Second International Workshop on

GEOSYNTHETICS AND MODERN MATERIALS IN

COASTAL PROTECTION AND RELATED

APPLICATIONS 4 –5 March 2013

organized jointly by

Department of Ocean Engineering, IIT Madras, India &

Universidad Anahuac Mayab, Yucatan, Mexico

in association with

Indian Geosynthetics Society and

The Indian Society of Hydraulics

THE ROLE OF GEOSYNTHETICS TECHNOLOGY AS PART OF A LONG TERM BEACH RESTORATION PROGRAM: THE EXPERIENCES IN YUCATAN, MEXICO

E. Alvarez-del-Rio1, M.B. González-Leija2

Abstract: The North coast of Yucatan Peninsula in Mexico has been in a regression process for decades. After Hurricane Gilbert (1987) and Isidore (2002) beach erosion increased dramatically. Human action 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 serious risk of destruction. Since 2004, AXIS INGENIERIA has developed several engineering solutions based on Geosynthetics.

The construction of submerged breakwaters (SBW) built with Geotextile Tubes (GT), engineered to reduce energy of incident waves over the beach, have had a very successful performance for coastline stabilization and restoration of lost beaches. The key factor has been the control of littoral drift along 60 km of coast, protecting critical eroded segments with SBW, without affecting downdrift beaches.

This paper presents an assessment of the experience from an integral viewpoint of a Coastal Management Program in Yucatan, where GT working as SBW have played a key role. It also covers the experimental activities performed during 2012 in order to optimize the structural and functional design of these structures, as wave energy attenuators.

Keywords: Coastline Stabilization; Submerged Breakwaters; Geotextile Tubes.

INTRODUCTION

The coast of the Yucatan Peninsula in Mexico consists on a fragile island barrier system (beach-dune-coastal lagoon) (Fig.1) and a coastal sea that has a wide and shallow continental shelf (nearly monotonic 1:1000 slope). Oceanographic conditions have allowed the development of this system, with a dominant westward alongshore sediment transport (Merino, 1997; Enriquez et al., 2010).

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 Yucatan’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).

1,2 Axis Ingenieria S.A. de C.V. Merida, Yucatán México, Email: enrique.alvarez@axisingenieria.com.mx

4-5 March 2013 Proceedings of Geosynthetics and Modern Materials In Coastal Protection and Related Applications

The role of Geosynthetics technology as part of a long term beach restoration program: the experience in Yucatan Mexico

Fig. 1 Map of Yucatan showing the segment of coast under study.

THE ROLE OF GEOTEXTILE TUBES

The first approach to offer a solution based on GT as SBW was implemented in 2004. The behavior of the structures accomplished the primary objective of reducing wave energy and restoring beach erosion by controlling alongshore sediment transport. The coast of Yucatan is known to be subject to low energy waves with Hs of less than 1 meter, periods of 4 to 6 seconds and a main incident direction coming from the E-NE, which generates westward alongshore sediment transport. These conditions determine the design parameters and the installation conditions of GT structures mainly as submerged detached breakwaters between 0.7—1 m depth or at –0.3 m (MLWL) referred to MSL. 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 Yucatan (Gonzalez et.al., 2012) (Fig.2).

a)

4-5 March 2013 Proceedings of Geosynthetics and Modern Materials In Coastal Protection and Related Applications

The role of Geosynthetics technology as part of a long term beach restoration program: the experience in Yucatan Mexico

b)

Fig. 2 a) Sequence of beach restoration using Geotextile Tube at Chicxulub, Yucatan (Gonzalez et.al., 2012), b) Sample of Restored beaches in Yucatán using Geotextile Tubes as Submerged Breakwaters

Despite the positive results, the geosynthetics technology used as beach protection measures still present limitations. It is well known that 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 of the problems that have surfaced may be classified related to:

a) Functional Design: the GT 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.

b) Structural Design: fragility of the geosynthetics fabrics when exposed for long terms to UV radiation; quick degradation of specific weak zones of the fabric in marine environment , such as seeming areas in ports, scour apron and anchorage tube; and finally, damages in the structure surface mainly by vandalism or boat propellers.

The limitations of the design methodology 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. On the other hand, attenuation measures were evaluated from a global perspective in order to define a long term solution for the stabilization of the complete system of beaches in Yucatan. 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 capacity to implement modifications and improve marine response of the structures at the lowest cost.

EFFORTS ON EXPERIMENTAL STUDIES WITH GEOSYNTHETICS

In order to solve some of the structural and functional design problems that arose in the past, and with the goal of promoting an iterative process of communication and exchange of information between research and coastal management agencies/stakeholders during 2012, AXIS INGENIERIA, funded by the National Council of Science and Technology (CONACYT) started collaborations with ANAHUAC-MAYAB University and the Engineering Institute of the National University (IINGEN-UNAM) in Mexico City to model different configurations of submerged breakwaters that will enhance the development of engineering solutions using geosynthetics technology.

Reference Point

Reference Point GT´s SBW

Beach width restored

4-5 March 2013 Proceedings of Geosynthetics and Modern Materials In Coastal Protection and Related Applications

The role of Geosynthetics technology as part of a long term beach restoration program: the experience in Yucatan Mexico

The physical model tests and experiments were carried out on IINGEN-UNAM´s facilities, in a 2D

wave flume with a length of 37 m, a width of 0.8 m and a depth of 1.20 m. The experiments were performed with the typical wave climate present in Yucatan, which was characterized using a 60 year wave hindcast (Silva, et. al., 2008) (Fig. 3).

a)

b) Fig. 3 Yucatan Coast annual wave climate in terms of H, T and wave Direction joint probability, b)

IINGEN-UNAM wave flume.

Three scales (1:5, 1:10 and 1:20) were used to model, as much as possible, sea states combining

two different Geotextile Tubes geometries: 1 GT model for 1:10 and 1:20 scales, and 3 GT for 1:20 scale (Fig.4). The analysis consisted on determination of incident and reflected waves, estimation of Hrms, and wave transmission coefficient (Kt) by Mansard & Funke (1980) and Goda (Goda & Suzuki, 1976) methods. A total of 288 trials were performed according with morphological conditions of the Yucatan´s coast.

Annual

Probability Probability

4-5 March 2013 Proceedings of Geosynthetics and Modern Materials In Coastal Protection and Related Applications

The role of Geosynthetics technology as part of a long term beach restoration program: the experience in Yucatan Mexico

Fig. 4 Geotextile Tube reducing wave energy a) Typical site at Yucatan b) Wave channel at II

UNAM and examples of sea states used during experiments RESULTS Wave transmission coefficient Kt was the key element to be evaluated so SBW could be engineered to accomplish a) coastline stabilization in critical eroded beaches and b) the maintenance of alongshore sediment transport, so erosion damages along adjacent beaches may be avoided. Figure 5 shows correlation between Kt=Ht/H versus Fr=Fb/H, where Ht is the wave height transmitted, H is the incident wave, Fb is the Free board measured as the distance between middle water level and the top of the structure, and Fr is the relative Free board .

Fig. 5 Kt transmission coefficient, as a function of Relative Free Board (Fb)

1:10 scale

4-5 March 2013 Proceedings of Geosynthetics and Modern Materials In Coastal Protection and Related Applications

The role of Geosynthetics technology as part of a long term beach restoration program: the experience in Yucatan Mexico

DISCUSSION AND CONCLUSIONS

The response of wave transmission to GT working as SBW is very similar to impermeable concrete structures that have been permanently used for coastal protection.

The different tests have shown that Kt is very sensitive to Free Board. It was verified that with

very high values of Fr (Fr > 2.5), wave energy transmission may increase. It is recommended then, for engineering purposes, to control Fr ≤ 2.0.

Based on empirical experience on site, by controlling KT ≥ 0.6, the alongshore sediment

transport reduction do not compromises stability on adjacent beaches. The model results, using local wave climate conditions, are a fundamental element to control Kt, when designing cross section of a SBW.

It is also recommended to continue physical modeling tests, varying crest width for same Free

Board conditions, to estimate its effect on Kt. This will improve the knowledge of all parameters involved when designing the geometrical sections of SBW.

The results from this scenario-analysis seem promising for the implementation of better

technologies in Yucatan, which will need to be integrated to sediment by-pass actions along the harbors. It is highly recommended, and based on the physical modeling of the SBW and numerical modeling in progress, to install an experimental field as part of the activities that will define an integral long term solution for the beaches of Yucatan northern coast.

GT technology permits a great flexibility to modify the geometry of a SBW once it is installed.

This allows for the adjustment of the Free Board as marine response demands for two mainly reasons: local beach restoration purposes, as well as for controlling littoral dynamics on the adjacent beaches. The lowest environmental impact based on this flexibility, offers a promising future for GT applications for coastal protection and beach restoration.

ACKNOWLEDGEMENTS

The authors wish to thank all the support received by the National Council of Science and Technology (CONACYT) for founding this work. Also thanks to the ANAHUAC MAYAB UNIVERSITY, the INSTITUTO DE INGENIERIA DE LA UNAM Campus Ciudad Universitaria, and the CENTRO DE INVESTIGACION CIENTIFICA DE YUCATAN, for all the assistance received along the project. REFERENCES Bezuijen A.; and Vastenburg, E. (2008). Possibilities and Limitations for Applications Hydraulic and

coastal applications, In: Proceedings of EuroGeo4, paper 282, 6p Enriquez-Ortiz, C., Mariño-Tapia, I., Herrera-Silveira, J. 2010. Dispersion in the Yucatan coastal zone:

Implications for red tide events. Continental Shelf Research 30, 127-137. Goda, Y . and Suzuki , Y. ,1976 . Estimation of Incident and Reflected Waves in Random Wave

Experiments. Proc. of the 15th Conf. on Coastal Eng., ASCE, pp. 828-845 González-Leija, M.and A. Solis-Pimentel. (2012) Coastal dune recovery and monitoring: the use of

geotextile tubes to prevent coastline retreat at Las Coloradas, Yucatán. 8th International

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The role of Geosynthetics technology as part of a long term beach restoration program: the experience in Yucatan Mexico

Conference on Coastal and Port Engineering in Developing Countries COPEDEC, Indian Institute of Madras, Chennai. 182-183 pp.

González-Leija, M, Solis-Pimentel, A., Alvarez del Rio, E y C. Wabi Peniche (2012) Coastal Management at Yucatan Mexico: Engineering efforts and experiences .33th International Conference on Coastal Engineering, Santander, Spain

Mansard E.P.D and E.R. Funke (1980) The measurement of incident and reflected spectra using a least squares method, Proceedings of 17th Conference on Coastal Engineering, Sydney, Australia

Merino, M., 1997. Upwelling on the Yucatan Shelf: hydrographic evidence. J. Mar. Syst. 13 (1–4), 101–121. Pilarczyck, K.W. (2000). Geosynthetics and Geosystems in Hydraulics and Coastal Engineering, A.A.

Balkema Publications, Rotterdam, 913p, ISBN 90-5809-302-6. Silva, et. al (2008) Atlas de Clima Marítimo de la Vertiente Atlàntica Mexicana, Universidad Nacional Autónoma de México