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Grass revetments

Grass revetments on dikes

To protect the Netherlands against flooding, a sound set of design and legal assessment instruments for primary flood defences such as dikes, dams, dunes, and hydraulic structures is necessary. Such knowledge is included in the Fundamentals of Flood Protection, Technical Reports and Software. In recent research projects and from practical experience, it appears that stairs, transitions, trees, and fences are often potential weak spots in flood defences. Examples of this include dike breaches by Hurricane Katrina near New Orleans in 2005 and the high water in Thailand in 2011.

Erosion near Jahncke Pump Station, Lakefront. Local scour was limited to the transition and stair construction; no breach erosion occurred (Katrina 2005)
Local scour at the top of Lakefront levee on Hayne Blvd, near Vincent Road (Katrina 2005)

Usually dikes are covered with grass revetments both on the outer and inner slopes. To determine the erodibility of these revetments, various prototype experiments were conducted from 2007 to 2015 on sea, lake, and river dikes in the Netherlands, Belgium, and Vietnam. These tests were performed with a wave overtopping simulator, wave impact simulator, and run-up generator. They showed that the expected value of grass strength is considerably higher than the wave overtopping flow of 0.1 to 1 l/s per m. Various sea dikes have withstood simulated storms with an overtopping flow of 50 l/s per m with no damage. Transitions, trees, staircases, and dike structures were also tested, which collapsed much earlier than the grass revetment due to higher local flow rates. These structures can therefore be considered potential weak spots on dikes.

Unlike grass revetments, erosion failure mechanism is one of the most complex processes due to the need to model both waves and the erodibility of the revetment, especially at transitions and obstacles where the flow is not uniform. Moreover, the heterogeneous composition of the cover is modelled inadequately. After the flood disaster of 1953, the impact of wave overtopping never played a major role in strengthening and assessing dikes, because the regulations were such that hardly any water could be expected over the dike (0.1 to 1 l/s per m). If the policy anticipates higher discharges (e.g., 5 to 10 l/s per m), the design and the legal assessment instruments should also be adjusted to the presence of dike structures.

Currently, there are no guidelines for dike structures, for example, stairs or narrow corridors between houses where the current is concentrated during wave overtopping. If the transition from an asphalt road to grass cover is not strengthened, a dike can be damaged as a result of overflow and, in the worst case, fail. This is worrying because primary flood defences at these locations could be incorrectly approved.

Erosion resistance of grass revetments on the crest and on the slopes, including dike structures, should be assessed in accordance with the law, but no calculation rules are yet available for dike structures. Moreover, not all aspects of the erosion process are yet fully understood. Hoffmans Advice can help you better implement erosion modelling.

Garden design on the inner slope of a dike
Decrease of the inner slope due to the asphalt road-grass transition near Klong Ta Nueng flood defence (Thailand 2011)

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