Water and Land Resources Division
Department of Natural Resources and Parks
King Street Center
201 South Jackson Street, Suite 704
Seattle, WA 98104-3855
206-477-4800 Fax 206-296-0192
TTY Relay: 711
T E C H N I C A L M E M O R A N D U M
November 15, 2019
TO:
Stormwater Interested Parties
FM:
Jeff Burkey, Hydrologist, Water and Land Resources Division, Department of Natural
Resources and Parks
RE:
Crowdsourcing Analyses to Evaluate Impacts of Climate Change on Stormwater BMP
Designs
Background
Recent analysis by the King County Department of Natural Resources and Parks, University of
Washington Climate Impacts Group, and Washington State Department of Ecology suggests that
projected changes in heavy rain events may require larger BMPs. In short, a suite of BMPs
evaluated for changes in rainfall at Sea-Tac were shown to need larger capacities to mitigate
future projections of rainfall using current design standards. However, the identified relative
increases needed were variable and counter-intuitive at times when considering soil infiltration
capacities, levels of development, and sources of rainfall. Moreover, aside from the orographic
effect of the Cascade Mountains on rainfall in the Puget Sound, a casual review of downscaled
rainfall in a few other locations within King County suggests results may be different than what
has been evaluated using projections associated with the Sea-Tac area. Additional research is
needed to develop a more robust set of scenarios and verify these results.
Why does it matter?
Regional stormwater facilities are typically designed to last several decades. This means that
stormwater infrastructure built today will experience changes in heavy precipitation due to
climate change. Understanding the range of possible outcomes is critical to ensuring that
stormwater systems can meet current and future service loads and mitigate future rainfall
projections. It may also help reduce the need for future retrofitting, which can be less efficient
and often insufficient and/or infeasible without incurring substantially more costs because of
surrounding constraints.
Jeff Burkey
November 15, 2019
Page 2
Crowdsourcing Proposal Overview
King County is proposing a crowdsourcing approach to increase the number of locations and
climate scenarios available for the stormwater analysis. Available now, there are a total of
thirteen downscaled climate scenarios (one using RCP 4.5 and twelve using RCP 8.5) at 145
locations spread across King and Thurston counties and Cities of Seattle and Everett.
Crowdsourcing can be a very efficient means to analyze conditions if analyses can be parallel
processed. For this effort, the steps needed to evaluate the impacts on BMP designs would be
repeated many times over.
The concept is to develop an informal collaboration where labor hours are shared among
participating members. The collaboration would establish templates to facilitate analyses by
defining scenarios that include:
• Best Management Practices (e.g., detention ponds, bioretention cells, etc)
• Geographic locations
• Levels of development to be mitigated (e.g., rural residential, urban residential,
commercial, etc)
• Background soil conditions (e.g., till, outwash, saturated, etc.)
• Global Climate Model scenarios (ACCESS1-0, GFDL-CM3, etc.)
To facilitate this sharing of effort, King County would set up a clearing house repository of
resources needed. This would include rainfall time series processed for inputs into Ecology’s
approved stormwater BMP design software, Western Washington Hydrology Model (WWHM)
2012. Among many materials, results would be summarized in a tabular format and posted for
display to the collaboration members. Sizing the BMPs using WWHM2012 does include some
elements of subjectivity. Hence, a quality control method would be established to capture
variability/biases among members performing the analyses. This may be something like a
common set of sizing scenarios to elucidate any tendencies among individuals when designing
BMPs.
Other Analyses
Other possible analyses of value could include:
• Looking at stormwater infrastructure and conveyance capacities and how projections of
future rainfall may exceed existing design standards.
• Looking into the sensitivities BMPs may have under certain characteristics of the rainfall
continuum.
• Downscaling climate model outputs over a broader geographic area.
Interest and capacity to conduct these additional assessments will be evaluated once the
collaboration is organized and operating.
Jeff Burkey
November 15, 2019
Page 3
Level of Effort
The amount of effort depends on the number of sizing scenarios performed. For a frame of
reference, sizing 100 BMPs would take about 40 hours of time. This is based on the assumption
that a person is familiar with WWHM and workbooks have already been setup to expedite the
design method processes that are not fully completed within WWHM. Participants are also asked
to participate in periodic meetings/conference calls and help with data analysis and summarizing
results. I would envision this effort occurring during the 2020 calendar year. Details will be
worked out with interested parties closer to launch of this effort.
Contact Info
If you are interested in participating in this collaboration, please contact me via email
([email protected], capitalization only for readability). We’ve already recruited a
few folks from the October 2019 workshop held in Redmond, but could use more. This effort
will start up in early 2020, probably with a kick-off workshop to go over and refine scope,
procedures, concerns, etc.
Regards,
Jeff Burkey
King County, Science and Technical Support, Water and Land Resources Division
cc:
Guillaume Mauger, Climate Expert, University of Washington, Climate Impacts Group
Peter Holte, Senior Program Manager, City of Redmond, Public Works Department
Laurie Larson, Program Manager, Washington State University Stormwater Center
Lara Whitely Binder, Climate Preparedness Specialist, King County
Mark Wilgus, Stormwater Expert, DNRP, Water and Land Resources Division (WLRD)
Jessica Engel, Program Manager, DNRP, WLRD