Vertical walls serve a vital role in safeguarding coastal areas and harbors from the extreme wave conditions. These structures are typically designed for installation in deep waters (often as caissons) where conventional wave overtopping prediction methods have proven effective. Vertical walls are more common as coastal protections. However, as waves approach shallower shores, the dynamics change significantly. The onset of wave shoaling and breaking introduces impulsive conditions that pose a challenge for existing prediction methods. This research aims to deepen our understanding of wave overtopping phenomena under extreme conditions, with a particular focus on investigating mean discharge and individual wave overtopping volumes. This paper will present the follow up of the work the WOW21 project (Wave Overtopping at Walls) presented at the last ICCE 2022 in Sydney.

Vertical walls are designed and built to protect coastal areas or harbours. Typically, these structures are installed in relatively deep water where incoming waves are reflected without breaking. In such cases the available prediction methods for wave overtopping discharge are well established. When the relative water depth h/Hm0 becomes smaller, waves start shoaling on the foreshore and breaking can occur leading to impulsive conditions. The present overtopping prediction methods (EurOtop, 2018) under impulsive conditions have been proposed by Van der Meer and Bruce (2014) including data from the VOWS project (Bruce et al.; 2001). Wave overtopping is strongly related to the relative freeboard Rc/Hm0, and for impulsive overtopping EurOtop suggests that for Rc/Hm0 greater than 3 there still might be significant overtopping but there is no data to validate such an extrapolation. Also the well-known overtopping graphs of Goda (2000) show that overtopping could be present for very large freeboards and impulsive waves. The objective of the present research is to investigate the mean discharge and volumes per wave from impulsive and violent overtopping at vertical walls with very large freeboards (up to Rc/Hm0 = 10).

Until about 1930, analysis of wave loads on vertical breakwaters was based on trial and error. Russell(1) noted it was unfortunate that “the young engineer …should be left to be guided entirely by circumstances, without the aid of any one general principle.” Stevenson(2) noted “the engineer has always a difficulty in estimating the force of the waves with which he has to contend.” Wave force formulae by Sainflou(3) and Goda(4) improved design methods, but were 50 to 100 years too late for many ‘old’ breakwaters, and do not apply to many composite breakwaters – ignoring the seminal influence of shoals or mounds on wave breaking and impulsive loadings. This paper presents case studies using empirical methods developed over the last 20-30 years to calculate loads and stability of example ‘old’ breakwaters including: Wick (failed before completion); Alderney (multiple breaches during construction, and only survives to 1/2 of its original length); and Dover (survives with substantial Factors of Safety). The screening analysis(5) used empirical methods developed over the previous 20 years(10), summarised in the case studies(6,7). Methods used here are empirical (no numerical modelling) so that the calculations can easily be repeated by local engineers.

A primary purpose of many coastal defences is to protect people from direct hazard due to wave overtopping. This is reflected by the use of an admissible overtopping as a key parameter in design or assessment of structures. Despite this long-established design driver, it is less than 20 years since guidance on admissible overtopping started to move from being based simply upon mean discharge to consider the volumes associated with individual wave overtopping events. Only in the past five years or so has attention zoomed in further, to associate the influences of direct hazard on the actual flow parameters (typically water depth and speed at the pedestrian). This paper will provide a developed view of the current state of the art and improved guidance, and the research from which it has emerged.

This paper presents a state of the art review of Floating Offshore Wind Turbines (FOWT) tank testing in wave basins from the perspective of understanding how different test methodologies currently deployed can be correlated to the development stage of the design being tested. This approach was already adopted by the wave energy sector, however, for the FOWT sector this is not fully developed, and only briefly mentioned in guidance documents. An open question in the application of aerodynamic loads within wave basin testing is, how complex does it need to be? For wave basin testing facilities, it is important to understand how a test program must be set up to meet the clients’ requirements. The designs being tested may be at different development stages, i.e., different Technology Readiness Levels (TRL) and it is important to understand how the test configuration will impact the clients’ objectives. Three main tank test campaigns will be performed at FloWave Ocean Energy Research Facility using the UMaine VolturnUS-S reference platform developed for the IEA wind 15-MW offshore reference wind turbine. It will be used as a basis for the development of a staged development approach for the FOWT sector. In preparation for these tests, the present paper explores and evaluates the appropriateness, with relation to TRL, of different methodologies for including aerodynamic loads in wave basin testing of FOWT.

Admissible wave overtopping is a key parameter in design specifications and also in safety assessments of the crest level of many coastal structures. This paper considers the hazard to people/pedestrians by post-wave overtopping flow over a horizontal surface, like a dike or breakwater crest, or a boulevard. Such flow is given by a flow velocity and a flow thickness. The most recent guideline is given in EurOtop (2018), where a maximum overtopping wave volume of 600 l/m is seen as the admissible or tolerable maximum. But no flow velocities or flow thicknesses are given. Previous work has been summarised by Sandoval and Bruce (2017) who brought existing fluvial tests on people or human subjects together with data derived from videos of actual overtopping hazard events available from the internet. A graph was developed with stable and unstable combinations of flow velocity and flow depth or thickness. The paper describes first tests in the Delta Flume of Deltares with a volunteer exposed to wave overtopping hazard on the crest of a dike with wave heights up to 1.8 m. Analysis determines flow velocities and flow thicknesses for stable and unstable situations. Additional tests with the wave overtopping simulator on the crest of a dike are described. In these tests, flow velocities and flow thicknesses were accurately recorded as well as the reaction of a volunteer, guarded by a safety line, on the crest of the dike as well as on the landward slope. These tests gave also stable and unstable situations with known flow velocities and flow thicknesses. The new data were added to the work of Sandoval and Bruce (2017) and a physically based as well as a simple guideline has been proposed for the transition between stable and unstable situations for people/pedestrians. In general overtopping velocities are allowed of 4 m/s with a flow thickness of 0.2 m, but also a large velocity of 7 m/s with only a flow thickness of 0.1 m. Flow thicknesses are always given without air entrainment.

Many historic breakwaters failed early in their life, leaving little information by which to analyse or understand their failures. As part of a wider analysis of ‘old breakwaters’, the first author has analysed the ‘stability’ of example vertical breakwaters using analytical methods developed over the past 20 years. This analysis is illustrated in this and the companion paper by three case studies: Wick (designed by Thomas Stevenson, failed 1870–1877); Alderney (damaged even during construction, lost its outer length 1865–1889); and Dover (still shows high stability after 110 years). In each of these case studies, representative cross-sections have been derived from historical records, as have the approach bathymetry. Representative wave conditions are transformed to the breakwater toes, including depth-limiting and impulsive breaking effects. Empirical formulae developed during and since the PROVERBS (Probabilistic Design Tools for Vertical Breakwaters) project have been used to explore incidence of wave impact loads, the main momentum loads and impulsive loads. Factors of safety against sliding and/or overturning have been determined for each example over a range of representative wave conditions.

This work aims to enhance the evidence base of extreme impact loads on the front (outer) wall and within the chamber of an oscillating water column (OWC) integrated into a vertical breakwater. Data are from large-scale physical model tests (at 1:9 scale) at the large wave channel (GWK). The paper shows that established prediction methods for nonimpulsive and impulsive wave loads for conventional vertical breakwaters can be applied to the front wall of OWC breakwaters. For the first time, impact loads within the OWC chamber have been quantified, on the rear in-chamber wall and on the chamber ceiling. These are found to reach magnitudes comparable to front wall wave impacts. Novel video records from within the OWC chamber provide new insight on water column behavior, including three different classifications identified as "single," "successional," and "water column" ceiling impacts. Conditions at risk of violent impacts within the chamber are also identified. These will be valuable in quantifying front (outside) design forces, in-chamber impacts, and in seeking to minimize in-chamber impacts occurrence, in construction and in operational phases.

Many historic breakwaters failed early in their life, leaving little information by which to analyse or understand their failures. As part of a wider analysis of ‘old breakwaters’, the first author has analysed the ‘stability’ of example breakwaters using analytical methods developed over the past 20 years with co-researchers. This analysis is illustrated by three case studies, the first covered in this paper and the second two in the companion paper: Wick (designed by Thomas Stevenson, failed 1870–1877)—Alderney (damaged even during construction, lost its outer length 1865–1889)—and Dover (still shows high stability after 110 years). In these case studies, representative cross-sections have been derived from historical records, as have the approach bathymetry. Representative wave conditions are transformed to the breakwater toes, including depth-limiting and impulsive breaking effects. Empirical formulae developed during and since the PROVERBS (Probabilistic Design Tools for Vertical Breakwaters) project have been used to explore the incidence of wave impact loads, the main momentum loads and the impulsive loads. Factors of safety against sliding and/or overturning have been determined for each example over a range of representative wave conditions and compared with reality.

Large floating structures, such as liquefied natural gas (LNG) ships, are subject to both internal and external fluid forces. The internal fluid forces may also be detrimental to a vessel's stability and cause excessive loading regimes when sloshing occurs. Whilst it is relatively easy to measure the motion of external free surface with conventional measurement techniques, the sloshing of the internal free surface is more difficult to capture. The location of the internal free surface is normally extrapolated from measuring the pressure acting on the internal walls of the vessel. In order to understand better the loading mechanisms of sloshing internal fluids, a method of capturing the transient inner free surface motion with negligible affect on the response of the fluid or structure is required. In this paper two methods will be demonstrated for this purpose. The first approach uses resistive wave gauges made of copper tape to quantify the water run-up height on the walls of the structure. The second approach extends the conventional use of optical motion tracking to report the position of randomly distributed free floating markers on the internal water surface. The methods simultaneously report the position of the internal free surface with good agreement under static conditions, with absolute variation in the measured water level of around 4 mm. This new combined approach provides a map of the free surface elevation under transient conditions. The experimental error is shown to be acceptable (low mm-range), proving that these experimental techniques are robust free surface tracking methods in a range of situations.

In the European research project HYDRALAB+ physical model tests have been performed in 2018 in the large wave basin of Deltares on crossing seas and wave overtopping over a 1:3 smooth slope. Crossing seas are defined as two sea states coming simultaneously from different directions. One could be a sea state developed by local winds, where the other one might be swell with a longer period coming from elsewhere from the ocean. The CrossOver project performed 170 tests on wave overtopping with different wave loadings (wave heights, periods, angles and spreading). Tests included “sea only” and “swell only” calibrations tests, and tests with sea and swell crossing, with sea obliquities ranging from -85° to +60° and swells from -75° to +60°. The influence of crossing seas on the distribution of overtopping wave volumes has been analysed using a Weibull distribution to describe such distribution and compared with the known theory for single sea states. The results agree well.

: Abstract Admissible wave overtopping is a key design parameter for coastal defences. For defences accessible to the public, it is likely that the admissible overtopping will be determined according to that which does not expose the public to direct hazard. Established methods base this admissible overtopping upon a combination of factors: mean overtopping discharge; individual maximum wave event volume, and the type of person exposed to the hazard, from unaware, easily frightened pedestrian through to trained and safety-aware personnel. The actual mechanism for the hazard – the actual flow causing a person to lose stability and fall – is less well explored. Literature studies, largely from the fluvial flooding community, are revisited and appraised against a simple analytical model for a person's stability in a flow. The recent 'resource' that are 'youtube' videos is then utilised, with three videos of actual overtopping accidents being identified and analysed quantitatively. Results are in broad agreement with the trends from the controlled studies. Significantly, however, the conditions found to be hazardous are under significantly shallower, faster flows than previously reported, with flows as shallow as 0.1 m shown to result in a person's fall. This work adds confidence to the use of the analytical model as an appropriate tool reflecting the actual physical process linking overtopped flow to direct pedestrian hazard.

Wave energy is one of the most promising marine energy resources in terms of the scale of the resource, but there remains little technology convergence and costs remain at near-prohibitive levels. Of many wave energy converter (WEC) concepts that have been developed over the years, the oscillating water column (OWC) stands out for its simplicity and low maintenance cost. Quite some experience of actual OWC operation has been gained to date from small, stand-alone pilot schemes. One way to reduce costs is the integration of an OWC-WEC into a breakwater, enabling some degree of cost-sharing between energy and harbour or coastal defence functions. A major problem encountered during the design of an OWC-WEC scheme remains the uncertainty in the wave loads, with their critical influence upon capital cost. A model to estimate forces acting on an OWC chamber in a caisson breakwater is proposed in this paper. Horizontal forces on the front (curtain) wall and the rear (in-chamber) wall are predicted. In addition, and unlike a conventional caisson breakwater, vertical forces acting on the caisson chamber ceiling will have considerable effect on sliding and overturning characteristics of the breakwater structure. The proposed model enables the prediction of chamber pressures which in turn influence the chamber vertical force. The new model has been compared with results from large scale physical model measurements from tests carried out in the very large wave channel, GWK, in Hannover (Germany). Forces under both regular and irregular wave conditions were measured. The comparisons show that the model fits well with the test results to the factor of 1 +/- 0.2 for the regular wave cases and to the factor of 0.8 +/- 0.2 for irregular wave cases. This model will enable the structural design of caisson breakwater-integrated OWCs to be approached with uncertainties reduced to those comparable with conventional caisson design.

It is common for the local sea state in coastal waters to be a complex combination of waves due to local and recent wind (the “sea”) and long period waves resulting from earlier weather systems, which have travelled many 100s of km with little attenuation of these very long waves (the “swell”). Sea and swell may have very different directions and periods. The CrossOver project was born out of the recognition that there is an absence of guidance on the influence of directionally bimodal (or bidirectional, or ‘crossing’) seas on wave overtopping at a coastal defence. Through a physical model study carried out in the Delta Basin, the “CrossOver” project is beginning to fill this gap in the knowledge.

This paper examines the impact of MetOcean conditions on weather window availability and duration for tidal energy operations and maintenance. Understanding these impacts at the initial planning stage will give a better estimation of project lifetime costs, and ensure that these costs are factored into the site selection methodology. Several sources of freely available data were input into the Delft3D modelling suite to produce spatially and temporally varying estimates of MetOcean data for the Surigao area in the Philippines. This data was validated where possible, with the generated tidal heights and flow speeds seeing a good fit to adjacent tide gauge and acoustic data. A Dijkstra’s Algorithm was applied to generate an optimum route to shore that accounted for depth restrictions. Weibull persistence statistics were successfully applied to the MetOcean characteristics at each point along this route, to calculate the probability of vessel limitations being exceeded. The number of probable access and waiting hours within a month, given a required weather window length and MetOcean threshold, was calculated. Flow is seen to be the most constraining, but also the most predictable MetOcean parameter and thus can be accounted for in operational planning. Wind is seen to impact little on transit but can be constraining for longer operations. Wave conditions are seen to constrain both transit and operations weather windows significantly under the limitations examined. Keywords— Tidal Stream, Site Selection, Weather Windows,

In the past, many coastal towns or villages constructed harbours for trade and/or fishing. These were protected by breakwaters, commonly rubble mounds to low-water, surmounted by vertical walls of dressed stone blocks (later concrete) with rubble core. Now, 100-150 years later, many such harbours have little or no income to maintain or repair their breakwaters, but meantime, the areas protected have been adopted for commercial and residential purposes, at increased risk of flooding if the 'orphan' breakwater were to collapse, Hampshire et al. (2013). Even after collapse, some wave reduction will however still be afforded by the relict structure. Tests by Allsop et al (2017) have modelled the collapse of several simplified breakwaters, and measured levels of wave transmission over the collapsed structures. This paper extends that work with more analysis of collapsed crest levels, and hence of likely wave transmission.

This paper investigates the accuracy of the Computational Fluid Dynamics (CFD) based Immersed Body Force (IBF) turbine modelling method for predicting the flow characteristics of a Momentum-Reversal-Lift type of tidal turbine. This empirically-based CFD model has been developed based on the actuator disc method enhanced with additional features to mimic the effect of the complex blade motion on the downstream wake, without the high computational costs of explicitly modelling the dynamic blade motion. The model has been calibrated against the flow characteristics data obtained from experiment and found to perform well, although there are few inconsistencies in the flow patterns which show some of the limitations of the IBF model compared to a full dynamic blade motion simulation. However, given the complexity and computational cost of modelling the detailed blade motion the limitations of the IBF model are acceptable and will be useful especially for optimisation of arrays of devices where there is a significant computational demand. Crown Copyright 2016 Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

In 2009, four of 16 chambers in the Mutriku breakwater-integrated Oscillating Water Column (OWC) were badly damaged by storms, probably due to breaking wave loads, and slam within the chamber. To minimize exposure of future plant to such risks, it is necessary to characterise wave conditions under which such an installation could experience impact loads. This characterisation can be crucial to controling the power-take off resistance to increase the survability of the device during extreme weather. Large scale physical model tests in the Grosse Wellenkanal (GWK) included a video camera installed inside the chamber facing the rear chamber wall. Pressure sensors in the ceiling of the chamber were utilised to quantify the water loads. In-chamber impact pressures of up to 8 ρgH were recorded on the chamber ceiling, associated with the ‘sloshing’ observed. The “sloshing” phenomenon is not uncommon and should be considered in design processes.