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Last modified
1/25/2010 7:12:56 PM
Creation date
10/5/2006 3:14:45 AM
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Floodplain Documents
County
Statewide
Basin
Statewide
Title
Nationwide Summary of US Geological Survey Regional Regression Equations for Estimating Magnitude and Frequency of Floods or Ungaged Ssites 1993
Date
1/1/1993
Prepared By
USGS
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />RURAL FLOOD-FREQUENCY ESTIMATING <br />TECHNIQUES <br /> <br />By W.O, Thomas, Jr. <br /> <br />INTRODUCTION <br /> <br />t <br /> <br />The National Flood Frequency (NFF) Program <br />provides equations for estimating the magnitude and <br />frequency of flood characteristics for rural, unregulated <br />watersheds in the 50 States and the Commonwealth of <br />Puerto Rico. The most current regression equations for <br />each State are included in NFF. These equations are <br />taken from reports that were published between 1973 <br />and September 1993, The purpose of this section is to <br />provide a brief overview of the rural regression equa. <br />tions that are presented in NFF. The regression equa- <br />tions for each State are documented later in the report <br />in the State summary section. <br /> <br />Watershed and Climatic Characteristics <br /> <br />The rural equations in NFF are based on water- <br />shed and climatic characteristics that can be obtained <br />from topographic maps or rainfall reports and atlases. <br />The USGS has published regression equations in many <br />States based on channel.geometry characteristics, such <br />as channel width, but these equations are not provided <br />in NFF because a site visit is required to obtain the <br />explanatory variables. The most frequently used water. <br />shed and climatic characteristics are drainage area, <br />main.channel slope, and mean annual precipitation. <br />The regression equations are generally reported in the <br />following form: <br /> <br />RQ-y=aAbScpd <br /> <br />where <br /> <br />RQ-y is the T-year rural flood.peak discharge, <br />A is the drainage area, <br />S is the channel slope, <br />P is the mean annual precipitation, and <br />a,b,c.d are regression coefficients. <br /> <br />The regression coefficients are normally com- <br />puted by taking the logarithms of the above variables <br />and using linear multiple regression techniques. In <br />instances where a variable could equal zero (such as <br />percentage of drainage area covered by lakes and <br />ponds), a constant is added to the variable prior to tak- <br />ing the logarithms. The frequency of use of the various <br />watershed!climatic characteristics in the rural regres. <br />sion equations given in NFF is summarized below. The <br />table below does not summarize the use of watershed! <br />climatic characteristics for regional studies, such as the <br />one by Thomas and others (1993). <br /> <br />Wotershed or cllmotlc choroctsrlst!c <br /> <br />Drainage area (square miles) <br />Main-channel slope (feet per mile) <br />Mean annual precipitation (inches) <br />Storage/area of lakes and ponds (percent) <br />Rainfall amount for a given duration (inches) <br />Elevation of watershed (feet) <br />Forest cover (percent) <br />Channel length (miles) <br />Minimum mean January temperature <br />(degrees F) <br />Basin shape ((length)2 per drainage area) <br />Soils characteristics <br />Mean basin slope (feet per foot or feet per <br />mile) <br />Mean annual snowfall <br />Area of stratified drift (percent) <br />High elevation index (percent basin above <br />6000 feet) <br />Relative relief (feet per mile) <br />Drainage frequency (number of first order <br />streams per square mile) <br /> <br />Number of SlStss <br /> <br />(Including <br />Puorlo Rico) <br />51 <br />27 <br />19 <br />16 <br />14 <br />13 <br />8 <br />6 <br />4 <br /> <br />4 <br />3 <br />2 <br /> <br />2 <br />I <br />I <br /> <br />There were 6 States in which drainage area was the <br />only explanatory variable in the regression equations. <br />In many States, 3 to 4 explanatory variables were used <br />in the equations, <br /> <br />6 Notlonwlde Summsry of U.S. Gsologlcol Survey Reglonol Regr..slon Equations for Estlmetlng Magnitude end Fraquency of <br />Flood, for Ungaged Slles, 1993 <br />
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