Protection against flooding from the river
Description​
The ecosystem service protection against flooding from the river concerns the temporary storage of water in areas that are relatively tolerant to flooding, so that flooding in sensitive areas (mainly urban areas and agricultural areas) is avoided. Historical changes in the landscape (drainage canals; deepening, diking and straightening of rivers and streams; increase in paved surface) have ensured that water is drained more quickly. During heavy rainfall, the increased and accelerated discharge of water downstream can cause flooding by overflowing the banks of watercourses. By restoring the water storage capacity in upstream areas, these floods can be reduced downstream. Tidal action during storms can also cause rivers to overflow their banks. In the case of protection against flooding due to tidal action, this can also work in reverse.
This service, although very important, cannot be put into simple rules to implement. It cannot therefore be included in the web tool.
Qualitative valuation​
For the qualitative assessment of areas that are potentially important for water storage, the physical storage capacity on the one hand and the type of land use on the other play a role. The scores for water storage are determined based on the physical suitability and desirability of flooding. This is very specific and cannot be put into a simple rule.
Quantitative valuation​
To determine the magnitude of the extent to which ecosystems contribute to flood protection, it is necessary to map, on the one hand, how much water these ecosystems store and, on the other hand, how much flooding volume downstream the ecosystems avoid. Flood models from, for example, the Hydraulic Engineering Laboratory are able to calculate flood maps that show the extent of the flood and the water depth. They do this for various possible floods, each with its own chance of occurrence.
Monetary valuation​
As shown in the SCBA of the Sigma Plan (IMDC, 2012), the benefits can potentially be very large. The total safety benefits of the optimal alternative between 2010 and 2100 were estimated in the SCBA Sigmaplan at €737 million. This is a reduction of the flood risk due to tidal action on the Scheldt by 78%. To compare this with other services, this equates to an annual benefit of approximately €30 million for the entire Sigma plan.
No generally applicable key figures are given in this study for this regulating service. After all, the benefits of protection against floods are very area-specific (depending on the damage).
More detailed models​
In Flanders, the flood risk methodology of the Hydraulic Engineering Laboratory (LATIS) is often applied (Deckers et al. 2013). In this methodology, the flood maps are combined with the land use to estimate the expected damage as a result of these floods.
In LATIS, the material damage that we can expect as a result of specific floods is estimated based on flood heights, damage functions (relationship between flood height and damage) and replacement values. By calculating the damage for different floods with different probabilities of occurrence, the flood risk can be determined. The difference between scenarios with and without additional water storage as a result of wetting or the creation of flood areas is a measure of the monetary benefits from protection against floods.
The results of these types of calculations are not available for the whole of Flanders and cannot be converted into a simple calculation function. It is best to check for navigable watercourses with the Hydraulic Engineering Laboratory (waterbouwkundiglabo@vlaanderen.be) or for non-navigable watercourses with the VMM (info@vmm.be) whether such an exercise has been carried out for your study area.