006, Proceedings of the First International Conference on the Application of Physical Modelling to Port and Coastal Protection. ISBN xxx-xxxx-xx-x ARTIFICIAL SURFING REEFS: THE PREPARATION OF PHYSICAL TESTS AND THE THEORY BEHIND MECHTELD TEN VOORDE (), JOSÉ SIMÃO ANTUNES DO CARMO (), MARIA DA GRAÇA NEVES (3) () Ph.D. student, LNEC, Av. do Brasil, 0, 700-066 Lisboa, Portugal. mvoorde@lnec.pt () Associate Professor, University of Coimbra, Faculty of science and technology, Department of Civil Engineering, 3000 Coimbra, Portugal. jsacarmo@dec.uc.pt (3) Research officer, LNEC, Av. do Brasil, 0, 700-066 Lisboa, Portugal. gneves@lnec.pt Abstract An artificial surfing reef is a submerged structure that is for several reasons interesting to use as a way to protect the coast. Experiments with such a structure are planned to be done in a wave basin. Based on theory an optimal reef design for the experiments is chosen. The test conditions for the experiments are determined by a case study that is planned to be executed for a small city at the west coast of Portugal.. Introduction Nowadays a large part of the population of the earth lives in the vicinity of the coastlines. The beach system at a coastline is not a static system, but a natural system. It is considered to be in dynamic equilibrium. This means that, without human interventions, sand is moved from one location to another but it does not leave the system. For example, winter storms may remove significant amounts of sand, creating steep, narrow beaches. In the summer, gentle waves return the sand, widening beaches and creating gentle slopes. Sometimes however equilibrium does not exist, causing structural erosion, for example due to a gradient in waves along the coastline. Besides natural unbalance, the unbalance can also be caused by humans who are transforming coastlines everywhere around the world. Some of the ongoing coastal modifications include building structures that regulate water and sediments (dams, seawalls, and jetties), ground water extraction leading to subsidence, and sea level rise induced by global warming. The increasing populations and development in coastal zones drive many of these changes. The consequences of such human behavior are evident all around the world. Sea-level rise for example continues to affect so many coasts, slowly flooding low-lying areas. In order to stop, or reduce, the erosion of the often occupied coasts, coastal protection is an important issue. There are several methods to protect a coast from seasonal and/or structural erosion: Breakwaters are long piers built offshore parallel to the shoreline, which are designed to provide calm anchorages in an area behind them called a wave shadow. They do that by
reducing the wave energy. Groins are lines of rock or pilings constructed perpendicular to the shoreline. They are designed to act as a partial barrier to littoral drift, trapping sand on the updrift side and causing erosion on the downdrift side. They don t reduce the wave energy like breakwaters do. Jetties are like groins lines of a certain material constructed perpendicular to the shoreline. They are often much larger than groins and there are often just one or two of them at river mouths and harbor entrances. They are designed to stabilize navigation channels at tidal inlet, river mouths and harbor entrances. Seawalls/Revetments are walls (vertical or under an angle) of a hard material to protect the area behind it. Mostly these structures are not well accepted by the public from an esthetical point of view. Artificial dunes are artificial dunes that have been built by bulldozing sand back from the beach or by placing snow fences to trap windblown sand. They are designed to maintain the dunes system in a certain area. Sand supply is the supply of sand on the beach itself by pumping sand directly onto the beach from interior or offshore zones. Sand supply is carried out in order to maintain the beach in a certain area. A relatively new way of protecting a coast is artificial surfing reefs. This are submerged breakwaters that have several goals. These goals are:. Coastal protection: Waves break over the reef by which a calm area is created behind it by the dissipation of wave energy.. Creating surfable waves: By a certain design of the submerged reef waves can break in such a way over it that surfers can make rides in this breaking waves. 3. Enlargement of environmental value: It has appeared from the Narrowneck reef in Australia that if the reef is built of geotextile sand containers the enlargement of environmental value is very big. Many new habitats have been grown on the reef. Artificial surfing reefs are a very attractive way of protecting the coast firstly because it is a hard measure to protect a coast with a relatively small visual impact. Secondly because it is in economical terms interesting to build. At the one hand have geotextile sand containers the tendency to be cheaper per unit of volume than rubble-mound structures and on the other hand can the surfing aspect attract the tourism, which is good for the local economy. Thirdly because the enlargement of the environmental value is a great benefit in these times, in which more and more nature is destroyed by the behavior of humans. There has been some research towards artificial surfing reefs. However, there has almost no research been done to the effect of oblique waves on the hydrodynamics around an artificial surfing reef. In order to fill this gap 3D physical test are planned to be done. The goals of these experiments are: Get a better understanding of the processes involved; Calibrate a numerical model; Obtain a new set of data for artificial surfing reefs. This paper will describe the preparation of those tests. Both test conditions and the geometry which will be used in the tests will be treated. The theory behind the artificial surfing reefs how to get surfable waves and what design qualities are theoretically determined is quite poor described in scientific papers. Because of that the theory behind artificial surfing reef regarding to surfability will also be described, since this determined the chosen geometry.
. Preparation of physical model tests The preparation of physical model tests will be treated in this paper. Different aspects had to be decided: Test conditions; Geometry of the reef in the basin; Position of the reef regarding to the shoreline.. Test conditions In the future a case study is planned to do for the coast of Leirosa, a small city at the west coast of Portugal. Based on the tide and wave regime of Leirosa s coast, the following conditions are chosen:. A tidal range of 3 meter. Irregular waves with a directional spread and with a: A significant wave height Hs varying from to 4 meter; A peak period Tp varying from 8 to 6 seconds; A wave angle varying from 0 to 30 degrees.. Geometry of reef in the basin In order to define the geometry of the reef to be set in the physical model, it s necessary to take into consideration what makes a wave surfable. Two things are important for the surfability of a wave: The peel angle; The breakertype (shape) of the wave... Peel angle The peel angle is the angle enclosed by the wave crest and the breakerline (fig. ). This is equal to the angle of the wave ray and the depth contours at the breakerline. Bar peel angle depth contours = breaking wave Figure. Peel angle wave direction Generally waves with a peel angle between 30 (fast ride) and 60 degrees are considered to be surfable (Couriel and Cox, 996). However, based on extensive field studies in New Zealand, Black et al. (997) noted that a peel angle less than 50 degrees would be a difficult 3
proposition for most surfers. Nevertheless, the peel angle can have a value of less then 50 degrees in certain parts (with a relatively small length) along the ride. However, the peel angle appears to have a maximum (Henriquez (004)). Figure shows that when the angle on deep water increases the angle at the breakerline increases, but just until a value of 66 degrees on deep water. After this value the angle at the breakerline decreases. In figure the peel angle is given when the reef starts at a waterdepth of 0 meter for a wave height on deep water of.8 meter and a period of 6 seconds. From the figure it can be seen that the peel angle at its maximum is not even 30 degrees. Figure. Peel angle as a function of the wave angle on deep water when the reef would start at a water depth of 0 m. Figure 3 shows what is the physical meaning of the fact that the peel angle has a maximum (at 66 degrees). The left values are the wave angles on deep water. The second column of values are the values of the wave angles at the line of the breakerline for the angle on deep water of 50 degrees (same wave conditions). From this can be seen that the wave angle grows at the same waterdepth when the wave angle on deep water grows (law of Snellius). The values in the third column are the values of the wave angle at the breakerline for the different angles on deep water. 50 66 9.63 9.63 0.49.50.4 9.5 Breakerline for phi0=50 Breakerline for phi0=66 BEACH Breakerline for phi0=80 80 Figure 3. Physical meaning of maximum peel angle of 66 degrees. 4
It is obvious from figure 3 that the bigger the wave angle on deep water becomes, the further the wave can travel towards the coastline before it breaks. In this way the wave can refract relatively longer. Apparently (fig. ) by this the angle at the breakerline becomes smaller when the wave angle on deep water is larger than 66 degrees. The theoretical explanation of the fact that the wave can travel longer to the coast when the angle on deep water grow is as follows: The lowering of the wave height (for the same water depth) when the angle on deep water grows is caused by both refraction and shoaling. The variation in the wave height for different water depths is given by Eq. []. H = H * Ks * Kr [] cg cosθ where Ks = is the shoaling factor and Kr = is the refraction factor. The wave travels c cosθ g from the location of wave height H towards the location of wave height H. Figure 4 shows Ks*Kr when the wave travels from deep to shallow water for an angle on deep water, phi0, of 45 and 75 degrees for the same wave conditions as mentioned before. This values of the angle on deep water are chosen to show in the figure because they are both on another side of the maximum of 66 degrees. Figure 5 shows the consequence for the wave height of what is demonstrated in figure 4. phi0=45 phi0=75 phi0=45 phi0=75 Figure 4. Multiplied factor of the shoaling and refraction factor Figure 5. Wave height as a function of the waterdepth Until now there is written about the maximum peel angle when the angle on deep water, phi0, grows. But the same situation occurs when a wave perpendicular to the coast that travels over either a shelf or a slope bottom meets a reef with a growing angle α (fig. 6). So also for the situation with the reef, there is maximum peel angle when α is 66 degrees. As a consequence the first design decision for the geometry is based on the fact that the peel angle has a maximum. There is chosen for a delta-form reef like demonstrated in figure 6. In figure 7 the first design decision is shown. Oblique waves are not taken into account here, because they will make the situation worse for the surfability. Nevertheless, in the tests oblique waves will be tested to see the effect on the hydrodynamics, that have a direct relation with the protection of the coastline. 5
Phi0 Beach Depth contours Shelf α Depth contours REEF Beach Figure 6. Coastal zone without and with a reef 66 4 REEF Figure 7. Design decision of half nose angle of delta-form of 4 degrees Another important point is the water depth where the reef starts. It has appeared that when the reef starts at a smaller water depth, an exponential grow can be seen for the peel angle. This is shown in figure 8. All the lines in figure 8 are for the same wave conditions as mentioned before. C B A Exponential grow A= Reef starts at 6 m water depth B= Reef starts at 8 m water depth C= Reef starts at 0 m water depth Figure 8. Exponential grow in the peel angle The reason that the peel angle experiences an exponential grow when the reef starts in shallower water is that the factor in the rectangle in Eq. [] is smaller in relatively shallower 6
water (with the water depth change, h-h, independent of the water depth). The wave travels from the location of the water depth h towards the location of the water depth h. So in fact the peel angle experiences this exponential grow because the refraction is larger in relatively shallower water. sinθ tanh( kh) tanh( kh) = * sin θ [] As a consequence of the importance that the reef starts at a small water depth the second design decision is the choice for a platform. The form of the platform is chosen in such a way that the volume of the platform will be as low possible (fig. 9). Just when the platform is at a depth of.4 m. (and so the reef starts at that depth) a peel angle of 50 degrees is achieved for the critical Hs and Tp, namely Hs=m. and Tp=6 s. So a depth of the platform of.4 meter under the lowest water level will be chosen (fig. 9). For the slope of the platform a value of :6 (fig. 9) will be chosen, because it is big enough for numerical simulations and as low as possible to restrict the volume of the platform. 66 4 = 90-66 DELTA PLATFORM delta 6.4 m platform Submergence d Figure 9. Design decision of a platform.. Breakertype In order to be able to surf a wave it has to be plunging. Henriquez (004) found, based on experimental results with a wave height of m, that the inshore Iribarren number, ξb (see Eq. [3]), should be between 0.6 and 0.9. 7
α tan ξ b = H b [3] L0 In this equation tan α is the slope that the wave experiences (instead of the slope of the normal on the reef contours (fig. 0)), Hb is the wave height at the breakpoint and L0 is the wave length on deep water. Way of the wave Crest Normal on the reef contours Shelf Figure 0. Way of the wave compared to the normal on the depth contours In order to see with which slopes the waves to be tested will be plunging some calculations are made for different wave fields. Figure shows the inshore Iribarren numbers for a slope (that the wave experiences) varying from :6 to :8, a wave height at breakpoint varying from to 4 meters and a wave period of s. The values that fall in the surf range (given by Henriquez (004)) are given in table. As can be seen only part of the combinations of slope and wave height give a plunging wave in the surf range. It has to be said that the surf range of 0.6-0.9 m. is found in experiments for a wave height of m at the wave maker. Here it is applied to a range of the wave height of -4 m at the breakpoint..7.4. SURF RANGE.8.5. 0.9 0.6 irribaren number 0.3 0 30 5 0 5 slope 0 5 0 Figure. Surf range according to Henriquez (004) 8
Table. Iribarren number for different slopes and wave heights slope /x 0 4 4 6 8 Ho 4 3 4 3 ξb 0.76 0.89 0.73 0.63 0.76 0.6 0.67 0.84 Because of this results a slope of the normal on the delta contours of :0 will be chosen (fig. ). It is expected that the wave will experience then a slope between :0 and :8. delta 6 Submergence 0 platform Figure. Cross-section of reef.3 Position of the reef Ranasinghe et al. (006) studied the influence of the distance from a submerged structure from the shoreline. They found that when the distance from the coast grows, 4 cells of currents will be created (fig. 3b) instead of (fig. 3a, where apex structure is seaward point of deltacrest). As a consequence of the formed 4 cells there will be sedimentation at the coastline instead of erosion with formed cells. As the distance becomes too large the effect of the structure on morphodynamic processes adjacent to the shoreline will start to diminish. a) b) shoreline Figure 3. Distance apex structure-coast 00 m. (left) and distance apex structure coast 50 m. (right) (Ranasinghe et al, 006) 9
Those authors have proposed an empirical relationship as a preliminary engineering tool to assess shoreline response to submerged structures. Shoreline accretion can be expected as Sa/SZW is larger than.5 and erosion can be expected as this factor is smaller than, where Sa is the distance from the undisturbed shoreline till the apex of the delta and SZW is the natural surf zone width. Based on the results of Ranasinghe et al. (006) for the experiments to be done a distance of 50 m. in the prototype between the apex of the structure and the coastline is chosen. 3. Conclusions 3D experiments have been prepared for artificial surfing reefs. For this preparation a theoretical study has been executed to find out what is important in order to create surfable waves. It has appeared that two things are very important for surfing and that are the peel angle and the breakertype. There can be drawn two main conclusions from the theoretical study. The first one is that the peel angle has a maximum, which is caused by the fact that the wave height lowers at the same water depth when the wave angle on deep water grows. The second one is that the peel angle experiences an exponential grow when the reef starts in less deep water, which is caused by the fact that the refraction is relatively larger in shallower water. Based on main conclusion from the theoretical study the geometry of the reef was chosen. Based on a study done before the position of the reef with regard to the shoreline was chosen. The chosen wave conditions are based on the wave regime of the west coast of Portugal that will be subject of a case study. Acknowledgments The authors gratefully acknowledge the financial sponsorship of M.Sc. ten Voorde's Ph.D. research by Instituto de Investigação Interdisciplinar, Coimbra, Portugal. References Black, K.P., Andrews, C., Green, M., Gorman, R., Healy, T., Hume, T., Hutt, J., Mead, S. and Sayce, A., 997. Wave Dynamics and Shoreline Response on and around Surfing Reefs. Proc. st International Surfing Reef Symposium, Sydney, Australia, University of Sydney, pp. Couriel, E.D. and Cox, R.J., 996. International Literature Review-Artificial Surfing. Report No. 95/39, Australian Water and Coastal Studies Pty. Ltd. Henriquez, M., 004. Artificial Surf Reefs, Technical University of Delft, Faculty of Civil Engineering. Ranasinghe, R., Turner, I. L., Symonds, G., 006. Shoreline response to multi-functional artificial surfing reefs: A numerical and physical modeling study. Journal of Coastal Engineering, in press. 0
PAPER TITLE: ARTIFICIAL SURFING REEFS: THE PREPARATION OF PHYSICAL TESTS AND THE THEORY BEHIND AUTHORS: M. TEN VOORDE, J.S. ANTUNES DO CARMO, M.G. NEVES KEYWORDS CoastLab06 Physical modeling Artificial surfing reefs Peel angle