REGIONAL EQUATIONS FOR ESTIMATION OF PEAK-STREAMFLOW FREQUENCY FOR NATURAL BASINS IN TEXAS U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 96–4307 b QT = cA PEAK DISCHARGE FOR STREAMFLOW-GAGING STATION STREAMFLOW-GAGING FOR DISCHARGE PEAK CONTRIBUTING DRAINAGE AREA Prepared in cooperation with the TEXAS DEPARTMENT OF TRANSPORTATION REGIONAL EQUATIONS FOR ESTIMATION OF PEAK-STREAMFLOW FREQUENCY FOR NATURAL BASINS IN TEXAS By William H. Asquith and Raymond M. Slade, Jr. U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 96–4307 Prepared in cooperation with the TEXAS DEPARTMENT OF TRANSPORTATION Austin, Texas 1997 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Acting Director Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information write to: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services 8011 Cameron Rd. Box 25286 Austin, TX 78754–3898 Denver, CO 80225–0286 ii CONTENTS Abstract ................................................................................................................................................................................ 1 Introduction .......................................................................................................................................................................... 1 Purpose and Scope .................................................................................................................................................... 1 Background ............................................................................................................................................................... 2 Estimation of Peak-Streamflow Frequency .......................................................................................................................... 4 Historical Peak Streamflows ..................................................................................................................................... 4 Low-Outlier Thresholds ............................................................................................................................................ 6 Skew Coefficients ..................................................................................................................................................... 6 Selected Basin Characteristics .................................................................................................................................. 7 Regional Equations for Estimation of Peak-Streamflow Frequency .................................................................................... 7 Regression Analysis .................................................................................................................................................. 7 Regression Equations ................................................................................................................................................ 10 Discussion of Regression Equations and Application .............................................................................................. 11 Summary .............................................................................................................................................................................. 14 Selected References ............................................................................................................................................................. 15 PLATE (Plate in pocket) 1. Map showing locations of selected stations with natural basins in and near Texas FIGURES 1. Map showing hydrologic regions of Texas ........................................................................................................... 3 2–3. Graphs showing: 2. Example of peak-streamflow frequency analysis for station 08171000, Blanco River at Wimberley, Texas .......................................................................................................................................................... 5 3. Relation between 100-year peak discharge and contributing drainage area for streamflow-gaging stations in regions 5 and 7 of Texas ........................................................................................................... 9 4–14. Graphs showing relation between basin characteristics for: 4. Region 1 ..................................................................................................................................................... 16 5. Region 2 ..................................................................................................................................................... 17 6. Region 3 ..................................................................................................................................................... 18 7. Region 4 ..................................................................................................................................................... 19 8. Region 5 ..................................................................................................................................................... 20 9. Region 6 ..................................................................................................................................................... 21 10. Region 7 ..................................................................................................................................................... 22 11. Region 8 ..................................................................................................................................................... 23 12. Region 9 ..................................................................................................................................................... 24 13. Region 10 ................................................................................................................................................... 25 14. Region 11 ................................................................................................................................................... 26 15. Graphs showing comparison of weighted least-squares regression analyses for region 5 ................................... 27 TABLES 1. Peak-streamflow frequency estimates, analysis information, and basin characteristics for stations with at least 8 years of annual peak-streamflow data from natural basins within and near Texas ....................... 28 2. Regression equations for estimation of peak-streamflow frequency for hydrologic regions of Texas ................ 62 CONTENTS iii 3. Critical values of the t-distribution for selected confidence limits for regression equations ............................... 67 4. Covariance matrix for regression equations ......................................................................................................... 67 ABBREVIATIONS ft3/s, cubic foot per second ft/mi, foot per mile in., inch mi, mile mi2, square mile iv Regional Equations for Estimation of Peak-Streamflow Frequency for Natural Basins in Texas By William H. Asquith and Raymond M. Slade, Jr. Abstract flood characteristics. The use and limitations of the regression equations also are discussed. Addition- Peak-streamflow frequency estimates ally, procedures are presented to compute the 50-, are needed for flood-plain management; for 67-, and 90-percent confidence limits for any esti- objective assessment of flood risk; and for cost- mation from the equations. Also, supplemental effective design of dams, levees, other flood- peak-streamflow frequency and basin characteris- control structures, roads, bridges, and culverts. tics for 105 selected stations bordering Texas are Peak-streamflow frequency represents the peak included in the report. This supplemental informa- discharges for recurrence intervals of 2, 5, 10, 25, tion will aid in interpretation of flood characteris- 50, and 100 years. The U.S. Geological Survey, tics for sites near the state borders of Texas. in cooperation with the Texas Department of Trans- portation, conducted an investigation to develop INTRODUCTION regional regression equations for the estimation of peak-streamflow frequency for ungaged sites in For more than 100 years, the U.S. Geological Survey (USGS) has been monitoring streamflow and natural basins in Texas. Peak-streamflow publishing streamflow data, including annual peak dis- frequency data from streamflow-gaging stations in charges, for stations throughout the United States. In natural drainage basins of Texas were used. Texas, peak-streamflow data are available for about 946 Peak-streamflow frequency for 559 active and inactive stations on regulated or unregulated Texas stations with natural (unregulated and streams that drain rural or urban basins. In addition to rural or nonurbanized) basins was estimated with this data, peak-streamflow frequency estimates are annual peak-streamflow data through 1993. The needed by planners and managers in Texas for flood- plain management, for objective assessment of flood peak-streamflow frequency and drainage-basin risk, and for cost-effective design of dams, levees, other characteristics for the Texas stations were used to flood-control structures, roads, bridges, and culverts. In develop 16 sets of equations to estimate peak- 1991, the USGS,