1

3.2 Innovative Submerged structures

Javier L. Lara, Inigo J. Losada, Rodolfo Silva, Edgar E. Mendoza, Shunqi Pan, Dave Simmonds, Maria Maza, Pablo Higuera

Abstract In this section we discuss the design and usage of various forms of submerged barrier structures. The aim of these is to afford a degree of protection to an exposed coastline through a reduction of the incident levels of wave energy. This is achieved through a manipulation of key hydrodynamic processes that creates a shielding of the coastline and often entails a corresponding modification of the sediment dynamics around these structures and also on the adjacent coast.

Thus we begin with a discussion of the key hydrodynamic and sediment dynamic processes around these coastal structures. We then outline a typology of current state of art structures and their relative merits. This is followed by a discussion of other design issues that include the need for evaluating their environmental impact with regard to the adjacent coastline, the choice of materials from which might be constructed and a consideration of the economics of these structures over their design lifetime.

Keywords Coastal defences, artificial reefs, submerged structures, V-shaped submerged breakwater, coastal protection

3.2.1. Introduction Among all the alternatives developed worldwide for coastal defence, those which have found best acceptance in recent years are the more environmentally friendly designs. Moreover, if the aesthetic impact is also considered (e.g. touristic industry), submerged structures, also known as reef breakwaters, seem to be a very profitable option regarding overall costs and construction time (Burcharth et al., 2007; Ranasinghe et al., 2010). The construction of this type of structure effects a reduction of onshore wave energy to landward. This may be desirable in order to prevent coastal erosion, to protect land and infrastructure from flooding, to protect from damage or remedy the poor design of existing coastal structures. They may also be used to increase the stability of beaches, possibly in combination with sand nourishments.

The use of submerged structures has been a common practice in the last two decades, especially in Europe, and more particularly along the Mediterranean coast. The term Low Crested Structures (LCS) traditionally refers to rubble-mound breakwaters whose crown elevation is near the still water level. LCS are usually constructed as “detached” structures, i.e. detached from the shoreline, and are aligned approximately parallel to the local shoreline in sites where partial protection from wave attack is required (Burcharth et al., 2007). These structures may be submerged, emerged, or both alternately, and are characterised by frequent and significant levels of wave overtopping. The main aim of LCS is to reduce the wave loads on the coast through a series of wave transformation processes occurring at, on or around the structure: namely wave reflection and energy dissipation due to both wave breaking on the crest and flows inside the breakwater body.

In the last decade, an interesting trend has been to explore the use of more innovative submerged structures that employ either natural features of the coastal zone, or some artificial representation of them. Examples of the features that might be considered suitable for their coastal protection function

2

include seagrass and oyster beds. The goal of these structures is the same as the LCS, but they are designed to reduce the environmental and economic impact at any specific location. These structures may or may be not considered as permanent solutions to beach erosion problems.

Generally speaking, permanent solutions are the more expensive; in fact they are sometimes around ten times higher in cost than other solutions although they may offer similar coastal protection, i.e. Woolmer et al. (2011). Wave climate, sediment properties, social and economic aspects need to be taken into account when selecting a solution (Baine, 2001), e.g. permanent solutions may be considered as optimal in places where intense storms are expected, but if the estimated damage to the area and reconstruction costs are not very high, a cheaper solution may be the best option, as well as offering a more ecological service.

This section will consider the use of innovative coastal structures as effective alternatives to the more traditional approaches employed in coastal protection, and is organized as follows. First, a short description of the more relevant physical processes when waves meet submerged structures is presented. Then, guidance is provided about design, cost and the morphological changes induced by these structures.

3.2.2. Physical Processes Submerged structures reduce the incoming wave energy across the structure by partially reflecting the waves at the toe, by triggering wave breaking at and on the structure, and by dissipation related to the wave induced porous flow through the structure. The main objective is reducing wave height at the coast and the long-shore sediment transport. No significant differences can be found in the use of innovative submerged structures compared with traditional LCS in terms of wave transformation and wave induced hydrodynamics at the coastline, although some processes are enhanced, mainly with the aim of increasing wave damping. The degree of protection generated by both LCS and SS depend on their geometries and maritime climate conditions, Pilarczyk (2003).

3.2.2.1 Wave hydrodynamics When the incident wave train impinges on a submerged structure, part of the energy is reflected back to the sea and part is transmitted in the leeside zone, Silva et al. (2002). Reflection is an important characteristic of submerged structures as this type of structure is generally narrow and built with relatively steep slopes, Losada et al. (1996a). The interferences between the incident and reflected waves give rise to standing wave patterns that affect the near-field flow and subsequently the stability of the structure, Lara et al. (2006).

Besides, as waves shoal on the rising front of the structure, significant nonlinear effects occur, resulting in the amplification of bound waves (phase-locked with the primary wave), which may result in an amplification of the surf beat at the beach. Most of the incident wave energy is lost on the crest of the structure, essentially by breaking, Gourlay (1996).

Part of the energy is also dissipated by air entrainment and friction at the solid skeleton interface and within the gaps created between the individual rocks, for rocky traditional LCS, or within the artificial holes created for that purpose, for ecologically compatible elements. For non-breaking waves, the flow resistance in the porous media is the main dissipation mechanism.

Significant nonlinear interactions occur in the zone of the structure crest between the various wave phases and some energy is transferred from the fundamental wave frequency to higher harmonics (Losada et al., 1996a,b). In the deeper waters of the leeside zone, the wave field is characterized by waves of complex

3

form, with lower height and lower mean period. In addition, wave interaction with submerged structures also gives rise to a series of three-dimensional phenomena, such as diffraction, 3D wave breaking and currents system generation, i.e. de Alegría-Arzaburu (2013) and Silva et al. (2013).

3.2.2.2 Currents and tide effects (water level) Submerged structures with a small freeboard typically induce wave breaking on their crest or at their off-shore or inshore slope leading to the generation of new current patterns (Johnson et al., 2005).

In the case of detached breakwater systems, or potentially any series of structures that provide shelter, wave breaking and transmission can drive current circulation in the shadows of the structures, and also induce fast flowing currents around the breakwater heads (ends) where there is potential for significant scour and a need for careful design (Johnson et al., 2005, Villani et al., 2012),.

Tidal setting and sea level rise, in different time scales, are also important when considering the design and performance of structures near the coastline. Tides and other water level variations determine the depth of water over or around a structure, thus influencing how strongly surface water waves and currents interact with the structure (Dean et al., 1997).

3.2.2.3 Morphological changes

It is generally understood that waves stir up sediment and currents transport this mobilised sediment. Thus, the deployment of a coastal structure or some other form of barrier will not only change existing currents and regions of wave breaking, it will also affect the sediment pathways and the areas of erosion and deposition, (Stauble, 2008). Where a barrier produces a wave shadow, the wave height is lower and deposition is likely to occur (Fairley et al., 2009). Waves can diffract around such barriers, approaching the shoreline obliquely, where they break and push sediment into the lee. Refraction over the deposited sediment in the lee reinforces this pattern of deposition.

Thus a barrier of finite extent will impact upon the shoreline, by initiating the growth of features referred to as a salient, double salients or eventually a tidal tombolo (Ranasinghe, 2010). These are tongues of sand that grow from the coast in the wave shadow, out towards the lee of the barrier. Tombolos, which resemble a permanent isthmus of sediment, will interrupt any littoral currents. This can result in up-drift accretion and down-drift starvation of sediment, which may be undesirable in certain locations.

Careful design guidance is needed to assist in the planning of any each type of structure or barrier. In the UK, guidance for the placement of submerged and surface piercing breakwaters in tidal sandy environments is available (Environment Agency, 2010). Complementary information can be found also in the Coastal Engineering Manual, U.S. Army Corps of Engineers, (2002) and the Japanese artificial reef design, Ministry of Construction (1992).

3.2.2.4 Additional issues Local scour is induced by the interaction of the wave and the elements that constitute the structure, particularly on the seaward side, but it may also be important in the leeside for plunging breakers (Sumer et al., 2005). One way to avoid this is by the installation of a geotextile carpet, with a toe of rocks on the seaward side.

4

Submerged coastal structures can induce downdrift erosion and updrift accretion on coastlines with a net-direction of sediment transport, but to a lesser degree than with surface piercing structures with crowns above the mean sea level

Most submerged structures give their best performance when placed over sand, where they can be partially buried and friction can enhance their stability. If they are placed over rock they can eventually be displaced by waves.

Land subsidence can cause structure settlement and therefore decrease the degree of coastal protection offered, similar to the effect of sea level rise. The expected sea level rise should be considered for structures with a useful life of 25 years or more. Indeed in the UK, for example, new schemes are appraised in terms of their performance and maintenance with consideration of the effects of subsidence and climate change over the next 100 years.

Submerged breakwaters may be effective tsunami control structures, Silva et al. (2000), thanks to the turbulence generated in the porous medium. Impermeable structures induce less energy dissipation and eventually the waves only shoal.

3.2.3. Examples of Submerged Structures Traditional hard engineered structures might comprise water surface-piercing, low crested structures, submerged reef structures and other forms of breakwater. "Softer" technologies that can also be considered are the use of submerged geotextile structures, or the installation of artificial dissipative blocks on the seabed to increase wave damping and to promote new habitats (Sundar and Sannasiraj, 2013). This section focuses on the hydrodynamic and morphological changes induced by LCS and rocky reefs, geo-bags and artificial reefs made of natural rock or geometric concrete units. Specific issues related to design and costs are pointed out for applicability and performance purposes.

Despite the extended use of submerged structures, certain hydraulic conditions must be satisfied for a submerged structure to operate adequately. Firstly, the protected beach must be capable of dealing with the transmitted waves. Because of the submerged nature of the barrier an important percentage of the waves will pass beyond the structure, which means that some agitation will be present. So, if the area intended to be protected is vulnerable even to these small waves, a submerged structure is not an option. Furthermore, the geometry and location of the structure must produce wave breaking for the mean wave regime and for a significant number of storms. If too many waves pass over the structure without breaking, the energy dissipation is nullified and the beach response may be even worse than it would be without such a structure. Another obvious factor to consider is the danger to navigation that submerged structures will cause.

3.2.3.1 Low crested structures and rocky reefs There are several reasons why LCS and rocky reefs are chosen for beach protection. Most importantly, their reduced visual intrusion makes them valuable, particularly in zones with important aesthetic constraints. Where economic constraints are foremost, low level structures, especially submerged ones, are generally more stable, and cheaper in that they use less material (d'Angremond, et al., 1996). Similarly, a further economic benefit is the possibility of rehabilitating other, existing structures just by reducing the incident wave conditions with a low-crested breakwater. Finally, due to the high level of transmission, LCS induce in-shore water renovation, which is desirable, particularly for recreational waters, as well as improving water oxygenation, valuable for animals and plants living in the protected area leeward of the breakwaters.

5

Burcharth et al. (2007) presented very comprehensive design guidelines for standard LCS providing methodological tools for the engineering design and for the prediction of performance and environmental impacts of this kind of structure, pointing out the importance in the selection of the material to be used, as well as the selection of the shape and location of the LCS.

Recently, V-shaped breakwaters have been suggested as an option, (Briggs, 2001). Geometrically similar to fishtail breakwaters, Figure 3.2.1, they protect the beach from a wider range of incident waves than parallel ones. Whilst, their performance on wave transformation and their impact on beach morphology are yet to be fully understood, a consideration of their performance can be modelled using an appropriate process-based modelling approach (see 3.2.4.1).

Figure 3.2.1. Beach profile (central section) and location of V-shaped submerged breakwater (insert: breakwater form).

3.2.3.2. Geobags Submerged or semi-submerged sand or gravel filled geotextile bags (geobags) are a semi-rigid structures that represent a reversible option that can be implemented in few weeks (Figure 3.2.2). The main advantage of these structures is that they are relatively inexpensive to install. Geobags have become increasingly popular as an alternative to conventional hard structures, especially in those situations where rapid implementation of stabilization measures are required.

The geotextile must be able to withstand the extreme environmental conditions where it is installed, including marine salinity, temperature fluctuations, and exposure to sunlight. The dimensions of the structure required influence the size of the geobags used. As a rule of thumb, the length of each geobag unit should be 5 to 25 times its height, and the larger the bag, the greater the mechanical stability offered against failures due to mechanisms such as sliding, overturning, etc. However if a large unit breaks, the area of coast left unprotected will be larger. The degree of wave energy dissipation is controlled by the relative crown width; the greater the width, the greater the degree of protection.

The hydrodynamic effects (e.g. reflection, transmission, diffraction, shoaling, breaking, etc.) induced by structures made of geobags are very similar to those induced by impermeable structures made of concrete. Although the geobags can reflect a large amount of incident wave energy, where the relative freeboard is

Submerged Breakwater

6

not large enough, waves are steepened and the breaking process is more intense than without the structure and therefore sediment transport is increased. For this reason it is recommended that the maximum freeboard must not exceed half of the annual mean significant wave height at the mean sea water level, see Silva et al. (2012).

Normally, scour protection must be considered at the toe of the structure on the seaward side to prevent the geobags sliding seaward as a result of the increment of wave energy from the reflection effect. With the presence of sand bars, it is advisable to locate these structures seaward of the bars (Short, 1992).

Local, natural (native) material is recommended for filling the geobags to reduce costs and contamination hazards in case of a break in the bags. The filling process must be completed in phases to prevent breakage of the geotextile and allow water drainage. When very fine material (clay and/or silt) is used as a filling, the weave of the geotextiles is a key parameter that should be selected to minimise the chance of leakage.

FIGURE 3.2.2. SUBMERGED GEOBAG STRUCTURE FOR TEMPORARY SHORE PROTECTION IN THE CARIBBEAN SEA

3.2.3.3 Artificial reefs This relatively new approach involves the emplacement of submerged permeable structures parallel to the coast and crowned at the mean sea level, to provide wave dissipation, mimicking the effects of a natural reef. These can be constructed from natural rock or from artificial f concrete units with specific functional geometries e.g. WAD© – Wave Attenuation Device – units or Reef Balls©. When designing these artificial reefs the following aspects must be addressed. Firstly, in storm conditions with associated storm surge, the effectiveness of an artificial reef decreases considerably. Stability (sliding and overturning), settlement and scour should be carefully considered during the design phase. Occasionally, sliding or overturning of the units occurs due to intense wave and current effects or settling deficiencies – in this case the units can strike one another and will eventually break. Therefore, to improve the stability of the pieces, the possibility of adding more weight to the bases of the pieces should be considered where local conditions call for this. For temporary deployment these protective structures have the advantage that they can be relocated with little technical effort. Where natural rocks are not used, special geometrical structural elements can be manufactured nearby and installed on site using a small barge crane Examples of these include WADs© (http://coastlinesolution.com/), ERCON (http://www.uni-due.de/nomatec/themen_ercon_inhalt_en.html), Florida Special-Artificial Reef (http://floridasportfishing.com/magazine/nautical-art-jewelry/reefmaker-artificial-reefs) and Reef Balls© (http://www.reefball.org/index.html). However, extra care is necessary during installation, as some designs, though effective when deployed, are susceptible to damage if

7

carelessly handled. Thus construction with these elements is straightforward provided that operations are carried out in calm conditions (see for example Figure 3.2.3).

FIGURE 3.2.3. AN EXAMPLE OF AN ARTIFICIAL REEF MADE OF WADS© IN THE CARIBBEAN SEA.

3.2.4 Further Design Considerations 3.2.4.1. Impact of structures on coastline In order to predict the effect a structure may have upon the adjacent coastline, the effective modification to the hydrodynamic and sediment dynamic processes needs to be considered. This is often achieved through numerical or physical modelling approaches, through which designs can be appraised or optimised

8

to achieve the desired level of protection and an acceptable level of morphodynamic adjustment. For instance, the design of a scheme of v-shaped breakwaters on an open tidal coast has been considered by Pan, et al. (2013). This required the use of a numerical model that incorporated the relevant processes – wave transformation, refraction, diffraction, wave breaking and the resulting current generation and sediment transport formulae for the sediment size in addition to a tidal modulation of the water level. Both sea-facing (VS) and land-facing (VL) configurations of the breakwaters were considered (Figures 3.2.4a,b). The geometry of the breakwater designs in these simulations is as follows: the arm of the v-shaped breakwater is about 100 m long and both ends are 50 m from its baseline, so that the angle between the two arms is 120 degrees. The breakwaters are sitting on a plane beach with a 1:20 bed slope, with breakwaters were located at approximately 10 m water depth, i.e. approximately 200 m from the shoreline at the mean tidal level, and 200 m apart from each other.

The results presented in figure 3.2.4c show the effect of the v-shaped breakwaters in comparison to the typical impact of constructing shore parallel segmented breakwaters. That is, the coastline responds with the formation of salient features behind the breakwaters, generated by the altered breaking patterns and currents in the lee of the structures and the ensuing pattern of sediment deposition.

(a) VS (b) VL (c) Shoreline positions

Figure 3.2.4. Computational domain and V-shaped breakwaters (a) Sea-facing (VS); (b) Land-facing (VL); (c) Shoreline position after 120 hours for VS (dotted), VL (dashed) and shore parallel (solid) breakwaters

This modelling has been used to show that the use of v-shaped breakwaters has a similar protective effect to that of shore parallel segmented breakwaters in a tidal setting but may be more cost effective in terms

Tides

Waves

50 m

173.2 m

9

of the amount of material required in their construction and in terms of the wider range of angles of wave attack for which they are effective.

3.2.4.2. Influence of choice of armour unit designs. Having decided upon the design of a submerged structure, the materials and choice of armour units from which the structure form is assembled needs careful consideration. This may be driven by the economics of the situation or may be informed by physical testing of candidate structures. An example of such tests comes from a programme of tests carried out at National University of Mexico (UNAM) by Mendoza, et al. (2011). These evaluated the efficiency of a submerged dyke design (Figure 3.2.5), constructed from either rock, concrete cubes, WADs© or geobags (Figure 3.2.6).

FIGURE 3.2.5. GENERAL SUBMERGED STRUCTURE CROSS SECTION.

FIGURE 3.2.6. SUBMERGED STRUCTURES TESTED (FROM LEFT TO RIGHT CUBES, GEOBAGS, WADS AND ROCKS).

The submerged structure was placed in front of an eroded beach profile (i.e. a profile that had been allowed to freely distort by a storm or similar event). The laboratory work included testing a non-protected profile which served as a control. A small group of significant wave heights and peak periods were selected to test the structures, that is 10 and 15 cm and 1 and 2 s (i.e. mean and storm wave conditions). All the tests were carried out using JONSWAP spectra with γ= 3.3. The dyke position was set to have the shallowest freeboard possible while still being considered as submerged, all in front of the same initial profile. A more detailed description of the experimental set-up and characteristics can be found in Mendoza, et al. (2011).

For the purpose of comparison, the ratio of the volume of sand removed, Volrem, above the still water level was compared to the initial volume of sand, Volini, above the still water level in the non-protected case and Volrem/Volini, was considered for each type of structure. The removed volume of sand above the still water level was evaluated as the difference between total volumes of sand before and after the tests.

From the experimental data, the equation obtained to estimate the ratio Volrem/Volini, is:

rem

ini

Vol Ha bVol L

= + (3.2.3)

where L is the local wave length, H the wave height, hs the height of the structure and h the still water level and the coefficients a and b can be computed from Table 3.2.1. Figure 3.2.7 shows the values of Volrem/Volini for the mean wave climate and for storm conditions.

10

Table 3.2.1 Values of the parameters a and b for different cases

No structure WADs© Cubes Rocks Geobags

a 1180.3 1217.9hsh

− +

3105.7 3060.8hsh

− +

1021.8 1050.3hsh

− +

1677.3 1646.6hsh

− +

1644.6 1633.1hsh

− +

b 27.7 27.9hsh−

192.9 187.3hs

h−

68.6 65.1hs

h− +

52.6 54hs

h−

57.1 65.3hs

h−

FIGURE 3.2.7. RELATIVE VOLUMES REMOVED FROM THE BEACH PROFILE VOLREM/VOLINI. A) FOR MEAN WAVE CLIMATE AND B)

FOR STORM CONDITIONS.

From Figure 3.2.7 it can be seen that in absence of structures, the beach profile continues to erode. An impermeable structure is efficient only for small wave lengths, i.e. under breaking wave conditions. The structure with highest porosity, the WAD©, provided the best performance, proving the relevance of dissipation due to interstitial friction. The structure made of rounded rocks, presented reduced friction which results in poor performance; it is thus recommended to use more angular shaped units. The structure constructed of conventional concrete cubes gives better beach protection than a submerged breakwater built of rocks or geobags.

3.2.4.3 Cost and lifetime When appraising competing coastal protection scheme designs, the lifetime cost of schemes, including construction and maintenance is often the main factor in addition to issues of buildability, and impact on environment and stakeholders. For instance, geobags are an important alternative approach to the construction of hard defence measures and particularly attractive due to their lower cost. In general terms, the cost of a structure made of geobags involves engineering design, materials, conveying or pumping of the filling material into the bags, installation and maintenance. As a reference, the total cost for 100 m of Geocylinder© of 4.5m width and 3.5m height is around €100,000 to €150,000. Depending on the

11

properties of the geotextile and the environmental conditions, geobags have a lifetime of between 3 and 5 years without maintenance, which could be extended to 30-50 years, with the replacement of damaged sections. It should be noted, however, that geobags are more vulnerable to vandalism and accidental damage than “harder” solutions.

The cost of artificial reefs involves engineering design, manufacturing, installation, monitoring and payment of patent rights. As an example, a submerged reef 100 m long, formed by two rows of WADs© of 2 m height and 1 m width, with each WAD© weighing 3.4 tons would cost about €100,000. Other types of elements, such as the reef ball© have similar costs to the WAD© elements. The lifetime of artificial reefs depends greatly on its performance in the first few years, but it may be up to 50 years, depending on the quality of the concrete (Metha, 2001). With time, as they accumulate biomass, they are more stable, given that they gain weight and settle better.

3.2.5 Conclusions Submerged structures and LCS are increasingly used to protect coastlines around the world given their aesthetic value and cost. LCS and submerged structures offer a valuable, increasingly popular solution at locations where complete protection from waves is not required and a moderate degree of energy transmission is even desirable. They can be an efficient protection for beach stability when used in combination with nourishment interventions and they are also used to protect harbour entrances and reduce siltation in entrance channels. The structural design of LCS and other, more innovative types of breakwaters should take various factors into consideration. The structure freeboard must be selected according to the amount of protection required and the tidal range. Then, the fact that the smallest wave heights have the highest transmission coefficients must be considered, remembering that higher wave heights have smaller transmission coefficients. Under extreme meteorological conditions it is possible that the effect of storm surge affects the functioning of the breakwater. These structures are usually considered as lying parallel to the coast, though the distance from the shore is relative to the currents generated by the waves. The material chosen for the construction of the breakwater also influences its degree of success. Cost factors and visual impact as well as environmental issues must all be weighed in selecting an appropriate type of structure at any given location.

References Baine, M. (2001) Artificial reefs: a review of their design, application, management and performance. Ocean and Coastal Management, 44 (2001), pp. 241–259

Briggs, M. J. (2001). Performance characteristics of a rapidly installed breakwater system (No. ERDC/CHL-TR-01-13). Engineer Research and Development Center Vicksburg Ms Coastal And Hydraulicslab.

Burcharth, H., Hawkins, S., Zanuttigh, B. and Lamberti, A. (2007). Environmental Design Guidelines for Low Crested Coastal Structures. Elsevier. The Netherlands. 400 pp.

d'Angremond, K., Van Der Meer, J. W., & De Jong, R. J. (1996). Wave transmission at low-crested structures. Coastal Engineering Proceedings,1(25).

de Alegría-Arzaburu, A. R., Mariño-Tapia, I., Enriquez, C., Silva, R., & González-Leija, M. (2013). The Role of Fringing Coral Reefs on Beach Morphodynamics. Geomorphology.

Dean, R. G., Chen, R., & Browder, A. E. (1997). Full scale monitoring study of a submerged breakwater, Palm Beach, Florida, USA. Coastal Engineering,29(3), 291-315.

12

Environment Agency 2010, Guidance for outline design of nearshore detached breakwaters on sandy macro-tidal coasts. Project: SC060026/R1. Environment Agency report SCHO0210BRYO-E-P. ISBN: 978-1-84911-178-2

Fairley, I., Davidson, M., & Kingston, K. (2009). The morpho-dynamics of a beach protected by detached breakwaters in a high energy tidal environment. Journal of Coastal Research, 56, 607-611.

Gourlay, M. R. (1996). Wave set-up on coral reefs. 1. Set-up and wave-generated flow on an idealised two dimensional horizontal reef. Coastal Engineering, 27(3), 161-193.

Johnson, H. K., Karambas, T. V., Avgeris, I., Zanuttigh, B., Gonzalez-Marco, D., & Caceres, I. (2005). Modelling of waves and currents around submerged breakwaters. Coastal Engineering, 52(10), 949-969.

Lara, J. L., Garcia, N., & Losada, I. J. (2006). RANS modelling applied to random wave interaction with submerged permeable structures. Coastal Engineering, 53(5), 395-417.

Losada, I.J., Silva, R., Losada, M.A., 1996a. 3-D non-breaking regular wave interaction with submerged breakwaters. Coastal Engineering 28, 229–248.

Losada, I.J., Silva, R., Losada, M.A., 1996b. Interaction of non- breaking directional random waves with submerged break- waters. Coastal Engineering 28, 249–266.

Mehta, P. K. (2001). Reducing the environmental impact of concrete. Concrete international, 23(10), 61-66.

Mendoza, E., Silva, R., Enriquez-Ortiz, C., Díaz-Hernández, G. and Lara, J.L. (2011). An Experimental Evaluation of Wave Energy dissipation Due to Submerged Structures. In Coastal Structures 2011. Japan.

Ministry of Construction. (1992). Artificial reef design manual , Coastal Branch, River Division, Tokyo, Japan (in Japanese).

Pan, S., Horrillo-Caraballo, J. M., Reeve, D. E., & Simmonds, D. (2013). Morphological Modelling of V-Shaped Submerged Breakwaters. Coasts, Marine Structures and Breakwaters. ICE, Edinburgh, UK.

Pilarczyk, K. W. (2003). Design of low-crested (submerged) structures–an overview. In 6th International Conference on Coastal and Port Engineering in Developing Countries, Colombo, Sri Lanka.

Ranasinghe, R., Larson, M. and Savioli, J. (2010). Shoreline Response to a Single Shore-parallel Submerged Breakwater. Coastal Engineering. Vol. 57, pp. 1006-1017.

Short, A. D. (1992). Beach systems of the central Netherlands coast: processes, morphology and structural impacts in a storm driven multi-bar system. Marine Geology, 107(1), 103-137.

Silva, R., Losada, I. J., & Losada, M. A. (2000). Reflection and transmission of tsunami waves by coastal structures. Applied Ocean Research, 22(4), 215-223.

Silva, R., Mendoza, E., & Losada, M. A. (2006). Modelling linear wave transformation induced by dissipative structures—Regular waves. Ocean Engineering, 33(16), 2150-2173.

Silva, R., Mendoza, E., Chávez, X. (2012). Diseño de prototipo de rompeolas sumergido para disipación de oleaje y recuperación de playas, mediante el modelado en laboratorio. Technical Report elaborated for AXIS Ingeniería, Mexico. 46 pp. (In Spanish).

13

Silva, R., Salles, P., & Palacio, A. (2002). Linear waves propagating over a rapidly varying finite porous bed. Coastal Engineering, 44(3), 239-260.

Stauble, D. K. (2008). Coastal Engineering Project Impact Assessment Using Long-Term Morphodynamic Change Analysis. In FSBPA Conference Proceedings. http://www. fsbpa. com/08Proceedings/02Stauble2008. pdf.

Sumer, B. M., Fredsøe, J., Lamberti, A., Zanuttigh, B., Dixen, M., Gislason, K., & Di Penta, A. F. (2005). Local scour at roundhead and along the trunk of low crested structures. Coastal engineering, 52(10), 995-1025.

Sundar, V., & Sannasiraj, S. A. (2013). Artificial Reefs: A Review. The International Journal of Ocean and Climate Systems, 4(2), 117-124.

U.S. Army Corps of Engineers. 2002. Coastal Engineering Manual. Engineer Manual 1110-2-1100, U.S. Army Corps of Engineers, Washington, D.C. (in 6 volumes).

Villani, M., Bosboom, J., Zijlema, M., & Stive, M. (2012). Circulation patterns and shoreline response induced by submerged breakwaters. Coastal Engineering Proceedings, 1(33), structures.25. doi:10.9753/icce.v33.structures.25

Woolmer, A.P., Syvret, M. & FitzGerald A., 2011. Restoration of Native Oyster, Ostrea edulis, in South Wales: Options and Approaches. CCW Contract Science Report No: 960, 93 pp.