Environment |
At grass roots level
A recent seminar presented by the British Dam Society gave an insight into the use of vegetation as an engineering material, and explained how grass cover on embankment dams and levees can help improve resilience to deal with increasing stresses
Above: Grass embankment dam
GRASS IS THE MOST commonly used plant in engineering and provides a cheap, versatile, durable surface to resist everyday wear and tear. It protects the soil surface from washout due to direct rainfall and surface run-off, while moderate damage does not last as the plant can regenerate. In addition, for dams and levees liable to overtopping and overflowing, grass also needs to provide a reliable long term revetment system to protect against the intermittent flow of water. Recent research has been undertaken on a purpose- built overflow generator to investigate the erosion potential of dikes. A Dutch and Belgian collaboration between Deltares, Delft University of Technology, and Flanders Hydraulic Research undertook destructive overflow testing at the Living Lab Hedwige- Prosperpolder.
Results showed that in the case of a high-quality grass cover, dikes can withstand high overflowing discharge for long periods. The test duration could be held for tens of hours (up to 30hours) without any critical damage occurring, despite measured flow velocities ranging from 2-4.5m/sec. It was also found that shorter grass (100mm) preformed less well, and where animal burrows, deformation or other anomalies
20 | January 2024 |
www.waterpowermagazine.com
were present, the erosion process initiated much earlier and progressed further.
Grass roots
Engineering with grass is described as being quite straightforward and the minimum requirements are for a good firm formation, plus a layer of quality topsoil and a surface that sheds water to prevent waterlogging.
When looking at the characteristics of grass consideration needs to be given to the sward – the parts of grass above and below ground characterised by a relatively short habit of growth and relatively continuous grass cover. Most grasses have a mass of fine fibrous roots and some also develop stems that run horizontally below ground (rhizomes). Healthy roots combined with strong rhizomatic growth ensures grasses are resistant to damages (reinforcing the surface and able to self-repair) and can colonise bare ground. Fibrous roots limit scour of soil at the surface and rhizomatic grasses are most suitable as their habit of growth continues to dense turf to resist erosion. Above ground there are grass blades and stolons – the stems that run horizontally above the surface.
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