Changes in the Magnitude and Frequency of Riverine Flooding Robert M. Hirsch Research Hydrologist May 16, 2018

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1 Changes in the Magnitude and Frequency of Riverine Flooding Robert M. Hirsch Research Hydrologist May 16, 2018 U.S. Department of the Interior U.S. Geological Survey

2 USGS involvement in floods Measure and report flood flows & water levels in real time Create the historical data base to assess flood risk throughout the Nation Enhance the understanding of flood risk to support public and private decisions

3 How do we deliver flood risk information? 8000 streamgages across the US Describe flood frequency at each site Develop regional statistical models that predict flood frequency at any location on any river in the US: streamstats

4 How is this information used? Floodplain zoning and planning (residential, commercial, industrial) Setting flood insurance rates Design of transportation infrastructure (streets, highways, railroads, pipeline) Design of water and wastewater facilities Note that the work being described here is about river flooding and not coastal

5 All of this analysis assumes stationarity Hydrologists have long-recognized the potential for trends in flooding, due to: Urbanization Deforestation (including fire) Artificial land drainage What about climate variation and change?

6 An example of the interest in this type of research: The problem statement by the National Cooperative Highway Research Program (Association of State Highway and Transportation Officials) Panel on Applying Climate Change Information to Hydrologic and Hydraulic Design of Transportation Infrastructure. "Incorporating the results of climate models will have very large cost implications for future infrastructure. Overestimating the magnitude of peak flows suggested by climate models can result in costly over sizing of drainage infrastructure, while underestimating may leave infrastructure vulnerable and their resultant flooding impacts on surrounding lands and structures inadequately addressed."

7 Freshwater flooding only: does not include storm surge source:

8 Records from 2 USGS streamgages Potomac River at Point of Rocks, Maryland NW Branch Anacostia River near Hyattsville, Maryland

9 No significant trend

10 Flood Frequency Analysis, using USGS Peakfq software

11 With the flood frequency analysis 1% Annual Exceedance 90% Confidence Interval

12 A big trend, but we understand the cause (urbanization) We can account for it

13 Do we understand it? Some combination of climate and land use drivers Red River of the North North Dakota South Dakota Minnesota

14 USGS has a long history of flood trend studies related to many change-drivers: In the last 20 years there have been several related to climate I will briefly describe 3 of the most recent ones

15 Has the magnitude of floods across the USA changed with global CO 2 levels? Hirsch and Ryberg, USGS streamgages 85 to 127 years Minimal reservoirs or urbanization What s the relation between each year s flood discharge and that year s global mean CO 2 concentration?

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17 Flood discharge versus global mean CO2 for each year

18 The relationship is weak, R 2 = 13%, but it is highly significant statistically The slope translates to an 11% increase in flood size for every 10 ppm increase in CO 2 Is this kind of relationship common?

19 Interpretation of the Magnitude Inflation Factor The percent change in flood magnitude per 10 ppm change in CO 2 Currently, CO 2 is increasing about 10 ppm every 5 years

20 medians The only region with a significant result was the SW (p = 0.002), and it was negative

21 Published conclusion What these results do indicate is that except for the decreased flood magnitudes observed in the SW there is no strong empirical evidence in any of the other 3 regions for increases or decreases in flood magnitudes in the face of the 32% increase in global mean CO 2 that has taken place over the study period. However, it is crucial that analysis of the empirical data be conducted repeatedly as greenhouse forcing changes over time because such empirical analyses are a valuable check on the results of theoretical or model-driven studies of this issue.

22 My further comment All approaches to understanding the flooding/greenhouse gas question have flaws. But we need to look at the data regularly and with diverse approaches to see what might be emerging.

23 Detecting changes in floods across the United States Analyses of changes in floods across the United States had generally focused on trends in the annual, instantaneous maximum flood. This implies that there is only one flood per year. In drought years, the annual flood may not actually have been considered a flood; it was merely the highest observed streamflow in that year These studies ignore the possibly that the frequency of within-year floods may have increased.

24 A peaks-over-threshold approach to quantify changes in floods across the United States (Archfield and Hirsch) Create a series of independent flood events that exceed a given threshold Duration of event Peak streamflow of event Volume of event Threshold Number of events per year 0 Time, in days 365

25 Example of the peaksover-threshold approach Oyster River near Durham, NH

26 345 sites

27 Is the number of regions with significant increasing trends as compared to the number of regions with significant decreasing trends more than what would have been expected by chance????????

28 Archfield, S.A., R.M. Hirsch, A. Viglione, and G. Blöschl, 2016, Fragmented patterns of flood change across the United States, Geophysical Research Letters, 43

29 Clustering flood change across the United States

30 Changes in the odds of a 25-year, 50-year, and 100-year floods: Hodgkins, Glenn A., et al., 2017, Climate-driven variability in the occurrence of major floods across North America and Europe, Journal of Hydrology.

31 Perspectives on flood change and attribution US National Climate Assessment, 2017 Detectable changes in some classes of flood frequency have occurred in parts of the United States and are a mix of increases and decreases.formal attribution approaches have not established a significant connection of increased riverine flooding to human-induced climate change, and the timing of any emergence of a future detectable anthropogenic change in flooding is unclear. (Medium confidence) IPCC, 2013 there continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods at a global scale. National Academies, 2015 science is currently unable to provide reliable forecasts of the types and direction of change that may occur on any river or in any given community.

32 Bulletin 17C Statement on climate Variability and Change There is much concern about changes in flood risk associated with climate variability and long-term climate change In those situations where there is sufficient scientific evidence to facilitate quantification of the impact of climate variability or change in flood risk, this knowledge should be incorporated in flood frequency analysis All such methods employed need to be thoroughly documented and justified. USGS efforts: Keep collecting flood data, including paleo data Keep our analyses up to date Continue to do research to understand the many drivers of change in flood risk Communicate with users about the uncertainty in flood hazard estimates

33 Contact: Robert Hirsch The only way to figure out what is happening to our planet is to measure it, and this means tracking changes decade after decade and poring over the records. Keeling, 2008, Recording Earth s vital signs, Science, p The scientific community needs to emphasize that the problem of flood losses is mostly about what we do on or to the landscape and that will be the case for decades to come [but] doing the right things should not depend on waiting for the answers to the greenhouse forcing flood issue. The continuation of empirical and model-based science and a no regrets strategy for limiting flood losses should be encouraged. Kundzewicz, Z.W., et al., Flood risk and climate change: global and regional perspectives. Hydrological Sciences Journal, 59 (1), 1 28.