Methods for peak-flow frequency analysis and reporting for streamgages in or near Montana based on data through water year 2015

Abstract

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Regional regression equations based on channel-width characteristics to estimate peak-flow frequencies at ungaged sites in Montana using peak-flow frequency data through water year 2011

“…The WLS approach assigns higher weights to sites with longer periods of record. Because many of the sites with longer periods of record in the Northwest hydrologic region experienced large floods in 1908, 1964, and 1975(Sando and McCarthy, 2018, the variability for the less frequent events is lower, leading to lower SEP, especially for the lower annual exceedance probabilities.…”

Flood-frequency estimation for very low annual exceedance probabilities using historical, paleoflood, and regional information with consideration of nonstationarity

“…Much greater detail about perception thresholds is provided in England and others (2019), including appendix 9, which has examples with data from streamgages and paleoflood estimates. As pointed out by Sando and McCarthy (2018), most peak-flow data have not been collected within a perception threshold framework, and specific protocols for defining and applying perception thresholds typically were not in place at the time the peak flows were recorded and electronically stored in a database. As a result, determining the appropriate thresholds for ungaged periods associated with some historical peaks can require considerable judgment (Parrett and others, 2011).…”

“…When dealing with large historical peaks, one often sets the lower perception threshold for an ungaged period to the magnitude of the lowest historical peak within that period. Some peak flows, with a code of 7, indicate historical peaks might not satisfy the requirement for reasonable confidence of nonexceedance during an ungaged period; such peaks might be considered "opportunistic" peaks (collected for some opportunistic reason other than the fact that they were particularly large; Sando and McCarthy, 2018). If the peak is an opportunistic peak, it is not clear that its magnitude should be the lower perception threshold; therefore, the peaks should be excluded from the frequency analysis.…”

“…The peaks not designated as historical, but outside a systematic period of record, did not fit the distribution of the other peaks. Therefore, they were treated as opportunistic peaks (Sando and McCarthy, 2018) and removed. Sando (1998) found that the generalized skew coefficient from the map in Bulletin 17B (U.S. Water Resources Council, 100,000 1976) was adequate for South Dakota streamgages; therefore, the regional skew used was 0.194 with a mean square error of 0.302.…”

“…That is, it was not clear that there was a particular threshold above which all peaks would be recorded during the historical period. The historical peaks are below the perception threshold for large peaks based on the paleoflood data and provide no additional information to the analysis; therefore, the historical peaks were treated as opportunistic peaks (Sando and McCarthy, 2018) and removed from the analysis. Sando (1998) found that the generalized skew coefficient from the map in Bulletin 17B (Interagency Advisory Committee on Water Data, 1982) was adequate for South Dakota streamgages; therefore, the regional skew used was 0.194 with a mean square error of 0.302.…”