Optical monitoring of sediment plumes from underwater explosive demolition

Underwater explosive demolition can remove a redundant pier, bridge support, intake structure or marine obstruction quickly, yet the event may disturb a substantial volume of settled material. Fine sediment released from the seabed can travel with tidal currents, reduce water clarity and carry contaminants beyond the immediate work zone. A well-designed optical monitoring programme provides the time resolution needed to understand that release as it develops.

This case study describes a practical monitoring approach for an Australian marine project involving the controlled demolition of submerged structural elements. It focuses on turbidity, suspended solids, plume movement, instrument deployment and data interpretation, with methods suitable for estuaries, ports, rivers and coastal construction sites. The example is representative rather than a report of one named demolition contract, so project-specific explosive, ecological and regulatory controls must always be developed by appropriately qualified specialists.

Project setting and monitoring objectives

The hypothetical project involved the removal of several reinforced-concrete piles and a short section of an obsolete wharf in a tidal estuary. The structure had been decommissioned, but mechanical removal alone would have required prolonged underwater cutting and heavy lifting. A controlled explosive sequence was selected to reduce diver exposure and shorten the period of in-water work. Before any charge was used, the project team needed to establish whether the surrounding sediment was clean, contaminated or likely to create a material water-quality risk.

The monitoring area included the demolition footprint, an upstream reference station, two downstream transects and a nearshore ecological receptor. In Australian conditions, station spacing must account for strong tidal reversals rather than assuming that “downstream” remains in one direction. A site near Port of Brisbane, for example, can experience vessel wakes, dredging-related currents and changing salinity in addition to the tidal signal. Similar factors apply around Sydney Harbour, the Swan River and industrial estuaries near Melbourne.

The principal objectives were to measure the background optical signal, capture the short-lived peak created by each blast, track the movement of the sediment plume and determine how quickly readings returned towards baseline. The team also wanted to distinguish demolition-related suspended material from natural fluctuations caused by wind, shipping, rainfall runoff or a passing vessel. That distinction is essential when results must be explained to regulators, port operators, Traditional Owners, contractors and local communities.

Selecting optical sensors for a short-lived plume

Optical turbidity monitors are well suited to this application because they can record at high frequency without requiring a water sample for every observation. A beam-attenuation or nephelometric sensor responds to particles in the measurement volume, producing a signal that can be expressed as turbidity units or related to suspended-solids concentration through site-specific calibration. The rapid response is valuable when a plume passes a station in minutes rather than hours.

A practical system may combine fixed turbidity monitors with suspended-solids sensors, pressure measurements and conductivity-temperature-depth observations. Pressure helps verify depth and identify movement of a mooring, while conductivity and temperature help interpret stratification and mixing. In a freshwater river, the optical response may be dominated by mineral particles; in an estuary, organic flocs, saline intrusion and changing particle size can alter the relationship between turbidity and mass concentration.

Sensor placement should represent the expected exposure without putting equipment directly inside the blast zone. Instruments can be mounted at several depths on a mooring, attached to a frame on the bed or positioned from a workboat during a mobile survey. The optical windows need protection from impact, entrained air and rapid fouling, while the frame must remain stable through currents and boat traffic. A sacrificial deployment design may be appropriate where debris or pressure waves could damage an instrument.

The measurement range deserves careful attention. A monitor configured for clear groundwater may saturate in a dense demolition plume, while a broad-range instrument may provide less resolution at background levels. The project team should record raw signal, diagnostic status, battery condition and quality flags alongside the processed turbidity value. Campbell Scientific support is relevant where D & A Instruments equipment is being integrated into a larger logging or telemetry system, particularly when the project requires unattended operation and reliable time synchronisation.

Establishing a defensible baseline

Baseline monitoring began several days before demolition and continued through representative tidal cycles. The aim was not simply to calculate an average turbidity value, but to describe the normal range, short-term variability and relationship between optical readings and local conditions. Measurements were compared with rainfall, wind, current direction, vessel movements and nearby construction activities. A reference station outside the expected plume corridor helped identify changes affecting the whole estuary.

The team collected discrete water samples over the optical range likely to occur during the work. Each sample was labelled with location, depth, time and tidal state, then analysed for total suspended solids using a documented laboratory method. These results were paired with sensor readings to develop a calibration relationship. Samples from only clear water would have produced an attractive but unreliable curve for the high-concentration portion of the event.

Optical fouling was controlled through a combination of cleaning, inspection and deployment duration. In warm Australian waters, biofilm can develop quickly, while fine sediment may settle on a sensor during slack water. Wipers or copper components can reduce fouling, but they do not replace field checks. The team also recorded zero checks and post-retrieval readings, allowing drift or a damaged optical window to be identified before the data were accepted.

Low-level measurements require a different discipline from plume monitoring. A useful reference for sensor behaviour, sample handling and uncertainty in clear water is the discussion of low-range calibration guidance, although the demolition project itself required a much wider concentration range. The same principle applies: calibration should reflect the particles, water matrix and operating range encountered in the field rather than relying solely on a factory relationship.

Capturing the demolition event

On the day of the operation, all stations were checked against a common clock and placed into a high-frequency recording mode. Logging intervals of one second to one minute may be justified during the active sequence, depending on the expected plume speed and the storage capacity of the data logger. A slower interval can miss a narrow peak as a plume front passes a sensor, especially in a fast tidal channel.

The baseline station continued to operate while the downstream stations were watched remotely or checked from a support vessel. The demolition contractor maintained the exclusion zone, controlled the firing sequence and confirmed that the environmental monitoring team was clear of the hazard area. Environmental instruments must never be used as a reason to compromise explosive safety, navigation controls or the requirements of the relevant Australian authorities.

The resulting time series typically showed a sharp increase at the station nearest the structure, followed by a delayed and broader response at more distant locations. The peak magnitude depended on sensor depth, current direction, charge configuration, sediment grain size and the amount of material lifted into suspension. A sudden optical spike at only one depth did not necessarily indicate a large plume throughout the water column; it could reflect a concentrated layer or a local disturbance at the mooring.

For interpretation, the project team compared each station with its pre-event baseline and with the reference station over the same period. Time of arrival, peak excess turbidity, duration above baseline and area under the excess curve were more informative than a single maximum value. The analysis also checked for saturation, bubbles and communication gaps. A sensor response that rose and fell in a few seconds, with no corresponding change at nearby depths, required examination before it was classified as sediment.

Interpreting plume transport and impacts

The data showed that the first plume lobe remained relatively close to the structure, while a weaker signal moved along the channel with the ebb tide. During the following flood, residual material was redistributed in the opposite direction. This pattern illustrates why a single downstream monitor can produce an incomplete assessment. Tidal monitoring should cover both directions, particularly around Australian ports where work windows and ecological constraints may coincide with spring tides.

Converting turbidity to suspended-solids concentration improved the engineering interpretation, but the conversion remained conditional. Particle size and composition can change during demolition, and freshly disturbed sediment may scatter light differently from settled material. The project therefore reported both the direct optical measurements and the calibrated concentration estimates, including uncertainty and the range over which the relationship had been verified.

The ecological assessment considered the duration and vertical distribution of the plume rather than treating every increase as equivalent. A brief surface signal may have a different significance from persistent high concentrations near the seabed, where benthic habitats, shellfish beds or fish-feeding areas may be exposed. If the project were near the Great Barrier Reef catchment, for instance, the monitoring design would also need to reflect sensitive coastal waters, seasonal rainfall and the consequences of sediment transport beyond the immediate worksite.

A complete evidence package included raw files, calibration records, sample results, deployment coordinates, depth information, maintenance notes and a time-synchronised event log. Photographs of the instruments before and after deployment helped explain anomalies. The report could then show whether the observed plume was short-lived, whether it reached a defined receptor and whether additional controls such as silt curtains, altered firing timing or a pause between charges were warranted.

Applying the findings to future marine works

The case demonstrates that optical monitoring is most useful when it is designed around the physical behaviour of a plume rather than added as a compliance exercise after the demolition plan is complete. A sound design begins with sediment characterisation, bathymetry, current measurements and an understanding of the structure’s footing. It then selects sensor range, depth, logging interval and station locations to resolve the expected event.

Vertical information is especially valuable in stratified estuaries and deep channels. A surface-only monitor may miss a dense near-bed layer, while a bottom sensor can be affected by resuspension caused by its own frame. Profiling systems and multi-depth moorings can reveal whether the plume rises, settles or follows a density interface. Related field considerations are discussed in vertical profiling methods, where the focus is groundwater rather than demolition, but the broader lesson about resolving conditions with depth is directly applicable.

For Australian contractors and asset owners, equipment support and data management can be as important as the sensor specification. A project may involve a marine construction company in Perth, a port authority in Queensland, an environmental consultant in New South Wales and a laboratory in another state. Clear responsibility for configuration, calibration, telemetry, retrieval and data custody prevents gaps when several organisations share the work.

Optical instruments cannot identify every contaminant and should not replace laboratory analysis where sediment chemistry is a concern. They provide a rapid indicator of particle disturbance, plume timing and transport, allowing field teams to respond while an event is still unfolding. Combined with targeted sampling, hydrodynamic observations and disciplined explosive-work controls, they create a defensible picture of how underwater demolition affects the surrounding waterway.