Real-time suspended-solids monitoring during a tidal harbour dredge

Harbour dredging in Australia is governed by a moving set of conditions. Tidal exchange can change water levels and current direction within hours, while vessel movements, wind, rainfall and propeller wash can redistribute fine sediment across a working area. A monitoring programme that relies only on occasional grab samples may miss the short-lived plume that matters most to an environmental approval.

This case study follows a representative maintenance-dredging project at a working New South Wales harbour, using optical turbidity and suspended-solids instruments to support decisions during a restricted tidal window. The project involved removing accumulated sediment from an approach channel while keeping commercial shipping, recreational boating and nearby aquatic habitats operating safely.

The monitoring team needed more than a continuous graph. It needed a defensible relationship between optical response, turbidity and suspended sediment concentration; reliable instruments that could withstand fouling and vessel activity; and clear trigger levels that the dredging supervisor could use without waiting for laboratory results.

The approach shows how real-time water-quality monitoring can make dredging more responsive. It also demonstrates why sensor placement, local calibration and tidal interpretation are as important as the instrument selected for the work.

Project setting and monitoring objective

The harbour supported a mix of bulk cargo, service vessels, fishing activity and smaller recreational craft. Sediment had accumulated in an approach area where safe under-keel clearance depended on maintaining the design depth. The dredge contractor was granted a limited operating window around suitable tides, with stoppages required when turbidity moved beyond the levels specified in the project’s environmental management plan.

The work was scheduled outside the busiest shipping periods, but the site could not be isolated. Harbour traffic continued under a permit system, and the dredging crew had to coordinate with the harbourmaster, port control and nearby operators. This is a common Australian constraint: the waterway is both a construction site and essential public infrastructure.

Before dredging began, the project team identified three monitoring zones. A reference station was positioned outside the expected influence of the plume, an operational station was installed near the dredge, and a compliance station was placed near the sensitive receiving area. The stations measured turbidity, water temperature and depth, while selected deployments also recorded conductivity and battery status.

The environmental approval defined action levels rather than a single universal limit. A first alert prompted inspection of the dredge and review of current direction. A higher trigger required a reduction in production rate or temporary suspension. These levels were interpreted against the reference station, since natural tidal resuspension could raise background turbidity even when the dredge was idle.

Turning optical readings into site decisions

The sensors detected changes in the way suspended particles scattered light. That optical signal was converted into turbidity units for operational display and then related to suspended-solids concentration through site-specific sampling. The distinction was important: turbidity is an optical property, while suspended solids are a mass concentration measured in milligrams per litre.

A calibration campaign was carried out before production dredging. Water samples were collected during low, moderate and high plume conditions, including samples close to the cutter or bucket activity and samples at the compliance station. The laboratory filtered, dried and weighed the retained material, allowing the project team to compare field readings with actual sediment loads.

The sediment was a mixture of fine clay, silt and coarser sand. As a result, the optical response varied with particle size, colour, shape and concentration. Guidance on mixed clay and sand was used when interpreting why two samples with similar suspended-solids mass did not always produce identical turbidity values.

Rather than forcing every observation into one perfect conversion equation, the team established an operating range and documented the uncertainty. The dashboard showed raw turbidity, the converted estimate of suspended solids, the reference-station offset and the duration of any exceedance. This gave the dredging supervisor enough context to distinguish a genuine plume response from a brief sensor disturbance.

Designing around tides and instruments

Tidal constraints shaped the entire monitoring design. During an ebb tide, the likely plume pathway moved towards the outer harbour, while a flood tide carried material back towards the inner basin. Current direction was confirmed with short-term hydrodynamic observations and vessel reports rather than inferred from the tide table alone.

The operational station was mounted on a protected frame away from the dredge’s direct suction and discharge effects. The compliance station was placed far enough from the work zone to represent potential exposure at the receiving area, yet close enough to detect a plume before it dispersed below the action level. Moorings were marked clearly and fitted with protective arrangements suitable for a commercial harbour.

Instrument maintenance was planned around Australian field conditions. Biofouling, floating debris, silt deposition and warm-water growth can degrade optical measurements, especially during long deployments. The crew inspected the optical window during each shift, compared readings against a handheld reference instrument and checked battery voltage, clock synchronisation and communications status.

Telemetry allowed the team to view readings from the site office while the instruments remained in the water. When a sudden increase appeared, the operator compared all stations, reviewed depth and tide information, and contacted the dredge master before changing production. A rise at every station suggested a natural event or vessel disturbance; a rise that began at the operational station and travelled with the current was more consistent with dredging.

This type of arrangement is also relevant to underwater construction beyond dredging. A separate submarine pipeline case illustrates how continuous optical data can help track sediment disturbance when marine works occur in a dynamic environment.

Field results and interpretation

During the first production shift, the dredge generated a short plume when the bucket entered a pocket of soft fine sediment. Turbidity rose rapidly at the operational station, followed by a smaller increase at the compliance station approximately one tidal phase later. The reference station remained comparatively stable. Because the system displayed the sequence in real time, the supervisor reduced the bucket fill and adjusted the working position before the compliance trigger was sustained.

Later in the campaign, a strong wind shifted surface water and caused natural resuspension along a shallow margin. Turbidity increased at both the reference and compliance stations, but the relative difference between them remained within the expected background range. Stopping the dredge solely because of the absolute value would have reduced productivity without improving environmental protection.

The monitoring record also identified a problem that routine spot sampling would probably have missed. At low tide, the operational sensor showed repeated narrow spikes at regular intervals. Inspection found that the mounting frame was vibrating when a nearby service vessel passed. The data were flagged, the frame was strengthened, and the affected measurements were excluded from the calibration dataset.

At the end of each shift, the team reviewed event logs alongside dredge position, production rate, tide stage and weather. This produced a clearer evidence trail than a collection of isolated samples. It showed when the plume formed, how far it travelled, how quickly it dissipated and which operational changes reduced the response.

Practical recommendations for Australian dredging programmes

A successful real-time suspended-solids programme combines instrumentation with procedures that people can follow during a busy shift. The monitoring plan should state who reviews the data, how often readings are checked, what constitutes a valid exceedance and which action follows each trigger. These responsibilities should be agreed before the dredge arrives, rather than negotiated during an incident.

The programme should also fit the local market and operating environment. Australian contractors may move equipment between ports in Queensland, New South Wales, Victoria or Western Australia, so calibration records, deployment hardware and spare parts need to be portable. Local technicians and suppliers can reduce downtime, while clear documentation helps an environmental auditor understand how field data were produced.

The following practices made the case-study system more reliable:

• Establish a reference station before dredging and use it to separate natural background variation from work-related changes.
• Calibrate optical readings against laboratory suspended-solids measurements collected from the actual harbour sediment.
• Place sensors according to tidal transport and sensitive-receptor risk, not simply at convenient access points.
• Inspect optical windows, moorings, batteries, telemetry and time settings at a frequency matched to fouling and vessel traffic.
• Display turbidity, estimated suspended solids, depth, station comparison and trigger duration on the same operational dashboard.
• Record dredge position, bucket or cutter activity, tide stage, weather and vessel movements alongside water-quality data.
• Define staged responses, such as checking equipment, reducing production, changing the working method or pausing operations.

The final report should preserve raw files as well as processed graphs. It should explain calibration limits, data gaps, flagged readings and the reason for each operational response. That transparency is valuable when results are reviewed by a port authority, state regulator, community representative or project insurer.

For Australian harbour projects, real-time optical monitoring is most effective when it becomes part of the work method rather than a separate compliance exercise. A well-designed system allows production to continue during suitable conditions, provides early warning when sediment transport changes, and gives the project team evidence for decisions made under tight tidal and commercial constraints.