Stormwater designs addressing climate change in the Pacific Northwest

Feb. 3, 2023

 Climate change has been a prominent topic of conversation in the stormwater world recently, prompting industry professionals across all levels to ask how tools like Western Washington’s Hydrology Model (WWHM ) will accurately model future conditions so stormwater designs effectively protect water quality, communities and habitat. 

Stormwater engineers design and size facilities use past hydrologic event models and observations. For example, western Washington uses precipitation records starting in 1948 through water year 2009 to estimate total rainfall and peak events, and while this calculation method was suitable in the past, the impact on stormwater runoff due to more recent changes in precipitation and air temperature suggest that drainage infrastructure designed to manage storms based on historical data may be undersized for future storm scenarios.  

Stormwater in Washington State

Stormwater management is heavily scrutinized in Washington state. The state helped jump-start the nation’s development of municipal stormwater permit programs in the 1990s and continues to investigate modern stormwater management approaches. The Washington State Department of Ecology (Ecology) issues water quality permits to cities, counties and the state’s department of transportation (WSDOT) to ensure that stormwater runoff is being regulated and managed. These public agencies are then responsible for making sure that projects comply with permit requirements through the development and enforcement of flow control and stormwater treatment standards. Stormwater experts routinely use hydrologic models (such as WWHM) to help permit applicants and public agencies monitor stormwater conditions and forecast how stormwater will behave in the future. 

Current Stormwater Calculation Methods in Washington State

From a stormwater perspective, Washington state is a tale of two lands. Western Washington (defined, regionally, as land west of the crest of the Cascade mountains) gets annual precipitation between 30 and 200 inches per year depending on location. Within this area is the Puget Sound region, home of the state’s densest population and urbanization. It receives a lot of rainfall, thus impacting runoff and stormwater management codes that influence water quality in receiving waters like the Puget Sound. Eastern Washington, the state’s more arid region, receives less than 10 to 60 inches per year depending on location. 

Due to the unique climate and rainfall pattern differences between western and eastern Washington, two separate stormwater design manuals were developed for each region outlining different stormwater calculation methods (i.e., continuous simulation (applying to western WA) vs. single-event hydrology (applying to eastern WA)). Analysis of precipitation data and patterns within Washington define these two methods: 

1. Rainfall in western Washington often occurs over longer (multi-day) time frames and with
relatively moderate intensities compared to eastern Washington. To accurately model and compare pre and post development runoff, it is important to account for the influence of preceding storm events and longer durations of continuous precipitation. Therefore, continuous simulation modeling is required for designing flow-based stormwater quality treatment systems and flow control systems in western Washington. Conveyance systems and some erosion and sediment control systems in western Washington can be sized using either continuous or single-event modeling.   

2. Rainfall in eastern Washington usually occurs in shorter (often under 1 day) time frames and with relatively high intensities (like during a thunderstorm) compared to western Washington. Therefore, single-event modelling is used to size all stormwater quality treatment systems and flow control systems in eastern Washington. Conveyance systems and erosion and sediment control systems in eastern Washington are typically sized using single-event modeling. 

Current Challenges to Adding Climate Change Assumptions to Stormwater Design

The design of our current drainage systems is often based on a traditional assumption that storm events are static or unchanging (also referred to, statistically speaking, as ‘stationarity’ where assumptions for future events are based on past data). However, recent studies conducted by the University of Washington Climate Impacts Group (UW CIG) indicate that storms are expected to become more severe with climate change. These projected changes in extreme precipitation in the Pacific Northwest due to a changing climate can be visualized using UW CIG’s online tool, as shown below. The example here shows that a 6-hour-long, 25-year storm event in Seattle will likely increase in intensity by 27% by the 2080s relative to the 1990s, assuming a high greenhouse gas scenario. 

To accurately capture changes in heavy rainfall events, engineers may consider the projected changes in extreme precipitation for single-event modeling. For use in a continuous stormwater simulation like WWHM, engineers may need to incorporate the climate model projections that have been bias-corrected to match the statistics of a particular rain gauge.  

What's Next for Stormwater Prediction Methods in Washington State 

Engineers attempting to design conveyance, treatment, and flow control systems to account for
future conditions are currently stuck in a gray area. With climate change, the assumption of stationarity and exclusive reliance on historical observations for estimating future conditions is questionable. 

While updates to continuous and single-event modeling techniques can be implemented using UW CIG’s regional climate model projections, software developers and public jurisdictions have limited resources and funding to quickly do so. Therefore, the current pace of change will likely continue to rely on public agencies (like Ecology and King County) to update guidance for stormwater design manuals, evaluate the capacity of their existing infrastructures to focus on locations to prioritize needed upgrades, and find ways to optimize the effects of retrofit facilities to help mitigate climate change impacts to stormwater. 

Cleo Pineda is a stormwater engineer with Aspect Consulting. She has five years of experience in helping clients address stormwater regulatory, drainage, flooding, water quality, planning and design challenges. She can be contacted at [email protected].

About the Author

Cleo Pineda | Stormwater Engineer

Cleo Pineda is a stormwater engineer with Aspect Consulting. She has 5 years of experience in helping clients address stormwater regulatory, drainage, flooding, water quality, planning and design challenges. She can be contacted at [email protected].