Navigation Structures: Breakwaters, Locks and Ship Lifts
Breakwaters, locks and ship lifts create and control navigable water; this domain tests armour stability, crest levels, and the hydraulics of transferring vessels between water levels.
Rubble-mound breakwaters
- Hudson formula: W = ρr H³ / (Δ³ KD cot θ), with Δ = ρr/ρw − 1. KD depends on armour type and acceptable damage: rock roughly 2–4, dolos 15–20 at 0–5% damage.
- Van der Meer's formula is preferred for long design lives and plunging waves because it accounts for wave period and number of waves, not just height.
- Crest level = design run-up + setup + freeboard. Overtopping limits are commonly 1 l/s/m for pedestrians and up to about 10 l/s/m where vehicles operate.
- Toe, underlayer and filter gradations follow standard ratios (underlayer about W/10, filter W/15 or geotextile) to prevent washout; trunk and head sections are checked separately.
Locks
- Capacity is set by lockage time: approach, mooring, filling/emptying, gate operation and exit. Filling time depends on culvert hydraulics and on allowable hawser forces, typically 5–10 kN per line.
- Chamber dimensions come from the design vessel formation, not one vessel; a 110 × 11.4 m chamber is the European Class Va standard, 190 × 12 m for Class Vb.
- Mitre gates suit low heads; sector, lift or sliding gates suit higher heads and shorter cycle times.
Ship lifts
- Caisson or trough lifts, counterweighted, suit high lifts (tens of metres) where lockage water loss is unacceptable; compare lift cycle time against a flight of locks when choosing.
Worked example
Armour stone ρr = 2,650 kg/m³, seawater 1,025 kg/m³, design H = 3.0 m, KD = 2, cot θ = 1.5. Δ = 2.65/1.025 − 1 = 1.585. W = 2,650 × 27/(1.585³ × 2 × 1.5) = 71,550/(3.98 × 3) ≈ 5,990 kg, so specify 6 t armour with a two-layer layer and a W/10 underlayer. A 3 t stone would be unstable at this wave height.
Traps
- Using Hs in Hudson without stating which design-wave convention the code requires.
- Judging lock capacity from chamber volume instead of the cycle time of the actual vessel formation.