Abstract
Abstract. Simulation methods for extreme flood estimation represent an important complement to statistical flood frequency analysis because a spectrum of catchment conditions potentially leading to extreme flows can be assessed. In this paper, stochastic, semi-continuous simulation is used to estimate extreme floods in three catchments located in Norway, all of which are characterised by flood regimes in which snowmelt often has a significant role. The simulations are based on SCHADEX, which couples a precipitation probabilistic model with a hydrological simulation such that an exhaustive set of catchment conditions and responses is simulated. The precipitation probabilistic model is conditioned by regional weather patterns, and a bottom–up classification procedure was used to define a set of weather patterns producing extreme precipitation in Norway. SCHADEX estimates for the 1000-year (Q1000) discharge are compared with those of several standard methods, including event-based and long-term simulations which use a single extreme precipitation sequence as input to a hydrological model, statistical flood frequency analysis based on the annual maximum series, and the GRADEX method. The comparison suggests that the combination of a precipitation probabilistic model with a long-term simulation of catchment conditions, including snowmelt, produces estimates for given return periods which are more in line with those based on statistical flood frequency analysis, as compared with the standard simulation methods, in two of the catchments. In the third case, the SCHADEX method gives higher estimates than statistical flood frequency analysis and further suggests that the seasonality of the most likely Q1000 events differs from that of the annual maximum flows. The semi-continuous stochastic simulation method highlights the importance of considering the joint probability of extreme precipitation, snowmelt rates and catchment saturation states when assigning return periods to floods estimated by precipitation-runoff methods. The SCHADEX methodology, as applied here, is dependent on observed discharge data for calibration of a hydrological model, and further study to extend its application to ungauged catchments would significantly enhance its versatility.
Highlights
Precipitation-runoff methods have a long history of application in design flood analyses and represent an important complement to statistical methods, for estimating floods with long return periods
At Atnasjø, the difference may well be related to the use of a 1000year precipitation sequence to produce a 1000-year discharge with PQRUT and HBV-Design Flood in a catchment which has a significant contribution from snowmelt
At Krinsvatn, the low values for the 1000-year discharge generated by the PQRUT and HBV-Design Flood simulations appear to result from 72 h design precipitation values which are too low, as indicated by a comparison with the observed precipitation during the highest recorded discharge
Summary
Precipitation-runoff methods have a long history of application in design flood analyses and represent an important complement to statistical methods, for estimating floods with long return periods. Alternative methods which consider catchment response to a range of precipitation events, either as an event-based model or in a long-term simulation mode, have clear advantages over the classical use of a design hyetograph with respect to assigning return periods to the simulated events (Cameron et al, 1999; Paquet et al, 2013). The results of the SCHADEX application are compared with other methods for design flood analysis in the Nordic region, including (1) a simple, event-based, method for estimating peak discharge in response to a predefined extreme precipitation sequence, which represents standard practice for design flood analysis in Norway; and (2) a long-term simulation using a calibrated hydrological model together with an extreme precipitation sequence that is iterated through a simulation period, which is similar to the simulation methods used for design flood analyses in Finland and Sweden. A comparison of all three precipitation-runoff methods is made with statistical flood frequency analysis based on observed discharge data and with the GRADEX method (Guillot, 1993) for flood estimation, which is widely applied outside of the Nordic region
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