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Flash floods and Urban flooding Sweden


Problems with efficient dewatering following heavy rainfall events are not uncommon already today, e.g. because of urbanization beyond the system capacity, system damages or misconstructions, and tougher demands on the required performance. Thus, sewer system upgrades are required in many cities, and a key issue in this situation is whether to take climate change into account and, if so, how (1).

According to Swedish design guidelines, the sewer system should be dimensioned so that

  • the water level resulting from a 1-year rainfall does not anywhere reach above pipe top, and
  • the water level resulting from a 10-year rainfall does not anywhere reach above ground surface or basement elevation (2).

From an ensemble of six climate projections for two 30-year periods (1961–1990 as a reference and 2071–2100 for the future projection), it was concluded that the 10-year 30-min rainfall intensity would increase between 8% and 28% (on average 24%). For all projections, the future increase for duration 1 hour is similar to the 30-min increase. It has been estimated that upgrading the sewer system to achieve a satisfactory performance for the future design storm would cost approximately twice as much as an upgrade based on today’s design storm (1).

Adaptation strategies

Two main strategies exist with respect to flood prevention and control; subsurface- and surface-oriented. In the former the network capacity is increased, generally by replacing pipes limiting the discharge with larger-diameter ones or by installing subsurface storages. In surface-oriented solutions, open waters such as ponds or channels are constructed, in which storm water may be stored and delayed following heavy rainfall events. Generally, pipe replacement in an urban area is very costly (1).


The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Sweden.

  1. Olsson et al. (2013)
  2. Svenskt Vatten (2004), in: Olsson et al. (2013)