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Experiments by Invited Researchers

 

EroGRASS: Failure of Grass Cover Layers at Seaward and Shoreward Dike Slopes

Project acronym: HyIII-FZK-20
Name of Group Leader: Thorsten Piontkowitz, Danish Coastal Authority
User-Project Title: EroGRASS: Failure of Grass Cover Layers at Seaward and Shoreward Dike Slopes
Facility: Large Wave Flume, GWK
Proceedings TA Project: Erograss: large-scale investigations of grass cover failure at sea dikes
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Summary:

The primary objective of the EroGRASS project is to perform large scale model tests to investigate in detail the erosion of a grassed clay dike cover due to (i) wave impact, (ii) wave run-up and rundown
flow and (iii) wave overtopping. The effects of wave impact as well as wave run-up and rundown flow on the grass cover are investigated on the seaward slope of the dike model. On the landward slope, grass cover erosion due to wave overtopping is investigated.

It is envisaged that the test results will improve the understanding of the erosion of grass cover layers due to wave forces. The clear physical understanding of the resistance of grass cover layers is regarded as indispensable, especially against the background of the enhanced focus on flood protection today and in future, also taking potential consequences of climate change into account.

The dike model has been built in the Large Wave Channel GWK with a crest height of 5.8 m above the bottom of the channel. The slope of the seaward side of the dike model is 1:4, the slope of the landward side is 1:3. The width of the dike crest is 2.2 m. In front of the dike model a foreland (slope 1:40) is built in order to generate higher waves and wave breaking.

The grass cover of the dike model has been established by 0.2 m thick grass sods, placed on a 0.6 m thick layer of clay, which covers the sand core of the dike. The grass sods are 2.35 m long and 1.25 m wide. In total, about 235 m2 of grass sods were transported from a natural dike near Ribe in Denmark to the GWK to be installed.

The installation of the 1.1 tons heavy grass sods on the clay layer was difficult and hard going, as no previous experiments could be used. Besides using a crane for transport, the cutting and installation of the grass sods were performed by additional handcraft. Experiences in building a prototype dike model in a large-scale research facility have been gained mainly in the way of handling the different building materials (clay, grass sods) and their properties.

Valuable measurements and observations have been obtained in investigating grass erosion due to breaking wave impact and wave up rush and down rush together with subsequent damage of the seaward slope covered with a grass clay layer. At the landward slope wave overtopping volumes and subsequent damage of the grass layer have been investigated.

The detailed analysis of the obtained data from the performed experiments focusses on:

  • The hydrodynamic processes associated with failure on outer slope and their implication for erosion. That includes the wave run-up and run-down processes, layer thickness, velocity as well as pressure caused by surging breaker and the implication on erosion and other failure.
  • The wave impact pressure that contains the effect of damping the impact by the water layer of preceding run-up. That includes further the analysis of the impact load time history, statistic analysis of impact pressure and force.
  • The hydrodynamic processes associated with failure and their implication of erosion on crest. The inner slope will be investigated based on the mean overtopping rate and the overtopping flow.

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