Adaptation of road infrastructure to climate change

Project 3
Assessing the dimensioning of road drainage facilities according to RAS-Ew and RiStWag against the background of changing temperature and rainfall events caused by climate change in Germany until 2100
Bildquelle: Bundesanstalt für Straßenwesen (BASt)


Climate models project for Germany a significant change of regional climates, including increased temperatures and changing precipitation properties that point to an increase in heavy rainfall. Road drainage systems therefore have a two-fold challenge ahead: On the one hand heavier rainfall poses the risk of hydraulic overload to the drainage facilities. On the other hand, long dry spells can lead to operational problems, e.g. if the vegetation in planted soil filters is damaged by drought stress.

So far, the aim of this research project was to monitor the current dimensioning approach of road drainage systems according to the „Guidelines for Road Design – Part: Road Drainage“ (RAS-Ew) and the „Guidelines for structural measures for roads in water reserve zones“ (RiStWag).

By assessing and further processing of several computation runs of the regional climate model CLM (Climate Local Model) for the near future (up to 2050) and the distant future (until 2100), the project has provided some knowledge as to whether the current dimensioning approaches are sufficient in terms of hydraulic performance and drainage safety whilst re-taining soil and water protection as well as in terms of constructive designing or whether appropriate adaptation measures would be necessary. The findings gained from this research should be considered for the revision of the RAS-Ew and the RiStWag.


Based on the regional differences of climate change impacts, the project assessed four model regions throughout Germany.

In order to purposefully estimate climate change impacts on road drainage, it is necessary to make statistical hypotheses regarding developments in rainfall intensity that have a temporal resolution of 5 minutes to 1 hour. Therefore, a statistical downscaling method was used in order to convey the results of the regional climate model CLM, featuring a temporal resolution of 1 day to maximum 1 hour, in higher resolution time series by using historic measuring station data.

In order to present what kind of impact predicted climate change will have on road drainage, hydrodynamic calculations and hydraulic long-term simulations were conducted on selected fictitious and real drainage situations.

The study encompassed the following drainage systems:

  1. Drainage facilities such as gutters, road gullies, canals and trenches
  2. Detention reservoirs, retention soil filters and recharge basins
  3. Sedimentation tanks, RiStWag systems


Following downscaling and statistical assessment of rain series, three of the four investigated model regions showed increases in heavy rainfall amounts that are predominantly signifi-cant in terms of their statistics.

Due to hydraulic calculations, there are currently no adaptation requirements foreseen for the dimensioning of drainage elements (e.g. canals) and tailings ponds. A simple dimensioning procedure in line with RAS-Ew already provides ample insurance that the impact of predicted increases in heavy rainfall is cushioned.

On the other hand, storage structures (e.g. stormwater retention tanks) have, following the study, shown a need for adaptation. This, for one, is due to the use of a simple dimensioning procedure as recommended by RAS-Ew and also due to the increase of rain inflow in relevant, statistical rain events.

Replacing the surcharge factor as in RAS-Ew with the simple dimensioning according to the guidelines in DWA worksheet 117 is recommended. Furthermore, introduction of a surcharge factor that takes predicted heavy rainfall increases into consideration is also recommended, that is if the overflow frequency of the systems is to be maintained in the future.

Because projection data do not point to any significant inreases of dry spells, no further action is needed.

Project data

Term of project: 15/02/2012 – 16/05/2014

Ingenieurgesellschaft für Stadthydrologie mbH, Hannover
hydro&meteo GmbH&Co. KG, Lübeck

BASt unit Z5
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