. Individual, site-specific Endangered Species Act and essential fish habitat consultation that would have added months to the project timeline.
SLOPES V is the 2013 revision to the Standard Local Operating Procedures for Endangered Species program reinitiated by the Portland district of the USACE. The program affects many types of development and construction projects, including those that impact wetlands and those that discharge stormwater into waters of the state below the ordinary high water mark.
SLOPES V contains stormwater quality standards that can be applied to existing or new developments in Oregon. The water quality storm event is higher than any required by the governing jurisdictions in the greater Portland area. For example, the SLOPES V water quality storm event can be over 1.2″ of rain (over 24-hours) in east Portland.
In contrast, City of Portland calls for up to 0.83″ of rain to be treated over 24-hours. Designing to City of Portland standards, treatment of 0.83″ of rain is considered equivalent to treating 90% of our annual runoff – over 32″ of rain! This is due to our frequent, low intensity storm events. Other local water quality events we design to can be less than 0.4″ of rain!
This industrial site was previously designed using vegetated swales to meet the City of Portland’s standards. When USACE review was triggered, their review letter stated that the swales need to comply with the SLOPES V standards (1.2” in this instance/project), or request formal consultation.
Our client didn’t have time for a formal consultation with an uncertain outcome. To comply with SLOPES V standards, the facilities must “accept and fully treat the volume of water equal to 50% of the cumulative rainfall from the 2-year, 24-hour storm for that site”. That amounts to 1.2″ of rainfall, but it was designed to treat 0.83″ based on Portland standards. One sure way to comply would be to redesign the facilities starting with a 1.2″ storm event.
However, I love challenges and was resistant to redesign the stormwater treatment facilities with these overly conservative requirements. I asked myself, does the SLOPES V rain event flow through the treatment facilities? And if so, do they adequately treat this larger event? With a little additional analysis it was found that the facilities do meet SLOPES V standards as designed! I’ll explain how: For this site, diversion/bypass manholes are used to direct small storm events into the swales for water quality treatment. Large storm events bypass the swales so that they do not damage vegetation.