Scour protections around monopile foundations of offshore wind turbines usually rely on loose rock as the main material, and on traditional construction methodologies. But how would a concrete prefabricated element perform instead of rock? This question, together with the particular focus on Xstream as main element, is at the heart of this master thesis. The research goal is to determine and describe the hydrodynamic stability of the Xstream elements, focusing on their application near monopile foundations under wave-only loading. The test program is mainly based on the conditions at the southern North Sea. While two main authors (Izbash (1935) and Shields (1936)) have set the foundation of the principles of stability of rocks under flows, the proliferation of offshore wind energy (and of submarine cables) motivated the creation of the JIP-HaSPro (Handbook of Scour and Cable Protection Methods), greatly based on studies by Broekema et al. (2023), and used in a considerable measure for this research. The present study was conducted as a physical modelling test program in the laboratory of BAM in Gouda, in a flume with a wave generator. A scale of 1:15.71 was used. Due to logistical reasons, two distorted scales were applied: the first (and main) distorted scale, with which correctly scaled orbital velocities near the bed were generated, while smaller water depths and wave heights (fitting in the flume’s height) could be applied; the second distorted scale consisted in the reduction of the pile diameter, so that it fitted in the flume’s width (this made the results somewhat conservative compared to the original prototype pile diameter of 10m); half monopile was adopted, adjacent to one of the flume walls. A rectangular form was adopted for the model scour protection, still conscious that a circular shape concentric with the monopile is the most suitable. The layer thicknesses were based on packing densities related to area. No sediment was used, i.e. the block layer was laid on a fixed bed. A group of 6 test series were carried out, each featuring a different layer thickness and two placing methods with different packing density as a result. Each test series consisted of 8 wave conditions with the same peak period (Tp,m = 3.2 s) and significant wave heights varying from 0.08m to 0.26m. The tests were 40 minutes long, aiming for a number of 1000 waves. Three types of movement were observed during the tests: rocking, internal displacements (i.e. all displacements larger than about 1 unit diameter), and removal from the protection (external displacements). The data collected consisted of a. wave data, measured by the wave gauges and processed in WaveLab; and b. block movement data, based on observations aimed at the three types of movement. The first feature noticed during the tests was the formation of patterns of erosion and deposition within the construction area, in the form of a trench and a mound, next to the half monopile and right behind it, respectively. The amount of blocks rocking grew gradually with the wave conditions, while both the internal and external displacements showed a delayed growing tendency with scattered peaks... ... Read more

Geotextiles are widely used in civil engineering, particularly in hydraulic engineering where they serve as filters and protect sand and clay from erosion. Their primary function is to prevent the washout of fine materials from the subsoil. Geotextiles are permeable fabrics and are usually made from plastics like polypropylene and polyethylene. However, due to environmental concerns surrounding synthetic geotextiles, there is a growing interest in moving away from plastics towards natural fibers. To facilitate this transition, understanding the engineering properties of these materials is essential. This study focuses on sandtightness, aiming to make applications more feasible.

Geotextiles can reduce the total thickness of a granular filter and thereby the costs. Because a geotextile can replace multiple granular filter layers, less material is required, which also results in lower emissions. Filters can be distinguished into two categories: geometrically open and geometrically closed filters. Sand particles cannot pass through a geometrically closed filter. The sandtightness is independent of the magnitude of the hydraulic load. Open filters prevent the erosion of the base material up to a certain hydraulic load. Open geotextile filters are not common used in hydraulic engineering.

Previous research has primarily focused on synthetic geotextiles, developing formulas to predict critical filter velocities based on their properties ((Van Der Knaap et al., 1986) and ( (Klein Breteler, 1988)). In contrast to these synthetic-focused studies, a study by Lemmens in 1996 on natural geotextiles, such as jute cloth, revealed a critical hydraulic gradient of 0.26, suggesting jute's potential as an alternative to synthetic materials. Despite these findings, stability criteria for natural geotextiles remain undefined, and newly developed geotextiles from natural materials have not yet been tested.
To address this gap, experiments were conducted in a flume set-up at the Fluid Mechanics Laboratory at TU Delft, using both woven and non-woven geotextiles made from natural materials such as jute, hemp, and wool. In the flow flume, a steady current was applied to a one meter long stretch of rock, closed at the top, with a geotextile-covered sand bed beneath it, allowing hydraulic gradients up to i = 1 to be exerted. This set-up enabled the determination of the critical load.

A total of 19 tests were conducted with 11 different configurations, involving variations with 7 different types of geotextile (4 woven jute geotextiles and 3 non-woven geotextiles made of hemp, jute, and wool). During all test steps of the 19 tests in the flow flume, measurements of water levels, pressures, and discharge were taken. Two endoscopes were installed at different locations in the filter layer, with holes drilled into two stones to secure the endoscopes. This allowed the camera to be positioned at the top of a pore, providing a view of the geotextile and detecting passing sand grains.

To determine the critical test step where the movement of base material begins, an innovative method of analyzing the erosion state was employed using endoscope images. These endoscopes are capable of detecting sand grains, allowing for the observation of erosion dynamics. In 4 of the 19 tests, the eroded sand was also suctioned after each test step to gain insight into the amount sediment transport. This indicates that even before the critical threshold level, there are small amounts of erosion immediately after increasing the hydraulic load. If the critical threshold level has not yet been reached, this erosion will return to zero during the rest of the test step. When determining the critical test step, two values were used for the critical load: a non-erosion criterion (Ho, 2007) and a criterion of 0.2 gr/s/m² (Klein Breteler, 1988), as used in previous studies to determine the start of movement. The non-erosion criterion is qualitative, whereas the Klein Breteler criterion is quantitative. It was found that these criteria are not equivalent, with a factor of 2 to 3 difference in the critical filter velocity.

This study concluded that the critical load for sediment transport is primarily determined by the filter velocity for open natural geotextiles. This contradicts previous studies that suggest a critical hydraulic gradient for open natural geotextiles. The critical filter velocity is influenced by the geotextile's characteristics, including thickness, opening size, and water permeability. Notably, the structure of the geotextile itself significantly affects the critical filter velocity. In this study, two woven jute geotextiles (J4: 422 gr/m² and O90 = 516.1 μm, J5: 518 gr/m² and O90 = 819.0 μm) with different structures were tested. The difference in opening size between the two geotextiles is approximately a factor of 1.5. However, the geotextile with much larger openings had a 1.75 times greater critical filter velocity with a start of movement criterion of 0.2 gr/s/m². Additionally, while the grading and size of the filter layer's grains influenced the critical hydraulic gradient, they had little to no effect on the critical filter velocity, emphasizing the significance of geotextile properties in determining their performance and stability under hydraulic load.

Ultimately, based on the tests, it can be concluded that the newly developed non-wovens could be a good alternative to synthetic geotextiles when considering sandtightness under parallel flow. All three non-wovens are reasonably stable at a hydraulic gradient of i ≈ 1, which is the maximum gradient that could be applied in our test set-up.

Recommendations for future research include testing with coarser sand, longer test durations, and synthetic counterparts for comparative analysis. Suggested improvements to the test set-up involve detailed height measurements of the sand bed and filter layer, and using high-resolution cameras for better sediment tracking. This sediment tracking by an endoscope can be improved by using a fixed pore for the endoscope for all tests. This can ensure that the pore volume is consistent across all tests and the size of the geotextile section captured in the image is also consistent. There is potential to use endoscope images for quantitative analysis of sediment transport. Further, the study emphasizes examining different flow conditions (perpendicular, and non-stationary) to enhance geotextile designs, particularly in dynamic environments like coastal revetments. ... Read more

The research, "An Integrated Approach to Identify Sustainable Materials and Strategies to Achieve a Carbon-Neutral Breakwater Design," aims to design breakwaters that maintain structural integrity and functional performance while minimizing environmental impact. The research addresses the urgent need to reduce carbon emissions in coastal engineering, aligning with global climate change mitigation goals.
The primary objective is to develop a comprehensive framework for designing carbon-neutral breakwaters... ... Read more

In 2008, a new concept to extract tidal energy was introduced called Dynamic Tidal Power (DTP). In this concept, the natural tidal wave propagating along a coastline is blocked using a long dam, inducing a water level difference between both sides of the dam. This water level difference, called the hydraulic head, can be exploited by placing turbines inside the dam. This is a promising concept, but the feasibility and suitable locations for DTP are largely unknown yet.

This study aimed to identify locations that have the highest potential to implement the DTP concept. By investigating the feasibility criteria and their influence and translating these criteria to spatial analysis, geographic regions with high potential were identified. Additionally, linkage opportunities were assessed and identified to possibly improve the feasibility of DTP.

Based on a sustainable feasibility study framework, the key aspects can be categorized into three main categories; technical, economic and social-ecological feasibility. These aspects were translated into criteria, whose influence was investigated using a hydraulic model. Considering the technical criteria, the tidal flow velocity proved to be more influential on the feasibility than the depth, finding weighting factors of 0.87 and 0.13, respectively. In order to define a threshold of the feasibility of DTP, the technical and economic criteria were combined by assessing the upper threshold of the Levelized Costs Of Energy (LCOE) of 0.1 €/kWh to find a minimum required average flow speed of 0.63 m/s. This threshold was used in the spatial model. The island linkage opportunity proved to be highly valuable. Connecting an island increases the effective blockage length, resulting in a hydraulic head and power output increase with approximately the same dam construction costs. This lowers the mean tidal current speed required for feasibility to 0.39 - 0.57 m/s.

The spatial analysis used a Multi-Criteria Weighted Overlay Analysis (MCWOA) approach, combining datasets and weighting factors for each criterion to find a total Potential Index (PI) score. In total 41 potentially feasible locations for DTP were identified. The geographic regions with the highest potential to implement DTP are the Yellow Sea, the seas in Southeast Asia, and the seas of Oceania. This is due to a combination of high tidal current velocities, relatively modest water depths and a good combination of secondary criteria (energy usage, population size & low shipping disruption). A high share of the island linkage opportunity was found in the locations with the highest potential (44%). It is recommended to investigate whether additional island linkage opportunities can be identified, by lowering the tidal current velocity threshold in the spatial model. Additionally, 233 locations were discarded, mostly due to their remote location. However, since these locations are technically feasible, it is recommended to investigate the possibility of transmitting the energy over long distances using Ultra High Voltage Direct Current transmission lines and study the total feasibility of DTP at these remote locations.

This study assessed the feasibility of DTP and identified that the Yellow Sea, the seas in Southeast Asia, and the seas of Oceania have the highest potential for a DTP project. Additionally, it provided new insights regarding the influence and thresholds of certain influential criteria, and it presents a hydraulic model and GIS tool that can be adjusted based on the preference of the user. These findings could be useful in future research on the DTP concept. ... Read more

Besides global warming, climate change is expected to result in (among others) an increased number of extreme precipitation events and a rising sea level. In the future, this can be the cause for an increased number of high river discharge events in combination with high water levels on the North Sea, which goes hand in hand with an increased risk of flooding in the Netherlands. However the rate of change in the future is hard to predict and therefore very uncertain.

One of the projects which seeks to be a (part of the) solution in climate change and flood prevention for the Netherlands is the Delta21 project. By combining a sea defence, energy storage lake, pumping station, overflow structure and storm surge barrier, the Delta21 project increases safety against flooding while providing a sustainable way for temporary energy storage.

In this thesis a design was created for the sea defence of the Delta21 project, during the design process the uncertainty of climate change was taken into account by the use of the adaptive pathway approach.

Three conceptual design variants are created which provide water safety until one meter of sea level rise occurs. When the sea level increases even more, the designs should be altered to the new circumstances. Various adaptation options are put together in the form of adaptive pathways which provide adaptation methods for up to five meters of sea level rise. The pathway schemes are evaluated on 4 evaluation criteria to select the preferred adaptation method for each of the three base design variants.

Using an analysis over the total lifetime of the sea defence measured in meters of sea level rise, the preferred conceptual design for the Delta21 sea defence is selected.

It turns out that the adaptive pathway approach can be used to integrate the uncertainty of climate change into the selection of a conceptual design variant for a sea defence. The use of the approach provides a methodological way of evaluating design options under uncertain circumstances. ... Read more

Breakwaters used to protect harbours and coastal areas worldwide are one of the most common coastal structures. The complexity of the physical processes associated with the design has led to the development of many empirical formulas while a standardized method for the selection of breakwaters’ design parameters in the EU does not yet exist. The most common approach to design such a structure nowadays entails using information and recommendations from design manuals and guidelines such as PIANC and the Eurotop Manual. A new Eurocode 1 which includes specific considerations for coastal structures such as breakwaters is in the development process. This study aims to compare the PIANC method with the method to derive actions/loads included in the Eurocode proposal prEN1991-1-8. To do so the following research question has been formulated:
“What differences between the PIANC method and the method proposed by the new EUROCODE in the design of a vertical wall breakwater can be identified, using the new breakwater at the Port of Genoa as a case study?“
As mentioned in the question, a case study is used. The Port of Genoa, one of the biggest ports in Italy plans to construct a new vertical wall breakwater. An initial design is openly available along with wave and water level data. This design is assessed using both methods and is further optimized. The aim is to gain insights into the differences between the PIANC and the new method.
At first, the failure mechanisms of such a structure are defined along with the safety factors and parameters. The data required to perform such an assessment is also an important aspect of the exercise. Most of the data are openly available during the consulting phase for the new breakwater in Genoa. In cases where extra data were necessary, they were based on the literature or on reasonable assumptions.
Based on the failure mechanisms and the retrieved data, the initial breakwater cross-section was assessed. The assessment both with the PIANC method and the new Eurocode proved that this design is sufficient and can be further optimized to decrease its costs. A high-level optimization is also conducted as part of this study in order to better understand the differences between the two methods. It can be concluded that the differences lay more in the method than in the actual result. For example, the proposed Eurocode creates a stable theoretical framework of how to choose a return period. The actual number may be very similar to the one that one would have used either way, but the choice can be argued in a better way.
On the other hand, the use of the new Eurocode revealed some problems and inconsistencies in the document which is confusing in certain parts. In addition, the new Eurocode which among others aims at standardizing the design process. However, parts like the combination of wave and water level actions and the choice of return period for the two main limit state functions are relatively clearer providing a solid base.
... Read more

The Eurocodes are design codes, widely accepted throughout Europe, that provide guidelines on how to deal with loads, resistance, and uncertainty. For most type of structures, loads and materials, the Eurocode has been well developed. However, for the loads on coastal structures, caused by wave, water level and current actions, no such Eurocode exists. Therefore, a new Eurocode is in progress. The preliminary version of this Eurocode is titled prEN1991-1-8, and will describe how to deal with hydraulic loads on coastal structures.
The Eurocode has a safety philosophy based upon partial factors and consequence classes, but the extension of this method to hydraulic engineering is not so straightforward. A premature introduction of the new Eurocode could mean that important design aspects related to sea condition parameters, such as extreme value analysis, dependence and wave transformation, are underexposed. In turn, this might give way to unsafe structures. Because of this, it is important to investigate how the semi-probabilistic approach (proposed by prEN1991-1-8) compares to existing design methods, both in terms of safety and in terms of ease of application. The goal of this thesis is to acquire knowledge on how the introduction of the new Eurocode influences the design of coastal structures, and address any inconsistencies and issues in prEN1991-1-8 before the preliminary version is adopted as the definitive version.

The main focus of the research is the re-design of a coastal structure following the instructions in prEN1991-1-8. The breakwaters of IJmuiden are used as a case study for this. Three different approaches are adopted for the design: a deterministic approach, a semi-probabilistic approach and a full probabilistic approach. The breakwater elements that have been extensively examined in this thesis are the armour layer, consisting of either rock or artificial units, the crest height and the crown wall.
Conclusions have been drawn based on any unclarities that are encountered in the design process, and based on comparisons between the semi-probabilistic approach and the two existing design methods, respectively.

The introduction of the new document prEN1991-1-8 should result in more conformity regarding the treatment of sea condition parameters and their accompanying uncertainties in design calculations. However, the new (draft) Eurocode in its current form does not seem to achieve this objective. This conclusion has been drawn based on the practical issues (i.e. room for interpretation in the document and poor descriptions of key concepts) that were encountered in the case study, and because the breakwater design that was arrived at by following the semi-probabilistic approach described in prEN1991-1-8, did not resemble the design outcome obtained with the deterministic and full probabilistic approach.

In addition, the semi-probabilistic approach is still a time-consuming method when applied to breakwater elements, while it is relatively easy to set up a full probabilistic calculation. It is recommended to shift (at least part of) the attention towards the full probabilistic approach, and explore the possibilities of making it the default approach for coastal structures, as it raises fewer questions than the semi-probabilistic approach and deals with uncertainties more extensively.

Nevertheless, it can be viewed as a positive development that there will be one general document to consult for the design of coastal structures. Most importantly, standardised levels of safety and return periods are now available with the introduction of prEN1991-1-8, even though their interpretations are not always straightforward. It is of course also still possible to improve the semi-probabilistic approach as it is currently proposed by prEN1991-1-8. When this is preferred, it is recommended to include a more systematic explanation of the DA-1 format in EN1991-1-8, describing the characteristic values, partial factors and safety margins to be adopted in design without any ambiguity.
... Read more

Since the behaviour of damage formation on steep slopes varies from mild slopes, a comprehensive definition of damage needs to be established for different situations. Knowledge about the transition of rock stability between steep and mild slopes is lacking. This study presents a reliable and efficient design method. The goal of this study is to describe the stability of rock slopes (1:6 - 1:10). Insight into governing processes and quantified damage characteristics are gained by analyzing wave attack in the transition zone of mild and steep slopes. The method is based on physical model test series with deep water wave conditions and perpendicular wave attack, which were performed to understand the stability of slope 1:6. The slopes were measured with stereophotogrammetry and processed in a 3D model with Agisoft Metashape software. Before wave attack, an initial profile is retrieved which can be compared with cumulative damaged profiles. Automatically detectable Ground Control Points are used as reference to assign scaling information and location recognition. The quantity and location of normative damage on mild slopes are identified for six damage parameters, based on different relations regarding erosion area and depth. The tests results have led to a number of key findings. Damage formations are developed by dominant downward (bar profile) or upward transport (berm profile). In general, a thicker layer and a milder slope give more upward transport. Besides the change in transportation direction, no significant differences in stability numbers were observed for various layer thicknesses with similar wave characteristics. For an increasing slope angle, the influence on stability significantly decreases between various wave steepnesses, due to the impact of a higher amount of more damaging plunging waves. The definition of initial, intermediate and failure damage limits are specified and the test profiles are matched per limit. Initial and intermediate damage are found at a constant value of _3_.3 for a varying layer thickness and slope angle based on the definition and matching profile analysis. Failure of the slope is defined by the event in which the impermeable layer or filter layer becomes visible, which occurs for a layer thickness of 2.53=50 at a damage level of _3_.3 = 1.5. Research at 1:6 slope shows that a higher failure limit is observed for layer thickness 53=50. Due to the lack of test results of other slopes (1:8 - 1:10), specific values cannot yet be indicated for the range 1:6 - 1:10. The lack of testing is also due to limits of the flume and wave machine. In order to remain at least 13=50 over the slope, an acceptable damage limit for a design with a layer thickness of 2.53=50 is found at _3_.3 = 0.9. For a layer thickness of 53=50, an acceptable value of _3_.3 = 1.5 holds a remaining layer of 2.53=50 due to the increase of bed mobility for higher damage numbers. The design formula of mild slopes is based on _3_.3 because this gives the lowest bias error, variability and measurement errors. The design formula for mild slopes is used to estimate stability, nominal rock diameter and acceptable damage for conditions within the applicability range of performed tests. This new method for mild slope design indicates that a higher allowable stability can be used compared to the previously used (extrapolated) method of Van der Meer (1988). A displacement is not necessarily linked to erosion due to alternating upward and downward transport on mild slopes. It can be discussed to what extent a rock, after displacement, contributes to the stability of the design. This can be investigated by the ratio of mobility and erosion over the entire slope. This study has shown that the damage parameters, based on displacements, are larger than the width-averaged erosion parameters. This appears to be a useful ratio for distinguishing mild slopes from steep slopes. In the latter these parameters are considered equal. Requirements for mild slope stability can also be imposed on the basis of the ratio between mobility and erosion as described in this research. ... Read more

Until now, an extensive design method for mild slopes has not been available. The aim of this thesis is to understand the stability of rock on mild slopes under wave attack for impermeable cores in order to optimize designs. Physical model tests have been executed to study this stability for a 1:8 slope. Mossinkoff (2019) executed physical model tests for a 1:10 slope and a re-analysis of these tests is included in this thesis as well. Damage caused by entrained rocks is quantified by damage parameters using stereophotogrammetry and coloured rocks in strips. In this study, the influence of several hydraulic and structural parameters on damage parameters has been investigated for mild slopes. A positive correlation has been found between the significant wave height and the damage parameters. Besides this, based on the analysis it can be concluded that the wave steepness and damage parameters are negatively correlated. An increase in layer thickness of the rocks does not seem to increase the stability of rock on mild slopes. However, the slope angle does have an effect on the stability as a milder slope is associated with less damage. Another conclusion is that more damage continues to be observed even after 15 000 waves. Based on the results of the physical model tests, a design formula is developed for mild slopes to be able to increase efficiency in designs of coastal structures for these mild slopes. This study also provides evidence that rocks on mild slopes have different characteristics of damage and damage development compared to steep slopes. The largest share of entrained rocks transport in upward direction and rocks on mild slopes seem to be more mobile compared to steep slopes. This suggested that it might be more efficient to study the moment when the filter layer or the core becomes visible instead of the static stability of rock within the armour layer itself. ... Read more

This research focuses on the estimation the failure probability of Shanghai sea-dikes system, taking into account several failure mechanisms. This method follows fully probabilistic approach, in which all relevant parameters for the resistances and the hydraulic loads vary according to specific distributions. ... Read more

The static stability of stones on mild slopes under wave attack is investigated by this research. First it was checked whether there are any numerical models that could be used to describe the stability of mild slopes. As such, there is no existing model at this moment that can accurately model the stability of mild slopes. To model this correctly, the model has to be able to model plunging and spilling waves, include bed sediment transport and to model the velocity and acceleration due to a breaking wave over multiple layers in depth. The existing models do comply mostly with one of these specifications. To get accurate results, the stability should be modelled with all of the specifications and not one model complied with this. Therefore, it was decided to investigate the stability of stones on mild slopes under wave attack with physical modelling tests at the research institute Deltares. The physical tests were executed with a constant slope angle of tan α = 1:10 and a constant nominal median stone diameter. The wave steepness (that determines the type of breaking for a constant slope angle), the significant wave height, the number of waves and the layer thickness are varied in between the tests to determine the influences of each parameter on the stability. Of each test an erosion profile is made by photogrammetry from which the profile-related damage parameters can be determined. In the bed strips with coloured stones are laid down to follow the transport of the stones. According to these erosion profiles three damage parameters were determined. The damage level, S, as used by Van der Meer (1988), the erosion depth, de, and the damage depth, E3. The damage depth, E3, is used to describe the damage for mild slopes. The range of the damage depth for start of damage to failure is from 0.5 to 2.3. These values are only applicable for a layer thickness of 2.5dn50 and a slope angle of tan α = 1:10. The development of the damage depth is investigated for multiple parameters. The breaker type gives a change in development of the damage depth. For a wave condition with plunging breakers the damage depth development is higher per increasing wave height than for a wave condition that includes both plunging and spilling breakers. For the transition zone from plunging to spilling breakers the damage depth increase is very small if the wave height increases. The damage depth is influenced by the number of waves, because after about 11,000 waves the development of the damage depth is still linearly increasing with the number of waves. The damage depth was not stabilized towards a maximum value after this number of waves. It can be concluded that the transport of the stones is mainly all below the still water level. This also followed from the damage zones. These indicated that the total area of the damage is in between the -2Hs and SWL. The maximum area location is around -0.8Hs w.r.t. the SWL. The transport is mostly upslope, which is attributed to the mechanism whereby wave forces are strong in upslope direction. The location where the most stones are picked up depend on the breaker type, the wave height and the layer thickness. Plunging breakers cause damage more downslope than spilling breakers. Plunging breakers cause more local damage and have a much smaller runup than the spilling breakers. Per increasing wave height, the location of damage is shifted more downslope due to the increase in breaker depth. ... Read more

“During the last safety assessment of Rijkswaterstaat in 2006 the Afsluitdijk failed on the current safety standards regarding flood protection. A new design was needed which ensured continuous protection against flooding in the future. Large scale and small scale tests are performed to optimize the design. Clear differences in the average wave overtopping discharge are observed between large and small scale. The new design of the Afsluitdijk has a complex geometry and consists of new types of elements of which not much information about the roughness of these elements is available. Also, the combined effect of roughness elements and berm has not been fully investigated yet. This asks for a more accurate method to predict the average wave overtopping discharge for composite slope. Various adjustments to current theories are proposed for this type of structure. “ ... Read more

Mangrove forests, which serve as a natural sea defence, have been degraded by human action during the last decades. To protect and recover these areas, different types of permeable structures consisting of bamboo have been proposed and applied. However, these structures are currently based on engineering judgement, as design rules are not available. The goal of this research is to make a step forward in the design optimization of permeable structures and gain a better understanding of the processes causing the wave dissipation inside the structure. In this study, scale experiments are conducted in the wave flume at Delft University of Technology. An array of aluminum cylinders is used as a schematization of the structure, with an element diameter of 4 cm and a minimum spacing of 2 cm. The tested wave height is 0.13 m,with a water depth of 0.6 m for the first set of experiments. These experiments evaluated wave transformation through a selected number of configurations. The water depth was 0.55 m for the second set, where the velocities and forces were also measured inside the structure. As the applied wave periods are short (T = 1 − 2 s), the tested wave conditions are in the range of small KC-numbers (4 < KC < 13). The first part of this research focuses on effect of different configurations and arrangements on the amount of energy dissipation, by measuring the incoming and reflected wave heights in front of and behind the structure. For the short waves, the horizontal arrangements dissipate more energy, as energy is dissipated by both the vertical and horizontal drag forces. However, the effect diminishes with increasing wave period. Considering the total dissipation and the amount of dissipation per element, the placement of the elements in rows perpendicular to the direction of wave propagation is found to be the most effective. To study the effect of the element diameter on the dissipation, the results are compared with previous research by Haage (2018) on a model with a diameter and spacing of 2 cm. As with the change in diameter also the structure porosity changed, a direct comparison was not possible and a comparison based on a simplified drag coefficient is done. No direct effect of the diameter is observed, as the obtained drag coefficients show the same trend and magnitude when plotted against an adapted Keulegan-Carpenter number (KC∗), which is based on the element spacing instead of diameter. Small KC∗-numbers result in large drag coefficients,which decrease when KC∗ increases. The second part of this research focuses on the processes inside the structure that cause the energy dissipation. A force and velocity sensor are applied at three locations inside the structure in separate experiments, to determine the relation of the force and velocity inside the structure with the undisturbed values. Two methods are applied for the analysis, based on two different principles. Method 1 is based on the assumption of a constant pair of force coefficients and an increase in velocity inside the structure, method 2 is based on the assumption of a constant velocity and an increase in force coefficients inside the structure.By comparing both methods, it is found that an increase in velocity is the most important factor for the increase in drag force, which is the driving factor for the energy dissipation. It is also found that the amplification factor for the velocity is dependent on both the structure porosity and frontal porosity. ... Read more

After more than 85 years, the iconic Afsluitdijk is in need of a reinforcement. During the latest safety assessment of Rijkswaterstaat the dike did not meet the standards at two components; the amount of wave overtopping (>10 l/m/s) and armour stability. This research goes into detail about the overtopping on the Afsluitdijk. In the new design a lot a of different factors influence the amount of wave overtopping: the roughness of several elements and a berm. The combination of these different factors makes it difficult to make a reliable estimation of the amount of wave overtopping. In the design new materials are used of which the effect on wave overtopping is unknown. The band width of the design is very narrow so an accurate prediction of the amount of overtopping in the prototype is necessary. Physical model tests of two different dike section are used in this research. These dike sections are tested on different scales (small to large) to validate the final design on prototype scale. Not much is known about the possible scale effects on wave overtopping so this research goes into more detail on this issue. ... Read more

Due to the removal of mangrove forests, coastal zones can suffer from severe erosion. One of the proposed solutions is the construction of permeable structures. This study aims to optimise the design of permeable (brushwood) structures in order to restore the sediment balance and encourage mangrove re-establishment on tropical mud coasts. Preferably wave transmission should be low in order to create a calmer climate behind the structure. In that way sediment is able to settle down, which could lead to a recovery of the mud profile. It is also preferred that reflection by the structure is low. High reflection rates cause scour holes that lead to instability of the structure. Furthermore, scour holes could hinder future mangrove re-establishment. Aiming to achieve low reflection and transmission rates, the dissipation inside the structure has to be as high as possible. Experiments were conducted in the 40 meter wave flume at the Environmental Fluid Mechanics Laboratory at TU Delft. The permeable structure was schematized as an array of cylinders. With the physical scale model various effects could be tested, including the porosity, structure width, arrangement, orientation, etc. The tests were done for 5 different wave cases, from which the wave energy distribution over reflection, dissipation and transmission was determined. The existing brushwood structures require intensive maintenance. This is partly due to the sinking of the material into the soft mud. Also, the brushwood material washes away often as it is lighter than water and difficult to constrain in vertical direction. An alternative design that requires less maintenance would be preferred. Therefore, it was interesting to see whether a comparable amount of wave dissipation could be achieved by using vertical elements only. One important finding is that in more shallow water regions, vertical and horizontal orientations have similar dissipation rates. In water regions that go more towards deep water, the horizontal structures have higher dissipation rates. This can be explained by the relative importance of the horizontal and vertical velocities due to the wave motion. In deep water vertical velocities are relatively high. As the horizontal elements have more exposure to this component in comparison to the vertical elements, they provide more dissipation. In shallow water the relative importance of the vertical velocities is lower, which explains the similar dissipation rates of the two orientations. The analytical model of Dalrymple (1984) was used to describe the energy dissipation through the structures. Drag coefficients were derived by using the calibration method. For KC<15 the drag coefficients start increasing. This is possibly due to the relative importance of the inertia force. Comparing the drag coefficients to the ones derived from direct force and velocity measurements in previous studies showed relative high values. This could be due to an underestimation of the horizontal velocity due to the wave motion. The velocity that is used is the undisturbed velocity in front of the structure. However, the velocity inside the structure might be higher as the flow accelerates in between the gaps of the elements. Furthermore, the wave cases in this research are in the Stokes 2nd and 3rd order region, indicating that the waves cannot be fully described by linear wave theory. The inertia, permeability and non-linear effects among other possible effects are not included in the analytical model of Dalrymple. Therefore, the drag coefficients do not only represent drag forces, but also other processes. To gain more insight on the physical mechanisms that affect the wave energy dissipation, it is recommended to test the same scale model with direct force and velocity measurements. ... Read more

Contrary to a breakwater outer slope, rear-slope revetment does not have to withstand breaking waves. However, generally, an equal dimensioned revetment is used on both the inner- and outer slope. Although with low-crested breakwaters severe overtopping occurs, the application of similarly sized armour on both sides still results in an over-dimensioned inner-slope revetment. An (economical) optimisation is possible in this regard.
The tests executed by Hellinga (2016) in this topic, showed large scatter in the found stability results of single-layer cube revetment at the rear slope. This scatter was party introduced by the large number of influencing factors in the test scope (Hellinga, 2016). By developing an overtopping simulator, the scope of the model-tests is delimited to solely the breakwater crest and rear slope. Moreover, larger scale tests are feasible in the same flume, reducing scaling errors.

The main objective of this thesis is a) determine if the approach of a (semi-) small scale overtopping experiment using an overtopping simulator is increasing the reliability of the test results, and (b) to increase the understanding of failure mechanisms of single-layer cube revetment on the crest and rear side of low-crested breakwaters.

To design and construct a simulator that is able to generate realistic waves, the relevant mechanisms of overtopping waves were determined and boundary conditions were composed. The behaviour of the simulator was tested in combination with a ‘mock’ breakwater. Seven types of realistic breakwater models were installed to investigate both the hydraulic behaviour of the overtopping volume and the stability behaviour of the revetment. The determination of the layout of a configuration is every time well substantiated by the results from the former configuration.

The water level inside the Wave Overtopping Simulators (WOS) reservoir showed a skewed drop during the release of an overtopping volume. Consequently, the water level measured at one single point was not accurate to determine the volume (change) inside the reservoir, and hence the discharge and velocity. Different test configurations failed, where others remained stable during exposure to simulated waves in the test range. Compared to stability numbers of the theoretical front side, the cubes are relatively stable. The transition of the crest and rear slope showed to be a weak point, and outwash of the rock material filling the gap is a significant possibility.

Improvements on the WOS are possible to enhance the ability to simulate the characteristics of a realistic overtopping wave volume and to give a more detailed evaluation of the magnitudes of these overtopping characteristics. The normal forces between the cubes at the slope are more important for the stability than the own weight of the cubes. The lack of interaction between the cubes at the crest results in a significantly lower resistance to hydraulic loads compared to the cubes at the rear slope. The layout of a sharp inner crest-line transition, if well designed, showed a stable alternative for a rounded transition.
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Block mattresses are widely used bed protection elements under non-uniform flows. Previous studies show that the turbulence of the flow plays a key role on the instability of the mattress. However, in the available design equations, the turbulence intensity is included as a magnification factor roughly and qualitatively defined. Consequently, the design equations are often used as a rule of thumb, where the engineering criteria of the designer is critical and large safety factor are used. In turbulent non-uniform flows, the available design equations were developed for uniforms flow and the turbulent intensity is included as a magnification factor, usually poorly defined. The stability reduction due to turbulence has been proved by several authors. However, there is not any study in previous literature that investigate the role of velocity fluctuations on block mattress failure. Nevertheless, the literature review showed that an innovative approach, which combines the mean velocity and the turbulence intensity to quantify the flow forces, has been largely study for loose rock by several authors. Following this trend, the option of applying this structure (u ̅+α√k) as a flow quantification on Pilarczyk equation was decided to study. For this purpose, a series of test were done under four different configurations. Two weir heights were used (15 and 18.5cm) and the distances corresponding to 3h and 4h were checked. Four failures were recorded for each configuration for a total of 16 failures. Additionally, a data base was recorded for each configuration where the flow discharge was related to mean velocity and the turbulence intensity. An analysis of the failure mechanism was done by synchronizing the velocity time series with the displacement of the blocks. The results showed that an episode of peak streamwise velocity and downwards (sweep) was present at the failure. However, the analysis of the velocity showed that higher combinations of streamwise velocity and shear stress were present before the failure on almost all the records. Therefore, the streamwise velocity and the induced shear stress cannot solely explain the failure of the block mattress. This conclusion is in line with the work done by Hofland (2005). However, the recorded data did not allow to describe completely the failure mechanism.
The turbulence magnification factor (α) at the stability equation was derived by two different approaches. The first approach was based on the peaks of velocity associated to failure while the second one was described as a fitting parameter. The analysis concluded that the α values were close to 3 for both cases, in line with the expected value from previous studies. Finally, the new developed equation was compared with the original Pilarczyk equation and the modification proposed by the Rock Manual (2007). The comparison shows that the new developed equation described the required thickness of the block more accurately than the available equations. Thus, the results show that the proposed approach described the effect of turbulence accurately on a design equation. Additionally, all the relevant derived conclusions for design propose were summarized in a design guideline. ... Read more

Downstream of hydraulic structures, the flow is normally of turbulence. This will cause considerable erosions on the gravel bed. To prevent this, block mattress is preferable in many cases. Its effective stabilization and feasibility on construction make it commonly used against scouring problems, protecting underwater pipelines and cables. The currently widely used design formula with dedicated parameters for block mattresses was derived by Pilarczyk. However, it is observed to be partly functioning under non-uniform flow condition, since the turbulence parameter is rather empirical. Therefore the main purpose of this thesis is to optimize the current design formula (Pilarczyk’s) for non-uniform flow condition. In this flow regime, turbulence is of importance for bed material entrainment. Thus this research focused on how to introduce turbulence effect into formula explicitly. To predict the failure of block mattress better, a detailed study on failure mechanism and impact of flow condition over the block mattress edge is necessary. ... Read more