Stormwater Around the World, Part 1


As a civil engineer working for Parametrix in Portland, OR, I can design a bioretention basin to remove pollutants from stormwater. I follow the guidance in the 2020 City of Portland Stormwater Management Manual, which requires the facility to treat a design storm of 1.61 inches of rainfall over a 24-hour period, which has been determined statistically to represent 90% of the average annual rainfall. It must have a minimum freeboard of 2 inches and a maximum ponding depth of 18 inches, among many other requirements. 

Bike (or drive) a couple of miles across the Columbia River on the Interstate Bridge to Vancouver, WA, and the same facility is called a bioretention cell, and I design it according to the 2019 Stormwater Management Manual for Western Washington. It is sized using a continuous model that computes runoff every 15 minutes for 50 or more years of historic rainfall data to treat 91% of stormwater runoff. It must have a minimum freeboard of 6 inches and a maximum ponding depth of 12 inches. 

Head east along the Columbia River for about 70 miles, and I reach the city of White Salmon in Klickitat County. There, I design it according to the 2019 Stormwater Management Manual for Eastern Washington. The same bioretention cell is sized for the 6-month, 3-hour event, which I must calculate by reading two separate maps, one table, and use an equation for the “Precipitation Magnitude and Frequency for Short-Duration Storms”. 

I have a job, in a sense, because stormwater regulations are both complicated but also extremely localized. These variations are often warranted. The hydrology is different in Western and Eastern Washington, for example, with more frequent, lower intensity storms in the former. There are variations in geology, with some places having better soils for infiltration practices or more sensitive groundwater aquifers. There are differences in surface water impairment and pollutants’ effects on aquatic species. And then there are a whole category of political, cultural, and historic differences that drive these variations. For example, why is the soil mix for bioretention different in Oregon and Washington, and why is the G-2 inlet in Oregon so different from a Catch Basin Type I in Washington.

What’s Next 

I am not going to try to answer those questions, and one could write forever about the differences between jurisdictions in the United States. Instead, I want to broaden our horizons and look at stormwater beyond our nation’s borders. If I peer abroad, will the regulations, practices, and terminology be so different as to be unrecognizable? Or will crossing the ocean be less impactful than crossing the river? Are other regions’ standards as hyper-localized as ours? These are the type of questions I hope to answer over this series of blog articles. 

Where should I look? My working hypothesis is that stormwater management practices will have advanced the most in places with three characteristics: they have very dense, highly urbanized places which generate more runoff and have fewer natural areas to disperse and infiltrate it; they have sensitive surface waters with vulnerable aquatic species and habitats; and there is political will to change practices. According to this theory, Washington has become a leader in stormwater management here in the Pacific Northwest because we have large, dense metropolitan areas like Seattle, anadromous salmonids, and voters and lawsuits that have driven change. Similarly, dense urban areas around the Edwards Aquifer in Texas and Chesapeake Bay in the Mid-Atlantic have been their region’s leaders. 

I am currently conducting interviews with experts in Italy and Australia, so please look forward to my reporting from them. I know interesting things are being done in England, China, Japan, and probably many other places. If you have places you’d like me to research, or better yet if you have contacts I can talk to, please let me know. Otherwise, look out for the next installment of “Stormwater Around the World”! 

Seth Sokol, P.E. 


Water Solutions Engineer 

503-420-5607 | direct 

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