Flooding risk assessment in Tårnby Municipality

The Danish BBR register

Climate change creates a risk for increased precipitation and sea level rises. Many low-laying buildings are in the risk zone of having flooded basements, which can have high financial cost to the owners, as well as destroy irreplaceable items.Therefore, it is relevant to investigate how many buildings lay in this risk zone. With Tånby Kommune as a case example, we will investigate the number of buildings with basements in low laying areas, as it is these that face the risk of being flooded. This will give us view of how big the problems is in the municipality, as well as a rather precise number of how many square meters of basement is in the risk zone.

Data

The input data for this exercise comes from the BBR Register, which has detailed information available for all addresses in Denmark. The Bygnings- og Boligregister (Building and Dwelling Register) has been in use since 1977 with the purpose to solve administrative functions. The entire database is available online in the form of an SQL database; for the present analysis, however, a smaller offline copy with selected data only for the municipality of Tårnby will be used.

The basement area calculations will be based on the floor area of each building.

Methods

The workflow of the operations can be outlined as follows:

  • A unified database of information relevant to the exercise was created by combining data from different sources (geography and floors of buildings). The entries located in Tårnby municipality were selected.
  • “Total basement area” data was incorporated to the aforementioned database and total basement area information was attributed to each of the selected buildings in Tårnby.
  • All buildings with basements below 2 m were selected and their total basement area was calculated.

Results

As the map shows, it is mostly buildings in the western part of the municipality that are under flood risk, with a small patch in the northeastern part facing the same risk as well. Most of the buildings in the municipality lay in the center, where the elevation is higher than two meters.

There are a total of 340 houses in the risk of being flooded and the total basement area in these zones is 31 847 square meters.

Map showing buildings under flood risk

Comments

The analysis has been simplified by choosing all houses with basements at elevations below two meters. This simplification may give considerably different estimates for a couple of reasons. First, not all basements are built at the same height. Where some basements are fully submerged below ground level while others are partially above. Secondly, local dips in elevation are not taken into consideration.

Workflow