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Improvements in wave transmission reduction for beach restoration
By Alfonso Solís Pimentel, González Leija Mariana, Aguilar Escalante Sergio
Abstract
The north coast of the Yucatán Peninsula in Mexico has been in a regression process for decades. After Hurricane Gilbert (1988) and Isidore (2002), beach erosion increased dramatically.
Human actions around the development of harbors and the unregulated construction of groins have accelerated this process and a consequence of this has been that beaches, infrastructure, and private properties are in permanent risk of destruction. Since 2004, Axis Ingenieria has developed several engineering solutions based on the use of geosynthetic materials.
The construction of submerged breakwaters (SBW) built with geotextile tubes (GTT), engineered to mitigate incident wave energy over the beach, have had a successful performance for coastline stabilization and restoration of lost beaches. The key factor is controlling littoral drift along 60km of coastline and protecting critical eroded segments with SBW without affecting down-drift beaches.
The basic parameter to identify when designing a SBW is the wave transmission coefficient Kt as the ratio between the transmitted wave height reduced due to presence of the structure and the incident wave height (Kt=Ht/H). This article summarizes the efforts to evaluate this parameter specifically for the local conditions in Yucatán, based on physical modeling testing, that may be replicated for different beaches governed by similar geomorphological characteristics and wave climate conditions.
Introduction
The coast of the Yucatán Peninsula in Mexico consists of a fragile island barrier system—beach-dune-coastal lagoon (Figure 1)—and a coastal sea that has a wide and shallow continental shelf nearly monotonic 1:1000 slope (Enriquez et al., 2010). Oceanographic conditions have promoted the development of this system, with a dominant westward along-shore sediment transport.
This component of sediment transport encounters natural or anthropogenic barriers that interrupt the natural flux by generating alternate erosion-accretion zones along the coast. As in many other coastal regions, the beach erosion phenomenon has turned into a considerable problem for Yucatán’s northern coast and has increased in the last two decades, causing environmental alterations and compromising coastal infrastructure, a fact that has substantive repercussions for coastal development (Gonzalez & Pimentel, 2012).
Geotextile tubes as submerged breakwaters
GTT as SBW have been applied in Yucatán since 2004. Just in the area of Progresso that corresponds to the most affected beaches due to uncontrolled infrastructure development, around 4.5km of SBW have been installed along 40km of coast. The basic function of the submerged structure is to reduce wave energy in critical segments of fully eroded beaches so along-shore sediment transport is controlled. In a shore system such as that of the northern coast of Yucatán, where beaches are connected along more than 300km with the same littoral dynamics, it is mandatory to be able to handle wave energy reduction so alterations to littoral drift that govern the stability of the coastline is maintained (Alvarez et al., 2013).
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Design aspects
The coast of Yucatán is subject to low energy waves with heights of less than 1m, periods of 4–6 seconds, and a main incident direction coming from the E-NE, which generates westward along-shore sediment transport.
These conditions determine the design parameters and the installation conditions of GTT structures mainly as submerged detached breakwaters between 0.7m–1m depth or at –0.3m mean low water levels (MLWL) referred to MSL (mean sea level). This cross section has proven to be a solution for the proper accumulation of sand and an effective coastal protection measure for various stretches of coast along Yucatán (Gonzalez et al., 2012)—see Figures 2, 3, 4, 5.
Even though the experience of 10 years of installation of GTT as SBW has had positive results, the geosynthetic technology used as beach protection measures still presents limitations. There is a lack of proper design criteria in comparison with rock or concrete conventional structures (e.g., Bezuijen and Vastenburg, 2008; Pilarczyk, 2000). Some problems that have surfaced may be classified related to:
Functional design—The GTT as SBW produced an excessive reduction of wave energy, causing erosion on other beaches under the same littoral dynamics, also known as the shadow effect. Under low tides, the outer side of the SBW caused wave reflection, eroding the profile at the base of the structure, leading sometimes to mechanical failures such as rotation.
Structural design—Observe the fragility of the geosynthetic fabric when exposed for long periods of time to UV radiation; and note the quick degradation of specific weak zones of the fabric in marine environment, such as seeming areas in ports, scour apron, and anchorage tube; and damages in the structure surface mainly by vandalism or boat propellers.
Design methodology limitations were addressed in subsequent years through an optimization process supported by local monitoring and analysis of its performance. By 2011, the methodology included environmental impacts around littoral dynamics caused by the structures. Attenuation measures were evaluated from a global perspective to define a long-term solution for the stabilization of the complete system of beaches in Yucatán.
Flexibility of the geosynthetics technology has become a key element, allowing a quick adaptation of structures already in place. Among others, one of the greatest advantages of this technology has been its adaptability to modifications required to improve marine response of the structures at the lowest cost.
Efforts on experimental studies with geosynthetics
Different configurations of GTT as SBW were defined to evaluate the performance of the structures related to wave transmission.
Physical model tests and experiments were carried out at the facilities of the Instituto de Ingenieria of the Universidad Nacional Autonoma de Mexico, in a 2-D wave flume with a length of 37m, a width of 0.8m, and a depth of 1.20m. The experiments were performed with the typical wave climate present in Yucatan, which was characterized using a 60-year wave hindcast (Figure 6), (Silva et al., 2008).
Test conditions
The wave characteristics used during the tests are representative of normal (H = 1m) and storm conditions (H > 1m) for the Yucatán coast. With this information the combinations of incident regular waves were established (Figures 7, 8) and wave transmission coefficient Kt was evaluated as a function of the freeboard. A full description of the test methodology is at González et al., 2014:
Where:
Hi = incident wave height
Ht = transmitted wave height
A full description of the test methodology is at Gonzalez et al., 2014.
Physical test results
(Gonzalez et al., 2014)
After investigating the transmission coefficient, it was concluded that the only dimensionless parameter that governs the wave transmission is the ratio of the submergence depth to the incident wave height.
- The experimental results and an empirical formula are presented in Figure 9.
- The results obtained have important implications for the structure design, which should be taken into consideration:
- Depending on the amount of protection and in function of the design wave height, the most appropriate freeboard is selected. Optimal freeboard should not exceed twice the design wave height.
- Once the design freeboard is selected, the fact that the smallest wave heights will have the highest transmission coefficients must be considered. However, higher wave heights will have smaller transmission coefficients; in other words, greater protection.
- In microtidal environments, the hydrodynamic behavior of the geotextile structures will be the same. However, in areas where the amplitude of the tides is important, the least favorable design condition—spring tide conditions—must be considered.
- Under extreme meteorological conditions, it is possible that the effect of storm surge arises. This should be analyzed and taken into account in defining freeboard of the structures.
Conclusions
The response of wave transmission to GTT working as a SBW is similar to impermeable concrete structures that have been permanently used for coastal protection.
The dimensionless parameter Kt is sensible to relative submergence.
Based on empirical experience on-site, by controlling Kt ≥ 0.6, the alongshore sediment transport reduction does not compromise stability on adjacent beaches. This applies exclusively to the beaches on the northern coast of Yucatán. However, in general terms, model results using local wave climate conditions, are a fundamental element to control Kt, when designing the cross section of a SBW.
Empirical formulation for Kt shows an increment of the incident wave for higher values of the relative freeboard due to the presence of the SBW. This has to be taken into consideration when defining submergence to ensure that is always Kt < 1. More tests are required to fully identify adequate submergence frontiers.
Further research must also include evaluation of the relative width of the crest for wave dissipation in combination with relative submergences to improve the approach to wave transmission coefficient Kt for designing GTT as SBW.
Acknowledgements
The authors thank the Instituto de Ingenieria of the Universidad Nacional Autonoma de Mexico for all the assistance received for this study and conducting the performance of the trials in its wave flume facilities. Also thanks to the environmental authorities for the state of Yucatán, Mexico, and for the support from Axis Ingenieria related to beach restoration activities.
References
Alvarez-del Rio, E. and González-Leija, M. (2013). “The role of geosynthetics technology as part of a long-term beach restoration program: The experience in Yucatán, México,” Second International Workshop on Geosynthetics and Modern Materials in Coastal Protection and Related Applications, IIT Madras, India.
Bezuijen, A. and Vastenburg, E. (2008). “Possibilities and limitations for applications hydraulic and coastal applications,” Proceedings of EuroGeo4, paper 282, 6p.
Enriquez-Ortiz, C., Mariño-Tapia, I., and Herrera-Silveira, J. (2010). “Dispersion in the Yucatán coastal zone: Implications for red tide events,” Continental Shelf Research 30, pp. 127–137.
González-Leija, M.and Solis-Pimentel, A. (2012). “Coastal dune recovery and monitoring: The use of geotextile tubes to prevent coastline retreat at Las Coloradas, Yucatán,” 8th International Conference on Coastal and Port Engineering in Developing Countries COPEDEC, Indian Institute of Technology, Madras, Chennai, pp. 182-183.
Gonzalez Leija, M., and Alvarez del Rio, E. (2012). “Coastal erosion management at Yucatán, Mexico: Engineering efforts and experiences,” Coastal Engineering Proceedings, 1(33), posters.8. doi:10.9753/icce.v33.posters.8, Santander, Spain.
González-Leija, M., Chávez, X.; Alvarez, E., Mendoza, E., and Silva, R. (2014). “Experimental study on geotextile tube applications as submerged breakwaters for beach protection in Yucatán, Mexico,” International Conference on Coastal Engineering, posters, Seoul, Korea.
Pilarczyk, K.W. (2000). “Geosynthetics and geosystems in hydraulics and coastal engineering,” A.A. Balkema Publications, Rotterdam, 913p.
Silva, et al. (2008), “Atlas de Clima Marítimo de la Vertiente Atlántica Mexicana,” Universidad Nacional Autónoma de México.