Guidelines for design flood estimation of large dams in Austria under climate emergency

Código

I-EBHE0033

Autores

Günter Blöschl

Tema

WG 1.10: Hydrologic Design - Solutions & Communication

Resumo

Almost all hydraulic structures, including dams, require an estimate of a design flood. This flood is normally defined as a flow rate associated with a fixed return period that depends on the type of structure. For example, in Austria, a return period of 5000 years is used for large dams (height above 15 m, volume above 500000 m³). There are three main problems with design flood estimation: extrapolation from low to high return periods, catchments without observed flow series, and climate change. A new guideline of the Austrian Ministry of Agriculture, Forestry, Environment and Water Management has been published, entitled ?Guidelines for the Safety of Dams from Floods?, for which the undersigned is responsible. This presentation summarises the main concepts of this guideline. The philosophy of design flood estimation is based on flood frequency hydrology, which involves combining statistics with additional information, namely causal information, spatial information and temporal information. Causal information includes rainfall characteristics and runoff coefficients as a function of return period, field observations and rainfall-runoff modelling. Spatial information includes that based on comparative hydrology (comparing two neighbouring catchments and reasoning about their hydrological differences) and various regionalisation methods. Temporal information includes longer systematic series in neighbouring catchments and historical information (e.g. historical photos, written documents from archives and interviews of residents). The statistical estimates can be combined with the additional information through non-quantitative methods (based on the degree of subjective belief) and quantitative methods (based on Bayesian statistics). The guideline stipulates two cases for demonstrating flood safety: the design flood (BHQ) equivalent to a 5,000-year event (Q5000) and the safety check flood (SHQ) representing an event significantly bigger than the BHQ. The safety check flood is estimated based on a sensitivity analysis with the rainfall-runoff model, taking into account the increases expected in a changing climate. For the conditions in Austria, an increase in short rainfall of at least 12% and an increase in long rainfall of at least 6% compared to the current design rainfall is foreseen. The guideline has entered into force in May 2024. Given its comprehensiveness, it can serve as an example for similar guidelines in other countries. References Merz and Blöschl (2008) Flood frequency hydrology: 1. Temporal, spatial, and causal expansion of information. Wat. Resour. Res. 44, W08432. Merz and Blöschl (2008) Flood frequency hydrology: 2. Combining data evidence. Wat. Resour. Res. 44, W08433. Viglione, Merz, Salinas and Blöschl (2013) Flood frequency hydrology: 3. A Bayesian analysis. Wat. Resour. Res. 49, WR010782 Blöschl und Merz (2008) Bestimmung von Bemessungshochwässern gegebener Jährlichkeit - Aspekte einer zeitgemäßen Strategie (Estimating design floods of a given return period - facets of a contemporary strategy). Wasserwirtschaft 98, 12-18.