Methods for Estimating Flood Frequency at Ungaged Sites in Montana
Three sets of estimation equations, based on (1) basin and climatic characteristics, (2) active-channel width, and (3) bankfull width were developed for eight flood regions. The eight flood regions are shown on the map below.
Estimation of flood discharges using the equations based on basin and climatic characteristics requires the user to measure drainage area and as many as two other characteristics from regional maps. Methods for making those measurements are described in the report. Estimation of flood discharges using the equations based on either active-channel width or bankfull width requires a site visit and measurement of the appropriate width at several locations. Methods for selecting a measurement site and making the width measurement also are described in the report. Limitations of the various equations also are described in the report, and users of the report and this web page are cautioned that they are responsible for all interpretations and assessments of calculated flood-frequency data.
The report also describes methods for weighting estimates from any two of or all three sets of equations to provide overall, weighted estimates that are at least as accurate as those from any of the single sets of equations. Methods for determining the standard error of prediction and confidence limits for any estimate also are described.