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!

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

 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!

Wisconsin: 14-Day Precipitation
Valid at 6/16/2008 1200 UTC
 Radar image of precipitation in June 2008

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

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

 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?
 p 3 = (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 !*p Y *(1-p) ( n -Y) / (Y!)*( n -Y)!
 25!*0.01
5 *(0.99) 20 / 5!*20! = 1 in 4.4 million
 Is this pretty rare?
 Consider what people are willing to gamble on

 Winning the Powerball Lottery: 1 in 195 million

 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 100year storm. Can they?”

 Mark Twain said, “The more you explain it, the more I don't understand it”
 Public more likely to respond this way

 Disbelief
 Disgust
 Cynicism
 Look for someone to blame
 Affects our credibility
 Less likely to take our advice in future

 Earthquakes
 Richter scale
 Measurement of
 Energy released
 Strength
 Duration of its seismic waves

 Tornadoes
 Fujita scale
(enhanced since
2007)
 Measurement of
 Intensity
 Area affected

 Hurricanes
 Saffir- Simpson scale
 Measurement of
 Barometric pressure
 Wind speed
 Storm surge

 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 100year storms
 Use a scale like other natural disasters

 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) = 2 6 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
G - 4
G - 5
G - 6
4 to 8 year
8 to 16 year
16 to 32 year
32 to 64 year
G - 7
G - 8
G - 9
64 to 128 year
128 to 256 year
> 256 year

Placing the Recurrence Interval
Storm into the New Rating System
Recurrence Interval
Rain Storm: RI = 2 (G-1) G - Factor
2-year 2
5-year
10-year
25-year
50-year
100-year
3.32
4.32
5.64
6.64
7.64
Category of
Storm
G - 2
G - 3
G - 4
G - 5
G - 6
G - 7

Simple G-Factor Table
Recurrence Interval
Storm
Duration 2-year 5-year 10-year 25-year 50-year 100-year
1 hour
2 hour
2
2
3
3
4
4
5
5
6
6
7
7
3 hour
6 hour
2
2
3
3
4
4
5
5
6
6
7
7
12 hour
24 hour
2 day
3 day
5 day
10 day
2
2
2
2
2
2
3
3
3
3
3
3
4
4
4
4
4
4
5
5
5
5
5
5
6
6
6
6
6
6
7
7
7
7
7
7

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

 Develop adjustment factors for G
 New formula: G
ADJ
= 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
1 hour
2 hour
3 hour
6 hour
12 hour
24 hour
2 day
3 day
5 day
10 day
2-year 5-year 10-year 25-year 50-year 100-year
0
1
1
1
1
2
2
2
3
3
3
4
1
1
1
1
1
1
2
2
1
1
1
2
2
2
3
4
2
2
2
2
3
3
4
5
3
3
3
4
5
5
5
7
3
4
5
5
6
6
6
8
7
9
6
7
7
7
5
6

 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!!!

 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?)

 Does not address:
 antecedent moisture,
 snowmelt,
 soil types,
 % of impervious cover,
 etc.
 Not intended to!
 These relate to runoff, not the rain storm event

 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

 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

 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

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

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