Last reviewed 30 Sept 2026 · 8 min read
Purpose of breakwaters
A breakwater is a structure built to protect a harbour or a shore from the action of waves. It reduces wave energy reaching the protected area so that vessels can moor safely, and it can also prevent siltation of the entrance and protect against the drift. It may also act as a quay on its inner face (a mole).
Types of breakwater
| Type | Description | Use |
|---|---|---|
| Rubble-mound (sloping) breakwater | A mound of quarry rock with an armour layer of large stones or concrete units; waves break on the slope, and their energy is dissipated in the voids | The most common; suitable for any depth up to about 30–40 m and on relatively weak foundations; flexible and repairable |
| Vertical-wall (monolithic) breakwater | A wall of concrete caissons, blocks or masonry; reflects waves | Deep water with a good foundation; less material but the foundation is heavily loaded and reflection causes scour |
| Composite breakwater | A rubble mound (base) with a vertical wall on top | Deep water; combines the advantages |
| Horizontally-composite / berm breakwater | A wide berm of stone on the seaward face, allowed to reshape | Stone-efficient in places with wide range of stone sizes |
| Floating breakwater | Pontoons or floating structures anchored in place | Sheltered, shallow, low-wave sites (marinas) |
| Special types — perforated (Jarlan), pneumatic, hydraulic | Wave-absorbing chambers, air bubbles, water jets | Rare |
Rubble-mound breakwater
Components
A typical cross-section, from the core outwards:
- Core — quarry run (small stones, 1–500 kg), the bulk of the volume; it is not exposed to the direct wave attack.
- Filter (under) layer — stones of intermediate weight, about one-tenth of the armour weight, preventing the core from being washed out through the armour.
- Armour layer — the outer layer of the largest stones or artificial concrete units, resisting the direct wave attack; normally two layers thick.
- Toe protection — a berm of stones at the foot of the armour that supports it and resists scour.
- Crest and crown wall — a concrete wall (superstructure) that provides access, resists overtopping and can carry a road or a pipeline.
- Rear-side protection — armour in the lee (for overtopping and the waves inside the harbour).
Armour units
Natural rock may not be large enough where the waves are high. Artificial concrete armour units — cubes, tetrapods, tribars, dolosse, Accropode, Core-loc, Xbloc — are cast in the required weight. Interlocking units (dolosse, tetrapods) are more efficient (a higher stability coefficient , so lighter units for the same wave) but they are slender and may break; simple cubes are robust but need to be heavier.
Hudson formula (armour weight)
where:
- = weight of an individual armour unit (N or kg × g),
- = unit weight of the armour material (rock ≈ 26 kN/m³; concrete ≈ 23–24 kN/m³),
- = design wave height (usually or at the structure),
- = specific gravity of the armour relative to seawater ( kN/m³),
- = angle of the slope with the horizontal,
- = stability coefficient depending on the unit shape, placement, damage level and the position on the structure (trunk or head — the head is more exposed and needs larger stones). Typical: rough angular quarry stone (2 layers): about 2–4; cubes: about 6–7; tetrapods: about 7–8; dolosse: about 16–30 or more (indicative values; use the applicable design manual).
The weight of stones in the successive layers is a fraction of : the filter layer is about to and the core even less.
Design wave height = 5 m, armour of concrete cubes with = 24 kN/m³, sea water 10.05 kN/m³, slope 1 (vertical):1.5 (horizontal), = 6.5.
; ; ; .
per cube.
The filter layer is then of the order of 1.0–1.2 tonnes each.
Crest level is fixed by the design water level plus wave run-up and allowance for overtopping. A wide crest (at least three armour units) permits construction and reduces overtopping. Side slopes typically 1:1.5 to 1:3.