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Environmental design of low crested coastal defence structures (DELOS)

Deliverables

Bioerosion is the erosion of hard substrata such as wood or rock by marine organisms. These are able to penetrate the substratum making holes by means of mechanical and chemical processes. Bioerosion in the sea occurs worldwide and can have detrimental consequences on natural ecosystems such as coral reefs but also endanger the stability of marine wooden structures. LCS however appears not to be particularly affected by bioerosion in Europe. This is because most of the structures are built in granite, concrete, i.e. a type of rock, which cannot be penetrated by boring organisms. Only structures made of carbonate rocks such as limestone are liable to erosion by borers. In addition, in temperate waters the presence of boring species seems limited only to certain geographical locations. No bioerosion was observed on several LCS and other coastal defence structures in the UK and Italy (Adriatic coast), although presence of various boring organisms (mainly date mussels) was recorded on LCS consisting of limestone blocks in the Gulf of La Spezia. However, also in this case the effect of bioerosion on stability was negligible, as holes made by bivalves were limited to the surface layer and not deeper than 10 cm. This result shows that bioerosion can be easily avoided by using granite and concrete instead of limestone rock as material for construction of LCS. The deployment of large building blocks would overcome any potential problems caused by bioerosion in carbonate rocks. Despite bioerosion has a small effect on stability of LCS, diversity of epibiota colonising the structures can be positively enhanced, especially if softer carbonate rocks are used as building material. Bioerosion and weathering increase the surface complexity of these rocks by creating crevices, pits and holes which provide more suitable habitats for a higher number of species.
When waves reach LCSs, they suffer transformation due to the processes of reflection, dissipation and transmission over the structure as well as diffraction around the heads. These changes modify the direction and magnitude of the energy flux reaching the beach, producing changes both in plan form and profile. To design functional LCS’s schemes, the designers need formulas for prediction of variables as wave reflection and transmission. Besides functional design, low crested structures should withstand the wave action during their usable life with damage below a determined level. The main damage mode of low crested rubble mound structures is related to the movement of armour units, so stability formulas for this damage mode should be provided for the design. The objective was to produce well-controlled closure relations for armour stability, wave transmission and reflection to be used in the functional design of LCS. Applied methodology, scientific achievements and main deliverables: - Deliverable D 43 complements DELOS deliverable D22 “Structural design preliminary report” oriented towards a revision of available formulae for the design of Low-crested rubble mound structures. - Deliverable D 22 presents a brief summary of the most used formulae for wave transmission and armour stability of LCS’s. - Deliverable D 43 includes the achievements of DELOS in these fields, specifically on wave reflection, transmission and LCS stability. As far as wave transformation over LCS’s is concerned, the DELOS project was focused on wave transmission. Wave reflection was not considered to be an important aspect and was only treated at the end of the project and some preliminary results obtained with DELOS data sets were presented in D43. For wave transmission, all previous data sets on wave transmission and results from DELOS laboratory experiments carried out at the University Cantabria and the Polytechnic University of Catalonia were gathered together (2337 tests). The full analysis was carried out by Briganti et al (2003). As a result two new formulae were proposed for narrow and wide -crest LCS’s, respectively. These formulas used three non-dimensional parameters to describe wave transmission. Besides this result, the DELOS database on wave transmission was used by Panizo et al (2003) to build a neural network. Using this network, the influence of additional non-dimensional parameters was analysed. The resulting neural network added three new parameters not included in the Briganti et al. (2003) proposed formulae. The shape of the transmitted spectra was analysed by Van der Meer (2003) who proposed a new methodology to define the shape of the transmitted spectrum. Using data from 3D basin experiments carried out within the DELOS project at Aalborg University, the oblique transmission was analysed, van der Meer et al. (2003). In relation to 3D LCS armour stability, two existing data sets Vidal et al (1992) and Burger (1995) were gathered together with data obtained in wave basin experiments within DELOS to develop a simplified stability formula for initiation of damage, Kramer and Burcharth (2003) based on the fact that most of LCS’s are built in shallow waters where wave height is depth-limited.Finally, based on the analysis of scour around LCS’s carried out within DELOS, new formulae were proposed for the width of the toe berm protection, for both the front slope and the head sections. New empirical formulae have been obtained for wave transmission, reflection and stability of LCS’s. These formulae improve or complement existing formulations, thus providing a way for a more reliable and cost-efficient design of the structure and the complete (beach - structure) LCS scheme.
The present results may facilitate informed decisions about the construction of coastal defence structures and the management of coastal areas. In particular, results have shown that: - Local environmental impacts can scale up in a non-linear manner. Both local and broad scales need to be considered in the design process, - Major alterations to the species identity and composition may result from the introduction of LCSs over large stretches of coast, - There is a risk that the LCSs may promote the expansion of introduced species, or of species that are a nuisance to beach tourism. For example, along the coasts of Emilia Romagna, the introduced species Codium has taken advantage of the availability of hard substrata along an exposed sandy coast, and in particular of sheltered habitats that seem to provide better conditions for its growth, - The spatial arrangement (i.e. location, relative proximity to natural reefs and other artificial structures) of LCSs is of great importance in influencing the type of hard-bottom species that will colonise any novel structure. The present results have been incorporated into the design guidelines. The potential end users will be mainly practitioners of coastal management. On a short term, the present original results are also expected to form the basis of some relevant publications on international scientific journals. In this case the end-user will be mainly the scientific community although some results could be disseminated to the general public. A first manuscript will be prepared within the end of the project to be submitted to a special issue of the International journal "Coastal Engineering". On a longer term, further collaboration will be carried on between the members of the consortium, to further develop some of the original ideas emerged from the project.
An inventory data bank containing structural data on LCS's and their morphological effects and performances is formulated. 175 sites mainly within EU are identified and brief information regarding the following subjects is available; LCS locations, main objective, impacts on bio-environment, socio-economic impacts, layout and cross-section geometry, and water level variations. The information is categorized in a databank and statistical information about the geometry of LCS's throughout Europe is derived. In addition, 14 sites in Europe were chosen for further inspection and detailed information regarding the following subjects is available; geometry and construction materials, local meteomarine conditions, sea bed and beach characteristics, structural and coastal protection performances, socio-economic aspects, and ecological aspects.
The effect of Low Crested Structures (LCS) on habitat complexity of the surrounding soft-bottoms is sometimes very low. However, different LCS designs and hydrodynamic conditions (e.g. tidal ranges) tend to give rise to different levels of response. The sediment descriptors (granulometry, organic matter content, chlorophyll-a content) usually do not show significant differences around the LCS because of the high within-treatment variability (nested Analysis of Variance “ANOVA” design). A multivariate analysis (Principal Component Analysis) usually provides better indications of recognizable between-treatments patterns of distinctiveness. The soft-bottom infaunal assemblages around the LCS also shows high within-treatment variability. Conversely, it is possible to demonstrate marked differences between the assemblages around the LCS and between them and the controls, independently of the tidal range (nested ANOVA design). The use of biological (number of species, abundance, biomass, diversity), as well as functional (trophic groups), descriptors are a better tool to record the impact caused by the LCS than the sedimentary characteristics. Multivariate analyses (such as Multi-Dimensional Scaling or Analysis of Similarity) show differences among treatments independent of the distinctiveness of the raw data. However, it is necessary to explore the results on the basis of either abundance or biomass and on the basis of either species or trophic groups, as the resolution may vary depending on the studied situation. The analyses confirmed the patterns obtained with the ANOVA approach, highlighting the fact that the largest dissimilarity always occurred between landward and control areas. The analyses of samples collected at successive distances from LCS (always designed according to the particular characteristics of each target site) are able to produce an integrated picture of the systems, that could be linked (easily) both to environmental trend and to dynamic models in order to assess the influence of hydro- and sediment dynamics on the soft-bottom assemblages. The effect of LCS on both sediment descriptors and infaunal communities was localised mainly in the proximity of the structures. The degree of exposition and the hydrodynamic regime at landward seem to be key features influencing the diversity of the infauna around the LCS, particularly stronger in the presence of additional structures (such as parallel groins) or after beach nourishment. As the presence of the LCS induces a disruption in the normal succession from shoreline to deep waters, changes in sediment characteristics and infaunal assemblages seem to be an inevitable consequence of the LCS construction, which always depend on the response of the peculiar assemblages inhabiting a given area. Thus, it is strictly necessary to know the composition of these assemblages before the construction, in order to be able to assess their possible subsequent evolution (e.g. species disappearance-from and/or colonization-of the new environment). Local and wide geographical areas should be studied to determine the possible sources of species able to respond to the new artificial conditions. The overall habitat diversity of the stretch of coast where the LCS is built usually tends to increase and, as a consequence, the biodiversity also tends to increase. To some extent, this could be regarded as a positive result. However, the ecological relevance strongly depends on the species responsible of the changes. Whether they are species accidentally coming from the newly added hard bottoms or species more or less associated to increasing disturbances, the increase in infaunal biodiversity must be considered as a negative transformation. Despite the join studies were carried out in very different coastal systems, the results obtained are consistent. This strengthens the findings on the effect of LCS on soft-bottoms and will allow the formulation of LCS design guidelines to be applied to different coastal systems in Europe. It seems feasible to introduce design criteria to facilitate a positive evolution of the assemblages (once the artificial situation tends to become stable) addressed, for instance, to try to avoid the development of insalubrious areas in the protected zone (e.g., by increasing the water flow through the LCS). Coastal managers from local and regional authorities, as well as private companies with interests in deployment of infrastructures affecting the littoral should take the above considerations into account. However, it must be pointed out that the effects of LCS on soft-bottoms should always be minimised, independently of any increase in biodiversity, as the principle is to maintain the most natural ecosystem conditions, protecting them from human interventions. For example, the number of LCS should be reduced to the minimum necessary to protect the coast, to avoid large-scale effects of habitat loss, fragmentation and community changes.
The design of structures to be built in the nearshore region generally involves the evaluation of different possible layouts, under the effects of local wave and current conditions, with the aim of minimizing costs and maximizing the desired results. In particular the design of low-crested structures involves optimisation of several parameters, which influence both the position and the shape of the structures. An alternative to physical models and attractive procedure is to employ suitable numerical and mathematical models. In principle, a very advanced numerical model, able to correctly simulate all the nearshore phenomena (turbulence, waves, currents, sediment transport, etc.) could be equivalent or even superior to a physical model. In practice, the numerical models currently employed in engineering activities, use several assumptions and simplifications: the phenomena that can be simulated strictly depend on the governing equations solved by the model. The objectives have been to improve our modelling capabilities of the hydrodynamics in nearshore areas including the presence of LCS. The two dimensional models are supposed to give a reliable prediction of wave and currents in areas adjacent to the LCS and to provide an improved description of the surf and swash zones. This hydrodynamic modelling should be extremely relevant for the analysis of the morphodynamics around LCS schemes. The results of this work are mainly due to contributions form the Universities of Rome and Thessalonica. The work carried out in Thessalonica has been mostly the development of a 2DH-Boussinesq-type model combined with a depth-averaged Darcy-Forchheimer equation for simulating wave propagation over submerged porous breakwaters. The model was tested against experimental measurements. Several data sets corresponding to different regular and irregular wave conditions were used for model verification. Scale effects assessment has been carried out with different experimental setups. The comparisons indicate that the model simulates quite well wave evolution at the regions before and over the breakwater. Present model behaviour is enhanced, compared with the behaviour of weakly non-linear Boussinesq-type models applied in the simulation of wave evolution over impermeable and permeable submerged bars. The work carried out by the University of Rome has been mostly devoted to the enhancement of 2DH models based on the extended Boussinesq equations in order to analyse consider improved applications in the surf and swash zones nearby LCS.Among the most relevant achievements are: a new shoreline boundary conditions for use in wave-resolving Boussinesq-type model; a new method for incorporating the swash zone into wave-averaged circulation models and a new method for simulating wave breaking effects into Boussinesq-type models. The technique developed for the new shoreline boundary conditions allows imposition of the appropriate value of water depth and velocity at the shoreward limit of the computational domain. Validation of the method against analytical solutions suggests good performance of the new boundary conditions. The new method for incorporating the swash zone into wave-averaged circulation models is based on an integral model. The technique has been validated against both numerical solution of the nonlinear shallow water equations and available experimental data. The new method for simulating wave breaking has been implemented in a numerical solver and tested with success. Comparison with available experimental data suggests very good performances of the new method. In order to improve wave breaking simulation, a new criterion for determining weather waves are breaking or not (breaking criterion) is being developed. The great advantage of numerical and mathematical models is that their application is usually much less expensive than physical ones: it is certainly more economic to modify computer files describing the bathymetry of the area under investigation than to rebuild a physical model layout. Therefore, a first economic implication is that the development of this kind of modelling may result in a lower a more reliable design costs for LCS. From the strategic point of view, the use of this kind of modelling should be considered as part of any guidelines for the design of LCS schemes.
Delft Hydraulics has provided for a link between the morphodynamic environment of the area surrounding a LCS and the prediction of the impact on local species communities. This link is based on application of Delft3D-FLOW and MOR (Lesser, 2002) and work done in the EU life BIOMAR project (Picton, 1998). Results indicate that a sensitive and realistic prediction of biotope changes caused by impacts of coastal infrastructure can be derived from this approach. The approach could prove valuable for utilisation in impact assessment related to the application of the EU bird and habitat directives. Developed modelling techniques are standard and directly available. Applicability is limited by the scope of the BioMar biotops system. LESSER, G.R., KESTER, J. VAN, ROELVINK, J.A., STELLING, G.S., 2002. Development and validation of a three-dimensional morphological model. WL/Delft Hydraulics, Delft University of Technology. PICTON, B.E. AND COSTELLO M. J. 1998. The BioMar biotope viewer: a guide to marine habitats, fauna and flora in Britain and Ireland, Environmental Sciences Unit, Trinity College, Dublin. Published by: Environmental Sciences Unit, Trinity College, Dublin. ISBN 0 9526 735 4 1.
New unique laboratory experiments on low-crested structures (LCS's) were performed within the DELOS project. The experiments were aiming at extending and completing existing available information with respect to a wide range of engineering design properties. 12 different structural set-ups in 3D were built in a wave basin at the laboratory at Aalborg University (AAU), and 325 tests were performed. In order to make ideal set-ups in the laboratory with respect to each subject it was necessary to separate testing with different purposes. In this way the wave basin experiments at AAU were grouped in stability tests, hydrodynamic tests and wave transmission tests. Tests were not performed at any specific scale, but a scale of 1:20 represent an approximate scale with respect to typical full scale configurations. Structure freeboard, wave height and wave period/steepness were varied in all tests. - 69 stability tests were performed to investigate structural damage to heads and trunks subject to 3D waves. Influence of wave obliquity was tested on 2 structural set-ups with different crest widths. - 88 hydrodynamic tests were completed to analyse wave and current flows near the structures, and to provide data for calibration of numerical models. 2 structural set-ups with a gap between the roundheads and 2 set-ups with oblique structures were tested in 2D and 3D waves. - 168 wave transmission tests were performed with the objective of studying influences of wave obliquities on transmitted wave energy, wave directions and spectral changes. 3 structural set-ups with rubble structures and 3 set-ups with smooth plywood structures were tested in 2D and 3D waves. The wave obliquity was one of the main parameters, which were studied in the wave basin experiments. The experiments provide unique information about the influences of this parameter where almost no research has been done before. The DELOS experiments have filled some gaps within existing knowledge providing valuable information for establishing design guidelines for low-crested structures.
New unique laboratory experiments on low-crested structures (LCS's) were performed within the DELOS project. The 2D experiments were carried out in two European laboratories aiming at extending and completing existing available information with respect to a wide range of engineering design properties. 174 wave channel tests in total were performed on two structural set-ups at small scale and two set-ups in a large scale facility. The small scale wave channel tests were performed at the University of Cantabria (UCA) in Santander, and large scale wave channel tests were completed at the Polytechnic University of Catalonia (UPC) in Barcelona. The near and far field 2D hydrodynamics were studied at small scale at UCA, and large scale 2D tests on wave transmission and reflection were performed at UPC. Tests were not performed at any specific scale, but the following scales represent an approximate scale with respect to typical full scale configurations. Structure freeboard, wave height and wave period/steepness were varied in all tests. 66 wave transmission and reflection tests were performed at UPC (large scale, about 1:4) to investigate the influence of crest width, structure slope, and scale effects on wave transmission and reflection coefficients for LCS's. 2 structural set-ups with different crest widths were tested in regular and irregular waves. 108 2D near and far field hydrodynamic tests were performed at UCA (small scale, about 1:10) to describe in detail the flow inside and over the structure. Wave transmission and run-up on the beach was also monitored. 2 structural set-ups with different crest widths were tested in regular and irregular waves. Flow velocities inside and close to the surface of structures was some of the parameters studied in the small scale wave flume tests. This subject is not only interesting with respect to engineering design properties but it provides also important ecological information on living conditions for lifeforms attached to the structure surface, because the flow velocities causes exposure to the lifeforms due to drag forces. The DELOS experiments have filled some gaps within existing knowledge providing valuable information for establishing design guidelines for low-crested structures.
It is well established that the distribution of intertidal organisms is correlated to wave exposure. Exposure is rarely well defined and a number of measures have been used to quantify exposure. Most measures involve different aspects of fetch and dominating wind directions. Rarely has the effect of waves been directly measured and often exposure is classified in terms of the biological community, introducing logical circularity. Surprisingly little effort has been focused on cause-effect relationships behind the observed correlations between wave exposure and the distribution of organisms. A few successful attempts to unravel the effects of wave action on the survival of shore organisms show that some organisms are indeed limited in their distribution by the hydrodynamic forces imposed by, in particular, breaking waves. A low-crested breakwater (LCS) introduces a strong gradient in wave exposure, mainly between the seaward and the landward side. This result focuses on detecting and predicting patterns of epibiota on LCS in relation to the hydrodynamic environment. Fieldstudies in UK (Elmer) and Italy (Ravenna) was conducted together with theoretical analyses. In field studies organism commonly found on the breakwaters were used, including barnacles, algae: long-lived (Fucus sp.) and ephemerals (Enteromorpha sp.) and the grazing molluscs Patella vulgata and Littorina littorea. Average flow speed was measured using the weight loss of discs cast in gypsum. Maximum force acting on an object in breaking waves was estimated using spring-loaded balls (Bell and Denny 1994). Studies were conducted to look at effects of topography (on larger and smaller scale) and scour on breakwater epibiota Theoretical models used were the mechanistic model of wave-induced detachment of epibiota, model of LCS interior flow and biological oxygen demand and model of LCS interior flow and nutrient depletion CONCLUSIONS From fieldwork together with statistical and theoretical analyses the following conclusions about identified relations between epibiotic patterns and hydrodynamic regime can be made: - The survival of epibiotic organisms may depend on rare events of maximum wave-induced forces. The maximum force acting on epibiota, and thus probability of detachment, is expected to be approximately linearly related to maximum breaking wave height. - Hydrodynamic forces, modelled and observed, acting on the epibiota on LCS can be sufficiently large to detach organisms or inflict tissue damages. - Topography on scales larger than epibiota will offer refuges increasing survival and biodiversity - Topography on scales smaller than epibiota may be important during recruitment and for adhesion strength. Very smooth surfaces, in particular in combination with high flow speeds, can be used to reduce the diversity and abundance of epibiota. - Scour on the observed LCS seems to be a problem for epibiota only in a zone close to the toe. - A model of flow within LCS indicates that the supply of oxygen is sufficient to support macro-fauna for pore sizes exceeding 0.2m.
The one-line model (ARIES), developed by MOD and improved within the DELOS project has been applied to study shoreline evolution behind a single detached breakwater by means of parametric tests. A number of these tests aimed to compare the predicted shoreline response with the one estimated with simple design rules largely used in engineering practice. Also the study case of Le Morge (Italy) has been simulated to test the model performance in predicting shoreline evolution in presence of multiple breakwaters. Also a procedure to obtain a description of the wave climate from directional wave recorder buoys has been proposed. It has been also shown how sensitive is the beach response to wave transmission which is the leading phenomenon for submerged breakwaters. This means that it is necessary to look critically to empirical design rules, which are derived considering only wave diffraction. The approach has been tested quite successfully to a real case of shore protection works by means of detached breakwaters together with a procedure for extracting from a wave climate a limited series of waves that can be considered equivalent to the complete time series in term of wave energy. The application of ARIES model with such improvements permits also to evaluate the efficiency of beach protection layouts. The example of Le Morge multiple breakwaters is significant in this sense.
Scale effects in physical models of rubble mound breakwaters appear because the ratios between the forces of interest, as present in prototype, cannot be maintained in a scaled model. None of the standard scaling laws for hydraulic models (Froude, Cauchy, Reynolds, Weber) provides accurate scaling for all processes of wave-related breakwater models. Scale effects are due to properties of breakers, wave impacts on armour blocks; run-up and overtopping; structure deformation; wave generated flow in the porous structure; flow forces on plants and organisms attached to the structure. Wave loads on breakwaters are compounded of impulsive and pulsating loads. No major scale effects are evident for pulsating loads. In this case Froude scaling is accurate to transfer model results to prototype scale. Impulsive loads induced by breaking waves involve compressibility of air during a very short time. The fluid becomes a mixture air/water and cannot be treated as incompressible. Froude law is not adequate for scaling model results of impulsive events. Cauchy law has proved to be inaccurate too. Surface tension can alter the deformation of the crest shape as the approaching waves break. Surface tension effects become increasingly important as wavelength and intensity of breakers decrease. Without surface tension the crest deforms to generate a jet at the crest that plunges into the wave face to start the turbulent spilling process. When surface tension becomes dominant, this jet is replaced by a bulge-capillary structure at the crest and turbulence is produced by separation at the point of high-upward surface curvature at the leading edge of the bulge. During the turbulent processwhen surface tension is unimportant, splashing motions occur: air bubbles are produced. When surface tension is dominant, surface fluctuations and turbulence are reduced, and a single continuous bumpy surface makes up the boundary between air and water. The different compressibility of water is due to the different process of bubble formation from fresh to sea water. Air bubbles are smaller in sea than in fresh water and are entrained in a breaking wave. Their size distribution is different from fresh to sea water. These differences are caused by: salt concentration, ionic structure, exudates of marine organisms, surface tension, temperature and viscosity. The effects of different compressibility of water from model to prototype is overestimation of pressure magnitude and underestimation of pressure rise time for impulsive loading, if Froude or Cauchy laws are used. In aerated sea water, the presence of air produces a cushioning effect, reducing the magnitude of the impact and extending its duration. As inertia forces scale as u^2, even a use of Weber law would lead to errors: surface tension does not scale from model to prototype. It is suggested to analyse wave loading by use of the impulse (integral of load over time during impulsive events). Scaling model impulses to prototype dimensions by use of Froude law gives accurate results even for impulsive events. Another parameter which can be used for impulsive events analysis is the integral of the ratio dp (pressure) to water density. This parameter is a function of percentage void ratio. Use of this parameter involves the need for measurements of air content in model and prototype. When comparing model tests with sea water to full scale field results, differences can be noted in maximum impulsive pressures. The different ambients (laboratory and field) play a decisive role and affect the ambient conditions of the experiment. Run-up seems to be influenced by scale effects. Overtopping scales down with Froude, even if for small values of discharge some scale effects might cause underestimation in small scale models. Porous flow in model is almost entirely laminar, whereas in field condition the motion is turbulent. Empirical methods exist for scale corrections. Forces on plants and organisms should be measured in the field. The transport mode in prototype should be maintained in sediment model. Dean parameter is used to asses which transport is dominating. In situations where both modes of transport (suspension / no suspension) occur, only qualitative results can be obtained as it is not possible to scale them simultaneously. A correct scaling for sediment preserves Dean parameter uses an undistorted geometrical scale and scales hydrodynamics with Froude. Such a model preserves similarity in wave form, sediment fall path, wave-induced velocity, break point, breaker type, wave decay. Scale effects can arise when viscous domain becomes dominant and in presence of ripples. Marine organisms attached to breakwaters increase structure roughness, reduce water flow through the structure and trap large quantity of sediment. All these effects affects flow through the structure.
LCS structures share several physical and biological features with natural rocky shores. The structures offer new hard substrate available for colonisation by algae and benthic organisms typical of natural rocky shores. As natural rocky shores, LCS are affected by same physical and biological processes, but differ in three major aspects: they have a limited horizontal and vertical extent, they are characterised by a lower habitat complexity and finally they experience a higher disturbance. As a result, habitat and species diversity on LCS is often lower than on rocky shores. Several environmental factors affecting epibiota are pre-determined and cannot be modified. These include: - Geographical variability, being determined by the species pool, wave and current regimes, tidal range and salinity; - Wave exposure and biological interactions; - Vertical gradients according to tidal height / depth; - Temporal variability, including colonisation processes, seasonal and long-term fluctuations; - Disturbance by sand scouring and siltation. These factors, although outside the control of engineers, need to be taken into consideration for the construction of LCS as they will have influence on the development of epibiota. LCS design features can be modified to minimise ecological impacts and to implement mitigation measures. Here are some simple rules that can be applied to obtain target effect or secondary management goals. Disturbance by sediment scouring and siltation can be reduced by the adding berms to the LCSs. Maintenance works can be minimised by increasing the stability of LCS and of the sea bed. More complex surfaces (e.g. textured or pitted block surfaces) and artificial rock pools can be integrated in the design of LCS to increase habitat and species diversity. Carbonate based building materials such as limestone become easily weathered and bioeroded, thus provide a variety of micro-habitats for to colonise. On macrotidal shores, LCSs that are located lower on the shorewill be colonised by a higher number of species. Complex habitats also promote living resources such as mussels, oysters and crustaceans. These effects can enhance the recreational and amenity use of these structures by public, such as food harvesting, sport fishing and appreciation of marine life. In conclusion, the environmental setting has an important influence on the colonisation and future development of epibiota, thus it needs to be taken into account by coastal engineers prior construction of LCS. In contrast, LCS design features are under the control of engineers and can be modified to minimise impacts and promote positive effects on recreational use and natural resources. Under the ecological viewpoint, low crested structures and other types of coastal defences lead, as most human interventions, to major modifications in the coastal ecosystem and their construction should be limited to the minimum. However, where these structures are a necessary measure against coastal erosion, attempts should be made to promote their integration in the coastal system, minimising ecological impacts and enhancing the socio-economical value. The findings summarised above are the result of extensive studies and experiments which were carried out in the Mediterranean (Spain, Italy), Atlantic (UK) and North Sea (Denmark). This guarantees the generality of the results and provides information that can be disseminated in form of European guidelines and recommendations for an environmentally sensitive design of coastal and sea defence structures. Several end users could benefit from the application of this result: - The private sector, such as engineering companies involved in the design and construction of LCS and surveyors involved in the Environmental Impact Assessment and monitoring; - The public sector, such as Environment Agencies and local councils involved in the decision making process and coastal management; - The scientific community, especially those in the research field of anthropogenic impacts on coasts and pollution. Taking into account the variety of target potential users, dissemination of this result will be through conference and seminars for scientific and wider audience, briefing of conservation agencies, distribution of technical reports to local councils and coastal managers, and peer reviewed international publications. Coastal defences are increasing exponentially along most European coasts, as a consequence of erosion and flooding. However, the ecological implications of LCS and other coastal structures have been considerably neglected, despite the acknowledgement of the need for a more sustainable coastal development, which must take into account the status, protection and preservation of the ecosystem. This result can help addressing stakeholders towards an integrated and more environmentally sound coastal management, as also required by the European Habitat and Water Framework Directive.
The construction of artificial rocky reefs (e.g. low crested defence structures, LCS) in a sandy beach environment creates virgin habitat for rocky shore organisms. Earlier isolated patches of natural habitat may become connected via a chain of “stepping-stones” such that gene flow is permitted and also the dynamics of some or all species is shifted from the original situation. Such a situation may promote the invasion of introduced or alien species. Depending on the actual positions of new artificial substrate the dispersal speeds, i.e., km/generation or similar, may differ. In this result we are assessing the large-scale effects of breakwaters on the distribution and abundance of species. For this result we have used Patella caerulea as a model organism. P. caerulea is a long lived prosobranch gastropod living in the intertidal. This marine snail grazes on the rock surface consuming the microfilm as well as recruits of barnacles and macroalgae. It has the potential of structuring its environment. Spatial ecology and spatial population dynamics are wide fields of research. However, it is still unknown what the general effect on originally isolated natural communities is when gene flow is established. This result tries to answer some questions about the effect of the positioning of LCS’s on the regional population dynamics of P. caerulea in the Ravenna region, Italy. A metapopulation is a population of populations. In effect this means that there exist several local populations that interact, via migration or dispersal. This interaction causes regional dynamics. Many people have helped in developing metapopulation models, and models exist for several different organisms, environments etc. Metapopulation models are used to calculate extinction risks for endangered species in fragmented landscapes, the design of nature reserves etc. We have constructed a metapopulation model that encompasses local dynamics. This is important for long-lived organisms such as P. caerulea. CONCLUSIONS We have modelled the maximum distance of dispersal to 89km. However, depending on variations in flow and behaviour of larvae dispersal may range between 10 and 89 km. This means that the probability of gene flow between the natural rocky reefs in the area was extremely low before the construction of breakwaters and harbours began. There are no longer any stretches of beaches without structures of 89km or longer. We conclude that although we cannot at present quantify the probability of gene flow, or migrants per generation, between the natural rocky reefs in the studied area, there exists a substantial transport of larvae along the Ravenna shoreline. Through the action of the structures as “stepping-stones” the natural reefs will undoubtedly experience gene flow. The position of a LCS has a great effect on the population dynamics following removal of structures in the model. Also, the removal of LCSs in the model will have different effects depending on flow regime, e.g., year-to-year variations. We have identified a number of key structures that with a high probability contribute to a substantial part of the regional population dynamics. That is, if new LCSs are added to this area their effect will be different depending on where they are placed. These clear large scale effects of LCSs on biota demands that planning of building new LCSs must be done on a regional or a country-wide scale.
The breaking process over the overtopped structure enhances the pumping of wave-induced mass fluxes over the low-crested detached structure. This results in an enhanced nearshore circulation and consequently the presence of the LCS modifies the sediment fluxes and morphodynamic evolution. Furthermore, the fluxes through the permeable structure may also contribute to additional modifications. Consequently circulation and morphodynamic models assessing morphodynamic evolution in the presence of LCS should include additional information regarding the fluxes on the top and through the structure. It is well established that hydrodynamic forces due to breaking waves are among the most important sources of shore organism distribution and mortality. Therefore, in order to interpret the biomechanical characteristics of the epibiota on a low-crested structure; stress levels resulting in tissue damages or complete dislodgement; average flow conditions to predict larval settlement and delivery of nutrients or critical periods of very low flow speed causing hypoxia; a feasible description of the flow is required. This is only possible based on an appropriate modelling of the velocity field and breaking processes in the near field of the structure and how those are affected by incident wave parameters, structure geometry and permeable material characteristics. However, it has to be pointed out that the different applications do not really require the knowledge of the turbulence fluctuations velocity components. The knowledge of the instantaneous mean flow, the seepage velocity, the maximum or minimum mean velocities is sufficient to fulfil most of the questions raised with regard to stability, functionality or ecological issues. Therefore, as part of the work carried out within WP2.1. a detailed analysis of the flow velocities in the surface region of LCS has been considered to be relevant. The main scientific and technological objective has been to provide reliable information on the velocity distribution around and inside LCS and how the velocity distribution is affected by the incident wave conditions, structure geometry and porous material characteristics. Furthermore, a second goal is to provide reliable tools to evaluate the magnitudes of the velocity fields under design conditions. This information is extremely important to evaluate LCS stability since some of the stability formulations are based on the evaluation of the velocity at the surface of the structure. The applied methodology consists of the combination of the analysis of the experimental data gathered in the frame of the project and the generation of numerical data using the 2DV numerical model based on the VARANS equations (deliverable D19). Based on both the numerical and experimental results the velocity distribution is studied. The influence of relevant magnitudes such as berm width, freeboard, incident wave conditions, model scale and structure material on the velocity distribution is considered. Comparison between measured and numerical velocities show a very good agreement, showing only small deviations very close to the surface of the structure where local effects are affecting the measurements. The main deliverable has been a document on flow velocities in the velocity of the surface of LCS. In this document the application of deliverable D19: Calibrated 2DV near-field flow model, to evaluate the velocity field around, on and inside LCS is explained. Several examples for the validation of the results are shown, comparing numerical and experimental data. The model is proven to reproduce with a high degree of agreement the velocity field around, on the surface and within the structure for different geometries, incident wave conditions and construction material. The shoaling and breaking effects in the seaward and crest zones, as well as the higher harmonics generation phenomenon in the transmission zone, are well simulated, whether the structure freeboard is positive, zero or negative.
The present result may be used to draw conclusions about the potential effects of coastal defence structures over large spatial scales. In particular, it has been shown that major alterations to coastal environments may result from the introduction of LCSs over large stretches of coast as a consequence of the increased availability of hard-bottom and sheltered habitats in areas where they do not occur naturally. Results suggest that, although defence structures become colonised by rocky-bottom species, their assemblages seem to differ from those occurring on nearby natural reefs. Further, LCSs seem to act by changing the patterns of distribution of locally abundant species rather than by increasing species diversity. This raises concern that LCSs may cause considerable change to the identity and/or abundance of epibiotic species within an area. The possible consequences of these changes to coastal assemblages should be taken into account when establishing coastal zone management plans covering large stretches of coastlines. The present results have been incorporated into the design guidelines. The potential end users will be mainly practitioners of coastal management. On a short term, the present original results are also expected to form the basis of some relevant publications on international scientific journals. In this case the end-user will be mainly the scientific community although some results could be disseminated to the general public. A first manuscript will be prepared within the end of the project to be submitted to a special issue of the International journal "Coastal Engineering". On a longer term, further collaboration will be carried on between the members of the consortium, to further develop some of the original ideas emerged from the project.
Over the past several years significant efforts have been devoted at DHI to develop advanced computational fluid dynamics (CFD) tools. The development has been centred around a so-called three-dimensional Navier-Stokes solver, NS3. The model has been developed with the aim of making it applicable to the analysis and investigation of as many flow and sediment transport problems as possible. Thus, the model includes a description of the instantaneous free surface in order to make it applicable to e.g. simulation of breaking waves in the surf zone. Since a reliable and ample description of the three dimensional flow around low-crested structures is nowadays only possible using physical models or field data, in the frame of DELOS the intention has been to apply NS3 to the simulation of flow over and around LCS. The term flow referring both to wave motion (e.g. wave breaking and overtopping) and to the wave-induced currents and over and around the structure. In order to simulate the vertical structure of the flow adequately, the NS3 model has to include a suitable description of the spatial and temporal structure of the turbulence under breaking and broken waves. Main scientific/technological objective has been to analyse the capability of the NS3 model to simulate the near-field flow around impermeable low-crested structures, especially three dimensional processes such as wave breaking under oblique incident waves or wave propagation around the head to the LCS. The Navier Stokes solver solves the instantaneous Navier-Stokes equations in three dimensions using a finite-volume approach on a multi-block grid. The spatial discretisation is based on the finite-volume approach on a multi-block grid. The time integration of the Navier-Stokes equations is performed by application of the fractional step method. The free surface is resolved using a Volume-of-Fluid (VOF) description. The waves are generated at an inlet boundary where Stokes waves up to 5th order, cnoidal waves up to 5th order or to use a stream function waves can be specified. At the beach, the remaining wave energy is absorbed by means of a sponge layer. The 3D Navier-Stokes solver includes several turbulence models: RANS (Reynolds Averaged Navier-Stokes equations) models such as k-e, k-w or LES models (Large-Eddy Simulation) with a Smagorinsky sub-grid scale model or a k-equation for the sub-grid scale turbulence. The model application for LCS has been validated with a limited set of cases provided by the DELOS experimental work carried out in the Aalborg wave basin, since the model is computationally expensive. Comparisons between numerical and free surface records show a very reasonable agreement especially in front and above the structure. Even if the model is still under development and not ready for standard engineering applications, results have shown that the potential applications of the model to analyse the three-dimensional flow around the structure are very high. In a few years, considering the increasing computer power capabilities, this kind of model will be a unique tool for LCS design. It is extremely relevant to consider that the number of physical model tests to be carried out will be considerably reduced. Furthermore, most of the limitations due to physical models will be overcome.
Field measurements in the area of interest were used for the calibration of the hydrodynamic model. The resultant tidal currents, in the area of the scheme, flow in a counter-clockwise direction reaching peak magnitude during high water. The associated net bed-load sediment transport over a spring-neap tidal cycle in the area has the same direction as the peak currents in the area. Tidal currents are entering the area behind the structures, through the gaps at the east of the scheme, flowing over the salient features accelerating, during high water conditions. This accelerated flow over a restricted depth area causes the mobility of the sediment and it is probably a controlling factor of the salient growth. The bay area between the breakwaters has low sediment mobility under tidal currents. Wave diffraction at the gap of the breakwaters was observed during all of weather conditions, as confirmed by the field measurements. Diffraction is the main process decreasing the incident energy on the coast. Wave induces sediment mobility, estimated using a Stokes 2nd order waves, demonstrate the ability of even mild waves to transport the sediment in the direction of propagation of the diffracted wave. The presence of the salient feature is an evidence of the above sediment transport. The sediment trends established for the area within the project are in good agreement with the estimated wave-induced sediment transport. For the case of mild energy conditions in the bay area both methods estimate the direction of sediment transport, to be in the onshore with sediment directed towards the salient in the lee of the structures. Results obtained here are applicable to other offshore breakwater schemes in the Eu and especially in the UK were the environmental conditions are similar to the present site.
The effect of manmade activities is primarily local but can extend far away from the location of intervention. This underlines the importance of establishing coastal zone management plans covering large stretches of coastlines. The present guidelines on Low Crested Structures (LCS¿s) attempts to provide methodological tools both for the engineering design of the structures and for the prediction of performance and environmental impacts of such structures. It is believed that the guidelines will provide valuable inputs to coastal zone management plans. The target audience for this set of guidelines is consulting engineers or engineering officers and officials of local authorities dealing with coastal protection schemes. The guidelines are also of relevance in providing a briefing of current best practice for local and national planning authorities, statutory agencies and other stakeholders in the coastal zone. The guidelines have been drafted in a generic way to be appropriate throughout the European Union taking into regard current European Commission policy and directives to promote sustainable development and integrated coastal zone management.
COBRAS provides near field kinematics over and inside LCS and will be used to assess velocities over the surface of the stones and to calibrate wave propagation models. The model provides and excellent tool to design coastal defence structures. It solves the two-dimensional Reynolds Average Navier Stokes equations on a finite difference scheme tracking the free surface using a volume of fluid approach. Therefore, it is able to overcome most of the limitations of most of the available numerical models for water wave and structure interaction. It is able to simulate the two dimensional interaction of transient non-linear waves with any kind of structure including porous media flow and turbulence outside and inside the structure. The porous media flow is simulated using the Volume Average Reynolds Average Navier Stokes equations. The model can be used to evaluate velocities, pressure, forces, turbulent intensity, and free surface evolution, wave breaking, etc. when waves interact with structures. The model is able to consider any kind of geometry and number of layers and therefore it can be used not only for low crested structures but also for conventional rubble mound breakwaters, vertical structures or composite breakwaters. Furthermore, other potential applications for the model are the design of non-conventional structures such as Jarlan type structures; vertical barriers, etc. for which the existing design methodology is very limited. Therefore, it can be said that the model can be used by the coastal and harbour engineering community and more specifically by consultants and administrations involved in the design, construction and conservation of coastal defence structures to provide a more reliable design. There is a very limited number of such kinds of models in the world due to the ability of the model to simulate most of the 2-D processes involved in the interaction of wave and structure interaction. The model works as a numerical wave flume and therefore is a perfect tool to complement other traditional techniques to design structures such as empirical formulations or numerical models. At its current stage it requires some additional work to validate further applications since most of the validation has been carried out for LCS and to provide a user's friendly interface to make it available for engineering consultants and public administrations. After this additional work has been completed the model is expected to have many different commercial success factors.

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