Flash floods and Urban flooding Switzerland
Landslides and flash floods already cause more and more damages to infrastructure and settlements (1)…. Growing demand on land increases the competition for the very limited resource (2). Areas at higher risk of flooding and land-slides are now used for housing and infrastructure.
Heavy precipitation can lead to backwater in sewers and thus to flooding of cellars or entire neighbourhoods. With the expected increase in heavy precipitation, this problem will occur more often and, in critical cases, require the laying of larger dimensioned sewer pipes and installation of backwater valves (3).
Surface water flooding
Urban flooding may be triggered by heavy precipitation without being related to watercourses, when precipitation is too intense for urban drainage and overland flow results. This kind of flooding is also called surface water flooding. In general, surface water floods include all rainfall-related (pluvial) floods: both pluvial flooding and flooding from sewer systems, small open channels, culverted watercourses or flooding from groundwater springs (9). The notable difference between a surface water flood and a fluvial flood is that in the former case, water is making its way towards a watercourse, whereas in the latter case flooding stems from a watercourse (8).
In Switzerland, of all damage due to surface water floods and fluvial floods between 1999 and 2013, surface water floods are responsible for at least 45 % of the flood damage to buildings and 23 % of the associated direct tangible losses (8). Direct tangible flood damages to buildings only constitute a portion of the total flood losses. Fluvial flood damage generally coincides with surface water floods (8). This can be explained by the fact that both flood types are generated by the same rainfall input. Damage caused by surface water floods in Switzerland occurs by far most frequently in summer in almost all regions. There is no significant upward trend in damage due to surface water floods in Switzerland between 1993 and 2013 (8).
New lakes in deglaciating high-mountain regions - opportunities and risks
Glaciers in most icy mountain ranges are shrinking rapidly. Part of the melt water forms new lakes by filling up topographic depressions that become exposed as glaciers vanish. These lakes will change the landscape. New opportunities arise for hydropower production, touristic developments, and freshwater supply. At the same time, risks arise of lake outburst floods, initiated by rock and ice avalanches when slopes become less stable. These opportunities and risks related to new lakes in de-glaciating areas have been inventoried for the Swiss Alps (4).
Hydropower: Large new lakes constitute possible new reservoirs with high potential energy for hydropower production. In fact, these lakes may be used as a ‘blue battery’ to store excess wind, solar or nuclear energy by pumping water to these reservoirs in times of energy surplus. Another potential synergy is combining hydropower and flood retention: with an adequate freeboard these lakes can absorb potential outburst floods from new lakes higher up in the catchment (4).
Tourism: Lakes can be important elements of landscape diversity. To some minor degree, the new lakes in de-glaciating areas may compensate for the loss of landscape diversity and attractiveness caused by vanishing glaciers (5), and thus contribute to the tourism industry.
Fresh water supply: With respect to the current situation, shrinking glaciers will increase the discharge of melt water on the short-term. On the long-term, however, the smaller glaciers will supply less melt water to settlements and agriculture downstream. The new lakes could partially replace the vanishing melt water supply in critical situations. The available water volume in naturally formed depressions is strongly limited, however. In the Swiss Alps, for instance, the total water volume of the anticipated future lakes is some 3 % of the presently existing glacier volume (6), and roughly corresponds to that of one average annual precipitation sum over the still glacier-covered surface. At best, small percentages of the lake volume can be used for freshwater supply, but even this limited use would require major infrastructure (tunnels, dams) and corresponding investment.
New lakes in potentially unstable, high-mountain regions are dangerous beauties, however, and call for protection against hazards when opened-up for tourists (4). These lakes form close to, or even directly at the foot of, destabilizing icy peaks. Rock and ice avalanches may trigger floods and debris flows that can affect down-valley areas over long distances and reach humans and their infrastructure in previously safe places. Once a new lake is being formed, hazards from destabilizing lateral valley walls as a result of glacial de-buttressing are likely to persist far into the future. Floods and debris flows from lake outbursts can cause disastrous damage and high numbers of fatalities (7). This risk can be managed, for instance, by lowering the level of the lake or by flood retention in flat lower parts of potential trajectories or at lakes situated in lower parts of the involved catchments (4).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Switzerland.
- Bader and Kunz (1998); Steininger and Weck-Hannemann (2002), both in: European Environment Agency (EEA) (2005)
- UBA (2004), in: European Environment Agency (EEA) (2005)
- OcCC/ProClim- (2007)
- Haeberli et al. (2016)
- Espiner and Becken (2014), in: Haeberli et al. (2016)
- Linsbauer et al. (2012), in: Haeberli et al. (2016)
- Carey et al. (2012a); Clague and O’Connor (2014), both in: Haeberli et al. (2016)
- Bernet et al. (2017)
- Hankin et al. (2008); Falconer et al. (2009), both in: Bernet et al. (2017)