Dredging, Reclamation, and Channel and Basin Design
Dredging questions test whether you can size a channel that stays navigable, select plant that matches the material, and manage the fill and disposal volumes that dominate cost.
Channel and basin dimensions
- Width follows PIANC guidance: one-way width of roughly 1.5–2 × beam in straight reaches, wider in bends and where cross-currents or cross-winds are strong; two-way width about 3–4 × beam including separation.
- Depth = draught + net under-keel clearance + squat + wave allowance + siltation allowance + dredging tolerance, less any tidal window credit.
- Squat in confined water can reach 10–15% of draught for large, full-form vessels at speed; it grows sharply with block coefficient.
- Bend radius should be at least 3–6 × LOA for large vessels, increased where currents set across the bend.
Dredging plant
- Trailing suction hopper dredger: self-propelled, works in open water and soft to medium material, limited by hopper volume and dredging depth.
- Cutter suction dredger: hard material and precise profiles, needs anchors and a pipeline; efficient on stiff clay and rock.
- Backhoe and grab dredgers: confined berths, obstructions and environmental precision, at lower production.
Production and reclamation
- Production = capacity × cycles per hour × fill factor × availability. Weather downtime of 20–30% is normal in exposed water and belongs in the programme.
- Reclaimed fill consolidates: compute settlement, then design surcharge and vertical drains to meet the handover date; residual settlement criteria are typically 100–300 mm over the design life.
- Dredged material: beneficial use first, confined disposal second, open-water disposal only under permit with turbidity and contaminant control.
Worked example
Channel for a 14.5 m draught vessel: UKC 1.45 m, squat 0.7 m, wave allowance 0.3 m, siltation 0.5 m, dredging tolerance 0.3 m, tidal window credit 1.0 m. Depth below chart datum = 14.5 + 1.45 + 0.7 + 0.3 + 0.5 + 0.3 − 1.0 = 16.75 m. With a 6,000 m³ hopper, 8 cycles per day, fill factor 0.85 and availability 0.75, daily production = 6,000 × 8 × 0.85 × 0.75 = 30,600 m³/day.
Traps
- Omitting dredging tolerance; the contractor is not paid to remove your tolerance.
- Mixing in-situ, hopper and barge measures without applying bulking factors of roughly 1.1–1.3.