New insights for real-time flood forecasting in Germany: Lessons learned from 2021 summer flood in Ahr river

We investigate the 2021 summer flood in Ahr catchment in West Germany, with the return period estimated preliminarily as 1 in more than 500 years [1]. A recent study has indicated that science did not fail to predict the flood event [2]. Yet, several scientific and administrative challenges are still to be addressed to improve existing flood forecasting systems for supporting local authorities to manage such extreme events. We bring some examples of what science and technology gaps need to be filled to address these issues. To do this, uncertainties associated with near-real time precipitation products with hourly and daily resolutions provided by the German weather service (DWD) have been investigated. The hydrological response of the catchment is tested to several high-resolution gridded precipitation observations and reanalysis data for post-assessment of the event. A new feature to read hourly meteorological input data was added to the mesoscale Hydrologic Model (mHM- www.ufz.de/mhm) to forced it with Radar-Online-Adjustment of hourly values measured at the precipitation stations (RADOLAN-mHM). Comparing the flood peak from RADOLAN-mHM with REGNIE-mHM at daily time steps provided valuable insights on development-orientation of near-real time and high-resolution flash flood analysis and forecast applications for Germany. Last but not least, the variability of maximum streamflow in the Ahr catchment was evaluated for future periods under climate change to check if such megafloods can be considered as new norms.

Fig 1. Boxplots of the annual maximum streamflow in Ahr river simulated by the mesoscale Hydrologic Model (mHM)
  for three periods between 1971-2000, 2000-2050 and 2051-2100. Simulation is conducted based on 21 ensembles under RCP 2.6 and 49 ensembles under RCP 8.5

References

[1] L. Samaniego, H. Najafi, O. Rakovec, P. Shrestha, S. Thober. (2021) High-resolution hydrologic forecasts were able to predict the 2021 German Floods: what failed?. AGU 2021 Fall Meeting, New Orleans.
[2] World weather attribution report, (2021) Rapid attribution of heavy rainfall events leading to the severe flooding in Western Europe during July 2021. https://www.worldweatherattribution.org/wp-content/uploads/Scientific-report-Western-Europe-floods-2021-attribution.pdf