flash flooding, stormwater, and decision...

Flash Flooding, Stormwater, and Decision Making

http://graham.umich.edu/climate

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.

PROJECT BRIEF: FLASH FLOODING, STORMWATER, AND DECISION MAKING

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http://graham.umich.edu/climate Last updated: 9/25/2015

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]

mailto:[email protected]

PROJECT BRIEF: FLASH FLOODING, STORMWATER, AND DECISION MAKING

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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.

https://www.ncdc.noaa.gov/stormevents/

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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.

http://water.epa.gov/polwaste/npdes/cso/

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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.

y = 0.0984x + 0.0098R² = 0.4798

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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.

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.

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.

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.

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.

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.

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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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.

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.

y = 0.0002x + 0.3122R² = 0.5165

y = 0.0003x + 0.2381R² = 0.3176

y = 0.0002x + 0.3426R² = 0.3019

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People Per Square Mile

Flash Flood Events by Population Density

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Linear (Combined Sewer)

Linear (Separate Sewer)

Linear (Adjusted Combined)

Combined

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y = 5E-05x + 0.4761R² = 0.2314

y = 0.0002x + 0.069R² = 0.3474

y = 0.0002x + 0.1316R² = 0.2427

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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.

y = 3E-05x + 0.3821R² = 0.0916

y = 1E-04x + 0.1101R² = 0.219

y = 0.0002x - 0.1324R² = 0.3907

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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).

y = 0.2049x - 0.0487R² = 0.2985

y = 0.0256x + 0.4324R² = 0.0104

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y = 0.1854x + 0.0269R² = 0.1482

y = 0.2851x - 0.5473R² = 0.3193

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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).

y = 0.3196x + 0.0502R² = 0.26

y = 0.0726x + 0.281R² = 0.0453

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y = 0.4137x - 0.0825R² = 0.233

y = 0.4275x - 0.2133R² = 0.5386

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Average Number of Flash Flood Events Per Number of Precipitation Days >1.75 Inches by Year (Highest Number of

Precipitation Days Discarded for Separate and Combined Sewers)

Combined Sewer

Separate Sewer



Combined

Separate

15


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.

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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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.

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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17


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.

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Average Damages per ThresholdSTORMWATER MANAGEMENT

Separate Sewer

Combined Sewer

18


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.

19


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