Experiences
Share your experiences with others so everyone can learn more about WSUD.
On october 21, 2014
Green roofs are gaining weight
A green roof is getting heavier, and you have to remember this from the start says Nykilde
The elements and the vegetation that are used in fully water-saturated condition regulate the weight of a green roof. This means, that you build up a test-model with all the elements : drain layer, growth layer and vegetation. This is watered until the water runs off. Then it is watered again until you are quite sure, that all the elements are completely water saturated. This gives a weight pr square meter that is used in the technical data for design of this roof.
Many people forget that a green roof over a period is collecting dust, leaves, vegetation e.g. and these things means, that the roof is gaining weight. Experiments over a 30-year period has shown that the roof is gaining 10 kg / square meter over that period. So when designing a green roof, you have to take this into account.
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Planting around rain gardens
Veg Tech has registered existing planting around rain gardens. The report was carried out by Lærke Kit Nielsen. Read the report here (in Danish): Planting report
Soakaways
Sludge trap
Sewer contractors have noted that soakaways for rainwater infiltration from agricultural washdown areas can pose a problem. If machinery is used extensively on soil with a high clay content, standard sludge traps cannot filter off material effectively, and soakaways quickly become clogged. Sludge traps must therefore be extra large.
Ventilation
Some people maintain that soakaways must be ventilated because it is difficult to force water through an air-filled space. Is that true?
Soil transfer
Is it true that “The Soil Contamination Act” requires contamination analyses when the excavated soil exceeds 1m3?
Earthworms and soakaways
Even though it is theoretically impossible to infiltrate in clay soil, it is often feasible in practice.
This is mainly due to the presence of tunnelling earthworms that increase soil aeration. There are approx. 500 earthworms per m2 soil and if they can be kept close to the soakaway, the solution will work well. Story says that when draining clay fields, farmers put straw at the bottom of the excavation to attract earthworms and thus improve drainage.
Earthworm tunnels, holes left by roots and other cracks are found at the top level (1-1½ metres down, i.e. in the brown clay soil). Below this layer, the clay is grey in colour and there are no cracks or crevices so the clay is dense. If soakaways are to be built in clay soil, they must be high up, well above the grey clay layer otherwise they will never drain properly.
It is also probably a good idea to collect earthworms to ensure a high level of activity when the soakaway has been built.
Geotextile is not just geotextile
Karup Airbase has numerous plastic soakaway units, and it has been something of a mystery as to why they have not worked better than they do. The initial theory was that the inefficiency was due to air pockets in the soakaways that stopped the water from entering, but on closer examination it was found that the geotextile surrounding the soakaway units was covered by a 2-3 mm layer of dirt on the inside, stopping the water from seeping from the soil.
Others have had similar experiences, namely that water cannot penetrate the geotextile - even when it is brand new.
Is it a bad idea to use geotextile around the soakaway units? Yes, if you use the cheapest kind and do not enquire about its properties. There are many types of geotextile, all of which have different properties. The most important property for geotextile used around soakaway units is water permeability, so ask the manufacturer and do not just buy the cheapest roll at your DIY centre.
As examples of geotextile requirements we can cite the Danish Environmental Protection Agency’s guidelines for sand filtering systems, which state that the geotextile used between the sand layer and collection layer for treated wastewater must meet the following standards: It must be a non-woven type with a water permeability of at least 20,000 litres/m2/per 24 hours and with an elongation at rupture of at least 25%.
To avoid using geotextile on soakaway units, engineer Christian Ramsgaard from the Karup Air Base has developed a hybrid soakaway that combines soakaway units with conventional gravel (that cannot penetrate the surface of the soakaway units). See more at www.hybridfaskine.dk (in Danish).
Geotextile around drainage pipes in connection with a perimeter drain gives rise to the same problems.