Perhaps the most significant impact on acoustic instruments during dredging is turbidity, both background turbidity and turbidity caused by the dredging operation itself. The type of sediment on the bottom is related to turbidity. Fluid mud (also referred to as cohesive sediment) can obscure the true seabed.

When operating on the seabed with fluid mud, higher sonar frequencies give better acoustic accuracy, while lower frequencies allow better penetration of fluid mud. High turbidity or fluid mud attenuate sonar energy and the reflected sound wave of higher frequencies may not indicate the true seabed. Lower-frequency sonars will penetrate fluid mud, but the data will be coarser with less resolution.

Mitigating turbidity impacts during dredging requires managing suspended solids released at site or stopping solids from entering sensitive areas. At present, monitoring of resuspended solids in the water column arising from dredging operations is performed by spot sampling over time. However, real-time monitoring can establish the baseline conditions prior to start of dredging, then continuously record sediment loading in the water column during dredging.

For turbidity, the key parameters to track are the size of sediment plume in the water column, the density and, if possible, the fractions of particles in the sediment plume. The direction and rate of dissipation of the sediment plume should also be tracked. Real-time monitoring of turbidity can be used to track these key parameters and enable timely management of suspended solids before resettlement sets the dredging operation back, or worse causes damage to sensitive environments.

Operator skill levels also impact efficiency. All industries, including dredge contractors, face a declining pool of candidates entering the workforce. There are more technical job vacancies than people to fill them. Any tool that increases efficiency for new employees who do not have years of practical experience is an asset.

Thus, effective training and tools increase production. It is possible for young unexperienced employees to learn faster with a real-time monitoring system, because they see what they are doing and how much progress they are making. They can literally see their efficiency with one view in the dredge-monitoring software (see Figure 2 for an example).

In addition, with fewer workers, contractors struggle to perform the same (or more) work to the expected quality standards. As a result, innovators are looking to reduce the number of workers required to do the job. By automating some dredging functions, such as vessel repositioning and anchor management, workers can instead focus on tasks requiring human judgement and intervention.

In the future, autonomous dredging may become practical, akin to autonomous ferry operation and container-vessel trials currently under way. Autonomous dredging would require skilled operators who are comfortable with the technology involved in offsite remote real-time dredge monitoring.

Benefits of real-time dredge monitoring

There are several benefits to monitoring dredges in real-time. These benefits include increased operator confidence, lower hydrographic-survey costs, accurate records of work performed, increased production, safer operation, reduced greenhouse gas emissions and improved fleet-asset utilisation. Operators gain increased confidence through the use of the visualisation software as they can see, based on actual soundings, what work has been completed.

If the dredge is outfitted with accurate sonars, sound-velocity and motion sensors, as well as accurate positioning equipment to fix and orient the dredge, daily surveys can be eliminated. Not only are project costs lowered, but the project manager also has accurate records of work performed. This record may prove useful for verifying performance to the client, even if local currents carry sediments back into the dredging area later.

Real-time dredge monitoring reduces the frequency of repositioning the dredge for rework. If the operator knows the area has been dredged to design by actual soundings, the operator can advance the dredge to the next area. This avoids having to stand by for a survey and if the survey results are not good, having to rework the previous position

Real-time monitoring also results in safer operation by reducing risk of slip-back and slope failure (North, 2022). Safety is of particular concern at port-deepening projects where the slope under existing berth decks was designed for shallower depths, or when shipping channels are being widened (Stainer et al., 2019).

Efficient dredging produces lower greenhouse gas emissions from optimised diesel engine run-time. For the foreseeable future, until alternative fuels become commonplace, dredges will operate on diesel. Efficient dredging without rework reduces both excess emissions and fuel costs.

When a dredging contract is executed on time and on budget, without extended time on site for rework, the contractor can commit that dredge to follow-on contracts. Knowing the asset can be deployed to a new project on time is vital when contractors have their reputation on the line.

Acoustic sensor options

A key piece of equipment in real-time dredge monitoring is the acoustic sensor. Four types of acoustic sensors can be used to obtain soundings: single-beam echo sounders, split-beam echo sounders, dual-axis sonars and multibeam echo sounders. Each have their own advantages and disadvantages.

Single-beam echo sounders

Single-beam echo sounders are low-cost sonars, available in wide-beam or narrow- beam versions. However, they produce downward soundings only. They do not provide area coverage, merely point soundings. To be of value to the operator, multiple sensors must be installed on a dredge to get sufficient data points of the seabed. Thus, system cost scales with the number of sensors employed.

In addition, single-beam echo sounders do not cover the worked area until the dredge has moved forward to the worked area. As one operator commented, they only know what they dredged four hours later and then they must still reposition the dredge and do the rework.

Split-beam echo sounders

Whereas single-beam echo sounders provide no information on target location, split-beam echo sounders use multiple transducers to cover a larger area and calculate target location in three dimensions. Split-beam echo sounders can therefore detect solids throughout the water column, quantify the amount and density of solids, and quantify what the material is.

With this functionality, the potential for split-beam echo sounders in real-time turbidity monitoring is promising. Split-beam echo sounders have already been successfully used to identify targets in the water column in other similar applications. For example, by ocean scientists to study biomass and in the Gulf of Mexico to quantify hydrocarbon seeps

The use of split-beam echo sounders for sediment detection is still being studied, but proof of concept is not far behind. Kongsberg’s Frank Reier Knudsen conducted a preliminary feasibility study to determine whether split-beam echo sounders can detect sediments in the water column, with good results.

Furthermore, preliminary controlled environment studies in an outdoor tank by Deltares and Ifremer indicate that split-beam echo sounders have potential to show sediment plume density, shape and rate of dispersion. Preliminary field work by Boskalis shows that a split-beam echo sounder can track and quantify resuspended sediments in an open water column, even in the presence of background sediments (Mech, 2023).

As mentioned above, the type of bottom and amount of turbidity may determine whether you pick a high- or low-frequency sonar. Split-beam echo sounders sweep through the broad band transmit frequency and by using different bands, the back-scatter amplitude of the returned signal allows users to quantify the amount and density of solids, and to quantify what the material is.

If mitigating turbidity is a major concern, split-beam echo sounders may be worth considering when putting together a real-time monitoring system. Understanding how much (and where) sediment is being transported and resettled is important, especially if there is concern of industrial pollutants or contaminants being transported downstream

Dual-axis sonars

A dual-axis sonar is comprised of a single narrow-beam transducer mounted on a precise two-axis rotator. The transducer is safely housed inside an oil-filled acoustically transparent dome, which isolates the transducer from the environment. A dual-axis sonar produces point-cloud data similar to a multibeam sonar. These soundings can be integrated into post-processing software, such as QPS Qinsy, Hypack, EIVA or Sonarwiz. The operator can select the best area coverage for a particular dredging site by adjusting the step pitch between pings to gain a fast, coarse measurement or a fine measurement over more time.

For example, a pitch of 7 degrees between pings provides 26 soundings in a 180-degree arc, while a pitch of 0.2 degrees provides 900 soundings in that same arc. Crucially, these soundings cover the area around the loosening tool, allowing for rework before the dredge is moved forward to advance the operation.

It is pertinent that the ASCE Manual of Practice 156 (Navigation Channel Sedimentation Solutions) speaks to the importance of field observations when developing models of sediment behaviour in shipping channels. Dual-axis sonars are emerging as a useful tool for collecting these field observations. Of particular importance is slope stability during and after deepening dredging for acceptance of larger vessels at existing berths.

Dual-axis sonars are steadily replacing single-beam echo sounders as the acoustic sensor of choice in real-time monitoring solutions. This fact is reflected in the fact that several of the case studies presented here use a dual-axis sonar.

Multibeam sonars

Multibeam sonars are available in imaging and point-cloud versions. Some multibeam sonars perform both functions in same sonar head. For dredge monitoring and visualisation, users should select point-cloud function sonars.

The main advantage of multibeam technology is speed. Multibeam sonars can provide soundings rapidly in a single sweep of the work site. Similar to dual-axis sonars, the soundings can be integrated into post-processing software, such as QPS Qinsy, Hypack, EIVA or Sonarwiz. However, processing the soundings causes a delay to visualising work progress.

Multibeam systems require a single or dual-axis rotator to sweep the beams over the work area. However, more moving parts means more potential points of failure and maintenance to address seal wear. They can be more susceptible to damage depending on the dredge type. Therefore, extra precautions must be taken to protect these systems from transducer damage due to solids in the water column.

Good-better-best

One framework to consider when deciding what level of instrumentation is appropriate for dredge operations is Good-Better-Best. These acoustic sensors run on a price- performance continuum where single-beam echo sounders are most economical, split-beam and dual-axis sonars are more expensive, and multibeam systems are most expensive. As the systems increase in performance and capability, the cost of ownership also increases due to maintenance and spares.

Regardless of which system is used, the benefit of all these acoustic-sensor options is that they reduce the amount of rework required by providing the operator with actual dredging performance, rather than imputed performance derived from inaccurate assumptions.

Case study 1: Cutter-suction dredge for maintenance dredging

In this case study, the sonar is mounted on the front of the dredge, so the beam pattern covers the area swept by the dredge as it pivots on its spud. The operator has a touchscreen in the cabin, which is used to run the sonar and see progress of the removed material to the design horizon.

Operators have found that relying only on the inclinometer readings to show dredging progress almost always showed more material dredged than was actually removed. As mentioned above, up to 20% of solids disturbed by the loosening tool resettle back onto the seabed. This resettlement can be clearly seen in the side elevation view shown in Figure 2.

In Figure 2, the 3D view at the top and 2D-elevation view at the bottom show the dredge and material removed from the baseline digital terrain map. The 2D-elevation view at the bottom shows the design horizon as a red line and the theoretical removed material based on ladder inclinometer in pink. The blue line shows the resettled sediments disturbed by the cutting tool from soundings.

Soundings can be taken during the dredging process, either when the dredging operation is stopped for anchor repositioning or at shift change. Each dredging operator needs to determine what method best suits their workflow. However, when the dredge is stationary, turbidity in the work area reduces as disturbed solids settle back to the seabed or are carried away by the current.

The sonar is set to work and the sonar soundings are processed in the visualisation software, showing dredged material removed from the baseline digital terrain map. This rapid validation of dredging progress, performed before the dredge is moved, eliminates the legacy method of moving the dredge, standing by for a survey vessel to map the worked area, processing the data and then repositioning the dredge for rework. This elimination of redundant work contributes directly to the dredge contractor’s profitability and schedule performance.

Case study 2: Backhoe dredge for precision/surgical dredging on a capital project

As in case study 1, the sonar is mounted on the front of the dredge platform (Figure 3) where the sonar beam pattern can cover the work area.

The dredge-monitoring system in this case is comprised of a dual-axis sonar (mounted on a pole on the pontoon – see front left of photo in Figure 3), and a monitor inside the operator’s console. The dual-axis sonar covers the work area dredged by the backhoe.

Some dredge-visualisation software uses inputs from position sensors located on the backhoe. The software assumes that each bucket scoop is 100% full. The visualisation software then removes material from the baseline digital terrain map per the position sensor data.

However, just as disturbed material in rotating cutter-suction dredges is not removed from the site, but resettles on the seabed, these assumptions mean that physical survey often shows the area is under-dredged and rework is required. Real-time dredge monitoring allows for surgical dredging when used in conjunction with positioning sensors, as shown by the example in Figure 4.

The point-cloud data provided by the dual-axis sonar provides the operator with the ability to surgically remove material where desired and leave material undisturbed when required for project construction to be successful. The operator can then touch up areas requiring removal before the pontoon is advanced further up the canal.

This dredging operation in a German canal required undisturbed areas to remain as anchor points to secure a concrete lining. The dredge operator was able to accurately position and remove material, leaving each pillar of approximately three metres on the canal bed. Efficiencies and accuracy combine to ensure quality work while performing on schedule and to budget.

Case study 3: Clamshell dredge during maintenance dredging

In this case study, silt was dredged from a dam reservoir. However, the dredging operation had to be conducted without causing damage to a concrete wall in the bottom of the reservoir. Precise operation by the dredge operator was required. The operator benefitted from being able to see progress via the dredge- visualisation software using actual soundings. Figures 4 and 5 show two elevations of dredging progress.

In Figure 5, the operation is in its beginning stage. Note the silt sloped along the concrete wall, which is the high point to left of work area. Suspended solids are seen in the water column (caused by the clamshell lifting to the surface) and disturbed solids are shown as a purple pile in the centre of the work area.

In Figure 6, material removal in the reservoir has progressed and the face of the concrete wall is now clearly visible. Note the heavy quantity of suspended solids in the water column (from sediment falling out of the clamshell as it lifts). If not using sonar, a theoretical assumption of 100% fill of the clamshell would have been incorrect.

Note that the sonar soundings reflect off suspended solids at various points in the water column from the seabed to the surface. This noise can be filtered out so that the operator is presented with data only in the dredged area, as seen in Figure 7.

Figure 7 shows a screenshot of dual-axis sonar data on the operator’s monitor in the dredge. The software applies filters to remove backscatter from suspended solids in the water column. The face of the concrete wall is clearly visible, giving the operator confidence in dredging with minimal damage to this structure or the dredge.

Again, as noted previously, if the dredge- visualisation software relies on positioning data only and assumes 100% material removed per bucket load, performance must be proved by moving the dredge, running a survey vessel over the work site and repositioning the dredge to perform rework clean-up.

Case study 4: Visualising solids in the water column with a split-beam echo sounder

Boskalis has done preliminary field work using a split-beam echo sounder in open water. The purpose of their study was to visualise solids in the water column and to collect data for further analysis. The set-up consisted of a wide-band transducer pole mounted over the side of a small survey vessel (Figure 8).

This vessel then ran multiple passes crossways, behind the path of a water- injection dredge operating at the Port of Rotterdam. Rotterdam has significant amounts of fluid mud. This has the potential to cause significant background sediment noise due to the number of large super container vessels (18,000 TEU and up) and large tugs to manoeuvre them in the port.

Figure 9 shows the survey vessel track behind the water-injection dredge. The range of the echo sounder was set to 30 metres. You can see the heat map of the resuspended sediment at the lower edge where the survey vessel was just behind the dredge. You can also see how the resuspended sediment drifts left of the dredge’s track over time due to local currents.

Figure 10 shows the water column data as the survey vessel crosses from side to side of the channel, across the dredge’s track. You can see how each time it crosses the track the resuspended solids occur through the entire depth of the water column. The data shows density and thickness of the plume.

It should be noted that while this field work is promising, more detailed study needs to be done before this can be considered proof of concept. It is important to emphasise that this solution generates real-time remote measurements and makes 3D visualisations possible.

Conclusions

Installing acoustic sensors on a dredge provides the operator with a real-time monitoring method that is far more efficient than dredging blindly. Real-time dredge monitoring brings tangible benefits to operators of cutter-suction dredges, clamshell dredges and mechanical backhoe dredges.

Although single-beam, split-beam, dual-axis and multibeam sonar systems have both advantages and disadvantages, the benefits of integrating it into dredge operations is clear. Real-time dredge monitoring improves operator economics and safety by giving the dredge operator situational awareness, including missed material, slip-back and potential slope failure.

Investment options range from the lowest cost option – single-beam echo sounders – to the most expensive multibeam systems. Before making an informed investment decision, contractors need to trade off the costs and benefits of each system, while considering maintenance and replacement costs for damaged equipment.

Operators should be mindful of local conditions that may impact the accuracy of soundings, such as variations in water temperature and salinity during operations. For the best results, they may need to adjust sonar software parameters and make sure that sound-velocity sensors, motion reference units, and Real-time Kinematic positioning (RTK) are being properly used. Operators must also consider the trade-offs between sonar frequency and data accuracy from soundings, which are impacted by turbidity and fluid mud (cohesive sediment) in the water column.

Dredging operations can potentially impact water quality and affect ecosystems, due to increased turbidity caused by sediment resuspension or contaminant mobilisation and transport. Sediment can also enter the water column when the dredge does not fully capture the dredged material. The dredge itself can disturb sediment, causing it to resuspend in the water column. For these reasons, real-time monitoring of resuspended sediment is also of particular concern.