Scientific Investigations Report 2008-5206
This report provides estimates of flood discharges at selected recurrence intervals for streamgages in and adjacent to New Hampshire and equations for estimating flood discharges at recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-, and 500-years for ungaged, unregulated, rural streams in New Hampshire. The equations were developed using generalized least-squares regression. Flood-frequency and drainage-basin characteristics from 117 streamgages were used in developing the equations. The drainage-basin characteristics used as explanatory variables in the regression equations include drainage area, mean April precipitation, percentage of wetland area, and main channel slope. The average standard error of prediction for estimating the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year recurrence interval flood discharges with these equations are 30.0, 30.8, 32.0, 34.2, 36.0, 38.1, and 43.4 percent, respectively.
Flood discharges at selected recurrence intervals for selected streamgages were computed following the guidelines in Bulletin 17B of the U.S. Interagency Advisory Committee on Water Data. To determine the flood-discharge exceedence probabilities at streamgages in New Hampshire, a new generalized skew coefficient map covering the State was developed. The standard error of the data on new map is 0.298. To improve estimates of flood discharges at selected recurrence intervals for 20 streamgages with short-term records (10 to 15 years), record extension using the two-station comparison technique was applied. The two-station comparison method uses data from a streamgage with long-term record to adjust the frequency characteristics at a streamgage with a short-term record.
A technique for adjusting a flood-discharge frequency curve computed from a streamgage record with
results from the regression equations is described in this report. Also, a technique is described for estimating flood discharge at a selected recurrence interval for an ungaged site upstream or downstream from a streamgage using a drainage-area adjustment.
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Olson, S.A., 2009, Estimation of flood discharges at selected recurrence intervals for streams in New Hampshire: U.S. Geological Survey Scientific Investigations Report 2008–5206, 57 p.
Purpose and Scope
Description of Study Area
Flood Discharges at Selected Recurrence Intervals for Streamgages
Peak Discharge Data Used in this Study
Determination of the Magnitude and Frequency of Flood Discharges for Streamgages
Combined Records of Nearby Streamgages
Magnitude and Frequency of Flood Discharges for Streamgages
Maximum Recorded Floods and Envelope Curves
Characteristics of Streamgage Drainage Basins
Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals for Ungaged Stream Sites
Regression Analysis and Final Regression Equations
Limitations and Sensitivity
Accuracy of the Regression Equations
Accuracy Analysis of Individual Estimates from the Regression Equations
Standard Error of Prediction
Equivalent Years of Record
Use of Regression Equations at or Near Streamgages
Drainage-Area-Only Regression Equations
New Hampshire StreamStats
Appendix 1. Mean April Precipitation
Appendix 2. Wetland Areas
Appendix 3. Example Application