Dike Model

The prototype of the selected dike model represented a typical sea dike of the German Bight coast. With exception of the relatively steep seaward slope, it was comparable to typical dike cross sections as usually built in The Netherlands, Germany and Denmark. In general, a sea dike consists of a sand core, a clay layer and a grass cover.

The slope of the landward side of the dike model was 1:3, as being recommended in several national design manuals for sea dikes. The slope of the seaward side of the dike model was 1:4. This relatively steep slope was chosen instead of 1:6 in order to investigate impact loads due to wave breaking on the seaward slope without the damping effect of the previous wave down rush.

The crest height of the dike model was 5.8m above the bottom of the channel. The width of the dike crest was 2.2m. A foreland (1:40) was built in front of the dike model in order to provoke higher waves and wave breaking. The following figure shows the cross section of the dike model.

The dike model consisted of a sand core covered by a layer of clay and a grass cover. About 480m3 of sand were needed to build the sand core. This sand was available at the Coastal Research Centre. On both slopes and on the dike crest a clay layer of 0.6m was installed. On top of the clay layer, 0.2m thick grass sods were placed to complete the dike model with a grass cover. By placing the grass sods, the entire clay layer was about 0.8m thick. In order to cover the dike model with a 0.6m thick clay layer, a total clay volume of about 150m³ was needed.

On top of the clay layer, 0.2m thick grass sods were placed to complete the dike model with a grass cover layer. Since it is not feasible to sow grass on the clay layer and wait for well-established grassland vegetation, grass sods were excavated at an existing dike and transported to the Coastal Research Centre by trucks. The grass sods were 2.35m long and 1.25m wide. In total, about 200m2 of grass sods were needed.

The dike model was constructed in the Large Wave Flume of the Coastal Research Centre (a joint centre of the Universities of Hannover and Braunschweig) in Hannover, Germany. The Large Wave Flume is 5m wide, 7m deep and 324m long. The maximum water depth in the flume is 5.0m. Regular waves can be generated with heights up to H ≈ 2.0m and irregular waves with significant heights up to Hs ≈ 1.3m.

The crest of the dike model was located at a distance of about 190m from the wave generator (approx. at 2/3 of the channel length) due to a window in the flume wall which allowed the observation of the wave impact on the seaward dike slope The flume area behind the dike model was needed as a reservoir for overtopping water.

In order to fulfil the objectives of this research project, the dike model had to be covered with a natural grass layer. The challenge was hereby to get the natural grass into the flume. Since it was not feasible to sow grass on the clay layer and wait for a well-established grass cover, grass sods were excavated from an existing sea dike near Ribe (Denmark) and transported to the wave flume for installation on the dike model. The grass sods were excavated from the southern wing dike which was reinforced in 1998. The composition of grass community of the southern wing dike was similar to those used in Germany and the Netherlands.

Excavation of the grass sods at the wing dike was possible since the wing dike was not directly facing the sea. Only if a dike breach occurs in the first defence line, the wing dike would be loaded by waves and currents. The dike construction and the growing grass species are however identical to the main sea dikes. Permission to extract the grass sods was given by the Ribe Dike Board.

The grass sods were excavated from the dike crest of the southern wing dike during the last week of January 2008. The size of one grass sod was 2.35m in length and 1.25m in width. The thickness of the grass sods varied from 17cm to 22cm.

In detail, a tractor was equipped with an attachment composed of one horizontal blade and two smaller vertical blades. The cutting width of the horizontal blade measured 1.25m. The attachment was assigned to cut underneath and at both sides of the grass sod. While pulling the attachment, a wooden plate connected to the attachment by two chains was pulled underneath the grass sod at the same time.