Breakwaters are often the largest investment in port construction and can have a significant impact on ongoing maintenance costs and schedule. Consequently, when evaluating the feasibility of a proposed port, it is essential to first determine whether the port will meet the throughput target if it remains exposed to waves, or if a sheltered berth is needed to avoid weather-related downtime.
To answer this question, the project needs to conduct a preliminary berth availability assessment, which consists of:
- Conducting a high-level metocean study and estimating operational conditions (e.g., wave, wind, current, water level) at the proposed berth location,
- Estimating operating thresholds considering allowable mooring line / fender forces and vessel motions -- which depend on the terminal type (e.g., container, bulk, oil and gas, cruise, or ferry) and function (e.g., import/export), and
- Computing the percentage of time that operating thresholds are exceeded, so the vessel cannot berth, or the operation must stop, and the vessel needs to leave the berth.
If the analysis results show that a sheltered berth is needed for the planned throughput, the design team will develop a breakwater layout plan for the project. Whether offshore or connected to the shore, the breakwater location and arrangement depend on geotechnical considerations, water depth, maneuverability of the design vessel(s), and level of wave attenuation needed. Another important parameter to consider is the potential impact of constructing a breakwater on the shoreline morphology and sedimentation, within the project battery limits or on neighboring areas upstream or downstream of the proposed project. There are many examples in which this aspect of the design has been overlooked, underestimated, or misrepresented, and the owner had to deal with resulting financial or legal consequences.
Five important parameters to consider when designing a breakwater
Once it is determined that a breakwater is necessary, there are five essential questions to ask:
1. Have I correctly determined my operational and extreme wave conditions?
Reliable wave climate information is key to a satisfactory project. This includes a good understanding of how day-to-day wave conditions affect design vessel(s) while maneuvering and at berth (and consequently breakwater layout), and proper extrapolation of extreme event(s) impacting breakwater cross section design. Look out for these potential shortcomings when establishing operational and extreme wave conditions:
- Over-simplification of wave conditions (e.g., using parametric data sets and ignoring wave directionality)
- Over-estimation of wave attenuation using desktop design tools
- Miscalculation of the design event (e.g., considering improper return period)
- Ignoring the storm profile and focusing on the design peak event
2. Have I selected the proper type of breakwater for my project?
In parallel with designing the breakwater layout, conceptual design of a typical breakwater cross sections is developed by considering the following alternatives:
- Conventional (rock armour or precast concrete units)
- Berm (or dynamically stable)
Other types of breakwater which may be considered, depending on the function of berthing facility and level of wave attenuation required, are low-crested (submerged) or floating breakwaters.
A typical rubble-mound structure consists of a few elements: a core of relatively small stones to build the breakwater structure; armour stones (or artificial concrete units) to protect the core from reshaping and damage by waves; and one or more underlayers placed between the core and the armour layer to prevent the core material escaping through the armour layer voids and destabilizing the structure. Other elements of typical breakwaters are: a toe to protect the breakwater seaward face from the action of waves running down its slope; a scour apron to be placed at the toe to stabilize the seabed; and the crest and rear slope.
A trade-off study is needed to define the preferred solution, considering cost, schedule, and risk. Parameters to be considered include site geotechnical conditions, availability of rock material (size, gradation, quality, and quantity), design wave conditions, constructability considerations (methodology/equipment/risks), metocean conditions affecting construction, and stakeholders’ priorities. For example, constructability considerations of a breakwater in Mediterranean Sea (with highly seasonal wave conditions) are very different from that in South Pacific Ocean (with year-round background swell).
Sensitivity of the design to geotechnical parameters and settlement, liquefaction, and/or slope failure vary with breakwater type and proposed cross-section design. For example, a caisson breakwater or a breakwater to be armoured by one layer of CORE-LOCTM units on a 4H:3V slope is more sensitive to differential settlement, in comparison with a berm breakwater with wider base and less interlocking requirements. Mitigation measures to consider range from relocating or revising the breakwater arrangement, to replacing seabed material with competent material, using geogrids, or developing soil improvement techniques.
Dynamically stable (berm) breakwaters which are gradually reshaped by wave action to an equilibrium or stable profile have been more recently considered as a good replacement for conventional breakwaters in several projects. A few advantages of this alternative are flexibility in design and construction (as it needs less stringent placement tolerance in wave action), efficiency in using available rock material, and adaptability with storms and extreme conditions. On the other hand, and in certain conditions such as relatively deeper water, a rubble-mound base with a caisson breakwater on top can be the preferred solution. This breakwater type also provides the option of a workable crest to install and/or operate port handling equipment and at the same time can form a berthing face with effective sheltering.
3. Do I have a reliable source of rock material and how can I optimize my quarry operation?
Sourcing rock material suitable to construct the breakwater is one of the initial design steps. At the onset of the project realization, a quarry investigation, in collaboration with geologists, is needed to assess if there are existing or potential quarries within a reasonable distance from the proposed project site to provide rock material suitable for the project needs. Parameters to consider are quality of rock, gradation or quarry yield, volume, and production rate.
Challenges in sourcing large size rock material for use as armour layer in conventional breakwaters may necessitate considering other alternatives, such as a berm breakwater or casting artificial concrete units to replace armour stones.
Potential quarry yield is needed in the design to maximize the use of available rock, considering cost and schedule associated with blasting and sorting, and quarry waste. This includes optimizing core design and using wider gradation in the design, where feasible. In case of a berm breakwater, it involves selecting an optimal combination of median size, gradation, and volume to maximize use of quarried material.
While the unit rate for sourcing, transporting, and placing core material is less than that of other components due to the use of smaller size rocks and wider gradation, the overall cost for the core will be significant, as it forms most of the breakwater volume. In some cases, however, a cost-effective option is for the bulk of the core to be constructed using dredged material placed in layers inside bunds made of rock or even geotextile tubes.
Ideally, the core needs to be designed based on the available quarry yield, after selecting and stockpiling rock material needed for the armour layer, the underlayer and the toe, to maximize the use of quarry production while considering a number of criterial parameters such as:
- Vulnerability and expected reshaping of unprotected or partially protected core in day-to-day wave action or storms with a high probability of occurrence during the construction period
- Percentage of fines and potential environmental impact (i.e., construction plume)
- Permeability of section impacting armour layer stability, efficiency in wave energy absorption, and wave transmission (which may affect operation behind the breakwater).
Because of the widening of the section and the wave impact reduction with depth, the breakwater performance is more sensitive to the core design at the upper (and seaside) section of the structure in comparison with the lower section. As a result, more relaxed criteria can be used when selecting material for the lower (and leeside) section of the core to reduce the cost and shorten the schedule.
4. How reliable are desktop design tools for designing a breakwater and how can I validate and optimize the design?
There are several guidelines, recommendations, and formulas derived from results of past physical (scale) model testing programs which can be used to develop the breakwater design. However, unlike an experiment-based approach, desktop design tools are not able to capture site-specific conditions and project-specific demands, and it has been recommended to use the latter only for the conceptual and basic design stages.
A model testing program can assess the performance of a breakwater design with respect to wave attenuation and impact of diffracted waves on ship motions or the coastline, 3D effects of breaking waves on the structure (particularly breakwater round-head and bends), stability of the transition section between different sizes of armour stones or rock/concrete armour units, cumulative damage of a reshaping structure, and stability of the rear slope.
Once a preferred breakwater concept is selected by the client, a model testing program should be developed to validate the layout (arrangement and length) and cross-section design in selected wave conditions and to optimize/right-size the structure if possible (by reducing size/volume of materials, managing risk, and facilitating construction).
I have been asked on several occasions about the necessity of model testing due to its schedule potentially delaying construction, and the associated costs (typically by being given examples of breakwaters constructed in the past without that step!). Based on my experience, verification and optimization of a breakwater design in a testing facility is money well spent. For example, one of my clients wanted to explore a design deviation by using unscreened core to reduce construction costs, and the model testing results showed the likely consequences of that change, which included large overtopping, extensive damage to the armour layer and failure of the crest. Validating the design safeguarded that project from likely expensive repairs in the future. In another instance, after observing failure of the initial desk-top based design in the testing facility, I had to develop and evaluate several variations of the round-head arrangement and the size of the toe rocks and armour units to arrive at a successful design.
Additionally, the testing program can be done in parallel with the engineering and early construction work, and can focus on areas which lie outside the project’s critical path if needed, such as the round-head, the upper section of the breakwater, the crest, and the rear slope. This will mitigate potential impact of model testing on the project schedule.
5. Is my breakwater constructable, what are the construction risks to consider and how can those be mitigated?
A preliminary constructability assessment is needed at the early design stage and as part of the trade-off study, to consider size, type and number of equipment needed, type of construction (land based, marine based or hybrid), stockpile and fabrication areas and accesses, rock load out facility, etc.
Typical questions to ask include:
- What is the risk of under-construction structure (submerged, low crested, or partially armoured) being reshaped or damaged by waves, and what do I need to do to mitigate?
- Considering wave climate during construction, how much weather-related downtime can I expect?
- What is the preferred construction approach considering volume of material and placement tolerance?
Construction risks and the plan in place to mitigate them are essential for the company who insures the project.
A physical model testing program is recommended to evaluate key constructability aspects of the breakwater. The model testing results can be used to:
- Assess feasibility of placing artificial concrete armour units in the selected wave conditions, considering acceptable placement tolerances. The outcome will be used to estimate operating thresholds in which placing units is feasible and associated construction downtime.
- Determine the extent of reshaping that a partially constructed structure may experience in the selected wave conditions (e.g., annual storm peak event). The outcome will be used to estimate the risk of damage/reconstruction and develop mitigation measures such as using larger rocks for the core, temporary protection for the core, or limiting the length of an uncovered section.
- Evaluate the remaining strength of the reshaped structure by the design wave conditions and assess if it can withstand the selected sea states prior to completion of maintenance or reconstruction. The outcome will be used to modify the structure, if needed, to prevent failure and jeopardizing the terminal operations.
Breakwaters can be essential to protect ports and terminals from incident waves. Despite being a critical item in the performance and a large component of the overall Capex of the project, occasionally their design is inefficient or not optimal. Answering the above-listed questions will support stakeholders and address key design parameters which affect the feasibility, operability, and success of a port project.