A Proposed Method of Categorizing Extreme Rainfall Events -

The 100-Year Storm Redefined

I can’t tell these people anymore that we had yet

ANOTHER 100-year storm!

History of Flooding in Brookfield

Historic flooding events: March 1897 June 1917 June 1940 March 1960 July 1964 September 1972 April 1973

Recent Flooding in Brookfield

Recent history of flooding in Brookfield August 6, 1986 – 6.8” in 24 hours June 20 - 21, 1997 – 6” in 26 hours August 6, 1998 – 11.35” in 8 hours June 7 - 8, 2008 – 5.8” in 24 hours June 19, 2009 – 4.8+” in 3 hours

All 100-year storms or larger!

Radar image of precipitation in June 2008

Wisconsin: 14-Day PrecipitationValid at 6/16/2008 1200 UTC

Why Call Them 100-Year Storms?

2” rains are different depending where you are Need something that can be used consistently

across the country Account for variability in these storms by region

100-Year Storms

Recurrence interval Based on probability of the storm occurring How can there be two 100-year storms in a

row?

Let’s Call Them Something Else

1% probability storm Aren’t we clever? Public knows 1% = 1/100 They figure out that

it’s the 100-year storm Now they feel like we tried

to deceive them – hiding behind probabilities

Probability of the 100-Year Storm

Probability of 100-year event: p = 0.01 100-year event occurring three years in a row? p3 = (0.01)3 = 0.000001 or 1 in a million Not always intuitive 100-year event occurring once in 100 years? 1-(1-p)100 = 1-(0.99)100 = 1-0.366 = 63.4% Public thinks it would occur once every 100

years

Binomial Probability Applied to Extreme Events

More complex relationships 100-year event occurs 5 times in 25 years?

n!*pY*(1-p)(n-Y) / (Y!)*(n-Y)! 25!*0.015*(0.99)20 / 5!*20! = 1 in 4.4 million Is this pretty rare? Consider what people are willing to gamble on

Probability and Statistics

Winning the Powerball Lottery: 1 in 195 million

One Fatal Flaw

People don’t understand statistics Public’s response to 100-year storm? “Oh good, we’re safe for another 99 years.” Two 100-year storms in a row? “This storm was different than last year’s

storm, so they both can’t be called the 100-year storm. Can they?”

Public Confusion

Mark Twain said, “The more you explain it, the more I don't understand it”

Public more likely to respond this way

Public Response

Disbelief Disgust Cynicism Look for someone to blame Affects our credibility Less likely to take our

advice in future

Rating Other Natural Disasters

Earthquakes Richter scale Measurement of

Energy released Strength Duration of its

seismic waves

Rating Other Natural Disasters

Tornadoes Fujita scale

(enhanced since 2007)

Measurement of Intensity Area affected

Rating Other Natural Disasters

Hurricanes Saffir- Simpson

scale Measurement of

Barometric pressure

Wind speed Storm surge

Rating Other Natural Disasters

None of these natural disasters are rated using probabilities or recurrence intervals

Something that implies severity of the storm, not rarity of the storm

Use a scale instead to rate the event Easily understood, the higher the number,

the worse the natural disaster

Revise Terminology for Communication with Public

Keep It Simple S_ _ _ _ _

Use recurrence intervals as basis

Don’t call them 100-year storms

Use a scale like other natural disasters

A New Rating System Proposed

RI = 2(G-1) RI = Recurrence Interval of the rain event G = the Category of storm Example: G-factor of 7 in the formula is 2(7-1) = 26

equals the 64-year event

Categorizing Storms Based on Recurrence Intervals: RI = 2(G-1)

Category of Storm Recurrence Interval Rain Storm

G - 2 2 to 4 year

G - 3 4 to 8 year

G - 4 8 to 16 year

G - 5 16 to 32 year

G - 6 32 to 64 year

G - 7 64 to 128 year

G - 8 128 to 256 year

G - 9 > 256 year

Placing the Recurrence Interval Storm into the New Rating System

Recurrence Interval Rain Storm: RI = 2(G-1) G - Factor

Category of Storm

2-year 2 G - 2

5-year 3.32 G - 3

10-year 4.32 G - 4

25-year 5.64 G - 5

50-year 6.64 G - 6

100-year 7.64 G - 7

Recurrence Interval

Storm Duration 2-year 5-year 10-year 25-year 50-year 100-year

1 hour 2 3 4 5 6 7

2 hour 2 3 4 5 6 7

3 hour 2 3 4 5 6 7

6 hour 2 3 4 5 6 7

12 hour 2 3 4 5 6 7

24 hour 2 3 4 5 6 7

2 day 2 3 4 5 6 7

3 day 2 3 4 5 6 7

5 day 2 3 4 5 6 7

10 day 2 3 4 5 6 7

Simple G-Factor Table

Different Durations of 100-Year Storms Have Different Effects

2.8 inches of rain in one hour will wash out roads, culverts, and “flood” streets for a time

7.5 inches of rain in ten days causes rivers to overtop with widespread flooding

Need to Account for Differences

Develop adjustment factors for G New formula: GADJ = G x (DAF) x (IAF)

G = the Category of storm Duration adjustment factor Intensity adjustment factor

Intensity & Duration Adjusted Category Storms Recurrence Interval

Storm Duration 2-year 5-year 10-year 25-year 50-year 100-year

1 hour 0 1 1 2 3 3

2 hour 1 1 2 2 3 4

3 hour 1 1 2 3 3 5

6 hour 1 1 2 3 4 6

12 hour 1 1 2 3 5 6

24 hour 1 2 2 4 5 7

2 day 1 2 3 5 6 7

3 day 1 2 3 5 6 7

5 day 2 3 4 5 6 7

10 day 2 4 5 7 8 9

Brookfield Flooding Revisited

August 6, 1986: 6.8” in 24 hours = G-8 June 20 - 21, 1997: 6” in 26 hours = G-7 August 6, 1998: 11.35” in 8 hours = G-10+ June 7 - 8, 2008: 5.8” in 24 hours = G-7 June 19, 2009: 4.8+” in 3 hours = G-6 No implication of rarity!!!

Criticism and Scrutiny

No continuous curve for each category storm

Interpret between durations to pick the right category storm

Or develop a look-up table or formula to interpolate (any grad students looking for a project?)

Criticism and Scrutiny

Does not address: antecedent moisture, snowmelt, soil types, % of impervious cover, etc.

Not intended to! These relate to runoff,

not the rain storm event

Criticism and Scrutiny

Just another name for the 100-year storm

Precisely. That’s the point!

Better way to communicate to public

Severity, not rarity, is key to public’s understanding

Advantages of a New System

Preserves the existing data and science Does not require overhaul of hydrology or

engineering Generally consistent with rating systems

for other natural disasters Easy to understand Bigger the number, the bigger the storm

Advantages of a New System

Provides a more appropriate scaling factor The 50-year storm is not 50% as big as the 100-year

storm as some might assume It is almost as big (Typically only 1 unit away)

Uses different terminology than for floodplains Avoids confusion that 100-year storm always

results in the 100-year flood Or that the 100-year flood can only occur when

there’s a 100-year storm

Advantages of a New System

Does not imply that big storms are rare Minimizes criticism, skepticism and

general disregard for hydrologic or engineering descriptions of the event

Makes it clear to people that these are big storms that we should take seriously

Incents people to protect themselves against larger event

Does This Make Sense?

What’s Next?

Does this process have merit? Academia support? NOAA, USACE HEC support? Anyone with me?

Your Turn for Feedback!

For copies of this presentation or white paper: email me at: grisa@ci.brookfield.wi.us