| Environment
Factors such as density, length and stiffness will influence grass effectiveness in reducing flow at the surface. Grass can resist erosion by reducing 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 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.
References
Grass cover: improving establishment and resilience to deal with increasing stresses. Event organised by The British Dam Society. 6 March 2023.
www.ice.org.uk/events/ past-events-and-recordings/ recorded-lectures/improving- establishment-and-resilience
Below: Grass is the most commonly used plant in engineering
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