portland to protect both watershed resources and infrastructure investments with every development or improvement.
Central City 2035 Plan and ecoroofs
Portland’s recently enacted Central City 2035 Plan (CC2035) includes a standard requiring ecoroofs on most new buildings in the Central City Plan District (see Zoning Code chapter 33.510). The standard is implemented by BDS and is intended to maximize ecoroof coverage in the Central City because of the multiple benefits they provide, including stormwater management.
An ecoroof constructed to meet the CC2035 zoning standard will also count toward the project’s stormwater management obligations reviewed by Environmental Services, but additional stormwater management may be required if the ecoroof coverage is insufficient to meet the site’s entire stormwater obligation.
Because the CC2035 standard will likely result in a significant number of partial ecoroofs (i.e. <100% total roof coverage), questions about stormwater requirements for the non-ecoroof area in these situations will likely arise. To address those questions, Environmental Services is clarifying some of our existing Stormwater Management Manual rules to fit this new regulatory context.
Although spurred by the adoption of a new plan that only applies to Portland’s central city, these Stormwater Management Manual clarifications apply City-wide. Specifically, for partial-coverage ecoroofs, the following clarifications should be considered in building and site design as applicable to individual sites.
Ecoroof coverage and infiltration
For all development, the Stormwater Management Manual currently requires applicants to investigate the feasibility of infiltrating post-development stormwater into native soils on-site; if feasible, infiltration facilities must be constructed. The SWMM also says that “Ecoroofs… can be used without consideration of onsite infiltration rates” and that managed stormwater from an ecoroof “ is allowed to overflow to an approvable offsite discharge location” (p. 1-26), but doesn’t say whether these allowances start at a certain coverage percentage or if they only apply to a 100% coverage ecoroof.
Environmental Services now clarifies that the investigation of onsite infiltration for a building is waived only when the ecoroof covers ≥ 60% of the building; if the ecoroof coverage is ˂60%, the feasibility of infiltrating building runoff onsite must be investigated per the Stormwater Management Manual.
Ground level impervious areas
For incidental ground-level impervious areas associated with an adjacent building that has ≥ 60% ecoroof, such as concrete areas that are inset to match building articulation, onsite infiltration does not need to be investigated.
However, larger impervious areas that allow sufficient space to install an infiltration facility, such as parking lots, must be investigated for the feasibility of infiltrating stormwater per the Stormwater Management Manual.
Pollution reduction must be provided for all non-ecoroof area when discharging to a storm-only system (Category 3 in the SWMM Infiltration and Discharge Hierarchy). Pollution reduction is not required in the combined system (Category 4).
Flow control must be provided for all non-ecoroof area. A partial ecoroof can contribute toward meeting the site’s overall flow control obligation, and in some cases a partial ecoroof may be all that is required. To assess this, the project engineer must calculate flow control using the Santa Barbara Urban Hydrograph method using a 5-minute time of concentration, an ecoroof curve number of 61 (from Stormwater Management Manual Table A-4), and a weighted curve number for the entire roof area per the formula provided in the handout linked to below.
Please refer to the new handout Guidance on CC2035 Ecoroof Requirements and the SWMM for more information about partial-coverage ecoroofs and the relationship between the Stormwater Management Manual and CC2035. Questions can be directed to the Environmental Services staff assigned to projects already in for City review or to Stephen Himes at 503-823-7875.