Estimating flood magnitude and frequency for unregulated streams in Wisconsin

Abstract

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Peak streamflow trends in Wisconsin and their relation to changes in climate, water years 1921–2020

“…For example, peak streamflows at Sugar River near Brodhead indicate a U-shaped trend (figure 11A). The estimated flood magnitude for the 1-percent annual exceedance probability for this streamgage for the period of record (from water year 1914 through water year 2020) is 15,970 ft 3 /s, with a 95 percent confidence interval of 12,680 to 22,500 (Levin and Sanocki, 2023). However, if only 20 years of data were available at this stream, the resulting estimated flood magnitude for the 1-percent annual exceedance probability could be as low as 8,700 ft 3 /s (confidence interval, 6,150 to 19,640 ft 3 /s) using data from 1975 through 1995) or as high as 24,400 ft 3 /s (confidence interval 15,680 to 58,020 ft 3 /s) using data from 1914 through 1934, a nearly threefold difference.…”

“…Additional reports in 1992, 2003(Krug and others, 1992Walker and Krug, 2003;Walker and others, 2017) updated flood-frequency estimates using statistical methods described in Bulletin 17B (Interagency Advisory Committee on Water Data, 1982), which outlined revised procedures for estimating skew, dealing with outliers, and computing confidence intervals. Most recently, new flood-frequency regions and equations have been developed (Levin and Sanocki, 2023) using the expected moments algorithm methodology as outlined in Bulletin 17C (England and others, 2018).…”

Peak streamflow trends in Michigan and their relation to changes in climate, water years 1921–2020

“…The presence of a trend in peak streamflow time series is particularly problematic at streamgages with short periods of record because their peak streamflows may not be representative of the full range of peak-streamflow magnitudes and may result in a biased estimate of the flood magnitude. For example, Levin and Sanocki (2023) showed that flood discharge estimate for the 1-percent annual exceedance probability at a streamgage in Wisconsin varied by a factor of three when using a subset 20-year data record on a stream location with a trend in peak streamflow. Regression equations currently used to estimate flood magnitudes at ungaged streams in Michigan were last published in 1984 (Holtschlag and Croskey, 1984).…”

Techniques for estimating the magnitude and frequency of peak flows on small streams in Minnesota, excluding the Rainy River Basin, based on data through water year 2019

“…Data from streamgages for which peak flows were affected by controlled storage or regulated releases also were excluded from this analysis. All data are available in Levin (2023) and the USGS National Water Information System database (USGS, 2019).…”

“…1. First, the estimate of the Q 1% flood for this site is obtained using the GLS regression equation in Levin (2023,…”