STORMWATER CLIMATE RESILIENCE IN WESTERN
WASHINGTON
REDMOND CITY HALL
OCTOBER 2, 2019
JEFF BURKEY, KING COUNTY, DEPARTMENT OF NATURAL RESOURCES AND PARKS, SEATTLE, WASHINGTON
Impacts of Rainfall
Projections Designing King
County Stormwater
Facilities
Rainfall Patterns
Cascade Mountains Have Influence
Example:
• Mean Annual =
33 – 173 inches per year
Downscaling Global to Local
RELATIVE ERROR FOR EACH MODEL/METRIC
COMBINED ERROR SCORE
Best
Worst
Stormwater
Stormwater is the most
significant source of
pollutants to our receiving
waterbodies!
TYPES OF
STORMWATER
FACILITIES EVALUATED :
• DETENTION PONDS
• INFILTRATION PONDS
• WATER QUALITY
WETPONDS
• SAND FILTERS
• BIOSWALES
• BIORETENTION
DOWNSCALED
LOCATIONS
24 WRF Locations
65 King County
20 Seattle
Sea-Tac Used
Sea-Tac
RCP 4.5
2050s small
differences
2080s Longer
slightly more
intense
RCP 8.5
2050s Very short
more intense
2080s less intense
relative to 2050s
Sizing BMP facilities: WWHM2012
RELATIVE % CHANGE TO 1990S
(LOW PERMEABILITY)
RELATIVE % CHANGE TO 1990S
(HIGH PERMEABILITY)
1200%
300+%
Why do we compare relative to 1990s?
Model Bias!!
RCP 8.5
RCP 4.5
Why do we compare relative to 1990s?
Model Bias!!
RCP 8.5
RCP 4.5
SUMMARY
• CHANGES IN EXTREME RAINFALL INTENSITIES GENERALLY INCREASE
• TILL SOILS – BMPS GENERALLY INCREASE 10-60%
• OUTWASH SOILS – BMPS GENERALLY INCREASE 20-80%
• RESULTS BETWEEN LOW AND HIGH SCENARIOS ARE SIMILAR
• A MORE DISTURBED LANDSCAPE HAS LESS EFFECT ON BMP DESIGNS
• RURAL RESIDENTIAL YIELDS CONSISTENT GREATER INCREASES THAN COMMERCIAL
• MODEL BIAS COMPARING MODELED 1990’S TO HISTORICAL 1990S RANGED
FROM -20% TO + 60%
• GFDL-CM3 (RCP 8.5) LESS BIAS THAN ACCESS1-0 (RCP 4.5)
Recommendations in Discussion
Evaluate larger ensemble of scenarios
Guidance for Stormwater Design Manual Updates (KCSWDM)
Require addressing projections of larger BMPs
Volumetric safety, or “goldilocks” time series
Implement Fee in Lieu
Consider raising minimum BMP implementation
standards
Eliminate some flow control and WQ redev
projects exemptions
Revisit climate scenarios for each manual update
Need to Know More
Evaluate using more climate
scenarios
Conveyance Capacities &
Critical infrastructure
Evaluate at more locations
Evaluate different Time
Periods
Evaluate more strategies
Design Adapt BMPs
Expansion of Overbank flooding
Reduced TIA
Tree Planting / Forest Retention
1980-
2010
2070-
2099
1980-
2010
2070-
2099
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Outwash
Till
Commercial
Detention (acre*feet)
Wetpond (acre*feet)
Basic Sand Filter (up) (sq. ft)
Basic Sand Filter (dn) (sq ft)
Wet biofiltration swale (up)
(sq. ft)
Wet biofiltration swale (dn)
(sq ft)
Bioretention Cell (cu. Ft)
RA-5
Detention (acre*feet)
Wetpond (acre*feet)
Basic Sand Filter (up) (sq. ft)
Basic Sand Filter (dn) (sq ft)
Wet biofiltration swale (up)
(sq. ft)
Wet biofiltration swale (dn)
(sq ft)
Bioretention Cell (cu. Ft)
Land Cover
BMP
Soil Type
Sea-Tac
Historical
RCP 45
RCP 85
Number of BMPs Designed
Using 1 Location
2 Land Cover
Scenarios
7 BMPs
2 Soil Types
= 140 BMPs Designed &
Evaluated
Residential
Commercial
×13 climate scenarios =
728 BMPs
per location!
WE WANT YOU!
ACKNOWLEDGEMENTS
Climate Science in
the Public Interest
QUESTIONS?
JEFF BURKEY, HYDROLOGIST
[email protected] 206-477-4658
KING COUNTY
DEPARTMENT OF NATURAL RESOURCES AND PARKS
WATER AND LAND RESOURCES DIVISION
SEATTLE, WASHINGTON