Assessment of design flood levels on the Danube using probabilistic simulations

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Tamás Krámer, Jenő Jakab, János Józsa, József Szilágyi, Péter Torma

Tuesday 30 june 2015

12:15 - 12:30h at South America (level 0)

Themes: (T) Flood risk management and adaptation, (ST) Flood risk assessment

Parallel session: 5J. Floodrisk - Assessment

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In Hungary the design of levees is regulated by 100-year flood levels published for major lowland rivers. Although the appropriate target of flood management is risk, a comprehensive analysis of this variable is often too costly for planning local improvements. Clear-cut design flood levels are a pragmatic simplification that can easily provide a legal basis for floodplain management regulations. Flood levels of the Hungarian reach of the Danube River, established in the 1970s, were recently updated to reflect changes in physical conditions, using better statistical estimates from longer hydrographs and probabilistic numerical simulations. It is hard to define a single design flood event that is characterised by a return period of 100 years in all reaches of a river system. Instead, the stochastic character of floods was resolved by Monte-Carlo simulation. Daily flow hydrographs were generated to major inflow boundaries for a period of thousands of years using a hybrid Markov chain-based time series model with fitted statistical properties. Using this hydrological forcing, floods were then routed through the river system using a 1D hydrodynamic model. The design flood level at a particular river section was determined as the highest level of all simulated years when the simulated peak discharge stayed below the 100-year streamflow derived from historical observations. This constraint was extended to a sufficiently long reach downstream of that river section to account for the backwater effect of tributaries' extreme floods. The design flood levels on the upper Hungarian Danube determined in this new study turned out to be 1 to 1.5 m higher than the old ones.