Marcel R.A. van Gent

• PhD
• Professor of Coastal Structures at Deltares & TU Delft

Simulating the flow depths and velocities occurring during wave overtopping events at coastal structures with a numerical model is challenging. Reproducing these variables is relevant since they can be used as design parameters if they can be predicted accurately. OpenFOAM® was used to analyse flow depths and velocities that occur during wave overt...

This study presents the stability and hydrodynamics of the Reef Enhancing Breakwater (REB) based on a physical model study. The REB is an innovative shoreline protection solution that uses interlocking ReefBlocks designed to enhance marine life while ensuring structural stability. Through three physical model testing campaigns conducted in 2020, 20...

Estimates of wave overtopping generally determine the required crest level of coastal structures. Wind affects wave overtopping, especially for coastal structures with crest walls because these can cause a vertical wave motion above the crest that is susceptible to wind. Within the relevant range of overtopping discharges, the expected wave overtop...

Due to the three-dimensional flow field formed by the diffraction and refraction effects of waves in breakwaters, the stability of the armour layer of roundheads is more critical than the trunk sections. For this reason, roundheads require heavier units or a gentler slope. In this research, the stability of breakwater roundheads consisting of high-...

Estimates of wave overtopping generally determine the required crest level of coastal structures. Wind affects wave overtopping, especially for coastal structures with crest walls because these can cause a vertical wave motion above the crest that is susceptible to wind. Within the relevant range of overtopping discharges, the expected wave overtop...

This study presents the stability and hydrodynamics of the Reef Enhancing Breakwater (REB) based on a physical model study. The REB is an innovative shoreline protection solution that uses interlocking ReefBlocks designed to enhance marine life while ensuring structural stability. Through three physical model testing campaigns conducted in 2020, 20...

The hydraulic stability of rock armour layers has been extensively discussed in the literature, with numerous formulae proposed for design purposes. However, limited attention has been given to armour stability under shallow water conditions, largely due to the scarcity of experimental data. This research aims to address this gap by providing new i...

Wave overtopping of coastal structures has been studied using physical model experiments with rubble mound breakwaters in shallow water. The mean overtopping discharge is determined for three different foreshore slopes and various hydrodynamic conditions. The hydrodynamic results confirm that energy is transferred to low-frequency waves in very sha...

Hard stabilization methods have traditionally been employed to mitigate coastal erosion. Concrete armour is widely used due to its high level of dependence, robustness, ease of production and cost effectiveness (Cooke et al., 2020; Pikey and Cooper, 2012). It is inevitable that coastline ‘armouring’ will continue to rise because of the growing huma...

Single layer randomly placed armour units are used in many rubble mound breakwaters around the world. For these armour layers, breakage of armour units due to rocking could be a major damage mechanism, but no good methods exist to evaluate and quantify rocking. The aim of the study is to quantify the rocking impact velocities for irregularly placed...

Wave overtopping at rubble mound structures is one of the most important phenomena affecting the hydraulic performance of these coastal structures. In addition to the design of coastal structures, also the climate adaptation of coastal structures has become more important due to sea level rise. Adding a crest wall to an existing structure, increasi...

Climate adaptation of coastal structures has become more important due to climate change, resulting in sea level rise and increased wave loading for coastal structures with depth-limited wave conditions. If sea level rise causes wave loading that becomes too severe, one of the options is to reduce the wave loading before the waves reach the existin...

Artificial or Engineered reefs are primarily developed for enhancing the ecological system. In Guidelines for the Placement of Artificial Reefs (UNEP, 2009) they are described as "An artificial reef is a submerged structure deliberately constructed or placed on the seabed to emulate some functions of a natural reef such as protecting, regenerating,...

How to adapt coastal structures to climate change has become an important topic of study, particularly given the fact that sea level rise can also increase wave loading on coastal structures that were designed under depth-limited wave conditions. One of the climate adaptation measures to ensure that existing coastal structures continue to perform t...

Rising sea levels caused by climate change are increasing the risk of overtopping on coastal structures. Moreover, there is a growing societal concern about the visual impact of these structures, which leads to the lowering of their crest freeboards. In previous studies, safety during overtopping events was assessed considering the overtopping laye...

During storms, ensuring the protection of people, vehicles and infrastructure on the crest of coastal structures from wave overtopping hazards is crucial. The thickness of the wave overtopping layer is a key variable used for assessing safety and maintaining a secure design. Traditionally, this parameter is associated with the height difference bet...

Physical model tests have been performed to study static stability of rock-armoured mild slopes. Current stability design formulae for steeper rock-armoured slopes focus on plunging and surging waves. Slopes of 1:6 and milder usually have more spilling breakers which decreases the load. Also, on mild slopes displaced rocks more often remain present...

Single layer randomly placed armour units are used in many rubble mound breakwaters around the world. For these armour layers, breakage of armour units due to rocking could be a major damage mechanism, but no good methods exist to evaluate and quantify rocking. This paper utilizes novel embedded Rocking Sensors to obtain the first measurements of r...

In the design of coastal structures future sea level rise (SLR) needs to be accounted for. A method is described to account for uncertain future SLR and applied for a rubble mound structure. The optimal sequence of adaptation measures appears to be quite insensitive to the climate scenario, amount of inflation and discount rate. Methods to estimate...

The crest level of coastal structures such as dikes and breakwaters is often based on estimates of the amount of wave overtopping. One of the important parameters affecting wave overtopping is the angle of the incident waves since oblique waves can significantly reduce the amount of wave overtopping compared to perpendicular wave attack. Based on 3...

The design of crest walls is often based on empirical formulations, physical model tests, numerical models and a fair amount of expert judgement. The present work validates the prediction of wave induced forces on the front face of crest walls on top of composite breakwaters in the numerical model OpenFOAM. The results show that OpenFOAM is able to...

Stability formulae for armour layers of rubble mound breakwaters are generally developed for perpendicular wave attack and do not include effects of oblique waves. Waves usually attack breakwater obliquely as the sea wave is three dimensional. Several studies have been performed to investigate the effect of wave angle (beta) on the armor stability....

Slope stability formulae for rubble mound structures are usually developed for head-on conditions. Often, the effects of oblique waves are neglected, mainly because it is assumed that for oblique wave attack, the reduction in damage compared to perpendicular wave attack is insignificant. When the incident waves are oblique, the required armour size...

Wave transmission at low-crested coastal structures has been studied, based on physical model tests with trapezoidal impermeable, permeable and perforated structures. The differences between wave transmission at impermeable and permeable structures are relatively limited. For a perforated hollow structure with an impermeable vertical screen in the...

Randomly placed breakwater armour units under wave loading can sometimes start rocking, which can lead to breakage of armour units. This failure mechanism can especially become important for single layer randomly placed armour units for which full displacement of units will only happen at higher stability numbers compared to older types of units, a...

The crest level of seawalls is often based on estimates of the amount of wave overtopping. Methods to estimate the mean overtopping discharge have been provided in several guidelines. One of the important parameters affecting wave overtopping is the wind. However, the effects of wind have not been accounted for in detail in present design guideline...

Numerical modelling of wave interaction with rock-armoured rubble mound breakwaters has been performed to study wave overtopping. The influences of the slope angle, a berm in the seaward slope, a protruding crest wall, a recurved parapet, and the wave steepness have been studied using a validated CFD model (OpenFOAM). The numerical modelling confir...

The hydraulic stability of the armour layer of rubble mound breakwaters has been widely discussed in literature and several empirical formulas have been proposed for design purposes. Nevertheless, few studies focused on the armour stability in case of water depth limited water conditions. The aim of the present research is to provide more insights...

For a safe design of a rubble mound seawall, overtopping characteristics such as the mean overtopping discharge (q) and the maximum individual overtopping volume (Vmax) should be limited. Unlike q, the estimation of Vmax is more complex and requires a wave-by-wave analysis of overtopping as well as a statistical analysis. The present study contribu...

The performance of high-density cubes in the armor layer of a breakwater was investigated through an experimental study. For this purpose, two different cross-sections, one with a conventional cross-section and one with a berm, were modelled in a wave flume. A total of ten tests were performed for both cross-sections. Different concrete densities o...

Estimation of the mean overtopping discharge is a major task in the design and assessment of the crest level of rubble mound structures such as breakwaters and seawalls. The tolerable mean overtopping rates are given based on the associated risk and wave characteristics. Several empirical formulas have been developed for the prediction of mean over...

Physical model tests have been performed to study wave overtopping at rubble mound breakwaters, including breakwaters with a crest wall, breakwaters with a berm, and breakwaters with a crest wall and a berm. For rubble mound structures with a protruding crest wall or with a stable berm, limited information is available in literature even though pro...

Seawalls play a significant role in protecting coastal areas against wave attack and flooding. The accurate estimation of wave overtopping at seawalls is therefore crucial to adequately protect people and infrastructure in these regions. In this study, the mean wave overtopping rate at rubble mound seawalls was investigated through 140 small-scale...

The mean overtopping discharge and the overtopping flow parameters related to individual overtopping events are often used to characterize the wave overtopping processes at dikes. Roughness, berms and oblique waves have significant effects on the wave overtopping processes at dikes while these effects are still not fully understood. A 2DV OpenFOAM®...

Accurate calculation of wave overtopping is important for determining the required crest height and geometry of a dike. Berms and roughness elements are widely used to reduce the average overtopping discharge at dikes while the reductive effects of berm and roughness are still not fully understood. Several empirical formulae are available to predic...

The average overtopping discharge is an important parameter for the design of flood defences. Several empirical formulas are available for predicting the overtopping discharge at dikes. However, these empirical formulas often have their specific applicable conditions. To complement with the empirical methods, a numerical model has been developed us...

The stability number for rubble mound breakwaters is a function of several parameters and depends on unit shape, placing method, slope angle, relative density, etc. In this study two different densities for cubes in breakwater armour layers were tested to determine the influence of the density on the stability. The experimental results show that th...

The amount of wave overtopping at coastal structures such as vertical caisson breakwaters is strongly dependent on the angle of wave attack. The reducing effects of oblique waves on wave overtopping compared to perpendicular wave attack has been studied by means of three-dimensional wave basin tests. In these physical model tests the caisson breakw...

Conventionally, allowable mean overtopping discharge is used as a design criterion for coastal structures. The mean overtopping discharge needs to be limited to ensure structural stability as well as the safety of people, vehicles, and properties behind the structure. Nowadays, limits for the maximum individual overtopping volumes are also specifie...

Coastal structures are often designed to a maximum allowable wave overtopping discharge, hence accurate prediction of the amount of wave overtopping is an important issue. Both empirical formulae and neural networks are among the commonly used prediction tools. In this work, a new model for the prediction of mean wave overtopping discharge is prese...

Coastal structures are often designed to a maximum allowable wave overtopping discharge, hence accurate prediction of the amount of wave overtopping is an important issue. Both empirical formulae and neural networks are among the commonly used prediction tools. In this work, a new model for the prediction of mean wave overtopping discharge is prese...

In this study, the stability of cube armour units in the lower slope and berm of rubble mound breakwaters with a berm was investigated for irregular and regular placement methods. Cubes are simple and cheap in production. However, the design conditions for the upper slope and lower slope may differ depending on the water level of the dominant storm...

Toe design is an important task for coastal engineers as it ensures the stability of the main armor layer and prevents scour in front of the armor slope. Several laboratory experiments have been conducted to investigate the toe stability using different testing approaches, i.e. damage due to a single test condition and cumulative damage due to a nu...

Average overtopping discharge is a crucial parameter for design and reinforcement of dikes. Rock armours and berms are widely used to reduce wave overtopping discharge by introducing slope roughness and energy dissipation. Since-methods for estimating the influence of a rock berm and roughness of rock armour at dikes on the average overtopping disc...

The average overtopping discharge is an important parameter for the design and reinforcement of dikes. Rock armour on the waterside slopes and berms of dikes is widely used to reduce the wave overtopping discharge by introducing slope roughness and dissipation of energy in the permeable armour layer. However, methods for estimating the influence of...

Wave overtopping is an important design criterion for coastal structures such as dikes, breakwaters and promenades. Hence, the prediction of the expected wave overtopping discharge is an important research topic. Existing prediction tools consist of empirical overtopping formulae, machine learning techniques like neural networks, and numerical mode...

Estimation of the required armor size is a major task in the design of coastal structures under wave loading such as breakwaters and revetments. Several semi-empirical formulas have been developed for this purpose. However, these formulas are often either limited to certain water depth conditions or do not incorporate the permeability of the struct...

Oblique wave attack can significantly reduce the amount of wave overtopping at coastal structures compared to perpendicular wave attack. Guidelines exist to estimate the influence of oblique waves on wave overtopping. Guidelines to estimate the influence of a berm in the seaward slope are also available. These guidelines for the reducing effects of...

In coastal engineering context, the use of video imagery is widespread. Especially in field settings along sandy coasts, many types of data have been derived from video imagery, often using non-learning analysis techniques. Recent advances in the field of computer vision and deep learning allow for the automation of image segmentation. In this pape...

Rubble mound breakwaters usually present a crest wall to increase the crest freeboards without a large increase of the consumption of material. Methods in the literature to design crest walls are based on estimates of the wave loads. These methods are focused on the maximum loading that attacks a single position of the crest wall. In practice, cres...

Projections of sea level rise (SLR) vary in time and go together with large uncertainties. Not only best estimates of SLR vary in time, also the estimated uncertainties in SLR vary in time. Not all processes that cause the SLR are fully understood, which is illustrated by recent studies that discuss the rapid Antarctic ice sheet mass loss with a po...

A reliable estimation of average wave overtopping discharge is important for dike design and safety assessment. Berms and roughness elements are widely applied to reduce the overtopping discharge over dikes. In this study, the effects of a berm and roughness on the wave overtopping discharge are investigated by means of physical model tests. New em...

The probability of overtopping over the crest of dikes increases significantly during extreme events. Wave overtopping over dikes can initiate breaching of the dike. Hence, a reliable estimation of the overtopping discharge is important for the dike design and safety assessment. Berms and roughness elements are widely used around the world to reduc...

At the crest of rubble mound breakwaters a crest wall is often present to reduce the consumption of material compared to breakwaters without a crest wall or to enable access to the breakwater. Several methods to design crest walls are available. These methods are developed for wave loading where waves approach breakwaters in perpendicular direction...

Toe structures are a common feature in coastal rock armoured structures, both safeguarding the structure against scour at the toe and providing support to the armour layer. The toe structure itself is often comprised of rocks as well. Hence, generally some form of scour protection, such as a filter layer or a geotextile, is applied underneath the t...

At the crest of rubble mound structures a crest wall is often present to reduce the consumption of material compared to breakwaters without a crest wall. To design the crest wall several methods based on estimates of the hydraulic wave loads are available. These methods are focussed on waves that approach breakwaters perpendicular. In practice, the...

Physical model tests were performed in a wave flume at Deltares with rock armoured slopes. A shallow foreshore was present. At deep water, the same wave conditions were used, but by applying different water levels, the wave loading on the rock armoured slopes increased considerably with increasing water levels. This allowed an assessment of the eff...

An accurate description of the maximum flow velocity across flood-protective structures is necessary in order to determine the stability of the landward slope and the amount of cover erosion. In this paper, two new formulas are derived to describe the change in the maximum overtopping flow velocity along the dike crest and the adjacent landward slo...

This paper describes a method of determining the reaction forces of a vertical structure with an overhang to impulsive wave impacts. The aim is to develop a method to design a hydraulic structure exposed to the impulsive wave impact. At present, there is a lack of guidelines on the designing and verification with such a purpose. The impulse of the...

In order to design reliable coastal structures, for present and future scenarios, universal and precise damage assessment methods are required. This study addresses this need, and presents improved damage characterization methods for coastal structures with rock armored slopes. The data used in this study were obtained from a test campaign carried...

In this work, a CFD model (OpenFOAM) is extended to allow for the exchange of sediment between the inside of porous structures and the outside. This exploratory extended model is applied to simulate the morphological change around and under toe structures, as are often used in rock armoured coastal structures. Both the dimensions of the toe structu...

Transitions in the dike revetment or in the grass cover can significantly affect the wave overtopping discharge and the dike cover erosion. At the University of Twente, two PhD students recently started on the challenge of quantifying the effect of (1) waterside transition on the wave overtopping discharge and (2) transitions in grass covered dikes...

Physical model tests on the stability of rock armoured slopes have been performed to demonstrate the importance of water level variations during a storm, due to a tide or a storm surge. For the stability of rock armoured slopes also the importance of the sequence of storms at various water levels has been studied. The test results indicate that a s...

For the sake of overtopping, stability and economy, rubble mound breakwaters have been built with a berm in the seaward slope. Here, a breakwater model with cube armour units in the lower slope and a horizontal berm, and rock in the upper slope was tested experimentally. The purpose of the study was to investigate the stability of berms for two dif...

Most guidelines on wave overtopping over coastal structures are based on conditions with waves from one direction only. Here, wave basin tests with oblique wave attack are presented where waves from one direction are combined with waves from another direction. This is especially important for locations where wind waves approach a coastal structure...

Physical model tests on the stability of rock armoured slopes have been performed to demonstrate the importance of water level variations during a storm, due to a tide or a storm surge. For the stability of rock armoured slopes also the importance of the sequence of storms at various water levels has been studied. The test results indicate that a s...

Wave loads on crest walls on top of rubble mound structures determine the size of these crest walls. For the design of crest walls some design guidelines exist, but their validity is limited to particular designs of the cross section (berm, no berm, toe, armour layout, protruding crest element, etc). The present work addresses the validation of Ope...

This paper describes a method to measure the rocking motion of lab-scale armour units. Sensors as found in mobile phones are used. These sensors, data-storage and battery are all embedded in the model units, such that they can be applied without wires attached to them. The technique is applied to double-layer units in order to compare the results t...

Physical model tests were performed in a wave flume at Deltares with rock armoured slopes. A shallow foreshore was present. At deep water the same wave conditions were used but, by applying different water levels, the wave loading on the rock armoured slopes increased considerably with increasing water levels. This allows the assessment of effects...

In order to design reliable coastal structures, for present and future scenarios, universal and precise damage assessment methods are required. This study addresses this need, and presents improved damage characterization methods for coastal structures with rock armoured slopes. The data used in this study was obtained from a test campaign carried...

Underneath rock slopes of rubble mound structures often one or more granular filter layers are present. These filter layers prevent base material washout. In contrast to traditional filters, geometrically open filters allow for some movement of base material. In order to design such open filters the amount of erosion and accretion of base material...

Cubes in a single armour layer of rubble mound breakwaters are feasible and potentially economically competitive compared to other types of armour layers. The use of a berm in the seaward slope of breakwaters leads to an important reduction in wave overtopping or crest elevation. Another advantage of a berm in the seaward slope is that relatively s...

Permeable coastal structures that consist of rock typically contain granular filters. The internal transitions between two layers are normally geometrically tight to prevent material washout. Applying open filters with only minimal loss of material through the layer above, may be a cheaper alternative (see also Van Gent and Wolters, 2015). Physical...

Rubble mound breakwaters and revetments typically contain granular filters in one or more layers. The transition from the armour layer to the filter layer, and transitions between other layers within the structure, are normally geometrically tight to prevent material washout. This requires a limited ratio of the material size of the upper layer and...

This paper treats the numerical modelling of the behaviour of a sand core covered by rocks and exposed to waves. The associated displacement of the rock is also studied. A design that allows for erosion and deposition of the sand core beneath a rock layer in a coastal structure requires an accurate prediction method to assure that the amount of ero...

Rubble mound breakwaters and revetments typically contain granular filters in one or more layers. The transition from the armour layer to the filter layer, and transitions between other layers within the structure, are normally geometrically tight to prevent material washout. This requires a limited ratio of the material size of the upper layer and...

When design wave conditions are high and available rock sizes are small, the application of concrete armour units for the design of breakwaters becomes nearly inevitable. Modern day design practice then shows that often single layer concrete armour units are applied. The concept of a single layer obviously results in a considerable cost reduction c...

For most existing coastal structures such as sea defences an estimate of the amount of future sea level rise has been taken into account in the design stage. Once estimates of sea level rise change, sea defences need to be reassessed and adapted to ensure that they provide the required safety under the envisaged new conditions. The evaluation of ex...

A method to quantify, analyse, and present the settlement of single-layer concrete armour layers of coastal structures is presented. The use of the image processing technique for settlement analysis is discussed based on various modelling studies performed over the years. The accuracy of the technique is assessed, and the application of the techniq...

Permeable coastal structures under wave loading typically contain granular filters in one or more layers. The transition from the armour layer to the filter layer, and transitions between other layers within the structure, are normally geometrically tight to prevent material washout. This requires a limited ratio of the material size of the upper l...

A new large-scale wave flume has been constructed in Delft. The new Delta Flume has a length of about 300m, a width of 5m and a height of 9.5m. The maximum wave heights that can be generated are about Hs = 2.2m. Maximum individual wave heights of about Hmax = 4.5m have been generated. This unique facility enables physical modelling at prototype-sca...

Permeable coastal structures typically contain granular filters in one or more layers. These filters are normally geometrically tight to prevent material washout. A cheaper alternative is a geometrically open filter where only minimal base material loss or settlement occurs. In Van Gent and Wolters (2015) physical model tests were performed to esti...

This paper will address the validation and application of a volume of fluid method for coastal structures under the influence of normal incident irregular wave fields. Several physical processes will be addressed as part of the validation process, namely: (i)wave reflection from permeable and impermeable structures, (ii) wave transformation over a...