Flood Frequency Hydrology with Limited Data for the Weser River Basin, Germany

This study investigates challenges of and solutions for estimating regional flood discharges from a limited number of stream gauge records with varying lengths. Routine application of standard analytical flood frequency analysis methods using available hydrologic data can lead to biased flood-hazard assessments. This paper shows that more analysis is needed for common flood-frequency methods (e.g., USGS regional regressions from at-site fitting, or regional frequency analysis using L-Moments) applied to the Weser River Basin ($\sim \mathrm{46,000}{\mathrm{km}}^2$) in northern Germany. Daily average discharges and the 10 largest instantaneous peak discharges were obtained for 19 stream gauges located along rivers draining areas more than $\mathrm{2,000}{\mathrm{km}}^2$. These discharge records are analyzed to identify temporal sampling biases and redundant gauges. The discharge data are processed further to (1) estimate instantaneous peaks from daily average discharges (the adjustments from daily to instantaneous discharges reach as high as 1.39 for gauges with smaller drainage areas); (2) eliminate unrepresentative data for cases in which reservoirs may have changed flow conditions, because mean annual maximum discharges were 21% lower after reservoir construction for the longest gauge record; and (3) pool the data to increase sample size (to 840 years of pooled data compared with 52–94 years for each of the gauges) for extreme value distribution fitting. The processed data are used for regional frequency analysis with L-moments to estimate flood discharges (i.e., 100-year and 500-year) for all river reaches draining more than $\mathrm{2,000}{\mathrm{km}}^2$ throughout the basin. These processing steps result in discharge estimates with differences ranging from $-31\%$ to 71% for 100-year discharges and from $-41\%$ to 107% for 500-year discharges compared with at-site estimates.