Entrance Siltation After a Breakwater Extension at Kestrel Bay
Kestrel Bay is a medium-sized port on a sandy coast where the net littoral drift is about 450,000 m³/yr to the north. Three years ago the northern breakwater was extended by 320 m and the 260 m wide entrance was shifted 90 m further offshore. Since then maintenance dredging in the 60 ha basin has risen from 170,000 m³/yr to about 410,000 m³/yr at a contract rate of USD 6.80/m³. Surveys show 0.5–0.8 m of new deposition concentrated in a lobe just inside the entrance and along the inner face of the extended breakwater; the inner berths carry only 0.1 m of soft mud. Entrance samples are well-sorted medium sand (D50 0.32 mm); inner-basin material is silt with 18% clay. The basin has a spring tidal prism of about 2.1 million m³, no river inflow, and only one entrance. The dredging contract is being re-tendered for five years and the authority must choose between continued dredging, a fixed sand-bypass system (capex USD 9.5M, opex USD 0.9M/yr) and modifying the breakwater. Traffic is 1,850 calls/yr and the entrance is the only access.
1. Separate the siltation into its component mechanisms and state what evidence would confirm or refute each.
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Two sediment populations, two mechanisms.
Entrance lobe - littoral sand. The material is well sorted medium sand, D50 0.32 mm, sitting in the lee of the new breakwater extension and along its inner face. That is the classic signature of drift entering the entrance on the flood and losing its transport capacity in the sheltered zone; the ebb cannot return it because the basin has no river outflow and a weak flushing velocity. Check the flushing: with a 2.1 × 10⁶ m³ spring prism over roughly 6 hours through a 260 m wide section about 10 m deep, the mean velocity is 2.1e6/(6×3600×2600) ≈ 0.04 m/s. Medium sand needs about 0.3–0.5 m/s to move. The entrance therefore cannot self-scour; anything that enters stays.
Inner basin - fines. Silt with 18% clay at 0.1 m over the berths is a separate, slow, low-volume problem (biological matter, fine material from outside, possibly shore runoff). It should not drive the decision.
Volume check. 0.6 m over the lobe area is of the same order as the annual dredged volume, and 410,000 m³ against a 450,000 m³ drift is a capture ratio near 0.9. A plain breakwater rarely traps 90% of the drift; the more likely reading is recirculation - sand enters, deposits, is dredged, and re-enters next season - because the relocated entrance now sits inside the drift stream.
Confirming evidence: grain-size and heavy-mineral trends along the lobe; bedform asymmetry from multibeam; ADCP measurements across the entrance on spring and neap tides; wave-refraction modelling of the pre- and post-extension layouts; and a closed sediment budget (drift in − bypass − dredged − accumulation ≈ 0). Cost allocation per dredge area is also diagnostic: if the sand lobe is ~85% of the volume, the sand problem is ~USD 2.4M/yr and the fines are minor.
2. Compare continued dredging, a fixed bypass system and breakwater modification on annual cost and effectiveness, and recommend a course of action.
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Continued dredging. 410,000 × 6.80 = USD 2.79M/yr, about USD 13.9M over five years undiscounted, with volume risk upward if the trap keeps growing. It treats the symptom and leaves the port exposed to a single point of failure at the only entrance.
Fixed bypass. Capital recovery at 6% over 20 years: CRF = 0.0872, so 9.5M × 0.0872 = 0.83M/yr, plus 0.9M opex = USD 1.73M/yr. If the sand fraction is ~350,000 m³ (USD 2.38M/yr of dredging), the bypass must intercept 1.73/6.80 ≈ 254,000 m³, i.e. about 73% of the sand, to break even. At 80% capture the saving is only ~USD 0.17M/yr - financially near-neutral, but it buys reliability and reduces closure risk, which matters at 1,850 calls/yr.
Breakwater modification. Notching, shortening or re-angling the spur could reduce trapping, but capex is likely USD 4–8M, the design needs physical modelling, and there is a real risk of worsening wave penetration into the only entrance. This is a parallel study, not a 2024 construction decision.
Recommendation.
- Do not sign a five-year fixed-volume dredging contract; tender 2+1+1 with quantity bands and re-measurement.
- Install the bypass, sited to intercept drift before it rounds the breakwater head, with a performance guarantee measured by pre/post survey rather than by equipment hours.
- Run a two-season sediment-budget monitoring programme and model a notched spur; only build it if the budget shows the bypass is not closing the gap.
- Reserve a contingency dredge allowance for the fines in the inner basin.
3. Set out the monitoring and trigger framework you would attach to whichever option is chosen.
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Purpose: prove whether the chosen measure works, and give the harbour master an early warning of depth loss before it becomes a navigation restriction.
Survey regime.
- Monthly multibeam over the entrance lobe and the berth approaches; full-basin survey quarterly and after any storm above the 1-year event.
- Fixed cross-sections at three stations across the entrance for direct volume comparison; all reduced to Chart Datum with stated vertical uncertainty.
- ADCP deployment at the entrance for a full spring-neap cycle each season, plus seabed drifters or tagged tracer sand to establish pathway direction.
Trigger levels (linked to the dredging contract).
- Amber: 0.25 m accumulation in the berth approach or net gain > 1,500 m³/day averaged over a month - schedule a dredge campaign.
- Red: 0.4 m in the approach or any reduction of declared depth below the advertised figure - mobilise, and notify port users.
- Bypass efficiency: if capture falls below 70% for two consecutive quarters, review the intake position.
Sediment budget closure. Each year, reconcile drift influx, bypass volume, dredged volume and surveyed accumulation. A residual larger than the survey error means a mechanism is missing and the diagnosis must be revisited.
Governance. Keep a single monitoring register owned by the port engineer, report quarterly to the authority, and make the dredging contractor's payment conditional on post-dredge survey acceptance rather than on hopper counts alone.