WUR Colloquium Marco Zieverink

Datum publicatie 25-08-2010

Abstract
In the past decades the use of vegetated roofs has become more and more common. With it also the demand for more knowledge about the used vegetation and substrates has grown. Although in the past many substrate and vegetation types have been tried, it is still not always known why or how good the vegetation performs. Therefore this research has treated the following aspects:
- Indentify vegetation types potentially suitable for use on vegetated roofs.
- The hydrological requirements (e.g. stress or water level) of vegetation types ecology.
- The determination of hydraulic properties of roof substrates in the laboratory.
- Simulating hydrological characteristics of a vegetated roof in Wageningen with the program SWAP.
- Matching vegetation hydrological demand to simulated rooftop water availability.

 

In the study on potential roof vegetation the focus was on vegetation native to Wageningen. Therefore the Dutch-Flemish ecotope system was used to select ecotopes present near Wageningen that are comparable to those on roofs. This resulted in ecotopes P62 and G62 as potentially viable on vegetated roofs. The other method used was to check threatened habitats for viability of vegetated roofs using the EUNIS system. This was combined with the Natura2000 areas (to check if the habitats can be classified as native) and resulted in the selection of two habitats: \"Stroomdalgrasland\" and \"Heischraal grasland\". The selected ecotopes are characterized as moderately drought stress tolerant.

 

In the second phase of the project the hydrological properties of different kinds of substrate were determined using the evaporation method (verdampingsmethode) as described in NEN 5791 (in Dutch). For use in this project five substrate types have been obtained from roof constructors.
With the results of the evaporation method the soil hydraulic parameters were determined with the use of the computer program Appia.

 

With these parameters and other input data the model SWAP was used to simulate a vegetated roof in Wageningen The simulated roof had a substrate layer of 20 cm on which grass is grown. The grass was able to extract water up to -12000 cm and was no longer able to extract water when the pressure head was higher than -10 cm. On the basis of these critical pressure head values the days of drought stress and oxygen stress were calculated. In terms of drought stress the differences between the substrates are relatively small. Results suggest that for a single substrate the vegetation will experience more stress caused by drought. According to the results oxygen stress gives more insight in the differences between the substrate types. However the knowledge about the relation between plant species and oxygen stress is limited.

 

To make a proper comparison between the drought stress of the selected habitats and the simulated drought stress on rooftops a second simulation has been performed. Here all conditions are similar to those of the data of the EUNIS system results. From the outcome it can be concluded that habitat \"Stroomdalgrasland\" is suitable for used on a vegetated roof with substrate L or T+. Habitat \"Heischraal grasland\" is less suitable.

Overall the conclusions are that a comparison between substrates based on simulated groundwater levels does not produce satisfactory results. Better results are obtained with a comparison based on drought stress and oxygen stress. In terms of of drought stress one substrate seems to be more prone to drought stress, and is therefore less suitable for the vegetation type assumed. Of the selected vegetation types habitat \"Stroomdalgrasland\" is found to be the most suitable for use on one of the other four substrate types.

 

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