EroGRASS: Failure of grass cover layers at seaward and shoreward dike slopes. design, construction and performance

EroGRASS: Failure of grass cover layers at seaward and shoreward dike slopes. design, construction and performance

Piontkowitz, T.

Verhagen, H.J.
Verheij, H.J.
Cao, T.M.
Dassanayake, D.
Roelvink, D.

Civil Engineering and Geosciences

Hydraulic Enginering

2009-04-01

A large number of the dikes in the North Sea and Baltic Sea regions are covered with grass that is exposed to hydraulic loading from waves and currents during storm surges. During previous storm surges the grass cover layers often showed large strength and remained undamaged. A clear physical understanding of the failure of grass cover layers due to different wave loads is therefore indispensable today, especially against the background of enhanced hydraulic impact due to climate change. The strength of the grass cover layer lies mainly in its ability to withstand three types of wave actions: • Wave impact due to wave breaking on the seaward slope • Wave run-up and run-down flow after wave breaking on the seaward slope • Down-slope flow on the landward slope caused by wave overtopping. The main objectives of this research project are therefore to perform large scale model tests to investigate in detail the failure of grass cover layers due to (i) wave impact, (ii) wave runup and run-down flow and (iii) wave overtopping. Wave impact as well as wave run-up and run-down flow may induce grass cover failure on the seaward dike slope. Wave overtopping causes failure of the grass cover at the dike crest and on the shoreward slope. Hence, this research project deals with the investigation of grass cover failure anywhere along a dike profile: seaward slope, dike crest and shoreward slope. It is envisaged that the proposed research and tests will improve the understanding of the failure of grass cover layers due to wave loading. To obtain the aforementioned research objectives, large scale model tests at a dike model have been performed in the Large Wave Flume of the Coastal Research Centre – a joint centre of the University of Hanover and the Technical University of Braunschweig, Germany. The dike model represents a typical sea dike. With exception of the seaward slope, it is comparable to typical cross sections of sea dikes built in The Netherlands, Germany and Denmark. This relatively steep seaward slope has been chosen to improve the generation of wave impact on the seaward slope. The crest height of the dike model is 5.8m above the bottom of the wave flume and the dike model consists of a sand core covered by a layer of clay and a grass layer. The 0.2m thick grass cover layer is constructed with grass sods that have been excavated at the existing Ribe sea defence in Denmark and transported to the Large Wave Channel in Hannover by trucks. The report presents a first reporting of the EroGRASS project including a description of the design and construction of the dike model in the Large Wave Flume, the measuring and observation techniques and the test programme together with examples of records from the performed tests. Focus of this report is put on providing a well-documented description of the aforementioned issues, whereas the data analysis and presentation of the results are not included in this report since these are in progress and will be reported later.

dike
revetment
grass

http://resolver.tudelft.nl/uuid:7c5a02bb-1358-4448-ac71-41bb5e06f4af

Danish Coastal Inspectorate

Erograss project, Hydralab III

Institutional Repository

report

EroGRASS