Abstract
In many cases flood frequency analysis needs to be carried out on mean daily flow (MDF) series without any available information on the instantaneous peak flow (IPF). We analyze the error of using MDFs instead of IPFs for flood quantile estimation on a German dataset and assess spatial patterns and factors that influence the deviation of MDF floods from their IPF counterparts. The main dependence could be found for catchment area but also gauge elevation appeared to have some influence. Based on the findings we propose simple linear models to correct both MDF flood peaks of individual flood events and overall MDF flood statistics. Key predictor in the models is the event-based ratio of flood peak and flood volume obtained directly from the daily flow records. This correction approach requires a minimum of data input, is easily applied, valid for the entire study area and successfully estimates IPF peaks and flood statistics. The models perform particularly well in smaller catchments, where other IPF estimation methods fall short. Still, the limit of the approach is reached for catchment sizes below 100 km2, where the hydrograph information from the daily series is no longer capable of approximating instantaneous flood dynamics.
Highlights
Common flood frequency analysis (FFA) is based on samples of maximum flows
As in other studies before, it could be shown that the instantaneous peak flow (IPF)-mean daily flow (MDF) relationship depends primarily on catchment size
This study has shown that hydrograph characteristics, like the peak-volume ratio of flood events can be used to estimate instantaneous peak flows when only average daily series are available
Summary
Common flood frequency analysis (FFA) is based on samples of maximum flows. The dimensions and variability of these maxima pose the baseline for the choice of probability distribution, the estimation of its parameters and eventually the 20 deduction of flood quantiles as design criteria. Embracing the true dimension of a peak requires continuous measurement of the flow on a high temporal resolution. Such data is rarely available and oftentimes FFA needs to be carried out on average daily flow records instead. The daily averaging naturally flattens the flood peak and the true maximum becomes unknowable. The degree of this smoothing, i.e. the 25 difference between the true instantaneous peak flow (IPF) and the maximum mean daily flow (MDF) depends on the response time of a system, which is controlled by a multitude of factors.
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