Flash Flooding, Stormwater, and Decision Making
Photo: Richard Deming Photography Flash Flooding, Stormwater, and Decision Making for Cities in the Great Lakes
Megan Krajewski Climate Center Research Assistant, Earth and Environmental Science Student University of Michigan
Daniel Brown Climatologist University of Michigan Climate Center
Elizabeth Gibbons Director University of Michigan Climate Center
This project was sponsored by the Climate Center and University of Michigan’s Undergraduate Research Opportunity Program.
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PROJECT BRIEF: FLASH FLOODING, STORMWATER, AND DECISION MAKING
Recommended Citation: Krajewski, M., D. Brown, E. Gibbons, 2015. Flash Flooding, Stormwater, and Decision Making for Cities in the Great Lakes. Available from the University of Michigan Climate Center.
For further questions, please contact Daniel Brown, [email protected]
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Contents Introduction ...... 3 Definitions:...... 5 Method ...... 5 Challenges ...... 5 Results...... 6 Population Density ...... Error! Bookmark not defined. Number of Flash Floods per Year/Average Precipitation of a Flash Flood by City ...... 7 Average Precipitation ...... 7 Difference between Combined and Separate Sewer Event Totals by Precipitation Threshold ...... 9 Sensitivity ...... 9 Statistical Tests for Population Density ...... 10 Statistical Tests for Sensitivity ...... 12 Damages ...... 15 Discussion ...... 18 Acknowledgments ...... 18 References ...... 18
COMBINED SEWER SEPARATE SEWER 1. Albany, NY 16. Akron, OH 2. Aurora, IL 17. Ann Arbor, MI 3. Buffalo, NY 18. Bloomington, IN 4. Chicago, IL 19. Duluth, MN 5. Cleveland, OH 20. Eau Claire, WI 6. Detroit, MI 21. Erie, PA 7. Fort Wayne, IN 22. Grand Rapids, MI 8. Harrisburg, PA 23. Green Bay, WI 9. Lafayette, IN 24. Madison, WI 10. Milwaukee, WI 25. Marquette, MI 11. Philadelphia, PA 26. Minneapolis, MN 12. Pittsburgh, PA 27. Oswego, NY 13. Saginaw, MI 28. Springfield, IL 14. South Bend, IN 29. St. Cloud, MN 15. Toledo, OH 30. Traverse City, MI
Flash flood data from 15 cities with combined sewer systems and 15 cities with separate sewer systems was analyzed for this report.
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Introduction Heavy precipitation events have been increasing in frequency and intensity over time. The amount of precipitation falling in the most intense 1% of precipitation events increased by 37% in the Midwest and 71% in the Northeast from 1958 through 2012 (Walsh et al., 2014). The amount of precipitation falling during week- long, once a year precipitation events has also increased by 25% to 100% across the Great Plains and Upper Midwest (Kunkel et al., 1999). While projections of future heavy precipitation events greatly vary, most models project that daily extreme precipitation events will continue to become more frequent and more intense for many areas of the Great Lakes region (Kunkel et al. 2013). Areas that are currently vulnerable to heavy precipitation events will likely become more vulnerable in the future.
This work evaluates trends in flash flooding and historical precipitation totals from 1996-2011 in the eight states that border the Great Lakes. The National Oceanic and Atmospheric Administration’s (NOAA) Storm Events Database (https://www.ncdc.noaa.gov/stormevents/) includes detailed records of flash floods organized by their county or location of occurrence since 1996. GLISA staff maintains quality-controlled NOAA NCEI Global Historical Climate Network-Daily observational data (GHCN-Daily) from the Great Lakes region in order to inform climate adaptation efforts at the local, state, and regional level. This data is currently available and in accessible formats to users of varying backgrounds. (http://glisa.umich.edu/resources/great-lakes-climate- stations) While GLISA staff and affiliates use this quality-controlled subset of GHCN-Daily data widely throughout the region to provide quantitative, locally-relevant references of historical climate for stakeholders near the observation sites, it remained unclear if daily precipitation totals included in this data could be used as a proxy for quantifying the relative vulnerability and sensitivity to damage from precipitation-related storm events for nearby communities across the region.
Decision-makers in the region are interested in where vulnerabilities to stormwater overflows and unplanned discharges are greatest. Cities with combined sewers are often assumed to have a greater risk of overflows, as they direct stormwater into the same conveyance system that carries untreated wastewater. During heavy precipitation, combined sewer systems operating near capacity are forced to discharge untreated wastewater at pre-determined points or risk an uncontrolled overflow elsewhere. Separate sewer systems convey stormwater through a separate conveyance system, reducing or eliminating the risk of wastewater being discharged and the associated public health risks. Combined sewers are most common in the Northeast and Great Lakes regions of the US in cities.
While the public health and stormwater management benefits of a separate system are well known, it remained unclear if GHCN-Daily data could be used in conjunction with the NOAA Storm Events database to quantify an increased capacity to cope with heavy precipitation in cities with separate sewer systems versus those with combined systems.
The 30 cities analyzed in this study were chosen with an equal number of separate and combined sewer cities. The cities also cover a wide area throughout the Great Lakes states and information about each of the city’s sewers was clearly available online. The chosen cities had either mostly separate or mostly combined sewer systems.
In this study, we attempted to 1) test the effectiveness of GHCN-Daily data in quantifying regional sensitivity to the frequency of flash flooding following heavy precipitation, and 2) test the effectiveness of GHCN-Daily data in quantifying potential increases in the capacity of separate sewer systems. Understanding how GHCN-Daily data can be used to identify past and future vulnerabilities will help decision makers plan for changing weather patterns in the region.
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Definitions: Flash flood (according to NOAA’s Storm Events Database): A rapid rise in water level in places that are normally dry or have much lower water levels Must pose a threat to life or property Must be within a 6 hour period of the causative event (e.g., intense rainfall, dam failure, ice jam-related) Cannot exist for 2-3 consecutive days
Combined Sewer: Water removal systems that collect precipitation runoff, domestic sewage, and industrial wastewater in the same conveyance system. Most of the time, combined sewer systems transport wastewater to a sewage treatment plant where it is treated and then discharged to a water body. During periods of heavy precipitation, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. In these cases, combined sewer systems are designed to overflow and discharge excess wastewater directly to water bodies. (EPA, http://water.epa.gov/polwaste/npdes/cso/)
Separate Sewer: Water removal systems that collect stormwater, domestic sewage, and industrial wastewater in separate conveyance systems. During periods of heavy precipitation, sanitary sewage and industrial wastewater flows remain unaffected, and only precipitation runoff is discharged into local water bodies.
Methods The code was written in Python to extract data for 30 cities from NOAA’s Storm Events Database and GLISA’s daily precipitation record for 1996-2011. For each of the chosen cities, the flash flood date and damage that was recorded for the city’s county by NOAA was paired with GLISA’s daily precipitation data. The damages were adjusted for inflation according to the US Bureau of Labor Statistics data. Great Lakes cities were chosen based on the quality of the matching precipitation data and the availability of information for each city’s sewer type. This data was further used for examination in Excel.
Challenges NOAA’s storm events data was given by county, but GHCN-Daily precipitation data is point-based. Daily precipitation data is rarely collected exactly where storms are most intense and does not capture fluctuations in precipitation rate throughout a given day. Complete data from the Storm Events Database was not available for 2012 or 2014, and 2013 was omitted for data continuity. NOAA’s damage records greatly vary between cities; some cities have hardly any damages recorded while others have damage for nearly every storm. This could be a product of different data recorders and methods. Flash flooding can be caused by snow melt or infrastructure failure, which are classified as a 0 inch precipitation threshold.
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Results Population Density There is a strong correlation between population density and how often a city has a flash flood. Additionally, many of the cities with highest population densities have combined sewers. The cities that flood the most are generally cities with population densities greater than 6000 people per square mile that would have a difficult and expensive time replacing their old, combined sewer system with a new, separate one.
In general, more densely-populated cities have more impervious land cover than less dense cities. Natural land cover (forests, grasslands, pervious soil or vegetation) has the ability to absorb rainfall with much less runoff than impervious land cover such as buildings or parking lots. In a setting with high impervious land cover, runoff is transported to streams much more quickly than it would be in a more permeable setting, which can result in more frequent and significant flooding (Perlman, 2015; Flinker, 2010). Sewers in bigger cities must be able to handle a significant amount water during a precipitation event to avoid sewer overflows.
One obstacle faced during this study was comparing and contrasting the cities with separate and combined sewers because of other variables such as impervious surface, age of infrastructure, topography, and soil type, could also impact precipitation impact in the city. The majority of cities in the Great Lakes region with high population densities also rely on combined sewer systems. Because of this commonality it is difficult to find cases to compare similarly dense cities with separated versus combined systems.
Population Density and Average Number of Flash Floods per Year People per square 18000 7.00 mile (combined 16000 6.00 sewer cities) 14000 People per square 5.00 12000 mile (separate sewer cities) 10000 4.00 Average flash 8000 3.00 floods per year 6000 2.00 4000 Linear (Average flash floods per 2000 1.00 year) 0 0.00 y = 0.0984x + 0.0098
Erie Erie R² = 0.4798
Population Density (people per squremile) per (people DensityPopulation
Akron
Average Numnber of Flash Floods Per Year Per FloodsFlash of Numnber Average
Duluth
Detroit
Toledo
Aurora
Albany
Buffalo
Chicago Oswego
Madison
Saginaw
St. Cloud St.
Ann Arbor Ann
Cleveland
Pittsburgh
Ft. Wayne Ft.
Marquette
Eau Claire Eau
Layfayette
Harrisburg
Milwaukee Springfield
Green Bay Green
Minneapolis
Philadelphia
South Bend South
Bloomington
Traverse City Traverse Grand Rapids Grand City
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Number of Flash Floods per Year/Average Precipitation of a Flash Flood by City In the 30 cities, the average number of flash floods per year in combined sewer cities is 2.02 floods versus 1.05 floods for separate sewer cities.
Average Number of Flash Floods Per Year 7 6 Combined Sewer 5 4 Separate Sewer 3 2 1
0
Erie Erie
Akron
Duluth
Detroit
Toledo
Aurora
Albany
Buffalo
Oswego Chicago
Madison
Saginaw
St. Cloud St.
Ann Arbor Ann
Cleveland
Marquette Pittsburgh
Ft. Wayne Ft.
Eau Claire Eau
Layfayette
Harrisburg
Springfield
Number of Flash Flood Events per Year per FloodEvents Flashof Number
Milwaukee
Green Bay Green
Minneapolis
Philadelphia
South Bend South
Bloomington Traverse City Traverse Grand Rapids Grand City
Each city can take a different average amount of precipitation before it floods. There are several possible factors that may affect this number including amount of impervious surface, age of infrastructure, topography in and around the city, sewer type, soil type, and soil saturation before the flood occurs (Holton, 2003).
In order to compare the frequency of flash flooding in all 30 cities, the average number of flash floods a city has on a yearly basis was divided by the average precipitation the city gets before it floods.
Number of Flash Floods per Year/Average Precipitation of a Flash Flood by City 9 8 7 Combined Sewer 6 5 Separate Sewer 4 3 2 1
0
of a a Flash Flood of
Erie Erie
Akron
Toledo Detroit
Aurora
Albany
Buffalo
Duluth
Number of Flash Flood Events Events Flash Flood of Number
Oswego Chicago
per Year / Average Precipitation Average / Year per
Madison
Saginaw Saginaw
St.Cloud Lafayette
Ann Arbor Ann
Cleveland
Marquette Pittsburgh
Eau Claire Eau Harrisburg
Springfield
Milwaukee
Green Bay Green
Fort Wayne Fort
South Bend South
Minneapolis
Philadelphia
Bloomington Bloomington Grand Rapids Grand Traverse City Traverse City
Cities that rank the lowest on this graph must either have less than 1 flood per year, on average, or be able to handle a high amount of precipitation before a flood occurs. In this graph, 8 out of 10 of the lowest ranking cities have separate sewers. On the other hand, the highest ranking cities flood relatively often at lower precipitation thresholds. Average Precipitation Many separate sewer cities have a low number of flash floods per year divided by their average precipitation. To try to explain this trend, the average precipitation of all combined and all separate sewer cities was investigated. Of the cities sampled, there was no significant difference in the average amount of rain a
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combined or separate sewer city receives before a flash flood occurs. However, it is important to note that combined sewer cities as a whole have more flash flood events than separate sewer cities.
Average Precipitation per Flash Flood Event 3.5 3 Combined Sewer 2.5 2 Separate Sewer 1.5 1 0.5
0
Precipitation (inches)Precipitation
Erie Erie
Akron
Toledo
Aurora
Detroit
Albany
Buffalo
Duluth
Oswego Chicago
Madison
Saginaw Saginaw
Lafayette
Ann Arbor Ann
St. Cloud St.Cloud
Cleveland
Marquette
Pittsburgh
Eau Claire Eau Harrisburg
Springfield Milwaukee
Green Bay Green
Fort Wayne Fort
South Bend South
Minneapolis
Philadelphia
Bloomington Bloomington Grand Rapids Grand City City Traverse
After analysis of precipitation totals for 1, 2, 3, and 7 days, the most statistically significant data comes from the 1 day total. Precipitation totals for 2 and 3 days are somewhat significant, but 7 day totals do not correlate well with how often a city has a flash flood.
Because flash floods must occur within hours of a storm, they are typically caused by more than 1 inch of precipitation on the same day as the flood. Duration and intensity of rainfall are important factors to flood causation and vary from city to city (Montz, 2002). Additionally, the topography in and around the city plays a role in how much rain can cause a flood. According to Kelsch et al. (2001), “High intensity rainfall is more important than the total accumulation on small, fast-response basins. Basin characteristics are easily as important as the rainfall characteristics for determining the nature of the runoff.”
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Difference between Combined and Separate Sewer Event Totals by Precipitation Threshold Nearly across the board, combined sewer cities have more flash flood events than separate sewer cities by threshold. Combined sewers appear to be less effective in handling any size precipitation event than combined sewers.
Difference Between Combined and Separate Sewer Event Totals by Precipitation Threshold
80 Total
- 70 60 50 40 30 20 10
Separate Sewer Events 0 DifferenceBetweenTotal
-10 Combined Sewer Events 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3+ Precipitation Threshold (Inches)
Percent difference of the combined minus the separate flash flood events divided by the total number of events per threshold shows a substantially larger percentage of flash flood events occurring in combined sewer cities than separate sewer cities in 11 of13 thresholds. The average of all the percent difference thresholds show combined cities have 26.98 percent more flash flood events than separate cities.
Percent Difference Between Combined and Separate Events by Threshold 70 60 50 40 30 20 10 0 Percent DifferencePercent -10 -20 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3+ Precipitation Threshold (Inches)
Sensitivity Flash flood sensitivity is how likely a city will flood if it reaches a certain precipitation threshold. In this study, the sensitivity was found by dividing the number of flash flood events in a precipitation threshold by the total number of days a city reaches that threshold. Then all of the combined sewer cities and separate sewer cities were averaged. The most significant results come from precipitation totals accumulated on the same day as the
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flood. Somewhat significant results were seen when looking at sensitivities when precipitation totals from the day of the flood and the day before. The 3 and 7 day precipitation totals did not provide significant sensitivity trends.
The most notable result found when looking at sensitivities was a significantly higher chance of flooding in combined sewer cities above the 2.25 inch precipitation threshold. This may indicate that sewer type does not have an impact on flash flooding until very large amounts of precipitation are accumulated in one day. Additionally, maximum sensitivities were higher for combined sewers when precipitation gets exceptionally high.
1 Day Sensitivity 3 Separate Sewer 2.5 Combined Sewer 2 Combined Cities Average 1.5 Combined Cities Maximum 1 Separate Cities Average
Separate Cities Maximum Days in Thresholdin Days
Events Per Threshold/ Per Events 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Precipitation Threshold (Inches) Statistical Tests for Population Density As a statistical check of the effect of population density on flash flooding, each city’s population density was divided by the total number of flash flood events for that city, the number of events caused by greater than 1.25 inches of precipitation in one day, and the number of events caused by greater than 1.75 inches of precipitation in one day.
Clay-based soil types typical of the Midwest, such as alfisols and mollisols, can absorb about 1.25 inches of precipitation in one day. More than 1.25 inches begins cause soil saturation and runoff (Takle, 2011). Heavily urban areas can absorb less precipitation, but also actively manage water. Less urban and rural areas with less or no water management system would begin to see flash floods occur near this 1.25 inch threshold.
At 1.75 inches of precipitation in 24 hours, green infrastructure begins to get overwhelmed (Cruce, 2011). Additionally, The Milwaukee Metropolitan Sewerage District estimated in 2011 that 1.75 inches of precipitation in 24 hours was the lower threshold at which combined sewer overflows into Lake Michigan began to occur (Cruce, 2011).
An issue with comparing the separate and combined sewer cities is that the top 6 densest cities in this study have combined sewers. These cities are Chicago, Pittsburgh, Buffalo, Philadelphia, Detroit, and Cleveland. For comparisons sake, trends were found for all separate cities, all combined cites, and combined cities within the range of population densities as separate cities (Adjusted Combined on graph). The trend of all 15 combined cities dramatically changed when data for the 6 highest population densities were discarded for floods caused by 1.25 inches or greater precipitation days. In order to get an accurate idea of how population density and number of flash flood events are related, more cities need to be added to the study.
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Flash Flood Events by Population Density 7 Combined Sewer 6 Separate Sewer 5 Linear (Combined Sewer) 4 Linear (Separate Sewer) 3 Linear (Adjusted Combined) y = 0.0002x + 0.3426
Events Per Year Per Events 2 R² = 0.3019 1 Combined y = 0.0002x + 0.3122 Average Number of Flash FloodFlash ofNumber Average 0 R² = 0.5165 0 5000 10000 15000 20000 Separate y = 0.0003x + 0.2381 People Per Square Mile R² = 0.3176
When looking the trendline for all storm events for separate, combined, and the adjusted combined sewer cities, there is no significant statistical difference for flash flood frequency.
Flash Flood Events >1.25 Inches by Population Density 2 Combined Sewer 1.8 1.6 Separate Sewer 1.4 Linear (Combined Sewer) 1.2 Linear (Separate Sewer) 1 Linear (Adjusted Combined) 0.8 y = 0.0002x + 0.1316 0.6 R² = 0.2427 0.4 Combined 0.2 y = 5E-05x + 0.4761 0 R² = 0.2314
Average Number of Flash Flood Events Events Per FloodFlash of Number Average 0 5000 10000 15000 20000 Separate Year Greater than Precipitation of Greater Inches 1.25 Year People Per Square Mile y = 0.0002x + 0.069 R² = 0.3474
As population density increases, combined sewer cities were less likely to flood than separate sewer cities for flash floods caused by more than 1.25 inches of precipitation in one day. After discarding the 6 highest population density cities, the adjusted combined trend was not significantly different than the separate sewer trend.
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Flash Flood Events >1.75 Inches by Population Density 1.6 Combined Sewer 1.4 Separate Sewer 1.2 Linear (Combined Sewer) 1 Linear (Separate Sewer) 0.8 Linear (Adjusted Combined) 0.6 y = 0.0002x - 0.1324 R² = 0.3907 0.4 Combined 0.2 y = 3E-05x + 0.3821 0 R² = 0.0916 Average Number of Flash Flood Events Events Flash of FloodPer Number Average Separate Year Greater than Greater Precipitation of Inches 1.75 Year 0 5000 10000 15000 20000 y = 1E-04x + 0.1101 People Per Square Mile R² = 0.219
Combined sewer cities are not as likely as separate sewer cities to flood as population increased for flash flood events caused by more than 1.75 inches of precipitation in one day. After discarding the 6 highest population density cities, the adjusted combined trend was more likely to flood as population density increased.
These graphs again suggest that sewer type does not have a huge impact on flash flooding frequency until a certain threshold of rain is reached. Statistical Tests for Sensitivity The 1.25 and 1.75 inch precipitation threshold sensitivities are important as the thresholds for beginning to cause runoff and overwhelming green infrastructure. After removing one outlier from each type of city for having a very high average number of precipitation days greater than 1.25 or 1.75 inches per year, the linear trends for separate and combined cities were found.
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Average Number of Flash Flood Events Per Number of Precipitation Days >1.25 Inches by Year 1.8 1.6 Combined Sewer 1.4 Separate Sewer 1.2 1 Linear (Combined Sewer) 0.8 0.6 Linear (Separate Sewer) 0.4 Combined 0.2 y = 0.2049x - 0.0487 0 R² = 0.2985
0 2 4 6 8 10 12 Separate Greater than 1.25 Inches Per Year Year Inches Per 1.25than Greater Average Number of Precipitation Days y = 0.0256x + 0.4324 Average Number of Flash Flood Events Events FloodFlash ofNumber Average Greater than 1.25 Inches Per Year R² = 0.0104
Average Number of Flash Flood Events Per Number of Precipitation Days >1.25 Inches by Year (Highest Number of Precipitation Days Discarded for Separate and Combined Sewers) 1.8 1.6 Combined Sewer 1.4 Separate Sewer 1.2 1 Linear (Combined Sewer) 0.8 0.6 Linear (Separate Sewer) 0.4 Combined 0.2 y = 0.1854x + 0.0269 0 R² = 0.1482 0 2 4 6 8
Greater than 1.25 Inches Per Year Year InchesPer 1.25than Greater Separate
Average Number of Flash Flood Events Events FloodFlash of Number Average Average Number of Precipitation Days y = 0.2851x - 0.5473 Greater than 1.25 Inches Per Year R² = 0.3193
Bloomington and Philadelphia were outliers that were discarded in this graph because they had a high average number of precipitation days greater than 1.25 inches per year. Without these cities, the combined sewer trendline changes slope from 0.2985 to 0.1482 and the separate sewer trendline changes slope from 0.026 to 0.285. The separate sewer trend with all of the cities has a lower sensitivity to flooding than combined. The combined sewer trend is more sensitive than the separate sewer trend. The separate sewers are less sensitive when a city has fewer average days of precipitation greater than 1.25 inches (around 3 days per year).
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Separate sewers and combined sewers have about the same sensitivities when a city has a larger number of precipitation days greater than 1.25 inches per year (around 6 days per year).
Average Number of Flash Flood Events Per Number of Precipitation Days >1.75 Inches by Year 2 1.8 Combined Sewer 1.6 1.4 Separate Sewer 1.2 1 Linear (Combined Sewer) 0.8 Linear (Separate Sewer) 0.6 0.4 Combined 0.2 y = 0.3196x + 0.0502 R² = 0.26 0
Greater than 1.75 Inches Per Year Year Inches Per 1.75thanGreater Separate 0 1 2 3 4 5
Average Number of Flash Flood Events Events FlashFlood ofNumber Average y = 0.0726x + 0.281 Average Number of Precipitation Days R² = 0.0453 Greater than 1.75 Inches Per Year
Average Number of Flash Flood Events Per Number of P recipitation Days >1.75 Inches by Year (Highest Number of Precipitation Days Discarded for Separate and Combined Sewers) 2 1.8 Combined Sewer 1.6 1.4 Separate Sewer 1.2 1 Linear (Combined Sewer) 0.8 Linear (Separate Sewer) 0.6 0.4 Combined 0.2 y = 0.4137x - 0.0825 R² = 0.233 Greater than 1.75 Inches Per Year Year Inches Per 1.75than Greater 0
Average Number of Flash Flood Events FlashFlood ofNumber Average 0 0.5 1 1.5 2 2.5 3 Separate Average Number of Precipitation Days y = 0.4275x - 0.2133 Greater than 1.75 Inches Per Year R² = 0.5386
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The data from Bloomington and Philadelphia were again removed in this graph because they were high outliers in average number of precipitation days greater than 1.75 inches per year. Combined sewers are slightly more sensitive than separate sewers greater than the 1.75 inch precipitation threshold.
Since the trends had a significant change in slope when one point from both categories was removed, more cities are needed to verify the 1.25 and 1.75 sensitivity trends are accurate. Damages Damages from all flash flood events were analyzed and adjusted the amounts for inflation. After a number of different analyses, no correlation was found between separate and combined sewers and damage.
When comparing the average damage per storm and sewer type, no correlation was found. Most cities had an average amount of less than $1 million per storm. A few storms had outrageous damages that made the average damage amount unrealistic for cities like Green Bay and Detroit, and therefore not a good comparison between all cities.
Average Damage per Storm 16 14 12 Combined Sewer 10 8 Separate Sewer 6 4
USD (Millions)USD 2
0
Erie Erie
Akron
Detroit
Toledo
Aurora
Albany
Buffalo
Duluth Duluth
Oswego Chicago
Madison
Saginaw
Lafayette
St. Cloud St.
Ann Arbor Ann
Cleveland
Pittsburgh
Marquette
Eau Claire Eau
Harrisburg
Milwaukee
Springfield
Green Bay Green
Fort Wayne Fort
South Bend South
Minneapolis
Philadelphia
Bloomington Grand Rapids Grand Traverse City City Traverse City
Storms that caused more than $1 million in damages represent exceptionally damaging storms, and this level of damage is not typical of a flash flood. Discarding these outlier storms, the trend for damages and sewer type still did not yield an obvious correlation.
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Average Damage for Floods with Less than $1 Million in Damage 160000 140000 Combined Sewer 120000 100000 Separate Sewer 80000 60000
40000 Damage(USD) 20000
0
Erie Erie
Akron
Duluth
Detroit
Toledo
Aurora
Albany
Buffalo
Oswego Chicago
Madison
Saginaw
St. Cloud St.
Lafayette
Ann Arbor Ann
Cleveland
Pittsburgh
Marquette Columbus
Eau Claire Eau
Harrisburg
Springfield Milwaukee
Green Bay Green
Fort Wayne Fort
Minneapolis
Philadelphia
South Bend South
Bloomington Traverse City Traverse
City Rapids Grand
Precipitation amounts larger than 2 inches are more likely to overwhelm a sewer system, whereas smaller amounts are more likely to be managed or cause a smaller flood and less damage. After examining flash floods caused by less than 2 inches of precipitation in one day, no correlation between sewer type and damage amount was found.
One factor to keep in mind is this graph does not take soil saturation into account. A city’s soil could be near saturation from a previous day of rain and a small amount of precipitation could cause a flash flood. However, in our analysis this is a rare occurrence.
Average Damage for Floods with Less than 2 Inch Precipitation 4000000 3500000 3000000 Combined Sewer 2500000 Separate Sewer 2000000 1500000
Damage(USD) 1000000 500000
0
Erie Erie
Akron
Duluth
Detroit
Toledo
Aurora
Albany
Buffalo
Oswego Chicago
Madison
Saginaw
St. Cloud St.
Lafayette
Ann Arbor Ann
Cleveland
Marquette Pittsburgh Columbus
Eau Claire Eau
Harrisburg
Springfield
Milwaukee
Green Bay Green
Fort Wayne Fort
Minneapolis
Philadelphia
South Bend South
Bloomington Traverse City Traverse Grand Rapids Grand City
To see if certain precipitation thresholds were handled better by either sewer type, average damages by precipitation threshold were analyzed. Initially, a few outlier cities were found that caused unrealistic average damage amounts that are not typical of a flash flood. Any storm that causes over $1 million in damages is a storm that likely was too big for any sewer system to manage. About 90 percent of the average damages per threshold were less than $1 million.
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Average Damages per Threshold DISASTERS 50 45 Separate Sewer 40 35 Combined Sewer 30 25
20 USD (millions) USD 15 10 5 0 0 0.5 1 1.5 2 2.5 3 3.5 Precipitation Threshold (Inches)
Looking only at the average damages per threshold that were less than $1 million, there appears to be no precipitation threshold where substantial damages are reported. There is no apparent trend in damages compared to precipitation. Additionally, there is no correlation between sewer type and damage amount.
Average Damages per Threshold STORMWATER MANAGEMENT 1 0.9 Separate Sewer 0.8 0.7 Combined Sewer 0.6 0.5 0.4
USD (millions) USD 0.3 0.2 0.1 0 0 0.5 1 1.5 2 2.5 3 3.5 Precipitation Threshold (Inches)
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Discussion There are a number of factors that can help predict when a city will have a flash flood and how severe that flash flood will be. Factors that appear to strongly impact flooding include population density and sensitivity to flooding at high precipitation thresholds. As population density increases, frequency of flooding increases, likely due to more impervious surface in large cities. Also, as cities receive over 2 inches of precipitation in one day, there is a reasonable chance a flash flood will occur, regardless of sewer type. Using the GHCN-Daily data to analyze regional sensitivity to frequency of flash flooding following heavy precipitation proved an effective source for data analysis.
On the other hand, using the GHCN-Daily data to quantify potential increases in the capacity of separate sewer systems is uncertain. There is no significant difference in the amount of precipitation an average combined or separate sewer city receives, yet combined sewers have a higher percent difference of flash floods by threshold for nearly all precipitation thresholds. Additionally, after more than 2.25 inches of precipitation in one day, combined sewers were more sensitive than separate sewers. However, these results are impacted by population density, which shows that sewer type probably does not play the most important role in flooding frequency. The combined sewer cities in this report have an average population density of 7,131 people per square mile, versus 3,047 people per square mile in the average separate sewer city. Many combined sewer cities have a high population density and flood more often due to higher impermeable surfaces that cause more runoff and are more likely to overwhelm a sewer system. Cities with separate sewers may not have as many events on average because they typically have lower population density and cover a smaller area. Lower population density cities have less impervious surface and allow more rainwater to enter the soil instead of a sewer system.
There were also some variables found that did not show correlation with flash floods. Damages had no connection between sewer type or precipitation threshold. Also, sensitivities generated with multi day precipitation did not yield a strong correlation to sewer type. One day sensitivities from floods caused by less than 2.25 inches of precipitation also did not differ much between combined and separate average sensitivities.
In order to be able to help cities determine their individual thresholds for reasonable stormwater management, there are additional variables that need to be taken into account. These include amount of impervious surface, age and capacity of infrastructure, topography in and around the city, soil type, and so on. More research needs to be done to determine how these additional factors impact each city and what each city can do to be the most prepared for heavy precipitation in the future.
In general, more cities need to be added to this research in order to get accurate statistics about all of the factors that impact flash flooding occurrences.
Acknowledgments The authors would like to thank the University of Michigan’s Undergraduate Research Opportunity Program, for the opportunity to begin this project. The authors would also like to thank the University of Michigan Climate Center and the Great Lakes Integrated Sciences and Assessments for continuing to supervise and support this project.
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