# A Proposed Method of Categorizing Extreme Rainfall Events - The 100-Year Storm Redefined I cant tell these people anymore that we had yet ANOTHER 100-year.

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A Proposed Method of Categorizing Extreme Rainfall Events - The 100-Year Storm Redefined I cant 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 Precipitation Valid 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?

Lets Call Them Something Else 1% probability storm Arent we clever? Public knows 1% = 1/100 They figure out that its 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

Probability and Statistics Winning the Powerball Lottery: 1 in 195 million

One Fatal Flaw People dont understand statistics Publics response to 100-year storm? Oh good, were safe for another 99 years. Two 100-year storms in a row? This storm was different than last years storm, so they both cant 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 Dont 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) = 2 6 equals the 64-year event

Categorizing Storms Based on Recurrence Intervals: RI = 2 (G-1) Category of StormRecurrence Interval Rain Storm G - 22 to 4 year G - 34 to 8 year G - 48 to 16 year G - 516 to 32 year G - 632 to 64 year G - 764 to 128 year G - 8128 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-year2G - 2 5-year3.32G - 3 10-year4.32G - 4 25-year5.64G - 5 50-year6.64G - 6 100-year7.64G - 7

Recurrence Interval Storm Duration2-year5-year10-year25-year50-year100-year 1 hour234567 2 hour234567 3 hour234567 6 hour234567 12 hour234567 24 hour234567 2 day234567 3 day234567 5 day234567 10 day234567 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: 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 Duration2-year5-year10-year25-year50-year100-year 1 hour011233 2 hour112234 3 hour112335 6 hour112346 12 hour112356 24 hour122457 2 day123567 3 day123567 5 day234567 10 day245789

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. Thats the point! Better way to communicate to public Severity, not rarity, is key to publics 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 theres 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?

Whats 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

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