Staff Memorandum - Laserfiche WebLink

Stormwater Development Standards, October 10, 2024, staff memo City of Eugene • 180 W 8th Avenue, Suite 200 • Eugene, OR 97401 • 541-682-5291 • www.eugene-or.gov First Priority - Infiltration The NPDES permit prioritizes infiltration facilities over all other stormwater quality treatment and mitigation methods. Infiltration is achieved through the construction of low impact development facilities, mostly green (vegetated) infrastructure, designed to infiltrate the runoff from the water quality design storm. Runoff volumes above the water quality design storm are allowed to overflow to the City stormwater system or another approved destination. An example infiltration facility is a stormwater planter where the rainwater runoff from a street, roof or other impervious surface infiltrates through the plants and soil into the ground with an overflow outlet for larger rain events connected to a piped system, ditch or waterway. The NPDES permit allows exemptions to the infiltration requirement where there are certain physical constraints. Geologic and hydrologic exemptions to the infiltration requirement are in both the current code and proposed code amendment and apply where there is low infiltrating soil, shallow bedrock, high groundwater or steep slopes. If a property or project qualifies for an exemption, it must include the next- highest priority method or show that it also qualifies for an exemption from that method as well. Second Priority - Extended Filtration Extended filtration is achieved through the construction of green infrastructure facilities designed to filter pollutants from the runoff of the water quality design storm through plants and soil. The main difference between infiltration (highest priority) and extended filtration facilities is that infiltration facilities are designed to fully infiltrate the runoff from the water quality design storm whereas extended filtration facilities have an underdrain system that drains the filtered water out to the City stormwater system or other approved destination. There are possible exemptions from the requirement for extended filtration. If a proposed development is otherwise consistent with all other applicable lot and development standards but the development site does not have sufficient land area to provide extended filtration facilities without reducing the size of the proposed development, the development may be exempted from the extended filtration requirement. An example is a zero-setback development that is consistent with all other lot and development standards, because such a development would have insufficient area for infiltration or filtration facilities. Third Priority - Off-site Water Quality Mitigation A development that is exempt from both the on-site infiltration (highest priority) and extended filtration (second priority) requirement is assumed to rely on an off-site City treatment system and therefore is subject to the Low Impact Development component of the City’s stormwater system development charge (“SDC”) improvement fee. In that case, however, the City’s SDC methodology allows the development to get an SDC credit if it chooses to install an approved on-site mechanical treatment facility. For More Information More information on stormwater facility types and design can be found in the Stormwater Management Manual at the link below: https://www.eugene-or.gov/477/Stormwater-Management-Manual More information on the Off-site LID Water Quality Mitigation program can be found in the Stormwater Systems Development Charges methodology at the link below: https://www.eugene-or.gov/DocumentCenter/View/66325/Appendix-D-Stormwater-SDC-Methodology- Resolution-No-5100