The SCHADEX method: A semi-continuous rainfall-runoff simulation for extreme flood estimation

- SCHADEX is a probabilistic method for extreme flood estimation.
- It has been applied on numerous catchments for dam design flood estimation.
- It provides complete distributions of discharges up to extreme quantiles.
- It combines an original rainfall probabilistic model and rainfall-runoff simulation.
- Main features and hypothesis are illustrated with the data of a French catchment.

SummaryThis paper presents the SCHADEX probabilistic method for extreme flood estimation, developed and applied since 2006 at Electricité de France (EDF) for dam spillway design. SCHADEX is based on a semi-continuous rainfall-runoff simulation process: a continuous hydrological simulation provides an exhaustive description of the possible hydrological states of the catchment, while floods are generated on an event basis. The method has been built around two models: the Multi-Exponential Weather Pattern (MEWP) distribution for rainfall probability estimation, and the MORDOR hydrological model. The simulation process allows the production of an estimated distribution of flood volumes at the study's time step, as well as a distribution of flood peaks based on a peak-to-volume ratio computed from observed hydrographs of significant floods. A wide range of rainy events are simulated on each hydrological state, generating an exhaustive set of crossings between precipitation and soil saturation hazards. Consequently, SCHADEX stands clearly apart from the “N-years flood is generated by a N-years rainfall” paradigm, as a great diversity of hydrological scenarios can generate a flood of a given return period, with many variables taken into account. Since its development, SCHADEX has been widely applied in France for industrial studies, to catchments from several to several thousand square kilometres. This has allowed the testing and improvement of the method.In this paper, the basic concepts and hypothesis of the method are given in detail and illustrated throughout with the example of the River Tarn at Millau (2170 km2, South of France). The different steps of the method are successively exposed, up to the estimated distribution of flood daily discharges and flood peaks and their comparison with observed values. Lastly, some methodological perspectives to enhance the method are presented.