Design And Construction Of Berm Breakwaters

Jentsje van der Meer, Sigurdur Sigurdarson

• Éditeur: WSPC
• Année: 2016
• ISBN:
  9789814749626
• Sujet: Biological Sciences
• Sous-sujet: Science General

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Modern design of berm breakwaters began about thirty years ago. However, to date, there has been a lack of a well-established, formal design methodology on berm breakwaters. The authors Dr Jentsje van der Meer and Sigurdur Sigurda

Chapter 1

History of Modern Berm Breakwaters

1.1 Time before modern berm breakwaters

Several nineteenth century breakwaters are usually reported as the origin of berm breakwaters, [Bruun and Johannesson, 1976], [Baird and Hall, 1983]. Many of these were exposed to waves too high in relation to the size of rock available for construction. Steep slopes resulted in severe damage, often repaired by a continuous supply of fairly small rock until an almost stable S-shaped equilibrium profile was reached. Among these are the breakwaters at Cherbourg in France, Plymouth in UK, Madras in India and Port Elliot in Australia.

In the 1960s, Priest et al. [1964] described a seaward profile, which is natural to the breakwater materials and the waves to which they are subjected. Experimental studies, often at a single water level, showed a stable, reshaped cross-section of an S-shape resulting in less wave action than on the initial steeper profile. It was concluded that a greater cross-sectional area was required for the breakwater forming the natural profile than for the conventional type cross-section. But, considering the possibility of using smaller stones than those indicated by conventional formulae, there might be instances where breakwaters with natural profile will compare favourably, in an economical sense, with those of conventional profiles.

In the late 1970s and early 1980s many researchers and engineers considered the idea of equilibrium slope and the importance of porosity or permeability, [Bruun and Johannesson, 1976], [Bruun, 1985]. The porosity of the armour, under-layers and core is a determining factor in the intensity of out and inflow which affects the stability of the structure. It was noted that the stability of rubble mound structures increases when “maturing”. That is, the structure adjusts to wave attack and reaches an equilibrium profile or an S-shape with relatively small stone considering the wave climate.

1.2 Developments in Australia, mass-armoured breakwater

Australia played an important role in the development of berm breakwaters with many innovative structures being built in the 1970s and early 1980s. The earlier mentioned breakwater at Port Elliott in South Australia was one of the 19^th century breakwaters constructed by dumping quarried rock into the sea and allowing it to take its own natural slope. At Grassy on King Island, Australia, a refinement of the dumped rock breakwater was achieved using available material from a nearby quarry and careful design, [Gourlay, 1996]. Here 95% of the material was less than 2 t and 5% was between 2 and 10 t. A core of quarry run material was pushed out to an offshore island in up to 18 m water depths, allowing waves to form it into a rocky beach before the large rock, 6-10 t, was placed on top of the reshaped profile for stabilisation.

During a cyclonic attack, the Rosslyn Bay breakwater in Queensland, Australia, suffered severe damage. At high tide the breakwater was heavily overtopped causing catastrophic failure. Material was displaced from the crest and deposited on the leeward slope, widening the profile while the crest was lowered by about 4 m. Still the reshaped breakwater showed a capability of protecting the harbour to some extent, [Foster et al., 1978]. The breakwater was repaired by using commonly available rock sizes intermixed with modified concrete cubes with a grading that had the highest possible permeability, [Bremner et al., 1987]. The construction procedures eliminated or reduced the use of a crane and simplified the construction by end-tipping with a minimum amount of trimming by dozer and backhoe. The design anticipated that natural wave action would reshape the seaward slope to the stable S-shape found in nature.

The experience from the Rosslyn Bay breakwater was used in the design of an offshore breakwater to protect a reclamation adjoining Townsville harbour in Queensland, [Bremner et al., 1980], [Gourlay, 1996]. A shore-parallel offshore breakwater was built with crest level above high tide level. It was expected to fail or reshape under extreme wave conditions to form a submerged structure limiting the waves reaching the revetment protecting the reclamation. Extensive model testing showed that the design concept provided a considerable degree of safety against the design conditions being exceeded. Cost savings of the order of 40% were achieved over a conventional design, partly due to relative ease on construction not requiring large cranes.

The design of the Hay Point tug harbour in Queensland used the experience from these structures, [Bremner et al., 1987]. Interpretation of preliminary quarry investigations and trial blasts in a nearby quarry assumed a maximum available rock size of 2-3 t. Further investigations, however, showed that it was possible to quarry armourstone of 3-7 t in large quantities. The development of design using these armourstone led to a definition of the mass-armoured breakwater that is designed and built in an initially unstable form, but with sufficient material provided to allow natural forces to modify its shape to a stable profile. Among the advantages of the mass-armoured breakwater is the use of natural rock in its available sizes.

1.3 Developments in Canada, modern berm breakwaters

Bill Baird and Kevin Hall from Canada initiated the design of what could now be called "modern berm breakwaters", [Hall et al., 1983] and [Baird and Hall, 1984]. The idea was simple and effective with respect to design, construction and costs.

According to Baird and Hall [1984]:

The basic principal involved in this concept is the use of locally available materials. It is established that the greater the thickness of the armour layer, the smaller the stones that are required to provide stable protection against wave action. Therefore, the thickness of the armour layer for a specific breakwater is determined by the gradation of the available armour stones and the incident wave climate. The final cross-section makes allowance for the practical considerations of breakwater construction. New concepts for breakwaters that have resulted from the use of this alternative design procedure are described. Construction of these breakwaters in 1983-84 has demonstrated that significant cost savings are obtained.

In principle, the full quarry yield was divided into two classes: core and armourstone. The armourstone was used to create a homogeneous and permeable berm, including crest, and was constructed just by putting rock into the sea, as seen in Figure 1.1. This created a very steep seaward slope, often close to the angle of repose. The rock class was fairly small compared to a conventional stable structure and the first storms would partly reshape the berm into a more stable S-profile. Designs storms would give more reshaping until a large part or the whole berm was eroded and a stable S-profile was established. The easy quarrying (only two rock classes), easy construction and use of fairly small rock instead of large rock, or even concrete units, led to substantial cost savings.

Figure 1.1. Principal sketch of first design of modern berm breakwaters in 1984.

Figure 1.2. Berm breakwater of Helguvik, Iceland. Designed in 1983 by Baird & Associates and constructed in 1986-1988 by Icelandic contractors.

By 1984, two berm breakwaters had already been constructed in Canada: Codroy in Newfoundland and North Bay in Ontario. The Helguvik breakwater in Iceland had also been designed by Baird & Associates, but was constructed a little later. Figure 1.2 shows the Helguvik breakwater more than twenty years after construction. Although the intention was to have a reshaping berm breakwater, the berm has hardly been reshaped during this period as quite some safety was used in the detailed design, mainly through the establishment of design wave height.

1.4 Contact between Canada and the Netherlands

The paper Baird and Hall [1984] was presented at the International Conference on Coastal Engineering in Houston. It was at the same conference where the first paper on new stability formulae of Van der Meer [Van der Meer and Pilarczyk, 1984], was presented, using the stability number H_s/ΔD_n50 (H_s = significant wave height, Δ = relative mass density and D_n50 = nominal diameter, see also Chapter 2). At that time, the research of Van der Meer was still in progress, with a focus for 1984 and 1985 on dynamically stable structures, like gravel or shingle and rock beaches.

The idea was to describe profile formation for dynamically stable structures, which indeed became possible (Van der Meer [1988-a]). There is a direct link to reshaping berm breakwaters, being the connection between full dynamically stable structures and statically stable conventional structures.

A week after the conference in Houston, Baird and Van der Meer met each other in Ottawa, discussing the berm breakwater concept and dynamic stability with profile formation. It led to restructuring of Van der Meer's research on rock slopes and gravel beaches, including some tests with berm profiles (Figure 1.3).

The focus of the research, however, was from gravel beaches towards reshaping berm breakwaters, not from static stability to more dynamic stability. Most of the berm profiles had stability numbers of H_s/ΔD_n50 = 3.8-6.0, still far from statically stable reshaped berm breakwaters, where the stability number should not be larger than H_s/ΔD_n50 = 3.0. Dynamically stable structures, like rock and gravel beaches, could well be described by the parameter H_oT_om, where T_om is a dimensionless mean wave period (see Chapter 2). Using this parameter means that a longer wave period will give a longer S-profile. It is this parameter H_oT_om, which still plays a role in description of the behaviour of dynamically stable structures, but less in berm breakwaters as the influence of the wave period is much smaller for berm breakwaters.

Figure 1.3. Berm-type profile in research on dynamic stability of [Van der Meer, 1988-a]. Test 388, including calculated profile by Breakwat; H_s/ΔD_n50 = 5.9.

1.5 Developments in Iceland

During the preparation phase and model testing of the Helguvik breakwater in 1982 to 1983, undertaken by National Research Council (NRC) in Ottawa, the Icelandic Harbour Authority came to know about the berm breakwater design procedure. It was recognised that this design was very well suited for Icelandic conditions. At this time, the general opinion among engineers, geologists and contractors, was that it was difficult to quarry large armourstone from the Icelandic basalt. Several breakwater projects had been delayed due to th...