02 - Height of Buildings Above Flood Water Level

The height the site is located above any nearby water body and the distance between them are obvious considerations. The values at risk, impact of the potential flood on the value chain as well as regional extent of flooding, which can impact the lifelines (utilities) to the site give an indication of the exposure.

Many regulatory authorities and regional planning agencies define exposure zones for natural hazards Issues to consider for rain-induced (surface runoff) flooding include large areas of paved surfaces, deforestation, high development in recent years, etc.


  • The site is located in a flood zone with short recurrence times, i.e. high probabilities of being exceeded (on average 1 in 50 years or more frequent). See box below for explanation on the relationship between flood probabilities and return periods.
  • High percentage of assets and insured values concentrated at the part of the site that would be flooded.
  • If no flood maps available: Site is located at low elevation or close to water bodies. High hazard is assumed if buildings / assets elevation is less than 3 m above a water body.
  • Inadequate consideration of intense precipitation leading to pluvial flooding, such as inadequate drainage capacity or insufficient maintenance (from roofs, balconies, areas around basement and ground floor openings, ventilation shafts, driveways and parking lots, etc.).


  • Located in a flood level with high recurrence times / low probabilities of being exceeded based on reliable flood maps.
  • Small percentage of assets and insured values at stake by flood, eg. by being raised or protected from flooding.
  • If no flood maps available: Site is located at higher elevation and away from water bodies, not exposed to expected flood levels.

Risk improvement ideas

  • If no site-specific flood assessment has been carried out yet, consider doing one. Depending on the topography and type of adjacent water bodies, ie. flooding source, the following types of flooding can occur: flash flood, surface water overflow, fluvial flood, tidal flood, storm surge, pluvial flooding. If no flood maps are available, an approximation should be made to determine the potential water height inside the site as well as extent of flooding in the region.
  • Identify and check possible watercourse restrictions and how they could be avoided. Blocked culverts and bridges are known to have locally increased the water depth in floods considerably, ie. from a 50 year to a 100 year event.
  • Regularly consult local authorities for changes in flood hazard and associated building zone designation. It is a good idea to proactively ask for the building regulations close to water bodies and to get involved early in hazard mapping and zoning changes.
  • Check with local authorities what the flood risk is for an area before considering new building or extension projects.

System outage this weekend

What is “return period”?

Return period is another way of expressing the probability of occurrence of an event of a certain magnitude within a specific time frame. For example, a 100 year flood is an event that has a 1% probability of occurrence in a given year or a 10% probability of occurrence within a 10 year period. Similarly, a 50 year flood has a 2% annual probability of exceedance or a 10% probability of exceedance in a 5 year period. This does not mean an event will occur once in the given time frame. Rather a 100-year flood event can happen more than once in any given year, and it can occur once a year over several years in a row. The water level of a 100-year event may be referred to as HW100 and the corresponding floodwater flow as HQ100.