European and Global Flooding: Role of Climate, Land Use, and Infrastructure

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Episode recorded June 30, 2023
Episode released on September 21, 2023

Günter Blöschl is Professor and Head of the Institute of Hydraulic Engineering and Water Resources Management  the Technical University of Vienna. His research focuses on floods and droughts and changes in hydrological cycle caused by climate change, land use change, and water management. 

Highlights  |  Transcript

1. Flooding in West Germany on July 12 – 15 2021 resulted in ~ 175 deaths with some areas inundated with several meters of water. The high impacts were attributed to the flood being much larger than expected, problems with communication, and with flood hazard maps. 
2. Recent flooding in Bologna (May 2 – 3, 2023) unexpected because it followed extreme flooding in this region (Emilia Romagna) in Sept. 2022. The rapid recurrence of extreme flooding was unanticipated. 
3. Recent flooding from atmospheric rivers in California (Dec. 2022 through Jan. 2023) had about a 1 week lead time as the movement of the ARs across the Pacific were monitored. 
4. Floods often end long-term droughts, challenging water resource managers. In Brisbane in 2011 floods (23 drowned in Towomba) followed the Millenium Drought and reservoir managers were reluctant to release water, exacerbating flood issues (van den Honert and MacAneney, 2011). 
5. Two types of flood forecasts or predictions:
   1. Flood forecasts with lead times of days to weeks, e.g. atmospheric rivers 
   2. Flood risk estimation, probability of flood, not exact timing
6. Climate change impacts on flooding:
   1. Thermodynamic effect of climate change, Clausius Clapeyron equation, increasing temperature (T) with climate change increases water holding capacity of atmosphere. One degree of warming will translate into a 7% increase in water holding capacity.
   2. Climate change impacts on atmospheric circulation, storm track changes related to differential heating, e.g., N Pole heating more rapidly than Equator.

Clausius Clapeyron may not always be directly applicable to flooding because atmosphere is not always saturated, increase rainfall from higher water holding capacity may not translate to flooding because of impacts of soil moisture deficits, increased evaporation, snow and other processes. 

1. Last 30 yr in Europe correspond to flood rich period relative to 500 yr archival record. Previous flood rich periods little Ice age (17^th – 18^th century). Previous flood rich periods were related to cooler temperatures than flood poor periods. (Blöschl et al., Nature, 2020).
2. What causes floods and how are causes linked to catchment size (Blöschl, HESS, 2022):
   1.  Land use change impacts on floods for small catchments (few ha, km²) and minor events, deforestation, afforestation, agricultural machinery, urbanization
   2.  Hydraulic structures, river training (levees) increase flood magnitude because of loss of floodplain (moderate sized events)
   3. Climate change: study of European flooding in last 60 yr: 
      1. NW EU, flood increases related to increases in storm magnitudes; 
      2. E EU, decreasing flooding, ↑ T, ↓snowmelt;
      3. S EU, (a) decreased flooding in med to large catchments, related to ↑ evaporation, ↓soil moisture; (b) increased flash floods, convective storms, small catchments.
3. Climate change impacts timing of EU floods based on observational data for past 50 yr (1960 -2010) (Blöschl et al., 2017): winter flooding British Isles, S EU; spring flooding, E EU, snowmelt; and summer flooding, central EU. Earlier flooding Britain from heavier storms and in E EU from earlier snowmelt, S EU, shifts due to higher evaporation. 
4. Solutions to flooding: 
   1. Engineering: 
      1. Small catchments, retention basins 
      2. Larger catchments: levees, efficiency depends on water level, not flood duration
   2. Nature based, Green Deal in EU: 
      1. Small catchments, afforestation, wetlands, microponds
5. Austrian study: hydrologic flood frequency analysis, 500 yr floods, hyper resolution modeling (spatial, 2 m; temporal, 2 sec), dynamic flood hazard zones à risk management; inundation (extent, water depth, flow velocity)
6. Sociohydrology: two-way coupling of people and water
   1. Levee effect, reduced flood risk, increased development, increased flood risk. 
7. Future outlook: positive, economic, environmental, social, adapt; equity issues will be increasingly important.