Coastal dune stabilization using geotextile tubes at Las Coloradas

By Sergio Aguilar Escalante and Alfonso Sol“s Pimentel

Abstract

Beaches and coastal dunes at Las Coloradas, Mexico, were severely affected after Hurricane Wilma stormed through the northern coast of the Yucatan Peninsula in October 2005. Dune destruction connected open sea with lagoon systems at some points, with disastrous consequences for the local ecosystems.

The regional economy depends on salt extraction, and therefore interactions between open sea water and salt-production lagoons brought enormous economical losses to the Las Coloradas community. To repair the damage caused by the hurricane, emergency actions were taken. Beaches were filled to close the breakages between sea and salt-production lagoons. However, those actions did not assure a reliable long-term solution. The probabilities of such meteorological events to recur in this area are high. Therefore, it was imperative to build a natural structure to fully guarantee entirely dune stabilization, avoiding littoral drift alterations.

A reinforced dune using geotextile tubes covered with sand for revegetation was built along 2.5km next to the coast during 2006, and increased to 3.9km by September 2007. The structure protected the salt-production lagoons without affecting coastal processes in the area and natural evolution of surrounding beaches. Dune stability, beach performance, and littoral processes have been continuously monitored since reconstruction started in May 2006.

This paper summarizes design criteria, some building process details, and results obtained up to September 2007. A comparison of the structure performance and beach evolution under 2 different wave conditions is also described.

It is concluded that the implemented structure is performing as expected and the dune stabilized shows no evident risk of collapsing under extreme climatological events. However, some critical areas where the geotextile tube is fully exposed to wave action must be permanently monitored. The technology described herein represents a reliable solution; however, it is expected that much experience will be gained to improve, in the near future, the use of geotextile tubes as elements for dune reinforcement.

Introduction

This article describes a successful case of geosynthetics application in coastal engineering, specifically as a shore protection element for dune reinforcement.

The project site is located off the northeast coast of the Yucatan Peninsula, specifically at Las Coloradas (Figure 1). This coastal zone is important environmentally because it has been declared a natural protected area due to its biodiversity. The local economy relies on marine salt production, an industry established there 50 years ago. Salt production typically reaches 500,000 tons annually—the second largest in Mexico.

After Hurricane Wilma struck through the Yucatan Peninsula in October 2005, beaches and coastal dunes at Las Coloradas, Yucatan were severally affected. At some points, dune destruction connected open sea with salt-production lagoon systems (Figure 2) with disastrous environmental and economical consequences.

To repair the damage, emergency actions were taken. Beach fills were performed to close the breakages between sea and salt- production lagoons. However, the actions taken were not a reliable long-term solution to the problem. Due to the increased risk of extreme meteorological events in the area (CNA, 2007), it was imperative to build an artificial structure to fully guarantee entire dune stabilization. In addition, proposed actions should be environmentally friendly to avoid littoral drift alterations and damages to surrounding beaches.

To fulfill those demands, a reinforced dune using geotextile tubes covered with sand for revegetation was built along 2.5km during 2006 and 1.4km by September 2007, parallel to the coast. This solution has worked satisfactorily as a coastline regression control, supporting wave attacks during extreme weather events.

Project description

The shoreline protection system that was adopted consisted of beach dune reinforcement using 30m-long and 1.2m-high geotextile tubes filled with sand. The dune has worked as coastline regression limit, avoiding dune failures and preventing sea water from entering salt-production lagoons during extreme events. Similar to other cases (North Carolina University, 1992), this alternative technology has proved to work satisfactorily.

Stage 1 of the project (March, 2006) included dune reinforcement with geotextile tubes along 1,000m of coastline. During the second stage (September 2006) an additional 1,500m of geotextile tubes were installed. In 2007, 1,400m of dune reinforcement was implemented with similar positive solutions. The final goal is to protect the lagoon border with at least 3 new kilometers of dune reinforcement during the next 2 years (Figure 3).

The reinforced dune section was designed to reach 2.5m above the mean low-water level and is described in the following section. However, more monitoring time is necessary to be conclusive about optimization of geometrical design.

Designed section and expected performance

Wave climate affecting this site was the most important parameter taken into account for dune-height design. Since no local wave-data records existed, 19-year offshore wave data (1976-1995) taken from the USACE’s wave station WIS #114 located north of the Yucatan Peninsula (21.75° N.lat., 89.00° W.long.) were analyzed (see Figure 4).

Data analysis shows that for an average year, 73% of the offshore waves approaching Yucatan’s coast are not higher than 0.5m; and during winter (November to March) is when occurrence of waves higher than 1m and with lower frequencies (Tp=> 6 s) is likely to increase (Figure 5).

According to National Oceanic Atmospheric Administration’s Saffir-Simpson Hurricane Scale (NOAA, 2006), a 4-5 category hurricane storm surge is generally 4-5.2m above mean water level, data that has been corroborated locally after hurricanes Gilbert (1988), Isidore (2002), and Wilma (2005). Taking this into account, an ideal design should consider the dune crest to reach 5m above mean water level, which is unfeasible since building costs would increase considerably. In addition, dragging the amount of sand required could have repercussion on littoral dynamics. Additional limitations, such as turtle nesting, made this solution environmentally unacceptable.

Based on these criteria, dune height was designed to reach 2.5m above low-water level. Table 1 describes the main variables taken into account for the designed section (Alvarez 2006). Definitive geometrical section designed for dune reinforcement is defined in Figure 6.

Once the geometrical section was designed, woven geotextile for manufacturing tubes was defined in terms of mechanical properties as shown in Table 2. Selected geotextile for the tubes was manufactured with high-tenacity polypropylene fibers containing 2% carbon black. Geotextile tubes were manufactured 30m long with 3 filling ports.

Expected performance of the structure is described in Figure 6. Under normal conditions, sand accumulation on the beach is expected. During low and moderate storms, coastline regression and sand deposits on the submerged area are likely. And in the case of extreme events (strong storms and hurricanes), reinforced dunes will perform as a maximum coastline regression limit protecting salt-production lagoons. Understanding of distribution of wave conditions through the year, as described in Figure 5, becomes mandatory for an efficient monitoring of performance of beach profiles as expected in Figure 7.

Installation process

A hopper method was adopted (Figure 8), which consisted of placing the hopper into the tube’s filling ports and placing sand directly into it using a track excavator. At the same time, sea water was pumped into the hopper to produce the slurry that distributes filling sand alongthe tubes.

Once tubes are filled, they are covered with sand to reach design height. The dune is reforested with native species to avoid wind erosion (Figure 9).

Monitoring andreal performance

Dune stability, beach performance, and littoral processes have been continuously monitored since reconstruction took place. After the 2006 storm season (October-December), some critical points were detected along the first 400m of the structure. The beach was severely eroded in that section, leaving the tubes fully exposed to wave attack, which resulted in scour apron settlements and tube rotation (Figure 9).

As observed in Figure 10, scour apron was working at over tension, a fact that compromised structural integrity. No complete failure or breakage has been detected so far. However, this is a key point to improve in looking forward for long-term, low-maintenance cost solutions.

Figure 11 shows the beach profile after a storm in late 2006. The reinforced dune is working as limit coastline regression and the eroded section is evident.

At the beginning of 2007, the critical zone near the beach started to recover naturally, reducing the risk of structural failure.

In order to observe the critical zone’s beach evolution, control points were defined. Figure 12 shows beach evolution from January to May 2007. During this period, sand accumulation and beach growth was registered.

On the other hand, no problems have been detected in the rest of the reinforced dune, which has performed much better than expected (Figure 13).

Wave climate and beach evolution: Comparison of storms of 2006 vs. Hurricane Dean 2007

By the end of August 2007, Category 5 Hurricane Dean affected the project site. Wind speed reached 120km/hr with wave heights of approximately 3m (TWC, 2007; Ocean Weather Inc., 2007). Figure 14 shows a satellite image and estimated wave heights registered during its path trough the Yucatan Peninsula.

Wave data analysis (Section 3) shows that under normal conditions, wave heights do not exceed 0.5m, and their associated periods range from 2-3 seconds. The highest waves (0.5–2m) occur during winter (November to March). Storms during this period, locally known as “Nortes,” usually cause beach erosion and create submerged sandbars. Monitoring of the beach after Hurricane Dean shows a general beach recovery along the study site. This behavior could be associated with wavelength increments. Under these conditions, it is expected that sand accumulated on the submerged bars is carried out to the beach. This concept is described in Figure 15. It is also concluded that due to storm surge, the wave overtopping was evident, which contributed to beach accumulation of sand.

Conclusions

Geotextile tubes represent a reliable element of reinforcing dunes and protecting coastal areas against extreme meteorological events. In terms of environmental impact—a priority concern in every coastal project—the flexibility of geotextile tubes represents a great alternative.

This project, as well as similar applications of geotextile tubes in the northern coast of Yucatan during recent years, demonstrates that even the evidence of good performance of solution-adopted geosynthetics for marine applications generates many questions, for long-term performance that requires permanent research.

Geotextile vulnerability to UV exposition after storms, puncture failures detected along exposed tubes because of rock impacts during extreme events, scour apron performance for assuring settlements control, and seam and port strength of manufactured tubes, are, in the opinion of the authors, and based on day-to-day site observations, issues that require full attention of different manufacturers, to improve the performance of geosynthetics technologies for coastal protection.

The authors are engineers with the firm Axis Ingeniería S.A., Mérida, México.

Acknowledgements

This project has been designed and conducted by Axis Ingenieria, with construction by Industria Salinera de Yucatan (ISYSA). The authors wish to thank ISYSA for fully supporting the project and monitoring activities. Also thanks to environmental authorities from the state of Yucatan for their collaboration along the development of the project.

References

Alvarez E, Rubio R, Ricalde H, “Shoreline restored with geotextile tubes as submerged breakwaters in Yucatán Méxcio.” Gesoynthetics magazine (Vol. 24, No.3, 2006)

CNA (2007), Comisión Nacional del Agua (National Water Commission Meteorological Service), México. Search by web. smn.cna.gob.mx

NOAA (2007). National Oceanic and Atmospheric Administration, The Saffir-Simpson Hurricane Scale, USA. Available at: www.nhc.noaa.gov/aboutsshs.shtml

North Carolina University (1992), North Carolina Erosion And Sediment Control Planning and Design Manual, 1992.

Ocean Weather Inc. (2007). Marine Current Data, USA. Available at: oceanweather.com/data/index.html.