This manual provides methods for estimating the magnitude and frequency of floods on unregulated and unurbanized streams in Indiana that drain at least 15 square miles (38.8 square kilometres). The methods provide the design engineer with a means of estimating flood frequencies without having to analyze the records at individual streamflow sites.
The estimating equations in this manual are based on relations between floods of specific return periods and selected watershed characteristics. The most significant factors for estimating flood peaks in Indiana were found to be drainage area and precipitation index. The shape of a watershed was also found very significant in development of the regional equations. Other variables used in the regional equations are physical characteristics that further explain differences in the magnitudes of floods from the watersheds.
The regional equations are multivariate regression equations that relate peak discharges of 2-, 5-, 10-, 25-, 50-, and 100-year recurrence intervals to watershed characteristics and are essentially for natural streams. In this study, if 25 percent or more of the drainage area of a stream is above a reservoir, it was considered to be regulated, and flood peaks from it were not included in the analysis unless it could be determined that flood peaks were not materially affected, as in the case of several streams below small water-supply reservoirs. The equations also do not apply to streams that are affected by a high degree of urbanization.
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USGS Numbered Series
Floods in Indiana: technical manual for estimating their magnitude and frequency