Prof. Luuk Dorren (School of Agricultural, Forest and Food, Bern University of Applied Sciences (BFH), Switzerland)
The damage and loss of life caused by natural hazards are continuously on the rise. In 2017, the global costs of natural disasters reached more than 300 billion Euro [1]. The increasing losses can firstly be attributed to growing populations and more capital that potentially can be damaged. Secondly, improved reporting better captures the extent of the damages [2]. Thirdly, there is mounting evidence that climate change is altering the frequency and intensity of extreme weather events such as heatwaves, droughts, heavy rainfall, and storms, which in turn, is leading to an increased risk of floods (e.g., the 2021 Pacific Northwest floods which caused more than 2 billion Euro of economic damage and the 2022 Pakistan floods that caused more than 1700 casualties and over 15 billion Euro of economic losses), landslides, and wildfires. Climate change further causes sea levels to rise, which increases the occurrence probability of coastal flooding, erosion, and storm surges. Finally, climate change is causing glaciers and permafrost to melt, which can destabilize rock slopes and again increase the risk of landslides. Data from the large reinsurance companies show that the majority of the natural hazard related losses are not covered by insurance, which means that millions of households and businesses face a large protection gap [3]. To counter the widening of this gap, prevention, mitigation and therefore reduction of natural hazard risks is essential since insurance alone is not a long-term viable strategy.
In Switzerland, since more than two decades now, the mitigation of the effects of natural hazards is done by means of an integrated risk management (IRM) approach [4]. Even though the foundations for natural hazard registers and maps have been in place since the beginning of last century [5], until well into the 1980s many in Switzerland assumed that gravitational natural hazards could be well controlled by civil engineered structures. Although these hazard mitigation structures are generally able to reduce the risk of natural hazards, it has become apparent that they generally do not offer absolute protection and in exceptional cases may even have adverse. This is usually due to the fact that hazard mitigation structures cannot be dimensioned for extreme events or for interacting natural hazards. This can lead to cascading effects (overlapping and interacting hazard processes that lead to a chain reaction), for example when heavy rainfall and shallow landslides carry large quantities of sediment and wood into the channels, which can lead to blockages and subsequent flooding at bridges and culverts with resulting bank erosion and collapse of dams at locations where the expected occurrence probably was very low.
The prevention measures as part of the IRM framework are prioritized as follows:
As a general principle, the priority is to use spatial planning based on hazard maps to try to avoid hazardous areas in the landscape or not to exacerbate existing risks. If avoidance using spatial planning is not possible, the measures which have the next priority are the biological ones which have an effect over large areas. These mainly concern the existing forests with a protective function (so called protection forests) as well bioengineering measures such as, for example, retaining structures made from timber in combination with reforestation. According to [6], approximately one third or 1.32 million ha of the Swiss national territory is covered with forests, 49 % of which are protection forests [7]. The forest thus forms a large-scale, green infrastructure, serving an important protective function against natural hazard processes.
As reviewed by [8], forests can prevent the release of snow avalanches and shallow landslides, and also protect against rockfall impacts. Moreover, forests reduce bank and surface erosion in the vicinity of torrents and thus also reduce debris flows. Depending upon the spatial and temporal distribution of precipitation duration and intensity and the size of the catchment area, forests can reduce both the probability of occurrence and the intensity of flood events. As such, forests contribute to reducing natural hazard risks to an acceptable level in many places. Thanks to their combination with protection forests, civil engineered interventions designed to meet higher protection requirements are often more cost-effective (lower installation or maintenance costs). In certain places, such interventions only make sense because of the additional protection provided by the forest [cf. 9].
It is evident that forests alone are not able to reduce the natural hazard risk to an acceptable level in all locations. This is primarily due to the hazard perimeter not being sufficiently stocked (e.g., in active avalanche corridors or debris flow gullies) or because the forest’s impact is locally insufficient or non-existent (e.g., in the case of flooding of areas along major rivers). In such places, the third type of prevention measures, i.e., civil engineered structures, come into play. Well-known examples are river dams, sediment retention basins, avalanche barriers, road galleries and flexible rockfall nets. If these are not cost-effective, organizational measures may be able to reduce the posed risk. This could mean, for example, monitoring hazard process in combination with road closures and evacuation of residential areas. Other examples would be temporal measures designed to directly protect built infrastructure such as flood barriers made of water-filled hoses along rivers, artificial avalanche triggering or the blasting of rock masses which have been monitored by radar.
In my presentation, I will explain the Swiss IRM method and use examples from our own and other published research to illustrate the state-of-the-art in the quantification of the role of some ecosystem-based solutions for natural hazard risk reduction.