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SUMMARY OF STATE FLOOD-FREQUENCY TECHNIQUES By H.C. Riggs and W.O. Thomas, Jr. The remainder of this report provides a summary of the applicable rural and urban flood-frequency reports on which the equations in National Flood Frequency (NFF) Program are based. For each State and metropolitan area, there is a summary of the data used in developing the regression equations and a ref- erence to the applicable report. A description of the explanatory variables and a range of standard errors are provided for the regression equations for each State or metropolitan area. All figures and maps that could be easily digitized are also included in this report. In some cases, the user will need to consult the original report to obtain some of the input variables for the regression equations. A few notes about designation of watershed characteristics are appropriate. In the State summaries that follow, different analysts used different symbols for the same variable, We have not tried to standardize notation here but have retained the notation used in the original reports. The most prominent example of this use of different notation (and terminology) is that for main-channel slope, also referred to as channel slope, or streambed slope, which is identified by a variety of symbols such as S, SL, Sc, Sb, and Sm, Unless other- wise noted, all these slopes are the slope between two points on the main channel, 85- and 10-percent of the channel length upstream from the gage or outlet of the watershed. Percentages, such as the percentage of the water- shed in forests or lakes and ponds, are generally deter- mined by a grid-sampling method using 20-80 points in the watershed. A transparent grid is overlain the outline of the watershed on the most appropriate topographic map. The grid should have from 20-80 nodes within the respective watershed boundary, the number of nodes overlying green (forest) or blue (lakes and ponds) is determined, and the percentage of forest or lakes and ponds is computed as the number of node intersections (with green or blue) divided by the total nodes within the watershed. Mean basin elevation is also generally determined by the same grid-sampling method averag- ing elevations for 20-80 points in the watershed. Sev- eral maps of variables such as mean annual precipitation, the 2-year 24-hour rainfalI, average annual snowfall and minimum mean January tempera- ture are needed and provided for some States. The maps provided in this report are, in most cases, of smalIer scale than the maps provided in the statewide flood-frequency report. In some instances, the user may want to refer to the more detailed maps provided in the statewide reports for a more accurate determination of the explanatory variables, The regression equations are provided in the same fonnat as in the original reports. In the applica- tion of these equations, it is often necessary to add a constant to an independent variable that might equal zero. These constants are not always shown in the equations. The user should enter the actual value of the variable and the necessary constants will be applied in the computer program. Precipitation frequencies, such as the 2-year 24- hour precipitation or rainfall, are used as explanatory variables in many of the statewide regression equa- tions. In some of the statewide reports, this variable is defined as the 24-hour 2-year precipitation or rainfall rather than the 2-year 24-hour value. For standardiza- tion in this report, we use the terminology 2-year 24- hour precipitation even if the original statewide report reversed the order of the adjectives. A brief description of each variable used in the regression equations is provided in the individual state- wide summaries, It is assumed that the user is knowl- edgeable with regard to determination of many of the routine watershed characteristics, such as drainage area and channel length, from topographic maps. The appli- cable range of all variables is given in the NFF program so the user will know if estimates are being made out- side the range of data used in developing the regression equations. The user is cautioned NOT to extrapolate the flood estimates beyond the data used to develop the equations. SUMMARY OF STATE FLOOD-FREQUENCY TECHNIQUES 23