Stability Of Armour Layer Research Articles

Rocking Of Single Layer Armour Units Measured By Embedded Sensors

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 single layer armour units. This study utilizes embedded Rocking Sensors to obtain the first measurements of rocking impact velocities of single layer armour units. More generally, the paper (Hofland et al. 2023) shows how novel measurement techniques can be used for the investigating the stability of single layer armour layers.

Study Of The Hydraulic Response Of A High Permeable Breakwater Using Physical Modelling

Physical modelling tests were conducted in the wave flume of Artelia’s hydraulic laboratory to study the hydraulic response/stability of a rubble mound breakwater made with a non-standard core composed of crushed concrete blocks (tetrapod). This design carried out by EDF, is aimed at having high permeability and fits in an eco-design approach, through the reuse of existing materials already on site. Eventually, the hydraulic efficiency of three different sections were tested and compared, all sections having the same armour layer and the same main dimensions but different {core; filter} systems : core only made with crushed tetrapod, core and underlayer made with crushed tetrapod (no filter layers with rocks) and baseline design (quarry run core and rock underlayer). Several responses were studied: armour layer stability, overtopping and transmission of the structure and head loss on both sides of the structure with an inflow/outflow system in the rear side basin set up in the wave flume. This case study illustrates 1) the importance of the physical modelling approach to testing unusual structures, as part of an eco-design approach, where the use of standard design formulas does not allow to verify the hydraulic behaviour of a high permeable breakwater and 2) also shows that unconventional design can lead to satisfactory hydraulic results.

Numerical study of wave interaction with armour layers using the resolved CFD-DEM coupling method

Coastal armour layers have been commonly used to protect coastal structures from wave erosion. Assessing the stability of armour layers under extreme wave loading is a challenge for engineering design. In this study, we propose a resolved CFD-DEM coupling procedure to study the interaction of waves and irregularly shaped armour units. This model is validated by comparing the numerical results and a flume wave erosion test. The wave surface profiles and wave pressure distribution are reliably reproduced in the simulation. This model can well capture the wave energy dissipation of irregular armour units. Further, wave erosion on two types of armour layers, the cuboid and Dolos armours, are studied in a full-scale prototype model. The motion of individual armour units and the localised failure process under wave impact is observed and analysed in the simulation. We also study the influence of armour shape on overtopping discharge, pressure distribution, and vortex structure. The CFD-DEM method provides an advanced numerical tool for stability analysis and risk assessment of armour layers under the wave impact.

STABILITY OF RUBBLE MOUND STRUCTURES UNDER OBLIQUE WAVE ATTACK

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. Galland (1994), Yu et al. (2002), Wolters and Van Gent (2010) and van Gent (2014) performed laboratory experiments to consider effects of oblique waves on the stability of armour layers. They performed tests with long-crested and/or short-crested waves on rock and concrete armours. The aim of this study is to find an appropriate and compatible reduction factor for EBV stability formulae.

A new method for designing cube armours for low-crested and submerged breakwaters

Recently, coastal structures designers have paid more attention to low-crested/submerged breakwaters due to their significant inherent advantages. Although many formulae can be found in the literature for the design of breakwater armour layer, the majority of them are proposed for high-crested breakwaters. Currently, there is no comprehensive formula for the design of the cube armour layer for low-crested/submerged breakwaters. Among the important parameters affecting the armour layer stability of low-crested/submerged breakwaters, the most effective ones are crest width, packing density and placement pattern of the armour units. These parameters, however, are not directly included in the existing design formulae. In this article a comprehensive and complete formula for design of the armour layer for low-crested/submerged breakwaters based on the results of a large number of experimental investigations is presented. The proposed formula contains the most important parameters such as the crest width, armour units packing density and placement pattern, slope, freeboard, as well as the wave characteristics. The comparison between the armour layer's hydraulic stability number obtained from this formula and the corresponding values recorded at the laboratory reveals its favorable accuracy indicating the applicability of the proposed formula for safe design of the cubic armour layer for low-crested/submerged breakwaters.

Wave RUN-UP Measurements under very oblique wave incidence

Under the scope of the HYDRALAB+ transnational access project, the so-called RODBreak experiment was conducted in the multidirectional wave basin at the Marienwerden facilities of the Leibniz University Hannover (LUH). A stretch of a rubble-mound breakwater was built in the wave basin with a very gentle slope. Its armour layer was made of Antifer cubes, at the roundhead and adjoining trunk, and of rock, at the rest of the trunk. A set of tests was carried out to extend the range of wave steepness values analysed in wave run-up, overtopping and armour layer stability studies, focusing on oblique extreme wave conditions, with incident wave angles from 40º to 90º. The present study focuses on the analysis of measured wave run-up values obtained in the tests and on their on their variability as well as the influence of the wave obliquity and directional spreading. Keywords: rubble-mound breakwaters; run-up; oblique waves; physical modelling; RODbreak.

WAVE RUN-UP AND OVERTOPPING IN RUBBLE-MOUND BREAKWATERS UNDER OBLIQUE WAVE INCIDENCE

The existing data gaps triggered the interest in developing the present experimental work, whose main goal is to contribute to a new whole understanding of the phenomena to mitigate future sea level rise impacts in European coastal structures, including the run-up and overtopping characterization on rough and permeable slopes. The key point is to extend the range of wave steepness values in run-up, overtopping and armour layer stability studies, focusing on oblique extreme wave conditions and on their effects on a sloping breakwater's trunk armour and roundhead. Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/sHxoaocYuL0

Wave parameters influence on breakwater stability

This research paper experimentally explored the wave parameters effects on the breakwater armour layer stability. Breakwater stability mainly depends on the armour layer that guards the inner layers against the wave attack and the wave condition affects the breakwater stability. The wave height, length and steepness effect were investigated during the test program. Different formulas were available in the literature to predict the breakwater stability. The laboratory results were thus employed to assess the applicability of existing design equations. Experimental results showed that under normal wave condition (Hs) the breakwater was stable, whereas under storm condition (Hs of 1.2 designed wave height) the roundhead reached the failure state. The results approved the impact of wave parameters on the breakwater stability and damage progression. Results revealed that the existing design formulas underestimate the structure stability.

3-D Scale Model Study of Wave Run-Up, Overtopping and Damage in a Rubble-Mound Breakwater Subject to Oblique Extreme Wave Conditions

A set of scale-model tests carried out to enlarge the range of wave steepness values analysed in run-up, overtopping and armour layer stability studies, focusing on oblique extreme wave conditions and on their effects on a gentler slope breakwater’s trunk armour and roundhead, is presented in this paper. A stretch of a rubble mound breakwater (head and part of the adjoining trunk, with a slope of 1(V):2(H)) was built in a wave basin at the Leibniz University Hannover to assess, under extreme wave conditions (wave steepness of 0.055) with different incident wave angles (from 40º to 90º), the structure behaviour in what concerns wave run-up, wave overtopping and damage progression of the armour layer. Two types of armour elements (rock and Antifer cubes) were tested. Non-intrusive methodologies including a new application of laser scanning technique for the assessment of both armour layer damage and wave run-up and overtopping were used. It is expected that such work will contribute also with data to improve empirical formulas as well as to validate complex numerical model for wave-structure interaction.

HYDRAULIC STABILITY AND OVERTOPPING PERFORMANCE OF A NEW TYPE OF REGULAR PLACED ARMOR UNIT

The XblocPlus is a new type of interlocking single layer armour units that is placed with uniform orientation. This is novel and different from all other single layer, interlocking armouring systems. The hydraulic stability of the XblocPlus breakwater armour unit was tested in 2D and 3D hydraulic model tests. Wave overtopping tests were performed to determine the roughness coefficients of the EurOtop overtopping formula for the XblocPlus. Model tests on a rubble mound breakwater with XblocPlus armour included 2D tests with a 1:30 seabed slope and with 1:2 and 3:4 breakwater slopes and 3D model tests with a flat seabed and with a 3:4 breakwater slope. Wave heights up to 150% of the design wave height were tested in the 2D tests and up to 200% with wave directions 0° to 60° in the 3D tests. No armour unit displacements were observed in 2D tests with 1:2 slope. In the 2D tests with 3:4 slope one armour unit was displaced when the wave height reached 159% of the design wave height. No damage to the XblocPlus armour layer was observed in the 3D tests. A roughness coefficient of 0.45 was deduced from overtopping tests with wave heights of 60% to 100% of the design wave height. The model test results indicate little or no influence of wave steepness on XblocPlus stability and no adverse influence of wave obliquity while the seabed slope in front of the breakwater may have some impact on the XblocPlus armour layer stability.

Numerical test for single concrete armour layer on breakwaters

The ability of concrete armour units for breakwaters to interlock and form an integral single layer is important for withstanding severe wave conditions. In reality, displacements take place under wave loading, whether they are small and insignificant or large and representing serious structural damage. In this work, a code that combines finite- and discrete-element methods which can simulate motion and interaction among units was used to conduct a numerical investigation. Various concrete armour layer structures were built using a carefully researched placement technique and then subjected to a boundary vibration. By analysing the displacements and assessing the number of units that were displaced by more than one-third their nominal size, the numerical test programme indicated clearly that the initial build packing density was the most important parameter influencing the stability of concrete armour layers under vibration. The size of the underlayer rock and the type of unit also affected the numerical performance of the single-layer concrete armour systems under vibration. The results presented are for full-scale systems and therefore add further insights into simple laboratory ‘shake tests’, although the oscillatory loading in this study is acknowledged to be profoundly different to wave action.

PENGARUH DURASI SERANGAN GELOMBANG TERHADAP TINGKAT KERUSAKAN LAPIS LINDUNG PEMECAH GELOMBANG

this research tetrapod have been utilized for breakwater cover layer or armour layer. The effect of wave duration on the armor layer tetrapod stability was studied. The models were run in the irregular wave flume, where Bretschneider spectrum may be generated attacking the breakwater. The waves were set to break before reaching the breakwater. The test was conducted until the damage level of armour layer reached more than 5 %. The result of the research showed that waves duration and the high of the wave attack affect the stability of armour layer, especially when the wave was Rayleigh distributed. The effect of waves duration reduced as more and more waves broken before reaching the structure of the breakwater

Influence of armour porosity on the hydraulic stability of cube armour layers

Armour placement and packing density directly affect construction costs and hydraulic performance of mound breakwaters. In this paper, the literature concerning the influence of armour porosity on the hydraulic stability of single- and double-layer armours is discussed. Qualitative and quantitative estimations for the influence of armour porosity and packing density on the hydraulic stability are given for the most common concrete armour units. The analysis focuses on specific 2D hydraulic stability tests of double-layer randomly-placed cube armours with different armour porosities and permeable core. The stability number showed a 1.2-power relationship with the packing density for double-layer randomly-placed cube armours considering armour unit extraction and Heterogeneous Packing. The literature review and experimental results with small-scale breakwater models protected with a variety of armour units clearly indicate that a significant increase in armour porosity above the recommended values substantially decreases armour hydraulic stability. To avoid uncontrolled model effects, packing density should be routinely measured in small-scale tests, and armour placement techniques should be monitored at prototype scale. The actual packing density obtained in small-scale models and prototypes has to be explicitly reported, because packing density significantly affects hydraulic stability during service time.

Double layer armor breakwater stability (case study: El Dikheila Port, Alexandria, Egypt)

This research aimed to investigate the armor layer stability of the new breakwater of El Dikheila Port. An undistorted physical hydraulic model with a scale of 1:39.73 was employed as a tool to simulate the existing and the proposed new breakwaters. The physical model was constructed in the wave basin of the Hydraulics Research Institute (HRI), Delta Barrages, Egypt. The armor layer of the new breakwater was tested with regular and random placement for the trunk and the roundhead. The damage occurs with wave height of 1.2Hs at the head and the trunk sections with the random placement case. The model results showed that regular placing exhibited higher stability for initial damage and gradual damage progression than the random placement of the armor units.

STABILITY OF INTERLOCKING ARMOUR UNITS ON A BREAKWATER CREST

The hydraulic stability of single layer, interlocking armour units on low crested and submerged breakwaters was investigated in 2D hydraulic model tests. Displacements of armour units and rocking were monitored and have been applied as indicators for the armour layer stability on the crest, front and rear slope. The effect of freeboard, packing density and wave steepness on the armour layer stability have been investigated. The stability of interlocking concrete armour units on low crested and submerged structures is qualitatively different from rock armour. About 40% to 50% larger armour units are required on the seaward slope and crest of low crested structures (as compared to conventional high crested breakwaters). About 35% larger armour units are required on the rear slope. Larger armour units are not required on submerged breakwaters if the water depth on the crest exceeds 50% of design wave height.

WAVE INTERACTION WITH LARGE ROUGHNESS ELEMENTS ON AN IMPERMEABLE SLOPING BED

The present paper presents the results of an experimental and numerical investigation of the flow between large roughness elements on a steep sloping impermeable bed during wave action. The setup is designed to resemble a breakwater structure. The work is part of a study where the focus is on the details in the porous core flow and the armour layer flow i.e. the interaction between the two flow domains and the effect on the armour layer stability. In order to isolate the processes involved with the flow in the porous core the investigations are first carried out with a completely impermeable bed and successively repeated with a porous bed. In this paper the focus is on the impermeable bed. Results are obtained experimentally for flow and turbulence between the roughness elements on the sloping bed. Numerical simulations have reproduced the experimental results with good agreements and can hereby add more details to the understanding of the fluid-structure interaction.

Alternative placement technique for antifer blocks used on breakwaters

In this study, a placement technique named as “alternative placement technique” was developed for antifer blocks and the results of its application to a breakwater model were presented. This placement technique was compared with the existing techniques such as the “regular placement technique”, the “irregular placement technique” and the “sloped wall placement technique” by experiments. The comparison was carried out considering armor layer stability, prototype placement, clarity of the placement technique’s definition, armor layer cost, and wave runup. As a result of this investigation the “alternative placement technique” was found to be superior to the other existing placement techniques.

Empirical formula for breakage of Dolosse and Tetrapods

The slender, complex types of armour units, such as Tetrapods and Dolosse are widely used for rubble mound breakwaters. Many failures of such breakwaters were caused by unforeseen early breakage of the units, thus revealing an inbalance between the strength (structural integrity) of the units and the hydraulic stability (resistance to displacements) of the armour layers. Breakage occurs when the stresses from the static, pulsating and impact loads exceeds the tensile strength of the concrete. While the hydraulic stability can be studied in Froude-scale hydraulic model tests, it is not possible to study armour unit stresses in small scale models. This is partly because the strain in model armour units are too small to be recorded, and partly because the scaling law for impact load generated stresses is nonlinear. The paper discusses the scaling laws related to type of stresses and presents a method which allows studies of armour unit stresses by means of a load-cell technique. The technique necessitates impact load response calibration of the load-cell mounted model armour units against the equivalent response of prototype or large scale armour units. The procedure followed was presented by Burcharth and Liu (Burcharth, H.F., Liu, Z., 1992. Design of Dolos armour units. In: Proceedings of the 23rd International Conference on Coastal Engineering, Venice, Italy.) and Burcharth (Burcharth, H.F., 1993. Structural integrity and hydraulic stability of Dolos armour layers. Series Paper 9, published by the Department of Civil Engineering, Aalborg University, Denmark, 1993.), who also presented design diagram for determination of breakage of Dolosse in trunk sections. The paper presentes an expansion of this work to include breakage of Dolosse in round-heads and Tetrapods in trunk sections. The paper presents a simple dimensional empirical formula instead of diagrams for the estimation of the number of broken Dolosse and Tetrapods in prototype situations, because probabilistic design of breakwaters requires failure mode formulae with the associated uncertainties.

A probabilistic model of rubble mound armor stability

Two fundamental problems of breakwater armor layer stability — the behavior of interlocking resistance and the random nature of breakwater stability — are examined through laboratory experiments and a probabilistic model. Based upon laboratory experiments of static pull-out tests, it is found that the interlocking resistance of artificial blocks when randomly placed is, more or less, proportional to the weight of the units. Only for tightly packed blocks, arranged in a pattern, does the interlocking resistance show a possible deviation from the weight proportionality. The resistance of the armor layer is shown to be a random property that can be adequately described by a log-normal distribution. A probabilistic stability equation is developed. The final equation contains six random variables; their behaviors are not all known at present. However, with the partial knowledge we have, the equation is used to bring out a number of salient features in breakwater design.

Stability of breakwater armour layers — Design formulae

Stability of breakwater armour layers — Design formulae