Flood mapping


Flood hazard and flood risk maps for the areas of potential significant flood risk in Finland are presented in the  flood map service. In addition, flood hazard maps for about 100 other areas and some historic flood maps are viewable in the service. Flood maps are prepared by the regional environment authorities (ELY-Centres) and the map service is provided by Finnish Environment Institute (SYKE) and Flood Centre.

Flood mapping is the process of identifying on a map areas at risk of flooding. It provides a good foundation for efficient flood-risk management. Flood maps can be used when drawing up flood-risk management plans, for preventing flood damages, in land use planning, for providing information on floods, in rescue operations and in determining what the lowest allowable construction elevation should be in order to avoid flood risk.

Different types of flood maps

A flood map can describe an observed flooded area (a historic flood map) or it can describe a simulated flooded area with an indication of the flood probability irrespective of whether a flood has occurred or not (an inundation map or a flood hazard map). 

A flooded area can be located from remote sensing data (aerial photographs or satellite images). Historic flood maps also can be produced near-real time using satellite images. Water levels of the flood can be measured later from the interpreted flood limit. 

A flood risk map is defined as a map showing the potential impacts of flooding. The risk is defined with an equation: Risk = Probability x Consequences, where Consequences = Hazard x Vulnerability (potential number of people at risk of being flooded, important operations, the buildings that are difficult to evacuate, potential economic damages and adverse consequences for the environment).

In the mapping of flooded areas, attention must be paid to the reliability and accuracy of the source information. The historic flood maps are based on observations and thus they are reliable, especially if the observed flooded area is derived from accurate aerial photographs or satellite images or from the field markings. However, there is often no aforementioned source information available or the observed flood extent does not stand for a flood of a magnitude that has been chosen. In this case the flooded area has to be modelled. In flood modelling, flood scenarios can be simulated for several different return periods. The degree of danger can also be determined (e.g. water depth or flow velocity).

In Finland, inundation maps, flood hazard maps or historic flood maps have already been produced for approximately a hundred different areas. Several more flood hazard maps will be produced in the next few years. Most of the flood maps produced have been published as PDF files on the website of Finland's environmental administration (see links at the end of the page).

Flood modelling

One challenge of flood modelling is estimating water levels of rare, major flood events. Many factors of uncertainty are included in the estimation because reliable hydrologic observations exist for only a short period. The discharges and water levels can be estimated using statistical methods or modelling of the hydrologic cycle (runoff models). Usually water levels are calculated in the river locations using one-dimensional hydraulic models. Two-dimensional models can be used in the complex reaches. The most important inputs of the models are the geometry of the river bed and discharge information.

In flood modelling, a digital elevation model (DEM) of the earth's surface is also needed in addition to the water-level information from the area to be examined. The digital elevation model describes the topography of the area, and its accuracy affects essentially the accuracy of the flood hazard mapping and the cost. An accurate digital elevation model can be produced, for example, from aerial photographs with photogrammetry or laser scanning. Both methods are fairly expensive. Inundated areas and water depths can be digitally modelled with the above-mentioned information using GIS software by reducing the digital elevation model of the earth's surface from the digital elevation model of the water surface (see picture below).

Flood modelling. Flood extent and water depth can be modelled by reducing the digital elevation model of the earth's surface from the digital elevation model of the water surface. Areas where the result of the subtraction is positive are inundated.

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Published 2015-01-19 at 9:32, updated 2020-11-02 at 8:45