Evaluation of Potential Climate
Change Impacts on Stormwater
Facility Size and Cost
November 2019
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Evaluation of Potential Climate
Change Impacts on Stormwater
Facility Size and Cost
Prepared for:
Washington State Department of Ecology for grant award WQC-2016-KCWLRD-00136.
Submitted by:
Jeff Burkey, King County Water and Land Resources (WLR) Division
Jeff Pray, King County WLR Division
Funding was provided by:
Washington State Department of Ecology Centennial Grant
King County Wastewater Treatment Division
King County Water and Land Resources Division
Evaluation of Potential Climate Change Impacts on Stormwater Facility Size and Cost
King County Science and Technical Support Section
November 2019
Acknowledgements
These analyses would not be possible if not for the effort of downscaling global climate
model rainfall led by Dr. Guillaume Mauger, from University of Washington, Climate
Impacts Group. The project team members (Jessica Engel, Lara Whitely Binder, and Mark
Wilgus) also provided insight and clarity to the outcomes presented.
Funding for this work was a combination of King County Department of Natural Resources
and Parks (DNRP) funding ($187,000) and a Washington State Department of Ecology
Centennial grant ($250,000) totaling $437,000. The DNRP funding was split between the
Wastewater Treatment Division and the Water and Land Resources Division.
Citation
King County. 2019. Evaluation of Potential Climate Change Impacts on Stormwater Facility
Size and Cost. Prepared by Jeff Burkey and Jeff Pray, Water and Land Resources
Division. Seattle, Washington.
Evaluation of Potential Climate Change Impacts on Stormwater Facility Size and Cost
King County Science and Technical Support Section
November 2019
Table of Contents
1.0
Introduction ……………………………………………………………………………………………………………………………. 1
1.1
Background ………………………………………………………………………………………………………………………… 1
1.2
Goals and Objectives …………………………………………………………………………………………………………. 2
2.0
Data and Methods ………………………………………………………………………………………………………………….. 3
2.1
Climate Data ……………………………………………………………………………………………………………………….. 3
2.1.1
Climate Scenarios …………………………………………………………………………………………………………. 3
2.1.2
Time Periods Used ……………………………………………………………………………………………………….. 3
2.2
Facilities Evaluated in this Study ……………………………………………………………………………………. 4
2.2.1
Unit Costs of Facilities …………………………………………………………………………………………………. 6
2.3
Modeling Scenarios Used To Develop the Facilities ……………………………………………………. 7
2.4
Design Standards Used To Develop the Facilities ……………………………………………………….. 8
2.4.1
Application of the Single-event (SBUH) Analysis …………………………………………………… 8
3.0
Modeling Results …………………………………………………………………………………………………………………..10
3.1
Time Periods Used for Comparisons …………………………………………………………………………….10
3.2
Comparisons of Storage Volumes and Surface Areas ………………………………………………..10
3.3
Differences Between Design Standards and Rainfall Scenarios ………………………………13
3.3.1
WWHM2012 Bias ………………………………………………………………………………………………………..14
3.3.2
Single-event vs. Continuous Hydrologic Simulation …………………………………………….18
3.4
Increased Cost of Facilities ……………………………………………………………………………………………..19
4.0
BMP Design Considerations………………………………………………………………………………………………..25
4.1
Challenges in Pond Optimization …………………………………………………………………………………..25
4.2
Using Sea-Tac Location ……………………………………………………………………………………………………25
4.3
Hydrologic Modeling ……………………………………………………………………………………………………….25
5.0
Summary ………………………………………………………………………………………………………………………………..26
6.0
Conclusion ……………………………………………………………………………………………………………………………..28
6.1
Recommendation ……………………………………………………………………………………………………………..28
7.0
References ……………………………………………………………………………………………………………………………..29
Evaluation of Potential Climate Change Impacts on Stormwater Facility Size and Cost
King County Science and Technical Support Section
iii
November 2019
Tables
Table 1.
Scenarios used for BMP sizing. ………………………………………………………………………………….. 4
Table 2.
Time periods used for BMP sizing. ……………………………………………………………………………. 4
Table 3.
Modeling methodology and facility design criteria. ………………………………………………. 5
Table 4.
Unit costs for modeled facilities (2015 dollars, unadjusted). ………………………………. 6
Table 5.
Landscape assumptions used in WWHM2012 for BMP sizing. ……………………………. 7
Table 6.
Soil types and infiltration rates used for modeling in WWHM2012. ………………….. 7
Table 7.
Relative percent change of facility sizes resulting from projected climate
data for the dry and wet climate scenarios. ……………………………………………………………11
Table 8.
The percent change bias between facilities sized using historical 1990s
rainfall and simulated GCM|RCP scenarios (wet and dry) for the same
1990s time period (WY 1980–2009). ……………………………………………………………………..15
Table 9.
Computed storage volumes and surface areas for BMPs evaluated. …………………16
Table 10.
The percent change in required facility sizes resulting from a transition
from single-event modeling (i.e., SBUH) to continuous time series modeling
(e.g. WWHM2012, KCRTS). ……………………………………………………………………………………….19
Table 11.
Capital, operations and maintenance (O&M), and total costs for residential
sized historical BMPs (2015 dollars, unadjusted). ……………………………………………….21
Table 12.
Future costs are based on applying the relative percent increases (and
decreases) in residential sized BMPs when comparing 2080s to 1990s for
both climate scenarios (wet and dry) (see Table 7 for RPDs) to the
historical costs. ……………………………………………………………………………………………………………22
Table 13.
Capital, operations and maintenance (O&M), and Total costs for
commercial sized historical BMPs (2015 dollars, unadjusted). …………………………23
Table 14.
Future costs are based on applying the relative percent increases (and
decreases) in commercial size of BMPs when comparing 2080s to 1990s for
both climate scenarios (dry and wet) (see Table 7 for RPDs) to the
Historical costs. ……………………………………………………………………………………………………………24
Evaluation of Potential Climate Change Impacts on Stormwater Facility Size and Cost
King County Science and Technical Support Section
November 2019
Figures
Figure 1.
Outwash (high infiltration soils) underlain facility size percentage increase
using the UW CIG climate scenarios (Dry and Wet). …………………………………………….12
Figure 2.
Till (low infiltration soils) underlain facility size percentage increase using
the UW CIG climate scenarios (Dry and Wet). ……………………………………………………….13
Figure 3.
Facility size percent change affected by the UW climate scenarios. ………………….17
Figure 4.
Facility size percent change affected by the UW climate scenarios. ………………….18
Evaluation of Potential Climate Change Impacts on Stormwater Facility Size and Cost
King County Science and Technical Support Section
November 2019
EXECUTIVE SUMMARY
Why are we doing this?
Recent studies evaluating historical and projected future rainfall indicate more
intense precipitation events in the Puget Sound region. This suggests that flooding
and stormwater facilities built using current design standards are likely to be
undersized for future conditions. This is an important consideration for local
jurisdictions and stormwater managers, as stormwater facilities are typically
intended to last several decades and mitigate projections of future rainfall.
What is the purpose of this study?
The goal of this study is to identify a range of impacts when designing stormwater
facilities in King County and support possible modifications for updates to King
County’s stormwater design manual. To achieve this goal, comparisons are made
evaluating changes in sizes of stormwater facilities using standard stormwater
design practices and future rainfall projections.
How did we predict future rainfall?
Global climate models and downscaling techniques have evolved over time to
support more refined assessments at a local scale. This project uses the latest global
greenhouse gas scenarios generated in 2011, the latest set of global climate models
(GCMs) obtained from Climate Model Intercomparison Project, phase 5 (aka CMIP5),
and a regional weather model (i.e., Weather Research and Forecasting model—
WRF) to develop detailed projections of changing precipitation in the Puget Sound
region. King County partnered with the University of Washington Climate Impacts
Group to develop these regional rainfall projections for use in evaluating impacts
when designing stormwater mitigation facilities.
Two global climate model ensembles were downscaled to convert very coarse
landscape and atmospheric conditions and more accurately reflect local rainfall
patterns within King County (Mauger, et al., 2018). The two ensembles are
described as dry (a drier climate scenario) and wet (a wetter climate scenario)
referring to the seasonal rainfall less than and more than the average climate model
projections for the future. Both GCM ensembles project greater rainfall intensities in
the future when compared to historical (i.e., 1990s) conditions. The GCM that
produces a smaller increase in seasonal rainfall volumes (ACCESS 1.0) was paired
with the lower emissions scenario (Representative Concentration Pathway [RCP] 4.5). The GCM producing a larger increase in future seasonal rainfall (GFDL CM3)
was paired with the higher emission scenario (RCP 8.5). Both GCM outputs were
used as input for the Weather Research Forecast (WRF) rainfall model to downscale
to local conditions for the Pacific Northwest. While the time periods available from
the downscaling spans from 1970 through 2099, the time periods used for
Evaluation of Potential Climate Change Impacts on Stormwater Facility Size and Cost
King County Science and Technical Support Section
November 2019
comparisons include 30-year windows—water years 1980-2009 (hereafter: 1990s)
and 2070-2099 (hereafter: 2080s). For further detail on methodology in
downscaling GCM rainfall, please refer to New Projections of changing Heavy
Precipitation in King County (Muager, et al., 2018).
How did we test the impacts of stormwater facility design alternatives?
We used a stormwater facility design software package (WWHM) to define the
amount of storage volume and/or capacity to achieve current King County flow
control and water quality treatment standards for six types of facilities: detention
ponds, infiltration ponds, wet ponds, sand filters, biofiltration swales, and
bioretention cells. Additionally, because so much of the existing stormwater
infrastructure in King County is based on less effective older design methods, the
two prevalent types of stormwater ponds (i.e., detention pond and wet pond) in
King County were also sized using these older design standards to evaluate how
much storage volume design criteria might need to be increased (i.e., through
retrofits) to meet current design standards using observed rainfall. In all, there were
136 Best Management Practices (BMPs) facilities sized to characterize the range of
outcomes when taking into account two land use templates (rural residential and
commercial), two soil infiltration rates (high and low), two time periods (1990s and
2080s), and three sources of rainfall (observed, dry, and wet).
What are the results?
We anticipated needing larger increases in BMP capacities for the wet scenario
when compared to the dry scenario. Instead, the projected changes in facility design
sizes in response to future climate conditions were varied and not consistent
between the two future scenarios. Some BMPs in the dry scenario had larger
increases than in the wet scenario and vice versa. However, among all BMPs and
climate scenarios, storage volumes and treatment capacities increased ranging from
10% to 100+%—depending on land use intensity, soil infiltration, facility type, and
GCM ensemble.
In summary, a few themes emerged when comparing volumes (or footprints) of
BMPs sized for the 2080s relative to the 1990s climate.
1) Increases in BMP storage volumes needed were similar between the dry and
wet scenarios;
2) Increases in BMP storage volumes for outwash soils were more pronounced
than for till soils;
3) On average, BMPs sized for commercial land use required smaller capacity
increases than for residential land uses in high soil-infiltration areas;
4) Cost estimates were assumed proportional to increases in BMP capacities,
thus costs increase anywhere from 10 to 100 percent to mitigate future
conditions;
Evaluation of Potential Climate Change Impacts on Stormwater Facility Size and…