Another 100-Year Storm October 26, 2016 Mark Dennis, PE, CFM
Agenda Are severe rainfall events becoming more frequent? Is there confusion about the 100-year storm that makes it seem like they happen all the time? Arcadis 2015
Storm Frequency Terminology 100-Year Storm Storm event that occurs on average every 100 years Average Recurrence Interval (ARI) ARI (and 100-year storm term) still used in NOAA Atlas 14 1% Annual Chance Storm Storm event that has a 1% chance of being exceeded in any year Average Exceedance Probability (AEP) FEMA & USGS have transitioned to annual chance storm terminology. Arcadis 2015
Are severe rainfall events becoming more frequent? Rainfall depths in published rainfall atlases Other rainfall statistical analyses in published scientific journals Climate change models Arcadis 2015 04 November 2016 4
Technical Paper 40 National Weather Service Published in 1961 Limited rainfall data: 200 1 st order stations (at least 20 years) 5,000 other stations (5 to 20 years) Arcadis 2015 04 November 2016 5
Rainfall Frequency Atlas of the Midwest Midwest Climate Center & Illinois State Water Survey Published in 1992 409 stations (275 stations > 50-year record) Covers Midwest Arcadis 2015 04 November 2016 6
Rainfall Frequency Atlas of the Midwest (a) Number of stations (b) Average length of record (c) Number of times exceeded (d) Number of times expected Ratio (c)/(d) Illinois 61 87 69 36 1.92 Indiana 41 64 17 20 0.85 Iowa 43 80 20 24 0.83 Kentucky 25 67 11 12 0.92 Michigan* 46 60 71 21 3.38 Minnesota 25 67 14 12 1.17 Missouri 44 62 4 20 0.2 Ohio 41 60 27 19 1.42 Wisconsin 13 78 13 7 1.86 Midwest 246 171 1.43 Table 1. Number of Times the 24-Hour, 100-Year Value from Technical Paper 40 Is Exceeded by State [T]he findings suggest that the assumption of a stationary time series for fitting statistical distributions to historical precipitation data may be invalid. Arcadis 2015 04 November 2016 7
NOAA Atlas 14, Volume 2 (Ohio River Valley), 2006 Precipitation Frequency Data Server: http://hdsc.nws.noaa.gov/hdsc/pfds/ Arcadis 2015 04 November 2016 8
NOAA Atlas 14, Volume 2 Station Data Daily Hourly N-minute No. of stations 2846 994 96 Longest record length (data yrs) 126 101 105 Average record length (data yrs) 63 40 67 State Daily Hourly N-min Delaware 12 2 1 Illinois 192 80 6 Indiana 156 75 5 Kentucky 166 59 5 Maryland 74 16 2 New Jersey 76 22 3 North Carolina 196 51 6 Ohio 225 104 9 Pennsylvania 278 139 8 South Carolina 107 25 3 Tennessee 166 47 5 Virginia 156 47 6 Washington DC 3 0 0 West Virginia 141 42 5 Border states* 898 285 32 Total 2846 994 96 Arcadis 2015 04 November 2016 9
Table A.3.1. Number of stations tested and linear trend test results by state. Arcadis 2015 04 November 2016 10
Figure A.3.1. Spatial distribution of linear trend results, where + indicates a station with a positive trend and - indicates a negative trend. Arcadis 2015 04 November 2016 11
NOAA Atlas 14, Volume 2 Conclusion Overall, the 1-day annual maximum time series were free from linear trends and from shifts in mean for most of the stations in the project area... since the results showed little observable or geographically consistent impact of change in the statistics used to estimate precipitation frequency, the entire historical time series was used in this Atlas. Arcadis 2015 04 November 2016 12
NOAA Atlas 14, Volume 8 (Midwest), 2013 Arcadis 2015 04 November 2016 13
Figure A.2.1. Spatial distribution of results of t-, Mann-Kendall, and Levene s tests for 1-hour AMS. Red color indicates positive trends, green no trend, and blue negative trends. Arcadis 2015 04 November 2016 14
NOAA Atlas 14, Volume 8 - Conclusion Because tests at both, the 1-hour and 1-day durations indicated no statistically significant trends in the data, the assumption of stationary AMS [Annual Maximum Series] was accepted for this project area and no adjustment to AMS data was recommended. Arcadis 2015 04 November 2016 15
Third National Climate Assessment, 2014 Predicts increased precipitation based on: Climate models Academic studies of rainfall trends Arcadis 2015 04 November 2016 16
Other Academic Studies of Rainfall Trends Figure 2.18. The map shows percent increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events) from 1958 to 2012 Arcadis 2015 04 November 2016 17
Other Academic Studies of Rainfall Trends Found statistically significant increase in rainfall from 1931-1996; The national trend for the United States is upward at a rate of 3% per decade. Long-Term Trends in Extreme Precipitation Events over the Conterminous United States and Canada, 1999 During the 20 th Century, there was a 50% increase in the frequency of days with precipitation over 4 in the upper Midwest Heavy precipitation and high streamflow in the contiguous United States: trends in the 20th century, Groisman et al., 2001 Arcadis 2015 04 November 2016 18
Other Academic Studies of Rainfall Trends The majority of stations exhibit no trend in any of the extreme metrics, but the statistically significant changes that are observed indicate a tendency towards increased values. Arcadis 2015 04 November 2016 19
Table A.3.1. Number of stations tested and linear trend test results by state. Arcadis 2015 04 November 2016 20
Rainfall Atlas Timeline Atlas of Midwest (1992) Atlas, Vol. 8 (2013) 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s Tech Paper 40 (1961) Atlas 14, Vol. 2 (2006) Arcadis 2015
Difference in Rainfall Values Between 1961 and 2006 Atlases in Ohio 24-Hour Rainfall Totals for Franklin County ARI (years) Technical Paper 40 (1961) NOAA Atlas 14, Vol. 2 (2006) % Increase 2 2.6 2.6 0% 5 3.2 3.2 0% 10 3.7 3.7 0% 25 4.3 4.4 2% 50 4.8 5.0 4% 100 5.0 5.6 12% 24-Hour Rainfall Totals for Summit County ARI Technical Paper 40 NOAA Atlas 14, Vol. 2 (years) (1961) (2006) % Increase 2 2.3 2.4 4% 5 3.0 3.0 0% 10 3.5 3.5 0% 25 3.9 4.2 8% 50 4.4 4.8 9% 100 4.6 5.5 20% Arcadis 2015 04 November 2016 22
Increasing Rainfall Summary Severe rainfall appears to be increasing (not statistically significant in all studies) Climate models predict future increases in severe rainfall NOAA Atlas based on most data & current statistical methods (225 Ohio stations; 63-year rainfall record on average) The results also suggest the need to update rainfall frequency relations more frequently. An update on the order of every 20 years would be appropriate to capture any substantial changes - Huff & Angel Arcadis 2015
Why does it seem like severe storms happen all the time? Severe rainfall is actually increasing Limited rainfall record High degree of inter-decade variability Misreporting of storm frequency (labeling all severe storms 100- year storms) High likelihood of severe storms over large geographic area Arcadis 2015
In order to accurately predict a 10-year recurrence frequency event, we would need 100 years of records. (Professor Ven Te Chow, 1977) We now have nearly 100 years of rainfall records Limited Rainfall Record NOAA Atlas 14 includes rainfall estimate up to the 1,000-year storm Stephen Colbert explains how we can characterize storms as 1,000-year storms thanks to surviving weather reports from Kiawah Indians Arcadis 2015 04 November 2016 25
Inter-Decade Variability Significant variability in frequency of severe rainfall (Long-Term Trends in Extreme Precipitation Events over the Conterminous United States and Canada, 1999) Decade Severe Rainfall Frequency 1930s 1940s 1950s 1960s 1970s 1980s 1990s LOW HIGH LOW NORMAL NORMAL HIGH HIGH Arcadis 2015 04 November 2016 26
Inter-Decade Variability Lessons from a 500-year Record of Flood Elevations by James H. Eychaner Danube River at Passau, Germany Flood level records since 1501 Flood events greater than the 50-year flood occurred at intervals of 4 to 192 years... Similarly, the intervals between 5-year floods since 1955 ranged from 5 months to 16 years. Arcadis 2015 04 November 2016 27
Misreporting of Storm Frequency The vast majority of floods that occur in Western Washington, or for that matter most of the U.S., are NOT 100 YEAR FLOODS. (emphasis in original) Arcadis 2015 04 November 2016 28
Misreporting of Storm Frequency The media necessarily simplify the story by reporting the greatest damage and largest recurrence interval estimated at any location. Lessons from a 500-year Record of Flood Elevations by James H. Eychaner Arcadis 2015 04 November 2016 29
High Likelihood of Severe Storms Probability Theory Binomial Distribution Probability of heads in a coin flip No. of Coin Probability of at Flips least on heads 1 50.0% 2 75.0% 3 87.5% Arcadis 2015 04 November 2016 30
High Likelihood of Severe Storms (a) Number of stations (b) Average length of record (c) Number of times exceeded (d) Number of times expected Ratio (c)/(d) Illinois 61 87 69 36 1.92 Indiana 41 64 17 20 0.85 Iowa 43 80 20 24 0.83 Kentucky 25 67 11 12 0.92 Michigan* 46 60 71 21 3.38 Minnesota 25 67 14 12 1.17 Missouri 44 62 4 20 0.2 Ohio 41 60 27 19 1.42 Wisconsin 13 78 13 7 1.86 Midwest 246 171 1.43 Table 1. Number of Times the 24-Hour, 100-Year Value from Technical Paper 40 Is Exceeded by State Arcadis 2015 04 November 2016 31
High Likelihood of Severe Storms Probability of 1% annual chance storm Probability of at No. of Years least one storm 1 1.0% 2 2.0% 30 26.0% 60 45.3% With 41 stations in Ohio and assuming storms at each station are independent: 45.3% x 41 stations = 19 storms Arcadis 2015 04 November 2016 32
High Likelihood of Severe Storms Probability of winning scratch off lottery 1% chance on each ticket Probability of No. of Years winning 1 1.0% 2 2.0% 30 26.0% 60 45.3% With 41 players each buying 60 tickets: 45.3% x 41 players = 19 winners Arcadis 2015 04 November 2016 33
High Likelihood of Severe Storms Understanding Extremes, 2007 Urban Drainage Flood Control District (Denver, CO) Greater Denver 1,600 square miles Parts of 6 counties 100-year storms can happen every year [I]n a typical year, it is not unusual for Colorado to experience between 100 and 150 precipitation events that exceed the 100-year mark. Arcadis 2015 04 November 2016 34
High Likelihood of Severe Storms Why do 100 year events happen so often? Australian Government Bureau of Meteorology Greater Melbourne 2,500 sq km (965 square miles) Points 5km apart may be independent for 1-hour storm [T]here is a good chance (63% assuming full independence of points) of a 1-hour, 1% AEP event occurring somewhere in the general Melbourne area in each calendar year. Arcadis 2015 04 November 2016 35
High Likelihood of Severe Storms Tucson Arizona 1 km 2 gridded rainfall data from radar corrected by rain gages Determined ARI at each grid for various durations Spatial distribution and frequency of precipitation during an extreme event, Water Resources Research, 2009 Arcadis 2015 04 November 2016 36
Why does it seem like severe storms happen all the time? Severe rainfall is actually increasing Limited rainfall record High degree of inter-decade variability Misreporting of storm frequency (labeling all severe storms 100- year storms) High likelihood of severe storms over large geographic area For every complex problem there is an answer that is clear, simple, and wrong. H.L. Mencken Arcadis 2015
Questions/Discussion Mark Dennis, PE, CPM 330.515.5666 mark.dennis@arcadis.com Arcadis 2015