Stress test city - how resilient are our cities? | Umweltbundesamt

In Chapter 2 of the report, the crisis of energy supply, heavy rain and thermal load are used as examples of stress-testing climate scenarios.

The selected stress scenarios represent possible negative changes to the city whose causes cannot be influenced at the municipal level. Essential selection criteria are the relevance for urban development, didactic ability (robustness and adaptability) as well as significance and intensity. However, a statistical risk assessment with regard to the extent of damage and probability of occurrence does not underlie the selection. Further criteria are the informational situation and the availability of data as well as the operationalizability within the functional city model for the selection and the description of the scenarios. Basically, the concept of the stress test can be extended beyond the selection made so far.

Resilience is assessed in selected areas of the city using descriptors (see Figure 4 of the report). To better understand the results, the most affected descriptors will be selected to assess the impact of various crises and disasters, and ultimately to assess resilience. Negative effects may affect the basic urban functions if the descriptors, which are operationalized with different indicators according to the stress scenario, are not sufficiently robust and adaptable.

Securing the energy supply

Securing the energy supply is essential for the functioning of a city. This concerns the power supply and the provision of heat for households as well as process heat for industry alike. As less and less energy raw materials are sourced in Germany and Europe, energy imports play a central role.

Security of supply therefore depends directly on the reliability of deliveries, mainly from countries outside the European Free Trade Association (EFTA). However, the energy supply in Germany is based to a considerable extent on fossil fuels. At the same time, the energy transition is being driven forward with adaptation requirements for supraregional as well as local networks. Even short-term power outages lead to high economic losses, longer-term power outages to crisis situations, as gradually all critical infrastructure systems fail (information and communication system, transport and transport system with all modes of transport, health system). The higher in a city the specific consumption values in the descriptors economic structure (e.g. share of energy-intensive industry, power consumption per capita), and technical infrastructure, especially in transport (e.g. modal split etc.), the stronger they would be of possible shortages and price increases of fossil fuels but also power outages affected.

For the stress scenario Crisis of energy supply it is assumed:

• As more and more "fluctuating amounts of electricity" from renewable energies are fed into the grid, the security of the power supply is reduced.
• Despite the currently high grid stability, future risks due to the transformation of the energy system cannot be ruled out. In particular Germany's existing dependence on gas, oil and hard coal supplies, especially from third countries including Russia, represents a strategic risk.
• Due to the necessary backup capacity by gas-fired power plants in the course of the energy transition, dependence on risky natural gas imports could increase further.

 Heavy rain

Heavy rain events usually last briefly and affect as opposed to flood usually a smaller area. Nevertheless, heavy rainfall can cause high levels of damage and, in urban areas, at least in the short term, seriously affect basic urban functions. In particular, the technical infrastructure (roads, local public transport and long-distance traffic) is not or only partially functional during a heavy rainfall event. In addition to this rather short-term stress, heavy rainfall leads to physical damage to the technical infrastructure and to real estate (residential buildings, public buildings), which leads to a financial burden on private and public households.

For the stress scenario heavy rainfall it is assumed:

• The rare (rainfall> = 10 mm / 1 hr. Or> = 20 mm / 6 hr.) But also the extraordinary heavy rainfall events (rainfall> = 25 mm / 1 hr. Or> = 35 mm / 6 hrs.) will increase in the future,
• Since the channel systems in Germany are only designed for normal precipitation (design rain, defined with a recurrence period of up to Tn = 5 years), rain events with a recurrence time of Tn> 5 years lead to an overflow of sewerage and consequently to flooding.
• Due to the predicted temperature increases in the context of climate change, both the probability of occurrence of heavy rainfall events (days with heavy precipitation> 10 mm) and the intensity of precipitation (mm per unit time) will increase significantly. In extreme cases, rain events that are currently classified as rare can become a "frequent rain event" in the future.

 Thermal stress

The natural hazard of the heatwave, which rather creeps into the heavy rain, has a major impact on the health of the city, on the health, well-being and productivity of the population. In particular, thermal stress affects seniors and toddlers as well as the corresponding socio-economic infrastructures (kindergarten, retirement homes, etc.). The function of "population health" is part of the descriptor environment. Indirectly, heat waves can also affect the electricity (through the expansion of air conditioning systems) and the water supply (through prolonged drought) of a city. 

For the stress scenario thermal load, it is assumed:

• Due to the urban climate, cities are particularly affected by temperature increases (heat islands in cities).
• The health, well-being and productivity of the population are demonstrably negatively affected even in the mid-range of temperature increases.
• Due to the predicted global warming, a maximum probability of> 90% is expected, an increase in the number of hot days as well as heat waves (conservative prognosis with temperature increases of approx. 2 degrees).
• In the event of extreme climate change (temperature increase of approx. 4 degrees Celsius) long-lasting dry summers and periods of drought are to be expected even in the middle latitudes.