Florida Integrated Science Center

Estimating the Magnitude and Frequency of Floods for Streams in West-Central Florida, 2001

Scientific Investigations Report 2005-5080
By K.M. Hammett and M.J. DelCharco

U.S. Department of the Interior
U.S. Geological Survey


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Front cover of SIR 2005-5080

Scientific Investigations Report 2005-5080

Appendix A: Basin characteristics and flood-discharge estimates for gaging stations

Appendix B: Annual peak discharge data for gaging stations,
list by station


Figure 1 with links to individual stations


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Abstract

Flood discharges were estimated for recurrence intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years for 94 streamflow stations in west-central Florida. Most of the stations are located within the 10,000 square-mile, 16-county area that forms the Southwest Florida Water Management District. All stations had at least 10 years of homogeneous record, and none have flood discharges that are significantly affected by regulation or urbanization.

Guidelines established by the U.S. Water Resources Council in Bulletin 17B were used to estimate flood discharges from gaging station records. Multiple linear regression analysis was then used to mathematically relate estimates of flood discharge for selected recurrence intervals to explanatory basin characteristics. Contributing drainage area, channel slope, and the percent of total drainage area covered by lakes (percent lake area) were the basin characteristics that provided the best regression estimates. The study area was subdivided into four geographic regions to further refine the regression equations.

Region 1 at the northern end of the study area includes large rivers that are characteristic of the rolling karst terrain of northern Florida. Only a small part of Region 1 lies within the boundaries of the Southwest Florida Water Management District. Contributing drainage area and percent lake area were the most statistically significant basin characteristics in Region 1; the prediction error of the regression equations varied with the recurrence interval and ranged from 57 to 69 percent.

In the three other regions of the study area, contributing drainage area, channel slope, and percent lake area were the most statistically significant basin characteristics, and are the three characteristics that can be used to best estimate the magnitude and frequency of floods on most streams within the Southwest Florida Water Management District. The Withlacoochee River Basin dominates Region 2; the prediction error of the regression models in the region ranged from 65 to 68 percent. The basins that drain into the northern part of Tampa Bay and the upper reaches of the Peace River Basin are in Region 3, which had prediction errors ranging from 54 to 74 percent. Region 4, at the southern end of the study area, had prediction errors that ranged from 40 to 56 percent.

Estimates of flood discharge become more accurate as longer periods of record are used for analyses; results of this study should be used in lieu of results from earlier U.S. Geological Survey studies of flood magnitude and frequency in west-central Florida. A comparison of current results with earlier studies indicates that use of a longer period of record with additional high-water events produces substantially higher flood-discharge estimates for many gaging stations. Another comparison indicates that the use of a computed, generalized skew in a previous study in 1979 tended to overestimate flood discharges.

CONTENTS

Abstract
Introduction
    Purpose and Scope
    Previous Studies
    Description of the Study Area
Data Used for Analyses
    Annual Peak Discharges
    Basin Characteristics
Estimation of Flood Magnitude and Frequency at Gaged Sites
    Log-Pearson Type III Frequency Analysis
      Generalized Skew Coefficient and Weighted Skew
      Sensitivity to Long-Term Trends in Data
      Sensitivity to Historic Events Outside the Systematic Record
    Regression Analysis
      Development of Multiple Regression Equations
      Regionalization of Regression Equations
      Generalized Least Squares Regression Analysis
      Sensitivity of Regression Equations
    Weighting of Log-Pearson Type III and Regression Analyses
    Comparison with Previously Published Analyses
Estimation of Flood Magnitude and Frequency at Ungaged Sites
    Sites with a Limited Period of Record or Without Discharge Data
    Sites Upstream or Downstream from a Gaging Staion
Summary
Selected References
Appendix A. Basin characteristics and flood-discharge estimates for gaging stations, west-central Florida
Appendix B. Annual peak discharge data for gaging stations, west-central Florida


FIGURE

1. Map showing the study area and location of gaging stations, west-central Florida.


TABLES

1. Statistical summary of basin characteristics for gaging stations, west-central Florida.
2. Significant trends in annual peak discharge data, west-central Florida.
3. Comparison of Bulletin 17B flood discharges computed from the continuous systematic record and with inclusion of the 1912 historic flood at the Peace River at Arcadia, Florida.
4. Regression equations for geographic regions, west-central Florida.
5. Ranges of basin characteristics for geographic regions, west-central Florida.
6. Sensitivity of the 25-year flood discharges to errors in basin characteristics.
7. Comparison of Bulletin 17B flood-discharge estimates from this study with Bulletin 17 or Bulletin 17A estimates from previous studies.
8. Comparison of regression flood-discharge estimates from this study with regression estimates from previous studies.
9. Comparison of weighted flood-discharge estimates from this study with weighted estimates from previous studies.


U.S. Department of the Interior, U.S. Geological Survey
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