Estimating very rare floods at multiple sites in a large river basin with comprehensive hydrometeorological simulations
Authors: Viviroli Daniel , Sikorska-Senoner Anna E. , Evin Guillaume , Staudinger Maria , Kauzlaric Martina , Chardon Jérémy , Favre Anne-Catherine , Hingray Benoit , Nicolet Gilles , Raynaud Damien , Seibert Jan , Weingartner Rolf , Whealton Calvin 
Published: February 2023

Rare to very rare floods (associated to return periods of 1'000&#8211;100'000 years) can cause extensive human and economic damage. Still, their estimation is limited by the comparatively short streamflow records available. Some of the limitations of commonly used estimation methods can be avoided by using continuous simulation (CS), which considers many simulated meteorological configurations and a conceptual representation of hydrological processes. CS also avoids assumptions about antecedent conditions and their spatial patterns.We present an implementation of CS to estimate rare and very rare floods at multiple sites in a large river basin (19 locations in the Aare River basin, Switzerland; area: 17'700&#8201;km&#178;), using exceedingly long simulations from a hydrometeorological model chain (Viviroli et al., 2022). The model chain consisted of three components: First, the multi-site stochastic weather generator GWEX provided 30 meteorological scenarios (precipitation and temperature) spanning 10'000 years each. Second, these weather generator simulations were used as input for the bucket-type hydrological model HBV, run at an hourly time step for 80 catchments covering the entire Aare River basin. Third, runoff simulations from the individual catchments were routed for a representation of the entire Aare River system using the routing system model RS Minerve, including a simplified representation of main river channels, major lakes and relevant floodplains. The final simulation outputs spanned about 300'000 years at hourly resolution and cover the Aare River outlet, critical points further upstream as well as the outlets of the hydrological catchments. The comprehensive evaluation over different temporal and spatial scales showed that the main features of the meteorological and hydrological observations were well represented. This implied that meaningful information on floods with low probability can be inferred. Although uncertainties were still considerable, the explicit consideration of important flood generating processes (snow accumulation, snowmelt, soil moisture storage) and routing (bank overflow, lake regulation, lake and floodplain retention) was a substantial advantage compared to common extrapolation of streamflow records.The suggested approach allows for comprehensively exploring possible but unobserved spatial and temporal patterns of hydrometeorological behaviour. This is particularly valuable in a large river basin where the complex interaction of flows from individual tributaries and lake regulations are typically not well represented in the streamflow records. The framework is also suitable for estimating more common, i.e., more frequently occurring floods.ReferenceViviroli D, Sikorska-Senoner AE, Evin G, Staudinger M, Kauzlaric M, Chardon J, Favre AC, Hingray B, Nicolet G, Raynaud D, Seibert J, Weingartner R, Whealton C, 2022. Comprehensive space-time hydrometeorological simulations for estimating very rare floods at multiple sites in a large river basin. Natural Hazards and Earth System Sciences, 22(9), 2891&#8211;2920, doi:10.5194/nhess-22-2891-2022