Methods for determining magnitude and frequency of floods in California, based on data through water year 2006

Abstract

Methods for estimating the magnitude and frequency of floods in California that are not substantially affected by regulation or diversions have been updated. Annual peak-flow data through water year 2006 were analyzed for 771 streamflow-gaging stations (streamgages) in California having 10 or more years of data. Flood-frequency estimates were computed for the streamgages by using the expected moments algorithm to fit a Pearson Type III distribution to logarithms of annual peak flows for each streamgage. Lowoutlier and historic information were incorporated into the flood-frequency analysis, and a generalized Grubbs-Beck test was used to detect multiple potentially influential low outliers. Special methods for fitting the distribution were developed for streamgages in the desert region in southeastern California. Additionally, basin characteristics for the streamgages were computed by using a geographical information system. Regional regression analysis, using generalized least squares regression, was used to develop a set of equations for estimating flows with 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities for ungaged basins in California that are outside of the southeastern desert region. Flood-frequency estimates and basin characteristics for 630 streamgages were combined to form the final database used in the regional regression analysis. Five hydrologic regions were developed for the area of California outside of the desert region. The final regional regression equations are functions of drainage area and mean annual precipitation for four of the five regions. In one region, the Sierra Nevada region, the final equations are functions of drainage area, mean basin elevation, and mean annual precipitation. Average

Mixed populations and annual flood frequency estimates in the western United States: The role of atmospheric rivers

“…The annual peaks in the Sierra Nevada region are generated from a mixture of winter rainfall and/or rain‐on‐snow (AR‐generated) as well as spring snowmelt (non‐AR generated) [ Gotvald et al ., ]. As shown in Figure a, the fractional number of ARs is strongly linked to the steep elevation change from the central valley of northern California to the crest and eastern side of the Sierra Nevada mountain range.…”

“…The peak flow records at lower‐elevation sites have a higher percentage of winter rainfall AR‐generated peaks, while the higher‐elevation sites have fewer AR‐generated peaks due to the more typical springtime snowmelt runoff. This elevation pattern and mixed population was found when developing the new regional skew values for flood frequency analysis in California [ Parrett et al ., ] and when developing regional regression equations for ungaged basins in the Sierra Nevada hydrologic region [ Gotvald et al ., ].…”

“…Mixed populations were recently documented at several high‐elevation U.S. Geological Survey (USGS) streamgaging stations in the Sierra Nevada leading to complications when determining at‐site flood frequency estimates used to develop regional skew and regional regression equations for ungaged basins in California [ Parrett et al ., ; Gotvald et al ., ]. A combination of several large winter‐storm rain peaks and the more typical, smaller magnitude springtime snowmelt peaks were common among these sites [ Gotvald et al ., ]. In western Oregon, rainfall combined with rapidly melting snow produced the largest peaks on record during the flood events of December 1964 and February 1996 [ Copper , ].…”

Mitigating Impact of Devils Lake Flooding on the Sheyenne River Sulfate Concentration

“…Knowledge of the frequency and magnitude of flood peak is important for assessing the flood risk. Flood frequency analysis is an approach to calculate the magnitude of flood‐peak discharge and the associated exceedance probability (Gotvald et al ). The log‐Pearson Type III method, as recommended by Bulletin 17B of the Interagency Advisory Committee on Water Data (1982), is commonly used for flood frequency analysis by USGS (Gotvald et al ).…”

“…Flood frequency analysis is an approach to calculate the magnitude of flood‐peak discharge and the associated exceedance probability (Gotvald et al ). The log‐Pearson Type III method, as recommended by Bulletin 17B of the Interagency Advisory Committee on Water Data (1982), is commonly used for flood frequency analysis by USGS (Gotvald et al ). PeakFQ software (Veilleux et al ) was used to calculate the magnitude of flood in the Sheyenne River for five recurrence intervals (2, 5, 10, 25, and 50 years) by applying the log‐Pearson Type III distribution to the dataset of annual maximum instantaneous discharges measured from 1950 to 2004 at gauging Stations 3 and 4 (Table ).…”