Works / Stormwater Retrofitting Multnomah County Stormwater Retrofits / Multnomah County, Oregon

Multnomah County Stormwater RetrofitsMultnomah County is in the process of researching potential facility management options in anticipation of an upcoming bond measure. To better understand their options, Multnomah County hired GreenWorks, P.C. for guidance on potential stormwater retrofits that will provide long-term cost savings and meet broader sustainability goals. As the owner and operator of a significant number of projects in the region, Multnomah County first needed help to prioritize projects with the highest potential opportunities and determine which properties the County planned on keeping long-term. The prioritized list of 35 sites includes offices, health centers, libraries, law-enforcement facilities, and parking facilities, all having dramatically different site configurations and potential.

GreenWorks analyzed these sites using aerial photography to determine impervious areas, and existing site opportunities. This information was used to determine the rate of stormwater fees, as well as the annual maximum savings potential. Using Clean River Rewards fee reduction as a basis, we developed a set of criteria for determining the proposed approach factoring in site difficulty, County goals, and visibility of sites for promoting sustainable living. The array of options likely to be implemented included rain gardens, swales, stormwater planters, eco-roofs and pervious pavement. Our analysis determined a preliminary cost projection, overall capital cost, and cost saving resulting in a potential return on investment. The results of the study proved that a variety of stormwater retrofit opportunities exist at various project sites throughout the region. Payback periods varied depending on the specifics of the site, but for stormwater alone, within the City of Portland, payback periods were between 15 and 51 years when factoring in the rate annual stormwater fee increases. Combined facility improvements to the entire system would cost $2.1 million dollars and result in a payback period of 29 years.