Folks,
Vince has asked several good questions on the impacts of climate change and we wanted to fully address them. Vince - thank you for your diligent focus on the incorporation of climate change impacts on the SSWWEP. We want to make sure that we are properly incorporating climate change into the analysis, and that you and the rest of the CAC are comfortable with the methodology, to support your recommendation.
The impact of climate change on rainfall is complex, and there is a lot of variability, depending on which scenario or even time of the year is considered. This variability has led to many generalizations in the CAC meetings, with varying ranges discussed. Because of this large variability, it is probably best to examine the data from the EPA and output from our hydrologic (flow) model directly to understand the impacts. I have outlined the results below. I believe that these show a strong basis for the CAC to recommend a 10% increase in peak flows to account for climate change.
EPA Rainfall Data
We computed the climate change impacts on rainfall using the EPA’s National Stormwater Calculator, which was recently updated to include climate variability based on the Intergovernmental Panel on Climate Change protocols. The program has a low and high range, which varies from projecting less rainfall for the region to more rainfall for the region. The rainfall data shows that the annual average precipitation ranges from a decrease of 6% to an increase of 8.4%, depending on which scenario is selected. Below is a summary of the output from the program.
- Attachment 1 – Fact Sheet on EPA’s National Stormwater Calculator – This is a reference for the program from EPA that we used to compute the climate change impacts on rainfall. Page 7 of the program was used to compute climate change impacts.
- Attachment 2 – Near-term (2020-2049) “wet/warm” scenario for the near-term (6 – 35 years) – This shows output from the EPA program for Ann Arbor for the near-term (6 – 35 years) using the wet/warm scenario, which is the high rainfall scenario. Page 2 of the document shows the annual rainfall changing from 34.70 to 36.28-inches, which is a 4.5% increase in annual precipitation. Note that 20-30 years is a common window for utility master planning.
- Attachment 3 – Far-term (2045-2074) “wet/warm” scenario – This shows output from the EPA program for Ann Arbor for the far-term (31 – 60 years) using the wet/warm scenario, which is the high rainfall scenario. Page 2 of the document shows the annual rainfall changing from 34.70 to 37.60-inches, which is an 8.4% increase in annual precipitation. Note that 30-60 years is beyond the common window used in utility master planning.
- Attachment 4 – Near-term (2020-2049) “hot/dry” scenario – This shows output from the EPA program for Ann Arbor for the near-term (6 – 35 years) using the hot/dry scenario, which is the low rainfall scenario. Page 2 of the document shows the annual rainfall changing from 34.70 to 33.55-inches, which is a 3.3% decrease in annual precipitation. This shows the wide variability in projected precipitation from the EPA program, depending on which scenario is used.
- Attachment 5 – Far-term (2045-2074) “hot/dry” scenario – This shows output from the EPA program for Ann Arbor for the far-term (31 – 60 years) using the hot/dry scenario, which is the low rainfall scenario. Page 2 of the document shows the annual rainfall changing from 34.70 to 32.61-inches, which is a 6.0% decrease in annual precipitation. This shows the wide variability in projected precipitation from the EPA program, depending on which scenario is used.
- Attachment 6 – Near-term (2020-2049) Monthly Change in Precipitation – This shows the output from the EPA program for Ann Arbor for the near-term change in monthly precipitation for the hot/dry scenario, the wet/warm scenario and the “median” scenario. Note that there is a large variability in the precipitation changes from month to month. For the wet/warm scenario, the monthly values vary from a 12% increase in March to a 3% decrease for August. For the hot/dry scenario, the monthly values vary from a 5% increase in March to a 13% decrease for August.
- Attachment 7 – Far-term (2045-2074) Monthly Change in Precipitation – This shows the output from the EPA program for Ann Arbor for the far-term change in monthly precipitation for the hot/dry scenario, the wet/warm scenario and the “median” scenario. Note that there is a large variability in the precipitation changes from month to month. For the wet/warm scenario, the monthly values vary from a 22% increase in March to a 6% decrease for August. For the hot/dry scenario, the monthly values vary from a 10% increase in March to a 24% decrease for August.
Impacts on Peak Flow in the Sanitary Sewer
There is a lot of variability in projected rainfall due to climate change. For the SSWWEP, we are most interested in how this change in rainfall may affect the peak flows in the sanitary sewer system. The sanitary sewer tends to have a larger reaction to rainfall in the spring when the ground is wet, and a smaller reaction in the summer when the ground is dry. This effect will tend to amplify the higher climate change rainfalls projects in the spring months.
For this reason, we re-ran our 60-year continuous hydrologic model of the system, but with the revised monthly rainfalls suggested by the EPA program (i.e. we changed each month in the 60-year historic rainfall by the percentage from the EPA program). To represent the worst-case scenario, we selected the scenario with the highest rainfall, which is the far-term, wet/warm scenario, even though the forecast period for this run (31-60 years) goes beyond the normal planning window for master plans. We did this to illustrate the worst-case impacts on peak flows, to give the CAC an upper-limit to consider for climate change impacts. The results are tabulated in Attachment 8, which is described below:
- Attachment 8 – Output from frequency analysis for far-term wet/warm scenario – This shows the output from 60-year continuous hydrologic model for the five downstream metering points. The impacts from climate change vary from meter to meter and for the various recurrence intervals. The bottom line total shows an increase of 10.4% for the 25-year flow and 11.4 % for the 50-year flow due to climate change. These increases in peak flows are greater than the annual increase in precipitation due to climate change (8.5% for the long-term wet/warm), because there is a greater increase in precipitation from climate change in the spring when the ground is wet, and the sanitary sewer has a larger reaction to rain in the spring.
Because the above peak flow increases represent the worst-case for the wet/warm scenario for the far term (31-60 year) period, and the increases are in the range of 10.4% to 11.4%, we recommend that the CAC consider a 10% increase in future peak flows for climate change. This represents a reasonable mid-range value for the far-term, and is most likely well beyond the worst-case scenario for the near term (6-35 years), which is a reasonable planning window for this study.
TOAG Review of Methodology and Concurrence
The TOAG's reviewed the climate change methodology used to evaluate risk for the SSWWEP. The TOAG issued their review comments on June 17, and they are contained in Attachment 9. Amongst other things, this memo outlines the TOAG's conclusions that:
- The methodology used to assess impacts of climate change seems appropriate and standard.
- The methodology used is relatively standard and is similar to other stormwater assessments that have been conducted.
- The other Wet Weather studies being undertaken by Ann Arbor should similarly acknowledge and recognize the importance of climate change as a factor, which affects the City’s infrastructure.
Conclusions
The methodology that we have used to assess the impacts of climate change on the sanitary sewer are based on sound science, that are consistent with the values published by EPA in their National Stormwater Calculator Program. The TOAG has reviewed the methodology and found it to be appropriate and standard. Based on these results, we are comfortable recommending that the CAC consider a 10% increase in peak flows to account for climate change. This value reflects the high-end of the ranges that have been published, and should provide a reasonable basis for evaluating the impacts of climate change on Ann Arbor’s sanitary sewer system.
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