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 30 hours) 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 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 the 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. Factors such as density, length and stiffness will influence grass effectiveness in reducing flow at the surface. Grass can resist erosion by reducing the velocity of flow at the bed and a dense even cover ensures effective flattening at high flows, with stolons providing an effective anchoring geotextile reinforcement.
Healthy swards can take four years to develop and establishing good grass cover can be hindered by various factors. Tussocky grass cover, broken and bare patches of grass, grazing animals, established weeds, slow-growing grass and boggy conditions were some of the problems highlighted from case studies across the UK in recent years. While maintenance considerations for grass cover include regularity of mowing, whether cuttings are left in place, and grass height. Although regular short-cutting results in thicker swards able to withstand surface abrasion, it may also impact resilience as swards becomes less diverse and the depth and density of roots are reduced. Other considerations also include whether to graze with sheep, cattle or horses, and the application of fertiliser.
Mix it up
The standardised approach of specifying grass-only seed mixes for grass cover at dams has recently been debated, as it is likely to lead to disappointment if unsuitable for local site conditions and not measured against a sufficiently robust specification. There is also concern that certain industry regulations may have underestimated the hydraulic performance of plain grasses.
More diverse seed mixes, including a small percentage of appropriately selected herbs or legumes, may be beneficial and help increase resilience if seed selection is based on observations of existing grass diversity in the embankment and adjacent areas.
For example, a multi-species mix of grass was proposed to improve resilience at a flood storage reservoir in southwest England in 2017. Upon inspection it was reported that the upstream dam face had generally good grass cover with a limited number of weeds, and the downstream face had reasonably dense grass cover with scattered weeds. As flood studies indicated overflowing would occur, consideration was given to specify a bespoke seed mix of species, based upon those already existing in the embankment where eight grass species, 33 other herbaceous and four types of legumes were found.
The Environment Agency (EA) recently commissioned a review of its industry specifications for grass-seeded areas, with the aim of providing recommendations for more easily enforceable definitions of what acceptable grass cover is. The EA’s landscape specifications did provide definitions of growth and development which mentioned healthy, vigorous grass swards, free from visible effects of pests, weeds, and disease, along with the appearance of closely knit, continuous ground cover of even density, height, and colour. Although these are described as being reasonable descriptions, they do rely on subjective assessment and clarification was needed to help resolve differences in contractor expectations for completion and those of qualified civil engineers for certification.
So, for density it was suggested that in general areas of the facility, at least 80% of quadrat subdivisions need to be filled with grass sward displaying continuous ground cover of even density, increasing to 88% field for spillways. It is also specified that grass seed mixes need to be provided by approved suppliers comprising species appropriate to asset use (ie an embankment and/or spillway) and the top and base soil conditions present.
Climate considerations
The impact of climate change on vegetation is not widely discussed by engineers and studies on its effects tend to be focused on agricultural grassland production. However, a recent assessment by Peak District National Park concluded in 2021 that their reservoirs are overall highly vulnerable and have high sensitivity and exposure to climate change variables, with moderate adaptive capacity.
Although some of the effects of climate change such as higher concentrations of CO2 combined with higher temperatures can increase grass yield, these will also be offset by extremes of drought, waterlogging and both high and low temperatures. Indeed, changing temperatures can also affect the lifecycle of pests, diseases, and weeds. With the net impact of climate change deemed difficult to predict, it is reasonable to assume that the increased frequency and magnitude of extreme weather events will have negative effects on grass cover on dams and levees.
Resilience
As the BDS seminar highlighted, maintaining healthy grass cover is necessary for safeguarding the resilience of embankment dams and levees and is critical if they are designed to overflow. Grass will tolerate a range of operational stresses ranging from drought to saturated ground if the type of grass is appropriately selected and is sufficiently diverse. And as weeds are inevitable, it is also considered to be more beneficial to select ones that can improve resilience without affecting performance. Care needs to be given to ensure proper grass establishment while quantitative assessment should help improve the quality of grass sward. And it goes without saying that sufficient attention must also be given to maintenance all year round.
This article first appeared in International Water Power magazine.
