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HomeMy WebLinkAboutAppendix B - Climate Change Impact Analysis Barr Engineering Co. 4300 MarketPointe Drive, Suite 200, Minneapolis, MN 55435 952.832.2600 www.barr.com Technical Memorandum To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix B - Climate Change Impact Analysis Date: March 30, 2020 Project: Edina Flood Risk Reduction Strategy Support (23271728.00) Precipitation totals have been increasing in the Twin Cities for decades. The total precipitation in 2019 was the highest amount of annual precipitation on record. Barr reviewed climatological data to evaluate changes and long-term trends in precipitation. As shown in Figure 1, the record for the highest annual precipitation recorded at the Minneapolis-St. Paul International Airport was in 2019 and was nearly 8% higher than the next highest year (2016). Figure 1 shows the top 10 wettest years (most annual precipitation) for the Twin Cities using the Minneapolis-St. Paul International Airport gage. It is worth noting that three of the years on this plot are within the past two decades (2002, 2016, and 2019), and the two highest years, 2016 and 2019, are very recent. The average annual precipitation total for the Twin Cities (at the Minneapolis-St. Paul International Airport) is 30.6 inches. The driest year on record (1910) had a precipitation total of 11.5 inches. The 2019 annual precipitation was over 40% higher than an average year. Figure 1 Top 10 wettest years in the Twin Cities (precipitation at Minneapolis-St. Paul International Airport) To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix B - Climate Change Impact Analysis Date: March 30, 2020 Page: 2 P:\Mpls\23 MN\27\23271728 Flood Risk Reduction Strategy\WorkFiles\General Support\FRRS Appendices\FRRS Appendix B - Climate Change Impact Analysis.docx Figure 2 shows annual precipitation totals for the past 50 years, including 2019. In the past 50 years, there has been an increasing trend in average annual rainfall totals at a rate of about 0.66 inches more precipitation per decade. Figure 2 Annual precipitation for Hennepin County from 1970 to 2019 (Source: MNDNR State Climatology Office) It is worth noting that the 1940s, 1950s, and 1960s were three consecutive decades with approximately average precipitation. This was a prolonged period of relatively stable conditions when much of the development in Edina occurred. Prior to this period, the 1930s was a dry decade; in fact, the driest on record. From the 1960s on, there has been a clear trend in the total precipitation, both on an annual basis (as shown in Figure 2) and by decade. Figure 3 shows the average annual precipitation depth per decade from the end of the 19th century to the 2010s. The 2010s are the wettest decade in Minnesota’s history. To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix B - Climate Change Impact Analysis Date: March 30, 2020 Page: 3 P:\Mpls\23 MN\27\23271728 Flood Risk Reduction Strategy\WorkFiles\General Support\FRRS Appendices\FRRS Appendix B - Climate Change Impact Analysis.docx Figure 3 Average annual precipitation by decade (Source: MNDNR State Climatology Office) In much of Edina, the stormwater infrastructure was designed and developed decades ago (in the 1950s and 1960s) using design storms. At the time, the design storms were estimated based decades-worth of precipitation prior to the design. This means that stormwater infrastructure was likely designed largely based on precipitation experienced in the first half of the 20th century, and since then, precipitation quantities have only increased. The City’s stormwater system was designed to convey a certain amount of water and protect against impacts at a certain level. This “level of protection” is based on the capacity of public infrastructure to handle stormwater and on the likelihood, or probability, that a storm will occur. When storms are bigger or more intense than the infrastructure is designed to handle, or when it clogs, there are consequences such as disruptions in services and facilities, or damage to property. The relationship between the probability of these storm events occurring (defined by climate and infrastructure) and the resultant consequences (defined by vulnerabilities of public or private infrastructure) determines the overall community flood risk. Risk is changing primarily because climate is changing and is increasing the probability, or chance, that large, flood-causing storms will occur. The level of protection for design is a moving target. Designs from the past are undersized for today and there is a growing realization in technical circles that even if designs were revised to reflect today’s probability of storm events they may quickly be obsolete due to the changing risk brought by climate change. The question is, should engineering designs be based on the climate models of today or on some predicted future condition? The trade-off for future-sizing a design so that we are better prepared for climate change would likely mean higher present costs. Figures 4, 5, and 6 show how the extent of flooding has changed in the Weber Pond area of the Morningside neighborhood over time and what it To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix B - Climate Change Impact Analysis Date: March 30, 2020 Page: 4 P:\Mpls\23 MN\27\23271728 Flood Risk Reduction Strategy\WorkFiles\General Support\FRRS Appendices\FRRS Appendix B - Climate Change Impact Analysis.docx may look like in the future. The flood inundation extents shown are based on model results of storm events using the City’s stormwater management model (XP-SWMM). Figure 4 Flood inundation for a predicted 1% annual chance flood event in the past (~6.0 inches over a 24-hour period, based on Technical Paper 40) Figure 5 Flood inundation for a predicted 1% annual chance flood event using more recent climate data (~7.5 inches over a 24-hour period, based on Atlas 14) To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix B - Climate Change Impact Analysis Date: March 30, 2020 Page: 5 P:\Mpls\23 MN\27\23271728 Flood Risk Reduction Strategy\WorkFiles\General Support\FRRS Appendices\FRRS Appendix B - Climate Change Impact Analysis.docx Figure 6 Flood inundation for a 1% annual chance flood event projected for the future (~10 inches over a 24-hour period) In the following figure (Figure 7), we attempt to visually show the effects of infrastructure projects and the impacts of climate change on the flood volumes stored in Weber Pond. In Figure 7, one blue rectangle represents 10 acre-feet of stormwater storage in Weber Pond. This volume is equivalent to the storage capacity available in Weber Pond before impacts to the adjacent park or homes would begin to occur. 10 acre-feet of water is not inconsequential. It can be thought of as one foot of water over a 10-acre area, or 10 feet of water over a 1-acre area, or more specifically, 3 feet of water over the approximately 3.3-acre footprint of Weber Pond. In the present climate, Weber Pond would actually need to store close to 40 acre-feet of stormwater in the 1%-annual-chance (100-year) design storm event to avoid impacts to the park or adjacent homes; 40 acre-feet is nearly four times the amount that can currently be stored in Weber Pond without impacting infrastructure or amenities. If directly connected imperviousness were reduced by 25% in the contributing watershed, it reduces the flood volume that needs to be stored in Weber Pond to avoid impacts, but the reduction is minor. There are other methods to alter the flood exposure, such as with large infrastructure projects (pipes, pumps, storage, etc.). As shown in Figure 7, Option 2b (from Appendix C) actually transfers risk downstream, reducing flooding in other areas of the Morningside neighborhood and increases the volume that would need to be stored in Weber Pond (requiring additional protection for homes adjacent to Weber Pond, for example, by constructing flood walls), while Option 7b (from Appendix C) shows the greatest benefit in reducing flood volumes. Coincidentally, the amount of stormwater that needs to be stored in Weber Pond with the large infrastructure project Option 7b looks a lot like the amount of water that needed to be stored in the 1%-annual-chance (100-year) design storm event used in the past. To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix B - Climate Change Impact Analysis Date: March 30, 2020 Page: 6 P:\Mpls\23 MN\27\23271728 Flood Risk Reduction Strategy\WorkFiles\General Support\FRRS Appendices\FRRS Appendix B - Climate Change Impact Analysis.docx Figure 7 Pictograph of effects and impacts on stormwater volumes in Weber Pond due to climate change and infrastructure projects To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix B - Climate Change Impact Analysis Date: March 30, 2020 Page: 7 P:\Mpls\23 MN\27\23271728 Flood Risk Reduction Strategy\WorkFiles\General Support\FRRS Appendices\FRRS Appendix B - Climate Change Impact Analysis.docx Finally, it is worth noting that in all of the conditions shown in Figure 7, Weber Pond is not able to store the flood volume generated by these large amounts of precipitation (i.e., all conditions exceed “1 Weber Pond Equivalent”). In other words, there will be impacts to infrastructure and amenities adjacent to Weber Pond during a 1%-annual-chance (100-yr) storm event, even with large infrastructure projects, and/or with decreases in imperviousness, due to system capacity constraints and climate change. The stormwater management target continues to change as precipitation amounts continue to get larger and larger.