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Flood Hazard and Flood Risk 2019

Methodology

By December 22, 2019, flood hazard maps and flood risk maps were developed or updated for areas for which flood risks had been identified in 2018. The flood hazard maps show which areas could be flooded according to different probabilities of occurence. In Berlin, the following three flood scenarios are considered based on the FRMD:

  1. flood with a low probability or extreme event (rare event, HQselten/ HQextrem)
  2. flood with a medium probability (medium event, HQmittel)
  3. flood with a high probability (frequent event, HQhäufig).

Table 1 defines the flood events and flood scenarios according to the definitions by the River Basin Community Elbe.

Tab. 1: Flood event/ flood scenario definitions

  • Event

    Description

  • Rare event
    (HQselten / HQextrem)

    HQselten or HQextrem events are statistically very rare. Extreme events are incidents caused by the failure of flood control facilities, an unfavourable combination of rare flood events or an unfavourable combination of rare flood events and discharge impairments due to structural or other reasons. For areas along river Elbe with flood control facilities in place, the extreme event caused by the failure of flood control facilities is shown by default. HQ200, which statistically occurs every 200 years, is used to represent flooded areas. In areas without flood control facilities, HQ200 is also displayed.

  • “Medium event
    (HQmittel)”

    As defined by the European FRMD, HQmittel represents a flood event that statistically occurs every 100 years (HQ100). This does not rule out, however, that such an event may occur multiple times in a hundred years. According to German water law, HQ100 is used to define flood areas.

  • Frequent event
    (HQhäufig)

    HQhäufig is a flood event that statistically occurs much more frequently than once in a hundred years. The probability of HQhäufig to recur at waters in the RBC Elbe is regarded to be either 5, 10 or 20 years.

  • HQ: Abbreviation of “high” and the discharge coefficient Q.

In Berlin, several methods were used to determine the flood hazard maps for the different flood scenarios. This required, on the one hand, a joint approach with the State of Brandenburg, as the bodies of water flow from Brandenburg into Berlin and as the Havel flows back from Berlin into Brandenburg. On the other hand, this involved adapting the methodology to suit the natural conditions and data availability. The chapter on the methodology for deriving flood areas (SenStadt 2019c) and relevant studies (IPS 2009, IWU 2014, 2015, Koenzen et al. 2011) outline the methodological approaches taken.