Background and Goals
In the past, heavy rainfall events have occurred to an increasing degree in Germany, some of which have even resulted in deaths (e.g. Münster 2014, Southern Germany 2016). In addition to personal injury, this has also resulted in serious damage to property, particularly to physical infrastructure. The more frequent occurrence and rise in rainfall intensity of such weather events as well as the occurrence of extreme storms, hot spells or droughts is, according to the German Strategy for Adaption to Climate Change (DAS), a result of climate change. A problem we face in our time is that the structures of German cities that have grown fail to account for this development and corresponding measures to manage this have been neglected. Knowledge of the vulnerability of physical infrastructure as well as taking measures to protect existing buildings or so-called adapted building that takes the local situation into account as well as special building materials are becoming increasingly important. This report will present the vulnerabilities of the individual building materials and describe measures to protect physical infrastructure. Special attention will also be paid to available areas within the urban domain for binding and using heavy rainfall or allowing it to infiltrate or evaporate. Worldwide, there is an increasing number of strategies employing a so-called sponge city, which handles precipitationwater in the city in a sustainable manner. A return to a more natural water regime in the city means that water will be retained in the area or in special reservoirs in order to use it e.g. for management of green space. Increased evaporation will also lead to a sustainable improvement in the microclimate as cities be cooled overall. At the same time, measures for creating a sponge city cannot only be evaluated and planned over longer timeframes, rather they can also have a positive effect in the event of brief heavy rainfall events. The concept of the sponge city is primarily relevant with respect to long-term, higher-level urban planning as the measures generally concern public areas. It becomes all the more necessary to also break down these concepts to the smallest possible division within a city: a property. Therefore, the term "sponge property" will be introduced in this report. In addition to the protection of buildings, measures for the storage, infiltration as well as the evaporation and transpiration of a defined heavy rainfall event have been devised at the level of the property. In a computer-based calculation model, the effects of the individual measures were demonstrated on a sample property and then evaluated with the expected costs (cost-benefit analysis). The measures are evaluated considering various local aspects such as topography or soil conditions to allow for transferability.
Exemplary property is located in Bonn.
Steps in the process of adaptation to climate change
Step 1: Understand and describe climate change
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.
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 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).
- Flash floods
- Altered rainfall patterns
- Higher average temperatures
- long term = to 2100 and beyond
Step 2b: Identify and assess risks - Vulnerability, risks and chances
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):
- 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.
Step 3: Develop and compare measures
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
- Evaporation and transpiration
- Removal of sealing
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.
- beyond 2100 (far future)
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.
Step 4: Plan and implement measures
Analysis of effectiveness:
As part of the report measures were simulated for example property. The analysis based on a model of the following sample property shows how effective the measures for binding heavy rainfall actually are. Laser scan data is used for the model and a surface runoff model has been developed. Subsequent sprinkling with artificial rainfall makes it possible to obtain information on how the initial state changes and how the respective measures show their effectiveness. In the process, it is shown that the retention of the precipitation in particular is a decisive factor in providing protection against heavy rainfall. It can also be carried out with relative ease and effectiveness on the sample property. A legal assessment of feasibility will also be provided.
Example calculations show the costs of heavy rainbows. Particularly efficient is the creation of hollows and trenches, with comparatively low costs per retained cubic meter of precipitation. Finally, on the sample property in the model, around 70% of the precipitation of a 100-year event can be held back for half the cost.
There are numerous laws and regulations that must be considered when implementation a sponge property. This concerns the duty to dispose of wastewater, flood mitigation, the duty to treat precipitation water, standards for sizing drainage systems and for using rain water. Outside the Water Resources Act, the Building Code following were also considered:
- separation order (treatment of rainwater),
- DIN EN 752 (dimensioning of public drainage systems),
- DWA-A 118 (flooding and overflow),
- DWA-M 199,
- DIN EN 12056 and DIN 1986-100 (property, site drainage within buildings),
- DIN 1989-1 (rainwater use)
A project of the research program "Future Building" of the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB) conducted by the Federal Institute for Building, Urban Affairs and Spatial Development (BBSR) at the Federal Office for Building and Regional Planning (BBR).
Federal Institute for Building, Urban and Regional Research (BBSR)
- Reinhard Beck GmbH & Co. KG
- University of Wuppertal (BUW)
- Kommunal Agentur NRW, Düsseldorf