Effects of heavy rainfall on construction-related infrastructure | Umweltbundesamt

There is now no question whatsoever that the climate – and consequently urban climate – will change. For North Rhine-Westphalia, an increase in the average temperature of approx. two degree Celsius is expected for the mid-21st century (compared to the years from 1961 to 1990). As warm air is capable of absorbing and transporting more water, the increase in temperature will also have effects on the rainfall rates. Analyses of data as well as climate models show that the global volume of precipitation will increase by approximately two per cent for every degree by which the temperature increases (Kreienkamp et al. 2016). Forecasts predict that heavy rainfall and flash floods will increase or at least remain the same depending on the forecast model (UBA, 2015b). It is undisputed that, over the past 15 years, so-called heavy rain events have occurred to an increasing degree, at least in some regions (see Figure 1). Adaptations responding to the resulting problems have been provided for in the German Strategy for Adaption to Climate Change whereupon hot spells and flooding are, in most cases, considered collectively. This guideline, however, mainly addresses heavy rainfall events in conjunction with water retention and reducing runoff on individual properties.

Urban Climate

In this respect, there are climactic effects known as urban climate in cities and metropolitan areas when compared to surrounding regions with little to no development. The urban climate is affected, among other things, by the building structure and development density, the share of sealed surfaces, vegetation and other factors such as traffic and industry (MKULNV, 2011). In this respect, the drainage systems and/or the type of drainage in cities have a negative impact on the natural water balance. As a result, for example, the increased runoff attributed to sealed surfaces can cause overloading of the sewer system and flooding in the event of heavy rainfall. Reduced evaporation and transpiration is one of the factors that causes cities to heat up during the summer.

Heavy Rainfall

Heavy rainfall is at hand if very large volumes of rain fall in within a short time in a spatially restricted area, mainly in combination with storm fronts from May to September. In the process, small creeks can turn into torrential rivers (BBK, 2015). The German Meteorological Office (DWD) makes this phenomenon more tangible on the basis of warning levels. There are three warning levels for heavy rainfall:

• Volumes of rain from 15 to 25 l/m² in one hour and/or 20 to 35 l/m² in 6 hours (significant weather warning)
• Volumes of rain > 25 l/m² in one hour and/or 35 l/m² in 6 hours (storm warning)
• Quantities of rain > 40 l/m² in one hour and/or 60 l/m² in 6 hours (extreme weather warning/DWD, 2016).

This definition is consistent for the entire federal territory where factors such as topography, the amount of sealing and the development density are not included in the evaluation. They are, however, crucial with respect to the effect and above all the vulnerabilty of areas when faced with heavy rainfall. In order to better accept and communicate risks, a dimensionless rainfall index has been developed. It is based on local heavy rainfall statistics for every locality in Germany according to KOSTRA (CoordinatedStorm Rainfall Regionalisation Analysis). In the calculation of the index, the annual incidence is considered to a greater degree than the duration of the events. This means that, in the case of a higher recurrence interval and shorter duration, a high heavy rainfall index is at hand, which is plausible due to potential damage caused by such events (Mudersbach, 2016).

1961-1990

Vulnerability is defined as following according to UBA 2015a: "the extent to which a system is susceptible and therefore incapable of handling adverse effects of climate change, including climate variability and extremes". This also includes climate variability and extremes like heavy rainfall, heat and hail (DAS, 2008). The term resilience is just as important in this context. It describes the capability of a system to remain functioning and to quickly regain the original characteristics when subjected to inner and outer effects such as, in this case, extreme weather events impacting the property (BBK). Vulnerability and resilience can be influenced by specific measures. They include measures on the property such as ground sills or depressions. It is also important to adapt the buildings themselves, for example, by elevating the entrances to prevent the ingress of water. If such measures or similar measures cannot be implemented, suitable building materials in particular are decisive for the resilience of the buildings.

Objects requiring protection (building materials and construction):

• construction
• Location (topography, relief, landscape, degree of sealing, development density) 
• Rainfall intensity and duration
• carrying capacity of the soil and the public drainage system
• extent of preventative measures to reduce risks and protective measures of municipalities, developers and citizens (BBK, 2015).

Table 3 of the report sets out the water sensitivity of building materials.

Following measures are being introduced:

• Structural measures for protecting against heavy rain: Channelling water away from the building, Sealing the outer walls, Infiltration through light shafts, windows and doors, Backwater from the sewer system
• Storage: retention in the sewer, Retention on the roof (green roof / blue roof), Underground resevoirs, retention basins, retention on the surface
• Infiltration
• Evaporation and transpiration
• Removal of sealing
• Drainage 

Evaluation: Every possible measure incurs other costs and offers different effects with respect to protecting against heavy rainfall as well as positive effects on the environment. Evaluation matrices in the form of tables compare these measures so that they can be compared.

Reciprocal effects of the measures: The measures for binding heavy rainfall also have positive effects on the environment. As a result, reduced sealing has a positive effect on the microclimate; the increased infiltration of water promotes the replenishment of ground water and the use of the precipitation water, for example, in gardens reduces the use of potable water.