Background & Objectives

A large number of 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. The 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 primary function of the grass cover is to protect the dike body against erosion caused by loading from waves and currents during storm surges. Normally, erosion processes are divided in two parts: (a) the detachment process in which soil particles are detached from the compact soil surface by water forces and (b) the transport process in which the detached soil particles are washed away. The main function of the dike grass cover is to avoid detachment.

Besides this primary function of preventing detachment, a number of other functions such as agricultural, ecological and recreational can be met. In this way, a grass cover revetment can fulfil several functions simultaneously: combining erosion protection with nature or recreation

The grass cover revetment comprises grassland vegetation rooted in soil. At many modern dikes, the grassland vegetation roots in clay as these dikes have a covering layer of clay on top of a sand core.

The thickness of the grass cover varies from dike to dike. The structure and division of the grass cover is illustrated further below The green leafy part of the grass is the sward. The turf is the root mat which provides the strength and erosion resistance to the cay layer. The roots keep the soil particles together and create a flexible and tough layer that offers significantly higher erosion resistance than a bare clay layer. Approximately 65% of the grass roots are located in the top soil layer (0-6cm depth). At the same time, the top soil is elastic in moist conditions that can deform without tearing. The subsoil is composed of heavier clay which is stiff, plastic and usually less permeable. Root density is less which causes less porous clay.

Since the grass cover revetment is composed of natural vegetation, the grassland cover can vary due to unequal plant growth. Hence, a grass cover revetment consists of a more inhomogeneous surface compared to an asphalt or concrete revetment. Nevertheless, the grass cover has to fulfil the main function of protecting the dike body against erosion due to wave attack and currents.

Since the grass cover revetment is composed of natural vegetation, the grassland cover can vary due to unequal plant growth. Hence, a grass cover revetment consists of a more inhomogeneous surface compared to an asphalt or concrete revetment. Nevertheless, the grass cover has to fulfil the main function of protecting the dike body against erosion due to wave attack and currents.

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

Moreover, the EroGRASS project intended to give insight into the relation between the grass erosion resistance, the quality of the vegetation and the management of the grass cover. Former investigations and experience have shown that the resistance of grass cover to erosion can be controlled by grassland management, as rooting becomes denser and the grass cover gets more resistance to erosion. Hence, a balanced management can lead to a well-rooted grass cover revetment.