Revised techniques for estimating magnitude and frequency of floods in Montana

Abstract

Relations for estimating the flood magnitudes for ungaged sites in Montana have been updated. The State was divided into eight regions and separate multiple-regression equations for each region were developed that relate logarithms of annual flood magnitude to logarithms of basin characteristics for exceedance probabilities of 50, 20, 10, 4, 2, and 1 percent. The standard errors of estimate for an exceedance probability of 1 percent ranged from 39 to 58 percent in the western and central parts of the State and from 47 to 83 percent in the eastern part. The standard errors of estimate indicate a substantial improvement over previous studies. Techniques for transferring annual flood-frequency information at gaged sites to ungaged sites on the same stream have been updated. Included are curves relating flood-frequency information to drainage area for eight major streams in the State. Maximum known flood peaks in Montana are compared with estimated 1-percent-chance flood peaks and with national maximum known flood peaks. Values of flood discharges for selected exceedance probabilities and values of significant basin characteristics for all gaging stations used in the analysis are tabulated. Included are data for 339 stations in Montana and 34 nearby stations in Canada and adjoining States. The purpose of this report is to present updated techniques for estimating flood magnitude for exceedance probabilities of 50, 20, 10, 4, 2, and 1 percent for unregulated streams in Montana. The relations presented herein provide more reliable predictions than those in previous studies because of more extensive streamflow-gaging records and improved analytical procedures. The report is based on gaging data from unregulated streams having at least 10 years of streamflow record. Included in the analysis are 339 streamflow-gaging sites in Montana, 8 in Canada, 14 in North Dakota, 4 in South Dakota, and 8 in Wyoming. Locations and station numbers of all gages used in the analysis are shown in figure 1. Some streamflow-gaging sites having more than 10 years record were excluded from the analysis because the data were considered to be unreliable or unrepresentative of the region.

Methods for estimating peak-flow frequencies at ungaged sites in Montana based on data through water year 2011: Chapter F in <i>Montana StreamStats</i>

“…In previous regional regression studies for Montana, basin characteristics were manually estimated using paper topographic maps and overlaying transparent gridded cells on the maps. In previous studies, the number of candidate basin characteristics has ranged from 2 (Berwick, 1958) to 12 (Parrett and Omang, 1981). For this study, 28 basin characteristics were selected as candidate variables in the regression analyses and are presented in table 2.…”

“…Basin characteristics investigated as potential explanatory variables in the regional regression analyses were selected based on previous studies (Berwick, 1958;Parrett and Omang, 1981;Omang and others, 1986;Omang, 1992;and Parrett and Johnson, 2004), theoretical relations with peak flows, and the ability to generate the characteristics using GIS analysis and digital datasets. In previous regional regression studies for Montana, basin characteristics were manually estimated using paper topographic maps and overlaying transparent gridded cells on the maps.…”

“…Previous reports of methods for estimating peak-flow frequencies at ungaged sites in Montana include Berwick (1958), Parrett and Omang (1981), Omang and others (1986), Omang (1992), and Parrett and Johnson (2004). The most recent report (Parrett and Johnson, 2004) was based on data through water year 1998.…”

Estimating current and future streamflow characteristics at ungaged sites, central and eastern Montana, with application to evaluating effects of climate change on fish populations

“…Drainage basin characteristics used as predictor variables were selected based on hydrologic importance determined from previous research (Parrett and Omang, 1981;Omang, 1992;Parrett and Johnson, 2004;Sando and others, 2016), as well as the availability of data. Drainage basin boundaries for PRMS nodes and fish sampling sites were delineated in ArcMap (Esri, 2014) using the 30-meter National Elevation Dataset (Gesch and others, 2002).…”

Potential effects of surface coal mining on the hydrology of the Snider Creek area, Rosebud and Ashland coal fields, southeastern Montana

“…The method was developed through regression analysis based on the correlation of streamflow with the dimensions of the channel (Hedman and Kastner, 1977). Estimates of flood peaks were made from an empirical equation, which uses the basin characteristics of drainage area and percentage of forest cover within the basin, combined with a geographical factor that is dependent on the location of the basin within the State (Parrett and Omang, 1981). Based on indirect methods, mean annual flow of Snider Creek is about 170 acre-feet per year; magnitudes of flood peaks are 790 ft-Vs for the 25-year flood, and 170 ft^/s for the 5-year flood.…”