Mound Structures Research Articles

Experimental modeling of the interaction between waves and submerged flexible mound breakwaters

A submerged flexible mound breakwater can be employed for wave control in shallow water as an advanced alternative to the conventional rigid submerged designs. This study presents a flexible breakwater with an innovative geometry based on model experimentation. Experimental studies were performed to compare the wave energy dissipation by the flexible mound and rigid structures, over a range of test conditions, for example, three different diameters of structure, three water depths, and different regular wave heights for three different beach slopes. Results indicate that the present submerged flexible mound breakwater is stable, being appropriate for most operational sea conditions. Large amplitude waves can induce significant motions of the structure; therefore, the interaction between radiating and scattering waves is highly contributing to the wave energy dissipation. The wave energy dissipation at breaking zone of the present submerged flexible mound breakwater is significantly greater than that of the other types of flexible and rigid formerly investigated structures.

Possibility of intrusive igneous body beneath the Cretaceous sequence in Abu Gharadig Basin, Egypt: integration of geophysical data interpretations

Based on the explanation of seismic data covering the middle part of Abu Gharadig Basin in northern Western Desert, a diapiric structure has been detected eastward of the study area. The resolution of this diapiric feature has been enhanced using different five seismic volume attributes including instantaneous bandwidth, reflection strength, instantaneous frequency and reflection intensity. The interpreted diapiric feature exhibits discordant, nearly vertical, almost symmetric and mounded structure with uplifted overburden and upturned host rocks. The magnetic map for the study area represents high anomaly directly above the interpreted diapiric feature in the eastern part of the area of study confirming the high density of the intrusive body. The area of study also delineated on a temperature gradient map for the northern Western Desert showing high temperature gradient over the interpreted diapiric feature towards the eastern portion of the study area. This suggests that the examined body is a subsurface source for high temperature. Accordingly, the detected diapiric feature has been interpreted to be large body of igneous intrusion forming domal masses beneath the Cretaceous succession in the study area. This igneous intrusion is expected to have positive or negative effect on the hydrocarbon potential in Abu Gharadig Basin.

Probabilistic Risk Evaluation for Overall Stability of Composite Caisson Breakwaters in Korea

In the present study, the overall stability of typical Korean composite caisson breakwaters that were initially designed following the conventional deterministic approach is investigated using reliability approaches. Therefore, the sensitivity of critical uncertainties regarding breakwater safety is analyzed. Uncertainty sources related to the structure, ocean conditions, and properties of the subsoil and rubble mound are considered in the reliability analysis. Sliding and overturning failures are presented as explicit equations, and three reliability methods, i.e., the mean value first-order second-moment, first-order reliability method, and Monte Carlo simulation, are applied in the evaluation process. Furthermore, the bearing capacity of the rubble mound and subsoil are analyzed using the discrete slice method conjugated with the Monte Carlo simulation. The results of this study establish that the sliding failure generally is the most frequent failure occurring among the above-mentioned overall stability failures (around 15 times more common than failures observed in the foundation). Additionally, it is found that the horizontal wave force primarily contributes to the sliding of the caisson body, whereas the friction coefficient is the main factor producing the resistance force. Furthermore, a much small probability of overturning failure implies that the overturning of a caisson around its heels uncommonly occurs during their lifetime, unlike other overall failure modes. Moreover, the failure in foundations may commonly encounter in the breakwater that has a high rubble mound structure compared with sliding mode. Particularly, the performance function of the all foundation bearing capacities presents a nonlinear behavior and positively skewed distribution when using the Monte Carlo simulation method. This phenomenon proves that simulation methods might be an appropriate approach to evaluate the bearing capacity of a breakwater foundation that can overcome several drawbacks of the conventional design approach.

On the stability of rock armored rubble mound structures

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 structure in an appropriate way. The main objectives of this study are to (a) develop a unified physically sound formula for the estimation of the required rock size in all relevant water depth conditions and (b) to relate the effects of the permeability of the structure directly to physical parameters. To achieve these, first a comprehensive data base of deep and shallow water experiments within the design conditions was built. Then physical reasoning along with a robust data mining approach, i.e. M5 model tree, were used to develop formulas for armor stability. In the stability formula, wave characteristics such as the significant wave height and spectral energy mean period (Tm-1,0) are invoked while the permeability is incorporated using the ratio between the size of the core material and the armor stones. Accuracy metrics such as discrepancy ratio and scatter index indicated high performance of the model in different conditions. Finally, a probabilistic formula and some guidelines are provided for practicing engineers.

How the thermal environment shapes the structure of termite mounds.

A computational model has been developed to predict the role of environment in the forms and functions of termite mounds. The proposed model considers the most relevant forces involved in the heat transfer process of termite mounds, while also reflecting their gas-exchange function. The method adopts a system configuration procedure to determine thermally optimized mound structures. The model successfully predicts the main architectural characteristics of typical Macrotermes michaelseni mounds for the environmental conditions they live in. The results indicate that the mound superstructure and internal condition strongly depend on the combined effect of environmental forces. It is noted that mounds being exposed to higher solar irradiances develop intricate lateral channels, inside, and taller and more pronounced spire tilt towards the Sun, outside. It is also found that the mounds' spire tilt angle depends on the geographical location, following the local average solar zenith angle for strong irradiances. Although wind does not influence the overall over-ground mound shape, it significantly affects the mound internal condition. The results of this study resonate with what is seen in nature. The proposed approach provides a broader view of the factors that are effective in the form and function of a naturally made structure.

Solenosmilia variabilis-bearing cold-water coral mounds off Brazil

Cold-water corals (CWC), dominantly Desmophyllum pertusum (previously Lophelia pertusa), and their mounds have been in the focus of marine research during the last two decades; however, little is known about the mound-forming capacity of other CWC species. Here, we present new 230Th/U age constraints of the relatively rarely studied framework-building CWC Solenosmilia variabilis from a mound structure off the Brazilian margin combined with computed tomography (CT) acquisition. Our results show that S. variabilis can also contribute to mound formation, but reveal coral-free intervals of hemipelagic sediment deposits, which is in contrast to most of the previously studied CWC mound structures. We demonstrate that S. variabilis only occurs in short episodes of < 4 kyr characterized by a coral content of up to 31 vol%. In particular, it is possible to identify distinct clusters of enhanced aggradation rates (AR) between 54 and 80 cm ka−1. The determined AR are close to the maximal growth rates of individual S. variabilis specimens, but are still up to one order of magnitude smaller than the AR of D. pertusum mounds. Periods of enhanced S. variabilis AR predominantly fall into glacial periods and glacial terminations that were characterized by a 60–90 m lower sea level. The formation of nearby D. pertusum mounds is also associated with the last glacial termination. We suggest that the short-term periods of coral growth and mound formation benefited from enhanced organic matter supply, either from the adjacent exposed shelf and coast and/or from enhanced sea-surface productivity. This organic matter became concentrated on a deeper water-mass boundary between South Atlantic Central Water and the Antarctic Intermediate Water and may have been distributed by a stronger hydrodynamic regime. Finally, periods of enhanced coral mound formation can also be linked to advection of nutrient-rich intermediate water masses that in turn might have (directly or indirectly) further facilitated coral growth and mound formation.

The Alpha Crucis Carbonate Ridge (ACCR): Discovery of a giant ring-shaped carbonate complex on the SW Atlantic margin

Recently acquired bathymetric and high-resolution seismic data from the upper slope of Santos Basin, southern Brazilian margin, reveal a major geomorphological feature in the SW Atlantic that is interpreted as a carbonate ridge - the Alpha Crucis Carbonate Ridge (ACCR). The ACCR is the first megastructure of this type described on the SW Atlantic margin. The ~17 × 11-km-wide ring-shaped ACCR features tens of >100-m-high steep-sided carbonate mounds protruding from the surrounding seabed and flanked by elongated depressions. Comet-like marks downstream of the mound structures indicate that the area is presently influenced by the northward flow of the Intermediate Western Boundary Current (IWBC), a branch of the Subtropical Gyre that transports Antarctic Intermediate Water. Abundant carbonate sands and gravels cover the mounds and are overlain by a biologically significant community of living and dead ramified corals and associated invertebrates. The IWBC acts as a hydrodynamic factor that is responsible for both shaping the bottom and transporting coral larvae. We contend that the ACCR was formed by upward fluid flow along active sub-surface faults and fractures that formed by lateral extension generated by the ascending movement of salt diapirs at depth. The ACCR provides an important modern and accessible analogue for a seabed carbonate build-up related to sub-surface hydrocarbon systems.

Spectral Induced Polarization of limestones: time domain field data, frequency domain laboratory data and physicochemical rock properties

Summary With advances in data acquisition and processing methods, spectral inversion of time domain induced polarization (IP) data is becoming more common. Geological interpretation of inverted spectral parameters, for instance Cole-Cole parameters, often relies on results from systematic laboratory measurements. These are most often carried out with frequency domain systems on sandstone samples. However, the two different methods of measuring the spectral IP response differ in both measurement technique and scale. One of the main objectives of the current study is, thus, to perform a direct comparison of inverted spectral parameters from time domain IP field data with frequency domain IP spectra from laboratory measurements. To achieve this, field measurements were carried out before a ∼50 m long rock core was drilled down along one of the measurement lines. Solid parts of the core were vacuum-sealed in plastic bags to preserve the natural groundwater in the samples, after which the core samples were measured with frequency domain spectral IP in laboratory. The results showed that the inverted Cole-Cole parameters closest to the borehole were comparable to the IP spectra measured at the core samples, despite differences in measurement techniques and scale. The field site chosen for the investigation was a limestone succession spanning over a well-known lithological boundary (the Cretaceous—Paleogene boundary). Little is known in previous research about varying spectral IP responses in limestones, and an additional objective of this study was, therefore, to investigate possible sources of these variations in the laboratory. The IP spectra were interpreted in light of measured lithological and physicochemical properties. The carbonate texture differed strongly across the Cretaceous—Paleogene boundary from fine-grained calcareous mudstone (Cretaceous) to more well-lithified and coarse-grained wackestone and packstone (Paleogene). Both laboratory and field spectral IP results showed that these differences cause a large shift in measured bulk conductivity across the boundary. Furthermore, carbonate mound structures with limestone grains consisting mainly of ∼cylindrical bryozoan fragments could be identified in the inverted Cole-Cole parameters as anomalies with high relaxation times. A general conclusion of this work is that limestones can give rise to a wide range of spectral responses. The carbonate texture and the dominant shape of the fossil grains seem to have important control over the electrical properties of the material. A main conclusion is that the inverted Cole-Cole parameters from the field scale time domain IP tomography were comparable to the magnitude and shape of frequency domain IP spectra at low frequencies. This opens up large interpretational possibilities, as the comprehensive knowledge about relationships between lithological properties and IP spectra from laboratory research can be used for field data interpretation.

Extension of shallow water rock armour stability formulae to nonlinear waves

The present paper presents new formulae for hydraulic stability of the main armour layer on rock armoured conventional rubble mound structures. In the present study, new stability tests were performed covering both mild (1:100) and steep (1:30) foreshores. The tests covered nonbreaking and breaking waves on the foreshore and also waves of very low steepness. The tests with the 1:30 foreshore showed the armour layer to be significantly more stable in the surging domain than predicted with the Van Gent et al. (2004) formulae. This was attributed to nonlinear waves with significantly larger H1/3 than Hm0. New stability formulae are presented and fitted to existing and present stability data. The new formulae provide a significant increase in the reliability for nonlinear waves compared to the Van Gent et al. (2004) formulae.

Influence of oblique wave attack on wave overtopping and forces on rubble mound breakwater crest walls

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 dominant wave conditions are often not perpendicular but approach the breakwater under an angle. Oblique waves can significantly reduce the amount of wave overtopping and reduce the wave loads on coastal structures compared to perpendicular waves. Therefore, physical model tests have been performed in a wave basin where the wave loads on crest walls and the wave overtopping discharges have been measured under oblique wave attack. Design guidelines have been developed to take the reduction due to oblique waves into account. Together with available methods to predict the wave forces and wave overtopping for perpendicular wave attack, the obtained results enable a more advanced design of crest walls.

Who's been using my burial mound? Radiocarbon dating and isotopic tracing of human diet and mobility at the collective burial site, Le Tumulus des Sables, southwest France

The burial mound of Le Tumulus des Sables, southwest France, contains archaeological artefacts spanning from the Neolithic to the Iron Age. Human remains have been found throughout the burial mound, however their highly fragmented state complicates the association between the burial mound structure and the archaeological material. Radiocarbon dating and isotopic analyses of human teeth have been used to investigate the chronology, diet and mobility of the occupants. Radiocarbon dating shows that the site was used for burials from the Neolithic to Iron Age, consistent with the range of archaeological artefacts recovered. δ13C and δ15N values (from dentine collagen) suggest a predominately terrestrial diet for the population, unchanging through time. 87Sr/86Sr (on enamel and dentine) and δ18O (on enamel) values are consistent with occupation of the surrounding region, with one individual having a δ18O value consistent with a childhood spent elsewhere, in a colder climate region. These results showcase the complex reuse of this burial mound by a mostly local population over a period of about 2000 years.

Hydrological characteristics and functions of termite mounds in areas with clear dry and rainy seasons

Many species of fungus-feeding termites collectively build massive mound structures that enclose a network of broad tunnels to provide a controlled microclimate in which to raise fungus and brood by managing heat, humidity and respiratory gas exchange. However, the mechanisms by which termites regulate the water content inside the termite mounds are currently unknown in regions in southern Asia with distinctive dry and wet seasons. Here, we investigated the spatio-temporal variations in the water content in the mound structures of O. yunnanensis and then used the dye tracer infiltration method to determine how the existing mound structure and/or termite activities contribute to regulating the moisture inside the mounds. We found that the outer walls could act as a shield to prevent water infiltration into the mound, and the network of broad tunnels could act as a drainage system to eliminate excess water when the outer walls are compromised during heavy rainfall events in the rainy season. Furthermore, the termites could repair or reshape the mound to control water flows inside the mound. During the dry season, the high-density mound wall effectively reduced evaporation, and the termites could actively transport water to maintain favourable microclimatic conditions. Our results demonstrated the internal water spatio-temporal distribution in the mound and its regulation mechanisms and provided clues for understanding mound morphogenesis and the organization of insect societies.

Statistical Analysis of the Stability of Rock Slopes

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 effects of sea level rise. Damage was measured by using digital stereo photography (DSP), which provides information on each individual stone that is displaced. Two test series were performed five times. This allowed for a statistical analysis of the damage to rock armoured slopes, which is uncommon due to the absence of statistical information based on a systematic repetition of test series. The statistical analysis demonstrates the need for taking the mean damage into account in the design of rock armoured slopes. This is important in addition to characterising the damage itself by erosion areas and erosion depths. The relation between damage parameters, such as the erosion area and erosion depth, was obtained from the tests. Besides tests with a straight slope, tests with a berm in the seaward slopes were also performed. A new method to take the so-called length effect into account is proposed to extrapolate results from physical model tests to real structures. This length effect is important, but is normally overlooked in the design of rubble mound structures. Standard deviations based on the presented model tests were used.

Morphogenesis of termite mounds

Several species of millimetric-sized termites across Africa, Asia, Australia, and South America collectively construct large, meter-sized, porous mound structures that serve to regulate mound temperature, humidity, and gas concentrations. These mounds display varied yet distinctive morphologies that range widely in size and shape. To explain this morphological diversity, we introduce a mathematical model that couples environmental physics to insect behavior: The advection and diffusion of heat and pheromones through a porous medium are modified by the mound geometry and, in turn, modify that geometry through a minimal characterization of termite behavior. Our model captures the range of naturally observed mound shapes in terms of a minimal set of dimensionless parameters and makes testable hypotheses for the response of mound morphology to external temperature oscillations and internal odors. Our approach also suggests mechanisms by which evolutionary changes in odor production rate and construction behavior coupled to simple physical laws can alter the characteristic mound morphology of termites.

Deep onshore reflection seismic imaging of the chalk group strata using a 45 kg accelerated weight-drop and combined recording systems with dense receiver spacing

The Chalk Group forms important hydrocarbon reservoirs offshore and water aquifers onshore Denmark. Within a day of fieldwork, a 450 m long reflection seismic profile was acquired onshore in an area in southeast Denmark, where the Chalk Group extends almost to the surface and is approximately 900 m thick. The main objective of the study was to image the complete Chalk Group in high resolution and to study the origin of reflectivity within the different chalk units. A 45 kg accelerated weight-drop source, in combination with dense receiver spacing using microelectromechanical sensors mounted on a streamer and 48 planted geophones, was used for data acquisition. The profile runs subparallel to the cliffs of Stevns, and the recorded signal reaches the base of the Chalk Group at approximately 600 ms. The fully cored 443 m-deep Stevns-1 borehole, which is located at the recorded seismic line, provides excellent control on lithologic and facies changes. Comparison with the borehole data demonstrates that our seismic data set provides a high-resolution image of the internal layering of the Chalk Group. We find that the internal reflection coefficients of the Chalk Group are, in general, small based on wireline-log data. However, the reflected amplitudes are just big enough to be recorded with the receiver setup used, even from the pure chalk beds of the Chalk Group. The reflectivity seen on the high-resolution seismic profile is influenced by occurrences of clay-enriched chalk layers. Flint bands consisting of numerous flint nodules are a characteristic of the uppermost part of the Chalk Group at Stevns. The flint nodules appear to produce significant scattering of the seismic signals, and flint-rich layers appear with diffuse internal reflectivity characteristics. Outcrop-scale mound structures in Danian and Upper Cretaceous outcrops are for the first time seismically resolved.

THE INFLUENCE OF A CROWN WALL ON WAVE OVERTOPPING OVER RUBBLE MOUND STRUCTURES

The crest of a breakwater is often equipped with a crown wall with multiple functions such as a border of the pedestrian walkway, support of the rubble mound or concrete armour units, and reduction of the wave overtopping over the structure in storm conditions. However, the knowledge on how a crown wall influences the overtopping discharge is yet still limited. EurOtop (2016) advises to take the maximum of the crest freeboard Rc and the armour freeboard Ac in case a crown wall is present, and to use the average between Ac and Rc when no crown wall is present. This advice is based on expert judgement rather than on experimental research. The goal of the present work is to carry out experimental research and confirm or update the advice on overtopping over a crown wall for rubble mound structures.

TIME-FREQUENCY ANALYSIS OF 3D SHIP-WAVE FIELDS IN MARTIME WATERWAYS

In the last 15 years an increased number of serious damages of protective structures such as groins and revetments have been observed in maritime waterways. The analyses of these damages have shown that the current design approaches for the rubble mound layers of such structures are not sufficient to ensure sufficient stability against ship-induced wave loads. Since these approaches are determined for wind-induced waves, they do not consider the long-period ship-wave components. Within a joint research project, the design loads of ship-induced long-period waves on rubble mound structures in maritime waterways are investigated. By means of first project results, the knowledge gaps with respect to the classification and parameterization of ship waves have been identified. The project has clearly shown the need for a 3D analysis of three-dimensional, nonlinear ship-wave fields. Therefore, another research project was initiated in order to generate the scientific knowledge for the parameterization of ship-induced 3D wave fields as required for the hydraulic design of rubble mound structures in maritime waterways with a special focus on the spatial and nonlinear properties of ship waves.

MODEL TESTS ON WAVE TRANSMISSION COEFFICIENT FOR RUBBLE MOUND STRUCTURES WITH SUPERSTRUCTURES

The coastal structures, such as breakwaters, are constructed to provide the calm basin for ships and to protect the harbor facilities. The adequate design and the evaluation of design parameters are indispensable. The determination of crest height of coastal structures is one of the most important design process among all procedures. The allowable wave overtopping, the relative crest height (Rc/Hs) and the wave transmission could be applied to design the crest height of structures. The previous studies on the wave transmission coefficients were mainly conducted about the low crested structures. The previous design method could not cover the conventional breakwaters with superstructures. In this study, the wave transmission coefficient for rubble mound structures with superstructures would be investigated with hydraulic model tests.

Virtual reconstruction of damaged archaeological sites based on Unmanned Aerial Vehicle Photogrammetry and 3D modelling. Study case of a southeastern Iberia production area in the Bronze Age

Knowledge about cultural and archaeological heritage can dissolve or even disappear with the passage of time, especially when the protection level of the archaeological site is weak.The recent development of techniques based on remote sensing from remotely controlled light platforms, so-called Unmanned Aerial Vehicles (UAV) or drones, carrying sensors in visible and other spectral ranges, allows to measure efficiently the current surface morphology of a damaged archaeological site.In this work, a deteriorated and unique archaeological site due its chronological and functional singularity was chosen as the study case. Mound structures made of stone, interconnected one to each other in a regular network, covered by vaults, and well adapted to a smooth slope topography have no known precedents in the Iberian Peninsula in the Bronze Age. Nowadays, this site is seriously damaged, and its protection level by the administrations is weak. Nevertheless, two archaeological campaigns were carried out recording interesting information.A UAV-Photogrammetry project based on Structure from Motion (SfM) and Multi-View Stereopsis (MVS) algorithms was applied to model the surface terrain where the structural basis and connection channels were built, obtaining a dense point cloud, an orthoimage and a Digital Surface Model (DSM). The topographic data covering altered areas were removed from the dense point cloud, and then a new interpolated surface was obtained representing the unaltered morphology.Finally, the information recorded in the archaeological campaigns was materialised in a virtual reconstruction located between both surfaces, measured by UAV-Photogrammetry and interpolated, and then the original architectural complex in its context was recreated and shared with the scientific community through Google Earth, contributing to recovering and preserving this cultural heritage, even after its disappearance.

Numerical prediction of integrated wave loads on crest walls on top of rubble mound structures

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 OpenFoam/waves2Foam for the prediction of integrated forces on crest wall elements against a new set of experimental data in order to obtain a numerical model that can be applied for a wider field of application than the existing empirical guidelines. One key concern for the accurate modelling of wave loads is the spurious entrapment of air between the water surface and structural elements. The solution developed for this problem is a boundary condition that allows for air ventilation, while enforcing a predefined head loss characteristic. Compared to the existing technique of introducing small meshed tubes through the structure, the new method does not lead to excessive time-step limitations and is therefore more efficient (a practical case was accelerated by a factor 20). The new boundary condition is validated against experimental data of forces on bridge decks with girders.Subsequently, the numerical model is validated against experimental data for loads on crest wall elements from new experiments conducted in a wave flume. The comparison between numerical and experimental data is made both in the time domain and as probability of exceedance. Special emphasis is given to the openness of the faces of the crest wall to mimic the effect of mixing of water and air during the wave impact. Finally, the validated model is applied to evaluate the forces on crest walls as a function of the elevation of the crest wall with respect to the still-water level. This effect is of interest, since the level of the crest wall element is only tested to a limited extent in laboratory experiments and the bottom face was mainly wetted or submerged during these tests (existing empirical formulations). The numerical results are compared to an empirical design formulation [Pedersen, 1996] and conclusions on the general applicability of this particular empirical design formulation are presented. The effect of the shape of the wave spectrum on the resulting forces is investigated in a preliminary fashion.