THE IMPORTANCE OF EFFECTIVE DREDGE MONITORING
Nearly all the major ports and waterways in the world have at some time required new dredging works (capital dredging), as technological developments and the requirement to improve cost effectiveness have lead to larger, more efficient ships, and expanding coastal infrastructure which more recently includes provisions for renewable energy developments. New port developments, and expansion of current facilities (including larger and deeper access channels, turning basins and appropriate water depths) are required along waterside facilities to accommodate these vessels. Recent and current examples include the London Gateway Port Development Project, the New Port Project in Doha, the expansion of Southampton docks in the UK and development of the Port of Hastings in Victoria, Western Australia. Capital dredging projects can be both extensive and expensive.
Many of these dredged sites later require maintenance dredging (removing any sediment which has accumulated in the bottom of the dredged channels and berth boxes). Maintenance dredging is necessary to maintain safe operational water depths for navigation, and to facilitate continued access to berths, docks, wharves and jetties for the large vessels engaged in domestic and international commerce by ensuring that they are of adequate dimensions. Maintenance dredging is often a regular, perhaps annual ongoing, long-term activity, but can also be required on an ad hoc basis following storms, or other hydrodynamic events.
Why do we monitor?
It is well understood that dredging operations can have significant negative impacts on the local environment. As a result of this fact local government authorities stipulate that monitoring must take place as part of the process for issuing licences for dredging and, as such, is a legal compliance issue. Responsibility for monitoring the impact from dredging operations often falls to the dredge operators, including contractors, and data obtained from monitoring stations is usually shared with all stakeholders.
Establishing baseline information
It is necessary to obtain background readings of water quality parameters (turbidity, temperature,Dissolved Oxygen, pH, etc.), currents/tides and bathymetric data well in advance of the commencement of any works, in order to establish baseline information over a variety of conditions and seasonal fluctuations, and to understand how the currents and tides will affect the movement of re-suspended sediments. For example during the London Gateway Port Development Project, UK, water quality, benthic and epibenthic surveys first commenced in the Thames Estuary well over 10 years before the 4 year long dredging work began. An extended period of background monitoring is in the best interests of all parties to ensure that the baseline information is correct and accounts for as many regular hydrodynamic events as possible.
Background monitoring is nearly always undertaken with a permanently moored monitoring station (or stations, depending on the size of the project site), such as the OSIL 1.2m Tern buoy platform equipped with a Nortek AquaDopp current profilers, Campbell Scientific OBS turbidity sensors and Hydrolab multiparameter sondes, which needs to be strategically located within the proposed dredge site to avoid erroneously high or low data readings.
Defining threshold limits
Prior to undertaking dredging and/or disposal projects, a careful assessment by a competent authority to identify potential effects and to determine their significance is necessary. The “environmental impact assessment” (EIA) should take into account the background data previously gathered, will identify all stakeholders, and will define and agree acceptable threshold limits for contractors and stakeholders alike.
For example, when planning and undertaking operations for the London Gateway Project various stakeholders and interest groups were identified, and were involved with setting the threshold limits for water quality parameters,including the Environment Agency, Marine Management Organisation, river leisure industries, Kent & Essex Sea Fisheries Committee, the Thames Estuary Partnership, dredging liaison groups, the RSPB, Essex Wildlife Trust, Natural England, the Port of London Authority, and English Heritage.
Managing the dredge
On completion of the EIA, a network of monitoring stations should be established (in suitable locations according to the site conditions) that surround the site to be dredged, which will report real-time data back to the dredging operator. The operator will then use this data to manage the dredge and vessel activity to ensure that operations can continue as efficiently as possible, without exceeding the pre-determined parameter threshold limits or having to temporarily cease dredging operations, as downtime can be extremely costly.
This may be achieved in a variety of ways, including moving dredging operations to another area within the site. Disposal of sediment may also impact on operations and it is crucial to ensure that this occurs in an area where it is not likely to be washed back into the dredged site if it is not being used in associated land reclamation projects.
Operators for the London Gateway Project established well over 20 real-time monitoring stations throughout the duration of the project. Ocean Scientific International Ltd (OSIL) was closely involved with the supply of integrated systems and monitoring equipment, such as the 1.2m Shearwater buoy, and other services (such as analysis of water and sediment samples) to the London Gateway Project for background data and monitoring of the dredge project through to completion. Other solutions from OSIL that have been supplied to dredge projects around the world include standalone monitoring stations for water quality or current measurement, instrumented data buoys with support frames for moorings in shallow tidal waters, and small re-locatable buoys that can follow dredging activity, all of which have been in use on the New Port Project development in Doha, Qatar.
Data collection and calibration
Accurate data ensures that dredging operations remain in compliance with set regulations. For accurate data, all sensors should be fully calibrated before the project begins. If using a buoy-based system, the platforms should be fully assembled on shore prior to deployment. This includes attaching any sensors, towers, solar panels, and additional ballast weights if needed. Furthermore the complete monitoring system (sensors, data logger, telemetry, software) should be tested before the buoy is put in the water. While this process ensures that all equipment is functioning within specifications, it also gives everyone the chance to familiarize themselves with the system prior to deployment. Issues are always easier to deal with before the buoy platform is deployed in the water. It is also recommended to cross-check sensor accuracy against a separate instrument. This cross-checked data can then be used to show compliance, if needed.
Regardless of the instruments chosen to monitor a dredging site, regular maintenance and calibration is required, often by legislation, to maintain accuracy and keep equipment functioning within specifications. Instrument maintenance includes cleaning the instruments (especially if a sensor is not self-wiping), and replacing any deteriorating o-rings to prevent water ingress. If using additional parameter sensors (such as conductivity, temperature, pH and DO, or light-based turbidity sensors), further field servicing may be required.
Maintenance intervals are largely dependent on site conditions and other variables, such as the potential for biofouling, water temperature and seasonal fluctuations. Common calibration and maintenance intervals are on a weekly, bi-weekly or monthly basis. In case of sensor failure or damage, it is useful to have a full complement of spare sensors and sondes on hand. These can be field swapped during calibration or routine maintenance checks. Having a spare turbidity sensor available will reduce downtime due to unforeseen sensor failure, which could cause critical and costly interruptions to safe dredging operations. A ratio of 2:1 for deployed units/sensors to spares held is recommended from long experience of extensive dredge projects.
The requirement for maintenance and spares is not restricted to sensor sand instrumentation, however. It is often advisable, particularly for long running projects, to hold a stock of spares for buoy components. Experience has shown that, robust though the buoy systems are, unforeseen circumstances such as thefts, vandalism and collisions with marine vessels (i.e. dredgers),can also cause costly interruptions to dredging operations if external components, such as solar panels, telemetry equipment or cables, are damaged. Regular checks on buoy components are also essential in order to identify potential issues before they become serious: cables and connectors should be checked for wear and tear on a bi-weekly basis; marine growth should be removed from sensor access areas every 2-4 weeks; solar panels need to be cleaned on a monthly basis as a minimum to remove any built up dirt that could reduce the charging efficiency of the system; moorings should be checked every 6 months and replaced as required depending on local conditions.
It is also advisable to install a GPS locator on the buoy(s): this can allow the operators to double check that the buoy is in the correct position, but can also allow for location and recovery of the buoy in the unlikely event of a mooring failure, or if a buoy and its mooring becomes entangled with a passing vessel and is dragged to a new location, which can be a problem in busy waterways such as the Thames Estuary. Regular checks and maintenance to all systems should ensure that dredging uptime of 98% is consistently achievable.
The data collected is often used pro-actively during the project by the dredge operator to manage the water quality levels in order to utilise dredging time to full efficiency. To be effective, measurement data should be provided to the operator in real-time. In most instances the easiest and most efficient way to do this is with buoy-based monitoring systems. This system can then securely transmit the data to the dredge operator in real-time (at regular intervals, often every 10 minutes), to ensure that the pre-established threshold limits are not exceeded and allow the operators to make adjustments to the dredging programme or vessel movements as required. Most commonly telemetry is via satellite or cellular networks, but can also be via radio, but the telemetry system used must be well planned: overloading of cellular networks at peak times has been known to interrupt communications, and network coverage in the correct areas is critical; Line of Sight is essential for radio telemetry, and large vessels moving within the dredged area may obstruct communications; satellite is arguably the most reliable form of communication, but can be very costly.
OSIL buoy systems are flexible and are customised to the specific project and client requirements, but usually comprise a rugged hull and mooring system, with a high visibility top frame holding solar panels, electronics and telemetry equipment. They can house as many sensors as needed (at multiple depths to provide more comprehensive data),and all equipment is powered via a marine battery and recharging solar panel system. Continuous real-time data may be reported to operators direct to a single computer or network, or via a secure website for access from any internet-ready device (i.e. tablets & smartphones) or by multiple users, with various alarms in place to warn when limits are being, or are close to being surpassed. Careful planning of telemetry and data reporting systems ensure that the dredging operations progress smoothly, and any control measures can be implemented immediately if parameter limits are exceeded.
By undertaking extensive dedicated specialist monitoring, coupled with live data being supplied to dredge vessels, the dredge contractor is better able to monitor the dredge and therefore operate the dredge in the most efficient and compliant way possible in terms of vessel movement and sediment disposal with minimal environmental impact.
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