U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 97-4279
The efficient design and management of many bridges, culverts, embankments, and flood-protection structures may require the estimation of time-of-inundation and (or) storage of floodwater relating to such structures. These estimates can be made on the basis of information derived from the peak-flow hydrograph. Average peak-flow hydrographs corresponding to a peak discharge of specific recurrence interval can be simulated for drainage basins having drainage areas less than 500 square miles in Maryland, using a direct technique of known accuracy. The technique uses dimensionless hydrographs in conjunction with estimates of basin lagtime and instantaneous peak flow.
Ordinary least-squares regression analysis was used to develop an equation for estimating basin lagtime in Maryland. Drainage area, main channel slope, forest cover, and impervious area were determined to be the significant explanatory variables necessary to estimate average basin lagtime at the 95-percent confidence interval. Qualitative variables included in the equation adequately correct for geographic bias across the State. The average standard error of prediction associated with the equation is approximated as plus or minus (+/-) 37.6 percent. Volume correction factors may be applied to the basin lagtime on the basis of a comparison between actual and estimated hydrograph volumes prior to hydrograph simulation.
Three dimensionless hydrographs were developed and tested using data collected during 278 significant rainfall-runoff events at 81 stream-gaging stations distributed throughout Maryland and Delaware. The data represent a range of drainage area sizes and basin conditions. The technique was verified by applying it to the simulation of 20 peak-flow events and comparing actual and simulated hydrograph widths at 50 and 75 percent of the observed peak-flow levels. The events chosen are considered extreme in that the average recurrence interval of the selected peak flows is 130 years. The average standard errors of prediction were +/- 61 and +/- 56 percent at the 50 and 75 percent of peak-flow hydrograph widths, respectively.
Glossary
Abstract
Introduction
Purpose and Scope
Description of Study Area
Previous investigation
Acknowledgments
Description of data base
Methods of study
Streamgaging station selection
Hyetograph development
Predictor variable identification
Analytical methods
Determination of lagtime
Basin lagtime
Regional analysis
Testing of lagtime equation
Verification by prediction error sum of squares (PRESS)
Variable bias by residual plots
Sensitivity to variable errors
Development of dimensionless hydrographs
Regionalization of dimensionless hydrographs
Testing of dimensionless hydrographs
Shape verification
Geographical and width bias
Sensitivity to peak flow and lagtime
Hydrograph-width relations
Adjustment for correct runoff volume
Verification of the simulation technique
Limitations of technique
Hydrograph simulation technique
Simulating a peak-flow hydrograph
Estimating peak flow
Estimating lagtime
Expanding the dimensionless hydrograph
Summary and conclusions
Selected references
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For more information about USGS activities in Maryland, Delaware and the District of Columbia contact:
Director
MD-DE-DC Water Science Center
U.S. Geological Survey
8987 Yellow Brick Road
Baltimore, MD 21237
Telephone: (410) 238-4200
Fax: (410) 238-4210
or access the USGS Water Resources of Maryland, Delaware, and District of Columbia home page at: http://md.water.usgs.gov/.
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