Flood Frequency Studies in Alaska

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Peak-flow special conditions 

Peak flows from selected peak-generating mechanisms can have a characteristic magnitude and frequency different from other peak flows. A table of selected special conditions affecting the magnitude of peak flows at USGS streamgages in Alaska is available in a data release (Curran, 2023). 

Basin characteristics

Basin characteristics are physical and climatic features used as independent variables for estimating streamflow. Basin characteristics for an ungaged site that are substituted in the estimating equations should be determined by the same methods as those used to develop the equations. USGS SIR 2016-5024 uses two basin characteristics for estimating flood frequency:

1. Drainage area (DRNAREA) is obtained from a drainage area boundary for the site in question. Drainage area delineations are based on the Watershed Boundary Dataset.

2. Basin average mean annual precipitation for 1971–2000 from the PRISM climate dataset (PRECPRIS00) is obtained for areas in Alaska from Gibson (2009) (tip: no password is required when accessed from Internet Explorer) and for areas in Canada from PRISM Climate Group (2002).

Basin characteristics have been computed for selected streamgages and can be downloaded as a table from USGS SIR 2016-5024 Appendix A , also available as an ESRI geodatabase.

Media

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Regional skew areas

Flood frequency estimates for a streamgage are improved by using a weighted average of the station skew and a regional skew. USGS SIR 2016-5024 presents regional skew and standard error of the regional skew for two new regional skew areas (see figure 5, table 5, and table 6). Areas outside the regional skew areas are considered skew exclusion areas where station skew is used.

Statistics/equations

Flood frequency statistics for streamgages in Alaska and conterminous basins in Canada with at least 10 years of record through water year 2012 are shown in table form in USGS SIR 2016-5024 table 4 and can be obtained from a map view in StreamStats.

Tools for estimating flood frequency statistics are available in spreadsheet form in the USGS SIR 2016-5024 Applications Tool and in the USGS National Streamflow Statistics (NSS) program. For sites within Cook Inlet Basin, StreamStats provides tool for delineating a drainage area, computing basin characteristics, and computing the flood frequency estimation equations.