Abstract
Flood frequency analysis (FFA) concentrates on peak flows of flood hydrographs. However, floods that last years devastated large parts of Poland lead us to revision of the views on the assessment of flood risk in Poland. It turned out that it is the prolonged exposure to high water on levees that causes floods, not only the water overflowing the levee crest. This is because, the levees are weakened by water and their disruption occurs when it seems that the danger is over, i.e. after passing culmination. Two main causes of inundation of embanked rivers, namely over-crest flow and wash out of the levees, are combined to assess the total risk of inundation. Therefore the risk of inundation is the total of risk of exceeding embankment crest by flood peak and risk of washout of levees. Hence, while modeling the flood events in addition to the maximum flow one should consider also the duration of high water in a river channel, Analysis of the frequency of annual peak flows based on annual maxima and peaks over threshold is the subject of countless publications. Therefore we will here mainly modeling the duration of high water levels. In the paper the two-component model of flood hydrograph shape i.e. “duration of flooding-discharge- probability of nonexceedance” (DqF), with the methodology of its parameters estimation for stationary case was developed as a completion to the classical FFA with possible extension to non stationary flood regime. The model combined with the technical evaluation of probability of levees breach due to the d-days duration of flow above alarm stage gives the annual probability of inundation caused by the embankment breaking. The results of theoretical research were supplemented by a practical example of the model application to the series for daily flow in the Vistula River in Szczucin. Regardless promising results, this method is still in its infancy despite its great cognitive potential and practical importance. Therefore, we would like to point to the usefulness and necessity of the DqF models to the one-dimensional analysis of the peak flood hydrographs and to flood risk analysis. This approach constitutes a new direction in FFA for embanked rivers.
Highlights
The most popular way of flood protection in Poland is the embankment of the rivers
The aim of this study is to introduce formal aspects of the Duration-flow-Frequency (DqF) modelling in stationary and non-stationary conditions, to use it to assess the inundation risk due to the levees breach and to combine it with the annual maximum (AM) flow model to get the cumulative probability of inundation
The frequency of annual maximum uninterrupted duration of flows, D, over the flood alarm stage (HA) (or equivalently over the alarm flow (QA)) but excluding floods pouring over the embankment crest serves to assess the inundation risk of flood spilling out of river channel caused by scouring the levees (Figure 2)
Summary
The most popular way of flood protection in Poland is the embankment of the rivers. In consequence of this passive way of protection, floods in Poland occur mostly due to the levee breach or very rarely to flow over the. Warning and alarm stages are assigned to the places where water levels are observed, i.e. to the hydrological stations, their determination procedures as well as other inundation risk characteristics take into account, inter alia, the elevation of the embankment system for the whole adjacent river reach. The frequency of annual maximum uninterrupted duration, D (in days), of flows over the flood alarm stage (Figure 1) can be used to assess the risk of flooding due to waning of the levees' strength. The frequency of annual maximum uninterrupted duration of flows, D (in hours, days, etc.), over the flood alarm stage (HA) (or equivalently over the alarm flow (QA)) but excluding floods pouring over the embankment crest (which corresponds to flows exceeding the overtopping flow QB) serves to assess the inundation risk of flood spilling out of river channel caused by scouring the levees (Figure 2).
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