Prepared in cooperation with the Department of Homeland Security Federal Emergency Management Agency Magnitude of Flood Flows for Selected Annual Exceedance Probabilities in Rhode Island Through 2010 Scientific Investigations Report 2012–5109 U.S. Department of the Interior U.S. Geological Survey Front cover. March–April 2010 flooding along the Woonasquatucket River at Valley Street in Providence, Rhode Island, looking northward toward Atwells Avenue (traffic light) from Helme Street. Back cover. Historical marker found along Simmons Brook at Simmonsville east of Providence, Rhode Island. Magnitude of Flood Flows for Selected Annual Exceedance Probabilities in Rhode Island Through 2010 By Phillip J. Zarriello, Elizabeth A. Ahearn, and Sara B. Levin Prepared in cooperation with the Department of Homeland Security Federal Emergency Management Agency Scientific Investigations Report 2012–5109 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Zarriello, P.J., Ahearn, E.A., and Levin, S.B., 2012, Magnitude of flood flows for selected annual exceedance probabilities in Rhode Island through 2010: U.S. Geological Survey Scientific Investigations Report 2012–5109, 81 p. (Also available at http://pubs.usgs.gov/sir/2012/5109.) iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Purpose and Scope ..............................................................................................................................2 Study Area..............................................................................................................................................2 Previous Studies ...................................................................................................................................2 Data Compilation ............................................................................................................................................6 Record Extension ..................................................................................................................................6 Historic Data ..........................................................................................................................................9 Magnitude of Flood Flows at Streamgages .............................................................................................10 Skew Coefficient .................................................................................................................................11 Low Outlier Adjustment ............................................................................................................11 High Outlier Adjustment ............................................................................................................13 Generalized Regional Skew .....................................................................................................13 Quantifying Uncertainty in Annual Peak Records .........................................................................15 Annual Exceedance Probability Flood Estimates ..........................................................................18 Uncertainty of Estimates ...................................................................................................................18 Magnitude of Flood Flows at Ungaged Streams ....................................................................................19 Basin Characteristics .........................................................................................................................19 Exploratory Analysis ...........................................................................................................................19 Regional Regression Equations ........................................................................................................19 Accuracy and Limitations ..................................................................................................................22 Uncertainty Estimates of Regionalized Equations .........................................................................25 Factors Affecting Flood Flow Estimates ...................................................................................................27 Urban Influence...................................................................................................................................27 Trends in Annual Peak Flows ............................................................................................................30 Application of Methods and Significance of Results ............................................................................35 Weighted Estimates of Flood Flows at Streamgages ...................................................................35 Flood Flows at Streamgages with Limited Record ........................................................................36 Flood Flows at an Ungaged Site on a Gaged Stream ...................................................................38 Comparison with a Previous Rhode Island Flood Study ...............................................................40 Annual Exceedance Probability of the 2010 Flood in Rhode Island ...........................................41 Summary and Conclusions .........................................................................................................................43 Acknowledgments .......................................................................................................................................44 Selected References ...................................................................................................................................44 Glossary .........................................................................................................................................................49 Tables 7, 13, and 15 ......................................................................................................................................51 Appendix 1. Basin Characteristics Considered for Use in the Regional Regression Analysis ...78 Appendix 2. Measurement of Regression Error ..................................................................................80 Appendix 3. Link to Spreadsheet RI_Flood-Flow-Equations.xls .......................................................81 iv Figures 1. Map showing general area and selected streamgages used in the Rhode Island flood frequency study ..................................................................................................................3 2. Map showing land cover in the study area in Rhode Island, Connecticut, and Massachusetts from the 2006 National Land Cover Database .............................................4 3. Map showing hurricane and hurricane related storm tracks from 1851 through 2008 in and near Rhode Island .............................................................................................................5 4. Map showing streamgage skews used to develop the regional skew at selected streamgages in Rhode Island, eastern Connecticut, and south-central and eastern Massachusetts ............................................................................................................................14 5. Graph showing upper stage-discharge rating and range of uncertainty of higher annual peak flows used in flood frequency analysis at Mount Hope River near Warrenville, CT (01121000) ........................................................................................................16 6. Graphs showing uncertainty of A, annual peak and historic flows used to compute B, flood frequency curve at Mount Hope River near Warrenville, CT (01121000) ............17 7. Boxplots showing selected streamgage basin characteristics in Rhode Island, Connecticut, and Massachusetts used in flood frequency analysis .................................20 8. Boxplot showing residuals of generalized least squares (GLS) regional regres- sion of floods for 20- to 0.2-percent annual exceedance probabilities at 41 selected streamgages in Rhode Island, Connecticut, and Massachusetts ......................................22 9. Map showing residuals of generalized least squares (GLS) regional regression and at-site 1-percent annual exceedance probability flood flows at 41 selected streamgages in Rhode Island, Connecticut, and Massachusetts