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Shoreline restored with geotextile tubes as submerged breakwaters

June 1st, 2006 / By: / Erosion Control, Erosion Control Materials, Feature

Back to the beach in Mexico.

Abstract

The beaches of the northern coast of Yucatan in Mexico have been in an erosion process that has dramatically increased in the past 15 years. Changes in the littoral dynamics, mainly due to human action, have generated a coastline regression rate, estimated at 1m per year and more.

In addition, this region is affected by all hurricanes that follow a path through the Gulf of Mexico. Risk of destruction due to extraordinary wave conditions is a permanent threat. Coastline stabilization required a carefully designed project for controlling beach erosion, reducing as much as possible any changes to littoral dynamics that would have negative consequences in the long term.

This paper describes the technical solution adopted using geotextile tubes, as low-crested structures, along 4km of beach. Proposed actions for improving knowledge of this application are also discussed.

Introduction

In 2001, federal authorities from the Ministry of the Environment in Yucatan initiated a Beach Rehabilitation Program. For a long-term solution, it was imperative that any restorative actions not affect the natural dynamic process that relates wave climate/bathymetry/sediments. The philosophy behind the solution was to generate a sand accumulation process without interrupting alongshore sediment transport. Also, the solution had to be as flexible as possible, avoiding any rigid structures, so it would easily absorb any physical media modification.

Under these conditions and for critical points, geosynthetics were considered optimal for the beach-restoration project. Woven polypropylene geotextile tubes were designed to work as low-crested submerged structures. Their main function was to reduce the incident wave energy on the beach, by controlling the wave-breaking process, to the required level that maintains the dynamic balance on the shoreline (Figure 1).

Based on these criteria and using a maritime field database processed by federal and state authorities, geotextile tubes were designed according to coastal engineering theories regarding wave propagation, the breaking process, and their relation to sediment transport. The main goal was to generate a balanced beach profile integrated to natural littoral dynamics. By September 2005, 4km of geotextile tubes were installed.

Initial situation

The case discussed in this paper refers to a beach system defined as Barrier Island, formed by alongshore transport of sediments running parallel to the coast. The most-developed zone, near the city of Progreso, was originally limited by a vigorous and balanced beach 30m to 100m wide. However, as a particularity of this beach system, there are not many natural sediment sources such as river discharges, so littoral balance may be easily broken by infrastructure such as small harbors, piers, and groins. The first attempts to control beach erosion were individual actions undertaken to retain sediment for beach stabilization without considering other consequences along the coast. By the end of 2002 the situation was critical, since many beaches were almost fully eroded (Photo 1) and many faced the risk of permanent destruction due to extreme waves induced by any hurricane with a path through the Gulf of Mexico.

Proposed solution

The combination of wave climate, currents, tides, and storm surges is the main cause of beach erosion. Based on local experience in the past 20 years, any beach restoration action has to be environmentally friendly, reducing as much as possible any changes to littoral dynamics and it also needs to consider possible negative impacts on adjacent beaches. Under these requirements, the final objective of the beach restoration project was to re-establish the natural conditions that govern the littoral drift.

Originally, the solution consisted of: (1) the elimination of structures perpendicular to shoreline (groins); (2) beach nourishment from inland material banks, so actions in the seabed were fully avoided; and (3) implementation of sediment bypass techniques for the various harbors along the coast.

With these actions, alongshore sediment transport was partially recovered to natural conditions. Evidently, the beaches never recovered their original dimensions but in some segments, they were naturally stabilized solely by the free sediment motion. In critical segments, free sediment motion was not enough.

The hydraulic load on the coast governs morphological response to wave climate and shoreline control. When it is too high, related to the littoral drift, the erosion process causes shoreline regression. In these cases, incident wave energy may be reduced to conditions that generate a balanced beach profile by submerged structures, so sand accumulates without interrupting alongshore transport. According to the U.S. Army Corps of Engineers (1984), wave energy reduction is defined in terms of the energy transmission coefficient: Equation 1

Where Kt is the wave transmission coefficient, Ht is the transmitted wave height shoreward side of the submerged structure, and Hi is the incident wave height on seaward side of the submerged structure. Full description of wave reduction by submerged structures can be found also in Pilarczyk (2003).

Among the various alternatives for submerged structures, geotextile tubes where chosen for their viability to cause wave dissipation and thus reduce energy, in a flexible adaptation to a media as dynamic as the maritime media. The possibility of quick modification of structures according to morphological response to structures, and low costs for initial installation and maintenance, were also considered.

Design parameters

A precise analysis of wave climate currents and tides, and its interaction with bathymetry and sediment characteristics, controls the littoral drift. Once all these parameters are evaluated and their interaction with littoral drift is estimated, the cross section of the balanced beach profile, including the geotextile tubes, may be defined. At the end, the main interest is to control sediment transport since this will govern the erosion/accretion processes. Another important cause of erosion processes are storm surges. As discussed later, the behavior of geotextile tubes for major storm surges generated by hurricanes is out of the scope of this paper. However, in a full evaluation of shore erosion mechanics, they must be considered.

The basic parameters for designing the cross section that maintains the dynamic balance of the shoreline were defined from database information provided by the Ministry of the Environment:

  • Breaking wave height Hb: Hb<1.0m (93% occurrence)
  • Wave period Tz: 4 seg<Tz<8 seg (93% occurrence)
  • Bathymetric profile slope tan θ: 0.1%<tan θ<0.5%
  • Tides range: 0.90m
  • Net alongshore current direction: E – W
  • Littoral drift Sl: 60,000m3/yr
  • Sediment diameter D50: 0.30mm

One of the primary requirements for an efficient submerged tube cross-section design is to define the crest high, in relation to the still water level (SWL) for all the tide ranges, since this will govern the wave breaking mechanism that controls wave energy reduction. Photo 2 shows this concept.

By evaluating the dimensionless surf similarity parameter, also known as Iribarren number, the breaker type may be defined, and then, energy reduction mechanism estimated: Equation 2

Where ξ is the Iribarren number, tan θ is the slope of the seabed, H is the incident wave high, and Lo is the deep-water wave length.

When computing the Iribarren number for different site conditions in terms of tan θ, H, and Lo, it is verified that the breaker type in this case is basically the one called “spilling” that is associated with smooth slopes and distributed wave reduction along space. This is usually positive for a beach restoration design since sand is carried in a shoreline direction. Then, the accumulation of sand at the shoreward of the tube becomes a viable task.

Full information of wave-breaking processes may be found in the U.S. Army Corps of Engineers (1984).

An important issue is to avoid wave reflection in all types of tide conditions. This phenomenon may cause major changes to current patterns that govern alongshore transport. As mentioned before, Yucatan beaches are all connected to the same littoral dynamics. Changes in current patterns may cause negative effects to littoral drift at adjacent beaches.

Based on these criteria, three important considerations were verified permanently:

  1. Breaking on bathymetric profiles at low tide conditions where a “spilling” type in relation to Iribarren number analysis.
  2. A scour apron had to be engineered to work as a first wave breaking at low tide conditions to avoid direct impact of waves on main tube so that reflection could be eliminated as much as possible.
  3. Tubes must be engineered crested at high tide conditions in such a way that they generate a smooth dissipation of wave energy. Designing the structure too high would lead to a sure wave reflection; and if too low, the effect of the tubes would disappear (see Photos 2, 3a, 3b).

Geotextile tubes cross section

Once the littoral process was evaluated, a tube cross section was designed and geosynthetic materials were defined in terms of their mechanical properties. The following considerations were mandatory:

  1. Stresses on geosynthetics are very sensitive to the slurry pumping pressure when the tubes are filled. This pressure governs the criteria design for defining the estimated force of the required geosynthetics, working under load conditions.
  2. Slurry pumping pressure does not have a significant influence on the final sectional area of the tubes.
  3. The apparent opening size of the geotextile is conditioned by sediment diameter D50.
  4. Inlets separations are defined in terms also of D50. The larger sediment diameter D50, the closer the inlets are.
  5. The ultimate strength of required geosynthetics must consider (Leshchinsky 1996), reduction factors for installation damage, chemical and biological degradation, treachery creep, and seam strength:Equation 3

Where Tult is the ultimate strength of the required geosynthetic, Twork is the calculated tensile force under load conditions and RFid, RFd, RFc, and RFss are the reduction factors for installation damage, chemical and biological degradation, creep, and seam strength.

Another important consideration with a complex evaluation is the behavior of the tubes under permanent contact with wave action, UV exposition, and frictional effects of littoral drift while the tubes are covered by sand. These topics require special attention and development based mainly on site monitoring.

During the installation of the 4km of geotextile tubes in Yucatan, mechanical and geometrical parameters for geosynthetics were permanently modified. Up to September 2005, parameters considered best according to design theories applied and local observation are referred to in Figure 2 and have the following values:

  • Mechanical parameters:
    • Tult circumferential direction: 90 Kn/m
    • Tult axial direction: 70 Kn/m
    • AOS: 0.35–0.425mm
    • Factory seam strength: 50 Kn/m
  • Geometrical parameters:
    • a: 1.85m
    • a1: 1.25m
    • b: 0.90m
    • A: 1.4m2 (70% full)
    • l: 2.0m
    • a’: 0.40m
    • b’: 0.20m
    • Seams orientation: Axial–Not exposed to wave attack.
    • Inlet separation: 15m

Construction procedure

As discussed, stresses in the encapsulating geosynthetics due to slurry pumping pressure, makes the installation procedure a task that must be carried out under extremely controlled conditions. Overpressure during filling of tubes may produce failure of geotextile. Most of the job was carried out with slurry pumps with 4-in. and 6-in. discharge diameters with volume discharge rates up to 1000 gpm of slurry with 10–30% of solids. Photo 4b shows slurry-pumping operations.

A very significant topic is that, since the philosophy behind the beach restoration project is to reestablish natural conditions for littoral drift, and even that volume to fill geotextile tubes is not significant in comparison to sediment transport rates, the use of offshore sand banks was reduced as much as possible. In the case of Yucatan, some tubes were filled with offshore banks, but whenever it was possible, inland sand was carried to the job site before pumping it into the tubes (Photo 4a). In the case of beach nourishment, when required, in critical segments because of damaged conditions, no offshore sand banks were allowed.

Solution performance, monitoring, and maintenance

By September 2005, nearly one year of monitoring had passed since the first geotextile tube was installed. Performance of the tubes is evaluated basically from two perspectives: marine processes response and geosynthetics materials behavior.

Marine processes response:

Geotextile tubes have been performing satisfactorily, working as parallel submerged breakwaters. As expected, energy dissipation is generated by wave breaking due to the presence of the tubes. Turbulence generated shoreward induces sand accumulation without interrupting littoral drift. Meanwhile, there are no changes of natural current patterns seaward of tubes (Photos 5a, 5b, and Photo 6).

Efficiency of design depends highly on how precise is the evaluation of wave transmission at the geotextile tubes. However, there is not much literature on this topic and the existing literature refers basically to studies made of submerged rubble-mound breakwaters that evaluate wave transmission in terms of deep water wave height (Ho), wave length (Lo), structure geometry, and freeboard between crest and still water level.

These studies only give a qualitative first approach. However, results may be out of reality when applied to geosynthetic materials and tubes geometry. For design techniques development, wave breaking and energy dissipation/reflection, must be studied, monitored and tested on geotextile tubes, considering all the parameters involved and referred to in Figure 3. Detailed information on wave transmission research may be found in Daemrich (2002) and Wamsley (2002).

For a shoreline response evaluation, the data that resulted from wave transmission analysis must interact with distance offshore, orientation angle and length of tubes, beach slope shoreward of tubes, wave direction distribution, littoral drift, and grain size.

Storm surges have an important influence on shore stabilization, especially in regions such as Yucatan where hurricane risk is permanent. Predicting sea level rise due to storm surge is a complex task. The effect of geotextile tubes as submerged structures on storm surge must be neglected, since the energy dissipation is reduced as sea level rises.

Finally, the geotextile tube is designed to be part of the elements that generate a balanced beach profile. Once the shore has been stabilized, a vegetated dune must be developed that works as a natural defense for extreme conditions. Vegetated dune formation must be considered a goal when designing any beach restoration project on eroded shores (Photos 8a-b).

Behavior of the geosynthetics materials:

As discussed, tensile strength for geosynthetics is conditioned mainly by slurry pumping pressure. Most of the tubes were filled with pumping equipment from inland and offshore sediments banks, with no overstressed geotextiles detected. However, the question when designing is also the long-term behavior to UV exposition while tubes are covered by littoral drift.

Many times, there is an unpredictable period for having tubes full confined by sand since it depends on external factors to the job site, mainly related to the alongshore sediment transport. This topic requires full discussion with manufacturers because, originally, geosynthetics were not designed to work exposed to UV action. As in the case of the Yucatan project, during the littoral process, tubes may be naturally covered by seaweed and marine flora, which are very welcome since they provide additional UV protection.

Finally, there are many questions around long-term behavior of seams. In the case of the Yucatan beaches, transversal seams have been avoided when a permanent interaction with wave action is expected. They seem to develop a premature lost of mechanical properties. Anyway, this is just a primary conclusion coming from one year of observation. Engineering for seaming conditions requires a precise analysis in conjunction with manufacturers.

Conclusions

Geotextile tubes performing as shore-parallel, low-crested structures have shown that they are an effective and environmentally friendly alternative for shore stabilization.

Among many variables, wave transmission at geotextile tubes is the leading parameter that controls shore response. The existing literature and studies on this topic basically refer to rubble-mound breakwaters, and provide only qualitative information when applied to geotextile tubes design. Future designing techniques of geotextile tubes, as submerged breakwaters, will require development on predicting models for wave energy transmission, as a function of wave parameters, tubes geometry, and relative submergence.

As in all human coastal actions, it is extremely important to control negative effects in adjacent beaches when a shore protection project is executed. Geotextile tubes offer an effective alternative when modifications to installed breakwaters, due to marine media response, are required in the shortest time and at the lowest cost.

In a physical media as dynamic as the maritime media, there are many questions about the behavior of geotextile tubes in the mid- and long-term. Basically, the questions deal with durability against unpredictable UV exposure periods, as well as behavior against direct interaction with stresses generated by continuous wave action and sediment motion. Full research on this topic is required.

In this paper, the intention is to show the great potential of these promising shore protection alternatives. Extensive monitoring and research must be stimulated among public and private organizations to create permanent improvement on designing techniques.

Ing. Enrique Álvarez del Rio, Axis Ingenieria S.A. de C.V.; www.axisingenieria.com. Ramiro Rubio and Herbert Ricalde, Ministry of the Environment of Mexico, Yucatan Office.

Editor’s Note:

This follow-up story is a sequel to an article published in the September 2005 issue of GFR (now Geosynthetics), “Saving Yucatan’s coast.” It is based on the paper by Messrs. Álvarez, Rubio, and Ricalde presented at the proceedings of the international symposium, “Tsunami Reconstruction with Geosynthetics—Protection, Mitigation, and Rehabilitation of Coastal and Waterway Erosion Control” in Bangkok, Thailand in December 2005.

Acknowledgements

The supplier of the geotextile tubes for the 2005 Beach Restoring Program was Ten Cate Nicolon. The authors wish to thank the Ministry of the Environment, Yucatan Office, for all the field information supplied and support during development of the 2001–2005 Beach Restoration Program; and special thanks to the government of the state of Yucatan for permanent support and investment on innovative techniques for coastal rehabilitation. We also want to thank the private investors who promoted the application of new technologies for protection of their coastal properties.

References

Daemrich, K., Mai, S. and Ohle N. (2002). Wave Transmission at Rubble Mound Structures, Proceedings of First German-Chinese Joint Symposium on Coastal and Ocean Engineering, Rostock, Germany, April 2002.

Leshchinsky, D., Leshchinsky, O., Ling, H.I. and Gilbert, P.A. (1996). Geosynthetic Tubes for Confining Pressurized Slurry: Some Design Aspects, Journal of Geotechnical Engineering, ASCE, Vol. 122, No 8, 1996, pp. 682-690.

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

U.S. Army Corps of Engineers (1984). Shore Protection Manual, Coastal Engineering Research Center, Vicksburg, Miss. USA.

Wamsley, H., Hanson, H. and Kraus, C. (2002). Wave Transmission at Detached Breakwaters for Shoreline Response Modeling, ERDC/CHL CHETN-II-45, U.S. Army Engineer Research and Development Center, Vicksburg, Miss. USA.

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