HYDRAULIC STABILITY FORMULAE AND NOMOGRAMS FOR COASTAL STRUCTURES MADE OF GEOTEXTILE SAND CONTAINERS

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1 Proceedings of the 7 th International Conference on Asian and Pacific Coasts (APAC 2013) Bali, Indonesia, September 24-26, 2013 HYDRAULIC STABILITY FORMULAE AND NOMOGRAMS FOR COASTAL STRUCTURES MADE OF GEOTEXTILE SAND CONTAINERS D. T. Dassanayake 1 and H. Oumeraci 1 ABSTRACT: Geotextile Sand Containers (GSCs) are getting increasingly popular as an alternative to conventional hard (rock, concrete) coastal structures. Nevertheless, the GSC is still an emerging technology and there are still no proper guidelines available to design GSC-structures for shore protection. Therefore, this research study focuses on developing simplified formulae for the design of GSC-structures subject to wave loads. This paper summarizes the results of small scale hydraulic stability tests conducted at Leichtweiß-Institute (LWI) of Technische Universität Braunschweig, Germany, to understand the factors influencing the hydraulic stability of submerged and low-crested GSC-structures. Moreover, new hydraulics stability curves/formulae for the design of GSC-structures for shore protection, which are derived from the aforementioned scale model tests are also presented. Keywords: Geotextile sand containers, hydraulic stability, submerged structures, low-crested structures. INTRODUCTION Settlements in coastal lowlands are especially vulnerable to sea level rise and storm surges associated with climate change. However, these lowlands are often densely settled and growing rapidly. When considering the area along the coast that is less than 10 metres above the mean sea level, this zone covers 2% of the world s land area but contains 10% of the world s population. This percentage is higher in least developing countries than developed countries (McGranahan et al. 2007). Further, the value of the coastal ecosystems represents almost 40% of the value of all marine and terrestrial ecosystems (Oumeraci 2000). On the other hand, the still increasing socio-economic pressure on the use of coastal zones with the subsequent increase of the needs for more infrastructures has led to an increasing conversion of these vital zones to a built environment. Therefore, sustainable development of these valuable and sensitive coastal zones is essential (Oumeraci 2004). Hence, there is an urgent need for cost effective, environmentally friendly solutions to mitigate the risk of disasters related to climate change in coastal settlements. More versatile materials and innovative solutions are required for the design of new, cost effective shore protection structures as well as for the reinforcement of existing threatened coastal barriers and structures, including dune reinforcement and scour protection (Oumeraci and Recio 2010). Geotextile Sand Containers (GSCs), which represent a comparatively low cost and environmentally friendly alternative to hard coastal structures made of concrete or rock, were constructed for the first time in 1957 at Plumpot, The Netherlands (Bezuijen and Vastenburg, 2008). Coastal structures built with GSCs are obtained by substituting rocks or concrete units with containers made of geotextile and filled with locally available sand. A range of successful coastal protection structures using GSCs have been constructed in many parts of the world, especially in Australia, Germany, and South Africa (Heerten et al. 2000, Restall et al. 2004, Saathof et al. 2007, Bleck and Werth 2012, Corbella and Stretch 2012) Many experimental and numerical studies have been carried out in order to investigate the applicability, the durability and the performance of GSC-structures for various coastal applications such as groynes, revetments and breakwaters. However, the GSC is still an emerging technology and there are still no proper design guidelines available for the design of GSC-structures for shore protection. Therefore, the current research study focuses on developing new hydraulics stability curves and formulae for submerged and low-crested GSC-structures. Once GSCs are submerged, it is expected that the durability of the GSCs will significantly be increased due to the rapid marine growth. Moreover, most of the previous research studies have been focusing on either high GSC-revetments or scour protection systems in deeper water. The findings of the current research study are expected to fill some knowledge gaps related to the hydraulic stability of low-crested and submerged GSCstructures. First, the present knowledge related to the engineering properties of GSCs and their effects on the stability of GSC-structures and existing hydraulic stability formulae for GSC-structures were critically 1 Leichtweiß-Institute, Technische Universität Braunschweig, Beethovenstr. 51a, Braunschweig, GERMANY 180

Second, four series of especially designed laboratory experiments, which allowed us to have an insight into the influence of the above mentioned properties on the stability of GSC-structures and also

2 D. T. Dassanayake and H. Oumeraci reviewed (see Dassanayake 2013). The most important engineering properties are the properties of the geotextile material, the sand fill ratio, and the interface friction. Second, four series of especially designed laboratory experiments, which allowed us to have an insight into the influence of the above mentioned properties on the stability of GSC-structures and also to obtain the required parameters for the numerical modelling of their stability were performed. Experimental investigations consisted of two types of laboratory experiments (drop tests and pullout tests), small scale wave flume tests (hydraulic stability tests) and hydraulic flume tests (permeability tests). Third, numerical modelling of GSCstructures was conducted using the weakly coupled RANS-VOF model and FEM-DEM models (Dassanayake and Oumeraci 2013a, 2013). This was the best feasible option to numerically model the hydraulic stability of GSC-structures in the framework of this research study. Finally, combining both the experimental and numerical results, new stability curves and a simple formula were developed for the hydraulic stability of crest GSCs. This newly developed stability curves and stability formulae are expected to foster the applications of GSC structures for coastal protection. Approximately 350 model tests were performed by varying the wave parameters over a wide range (H = 0.05~0.24 m, T = 1.0~4.6 s, Rc = -0.25~+0.1 m, ξₒ = 3~20, in model scale) as well as the engineering properties of the GSCs such as the sand fill ratio (80% and 100%), the type of geotextile material (woven and nonwoven), and the inclination angle of GSCs (0 and +15⁰ to horizontal direction). In the model tests, the incident and transmitted wave parameters, the behaviour of the structure (high speed video records), the flow velocity around the structure, and the pressure variations at the crest of the GSCs structure were systematically recorded. Fig 1 shows the model setup in the 2 m wide wave flume and Fig 2 shows different model configurations tested in the wave flume. A total of 20 wave gauges including 3 arrays were used for wave measurements. The wave gauge arrays in front of the structure were used for the wave reflection analysis, while the gauge array behind the structure was used to determine wave transmission. NEW HYDRAULIC STABILITY TESTS Small scale hydraulic stability tests (Fig 1 and Fig 2) were conducted in the 2 m wide wave flume at Leichtweiss-Institute (LWI), Braunschweig, Germany to investigate the effect of the above mentioned engineering properties on the processes that govern the hydraulic stability of GSCs. Fig. 2 Model setup for the hydraulic stability tests in the 2m wide wave flume of LWI: (a) nonwoven, 80% filled, horizontal GSCs; (b) nonwoven, 100% filled, horizontal GSCs, (c) woven and nonwoven, 80% filled horizontal GSC, Fig. 1 Model setup for the hydraulic stability tests in the 2 m wide wave flume of LWI Nonwoven, 80% filled, horizontal GSCs (Test series NW80H) were considered as the basis for comparison of the effect of different parameters. According to the literature, (except in Australia) typical GSCs used in the field are often 80% filled GSCs (Pilarczyk, 2000, Recio, 2007, PIANC 2011). Then, the length; l c of a 80% filled GSC, is generally twice as large as its width and five 181

As the damage classification available for conventional coastal structures (e.g. rubble mound breakwaters) is not applicable to GSC-structures, a new method of damage classification is introduced for GSCs coastal structures in this study (Table 1).

3 Hydraulic Stability Formulae And Nomograms For Coastal Structures Made Of Geotextile Sand Containers times as large as its height. The geometry of model GSCs were selected such a way that they satisfy the above mentioned geometrical relationship once they are 80% filled. As the damage classification available for conventional coastal structures (e.g. rubble mound breakwaters) is not applicable to GSC-structures, a new method of damage classification is introduced for GSCs coastal structures in this study (Table 1). If a GSC is displaced less than 10% of its length or shows a rotational motion less than 10⁰, then it is considered as stable. If a GSC shows a greater displacement than 10% of its length or greater angular motions than 10⁰ it is considered either as an incipient motion or a displacement depending on the magnitude as shown in Table 1. By considering the critical GSC-layers of a GSC-structure the level of damage was then classified into five categories from no damage (DC 0) to total failure (DC 4). curve which distinguishes the stable and the incipient motion cases was first drawn (Figure 3). Here, stability numbers were plotted against modified relative crest freeboard (i.e. Rc * mod = Rc * /ξ 0, with Rc * = Rc/H m ). Each regular wave test consists of 100 waves and the mean wave height H m and mean wave period T m from the time domain analysis were considered when calculating the stability number N s and the wave steepens S 0. Table 1 New damage classification for individual GSCs and for the entire GSC-structure (modified from Dassanayake et al. 2011) Fig. 3 Hydraulic stability curve for low-crested and submerged GSC-structures made of nonwoven, 80% filled GSCs (series NW80H with Rc = m ~ +0.1 m / regular wave tests with H m ). Ideally, a single hydraulic stability curve should be found which can describe the behaviour of both submerged/low-crested GSC-structures with different positive and negative crest freeboards. Vidal et al. (1992) showed a relationship with the relative freeboard (Rc * = Rc/D 50 ) and the stability number (N s =H s /ΔD 50 ) of lowcrested rubble mound breakwaters by considering four different damage categories. The possibility of developing a similar relationship for the submerged/lowcrested GSC structures was also examined in the current research study. Based on the above mentioned new damage classification (Table 1), new hydraulic stability curves and a formula were developed which relate stability number N s and a modified relative freeboard Rc * mod=rc * /ξ 0 where Rc * = Rc/H m represents the common freeboard and ξ 0 the surf similarity parameter. Based on the hydraulic stability results from regular wave tests, a Finally, instead of defining two separate stability curves for the tests with regular waves and irregular waves, the possibility of using the same stability curves (Figs 3 and 8) for both types of waves were studied. Here, the irregular wave test data (test series NW80H) are plotted in Fig 8 by using the characteristic wave height H 2%, which is the mean of the highest 2% of the waves in the time series (mean of the highest 20 incident waves from the wave reflection analysis and for a Rayleigh distribution of the values H 2% = 1.4 H s ). The results show a better agreement with the initial curve developed for regular wave test. PREVIOUS HYDRAULIC STABILITY TESTS Moreover, the hydraulic stability curve for 80% nonwoven GSCs is validated by comparing the data from two previous model tests conducted by Oumeraci et al. (2002a and 2002b) in a small scale model (2 m wide wave flume of LWI, Braunschweig, Germany) and in a large scale model (large wave flume GWK, Hannover, Germany) to study the hydraulic stability and the 182

Two different types of structures were tested: low-crested reef structures and revetments (Fig 4 and 5).

4 D. T. Dassanayake and H. Oumeraci hydraulic performances of low-crested GSC-structures. One of the main objectives of these experiments was to identify the most relevant parameters for the hydraulic stability of high overtopping GSC-structures. Two different types of structures were tested: low-crested reef structures and revetments (Fig 4 and 5). The applicability of new formula (Fig 6) for nonwoven 80% filled GSCs was verified using the hydraulic stability results from these two scale model studies. wave tests from the current study cover a wide range from -3.0<(Rc/H s )< However, most of the tests were performed with negative crest freeboards. Therefore, the results from Oumeraci et al. (2002a and 2002b) are useful to extend the validity of the formula for low-crested structure. Even though the damage classification system used by Oumeraci et al. (2002a and 2002b) is different from what was proposed in Table 1, damage definitions; stable and little displacement in the two former studies are considered as equivalent to the damage level no damage (DC 0) and incipient motion (DC 1) in the current study. Fig. 4 Small (LWI) scale model tests on the hydraulic stability of crest GSCs by Oumeraci et al. (2002a) The main reasons for the selection of these two specific model studies are; (i) the current research study is focused on submerged/low-crested GSC-structures and the data sets from these two studies are most relevant as in both cases the tests were performed with 80% filled nonwoven GSC and with 1:1 seaward slope. (ii) Since most of the model tests were performed either with zero crest freeboard or with negative crest freeboards, with the help of these data sets, it is possible to extend the range of validity of the new formula. Figs 4 and 5 provides an impression of the tested relative crest freeboards (R c* =Rc/H) with irregular tests performed in the current research study and from the previous hydraulic stability tests by Oumeraci et al. (2002a and 2002b). The crest freeboards of irregular Fig. 5 Large (GWK) scale model tests on the hydraulic stability of crest GSCs by Oumeraci et al. (2002b) NEW HYDRAULIC STABILITY FORMULAE A new hydraulic stability formula for incipient motion of crest GSCs in a low-crested/submerged GSCstructure was developed using the stability curves drawn in Figs 3, 7 and 8. This formula can be written as; As shown in Fig 7, the hydraulic stability curves for two different geotextile materials (woven and nonwoven) tend to converge as the submergence depth increases. 183

The differences between woven and nonwoven GSCs are reduced in terms of the hydraulic stability, mainly because of different dominant failure mechanisms (see Dassanayake and Oumeraci 2012b). Fig.

5 Hydraulic Stability Formulae And Nomograms For Coastal Structures Made Of Geotextile Sand Containers Fig. 6 New hydraulic stability formula for the incipient motion of crest GSCs in low-crested or submerged GSCstructures (Dassanayake 2013). The differences between woven and nonwoven GSCs are reduced in terms of the hydraulic stability, mainly because of different dominant failure mechanisms (see Dassanayake and Oumeraci 2012b). Fig. 8 Hydraulic stability test data for lowcrested/submerged GSC-structures made of nonwoven, 80% filled GSCs (series NW80H with Rc = m~ +0.1 m / Irregular wave tests with Hs) Fig. 7 Incipient motion curves derived from regular wave tests Furthermore, 100% filled GSCs show smaller overlapping lengths compared to 80% filled GSCs. The overlapping length is important for the overall stability of low-crested structures as the possibility of pulling out or overturning of slope elements could be higher. However, as the submergence depth increases, only the crest elements govern the stability of the entire structure. Therefore, nonwoven 80% and 100% filled curves tend to diverge as the submergence depth increases (see Dassanayake 2013 and Fig 7). CONCLUSION One of the main objectives of this study is to develop new simplified hydraulic stability formulae based on recent and past experimental investigations and numerical simulations. The existing formulae and nomograms were critically reviewed/analysed with a summary of the results can be found in Dassanayake and Oumeraci (2012b). Most of the knowledge gaps were found to be rather related to the hydraulic stability of submerged and low-crest GSCstructures as compared to sufficiently high surfacepiercing structures (i.e. without excessive wave overtopping). Therefore, systematic investigations were conducted, mainly to identify the parameters relevant for the hydraulic stability. The main results can be summarizes as: Then new hydraulic stability nomograms were developed for horizontally placed, nonwoven 80% filled GSCs, nonwoven 100% filled GSCs and woven 80% filled GSCs by combining experimental results from regular wave tests and from numerical simulations (Fig 7). The stability curve for 80% filled nonwoven GSCs was extended to irregular waves using the results of the current and past experimental studies with wave spectra. It was then shown that the stability curves developed using regular wave test data can also be applied for irregular waves by substituting the mean wave height Hm of regular waves by characteristic wave height H 2%. This stability curve is validated for low-crested structure using the experimental data from small and large scale tests from Oumeraci et al. (2002a, 2002b) Finally a new formula was proposed for the hydraulic stability of submerged/low-crest GSC-structures. In most 184

6 D. T. Dassanayake and H. Oumeraci of those structure, for which the critical element are the GSCs at the crest. Therefore, this formula applies only for the incipient motion of crest GSCs (Fig 3 and Fig 8). For sufficiently high structures, Hudson like stability formula from Oumeraci et al. (2002b) can be applied as the formula is developed and validated with both small and large scale experimental data. ACKNOWLEDGMENTS The PhD fellowship granted for the first author by the German Academic Exchange Service (DAAD) is greatly acknowledged. Leichtweiss Institute, Technische Universität Braunschweig, Germany, and NAUE GmbH & Co. KG are gratefully acknowledged for their financial support to conduct the laboratory experiments. REFERENCES Bezuijen, A. and Vastenburg, E. (2008), Geosystems, Possibilities and Limitations for Applications, Proceeding of EuroGeo4; Fourth European Geosynthetics Conference, Edinburgh, Scotland, UK. Bleck M. and Werth K. (2012), Geotextile sand-filled containers as structures against erosion of sandy coasts - a comparison to rock material relating design, functionality, eco-effectiveness and costs, 12th Baltic Sea Geotechnical Conference, Germany Corbella, S. and Stretch, D.D. (2012), Geotextile sand filled containers as coastal defence: South African experience, Geotextiles and Geomembranes Journal, Vol. 17, Elsevier, pp Dassanayake, D.T. (2013) Experimental and numerical modelling of geotextile sand container structures, Ph.D. thesis (In preparation), Technische Universitaet Braunschweig, Germany. Dassanayake, D.T. and Oumeraci, H. (2013), Experimental and numerical modelling of the hydraulic stability of geotextile sand containers under wave loads, Proceedings of 9. FZK-Kolloquium "Modellierung im Seebau und Küsteningenieurwesen", Hannover, Germany Dassanayake, D.T., Oumeraci, H. (2012a) Engineering properties of geotextile sand containers and their effect on hydraulic stability and damage development of low-crested / submerged structures, The International Journal of Ocean and Climate Systems, Vol. 3, Issue 3, Multi Science Publishing, Essex, UK, Dassanayake, D.T., Soysa, A., Munith-Hadi, F., Oumeraci, H. (2011) Hydraulic stability of lowcrested GSC-structures, Internal report, Leichtweiß- Institute for Hydraulic Engineering and Water Resources, Braunschweig, Germany. Dassanayake, D.T.; Oumeraci, H. (2012b): Hydraulic Stability of Coastal Structures Made of Geotextile Sand Containers (GSCs): Effect of Engineering Properties of GSCs. Proceedings 33rd International Conference Coastal Engineering (ICCE), ASCE, Vol. 1, Santander, Spain, p 14. Heerten, G., Jackson, A., Restall, S. and Saathoff, F. (2000), New Developments with Mega Sand Containers of Nonwoven Needle-Puncture Geotextile for the Construction of Coastal Structures, Proceedings of International Conference on Coastal Engineering 2000, Australia McGranahan, G., Balk, D. and Anderson, B. (2007), The rising tide: assessing the risks of climate change and human settlements in low elevation coastal zones, Environment and Urbanization, Vol. 19-1, pp Mori, E. (2009), Coastal structures made of geotextile elements filled with sand: field and experimental research, PhD Thesis, Università degli Studi di Firenze, Florence, Italy Oumeraci, H. (2000), The Sustainability Challenge in Coastal Engineering (Keynote Lecture). Proceedings 4th International Conference of Hydrodynamics Oumeraci, H. (2004), Sustainable coastal flood defences: scientific and modelling challenges towards an integrated risk-based design concept. Keynote Lecture. Proc. First IMA International Conference on Flood Risk Assessment. Bath, UK, IMA - Institute of Mathematics and its Applications: pp Oumeraci, H., Bleck, M., Hinz, M. (2002a) Untersuchungen zur Funktionalität geotextiler Sandcontainer. Leichweiss-Institute for Hydraulic Engineering and Water Resources, Report Nr. 874, Braunschweig, Germany, (in German). Oumeraci, H., Bleck, M., Hinz, M., Kübler, S. (2002b) Großmaßstäbliche Untersuchungen zur hydraulischen Stabilität geotextiler Sandcontainer unter Wellenbelastung. Leichweiss-Institute for Hydraulic Engineering and Water Resources, Report Nr. 878, Braunschweig, Germany, (in German). Oumeraci, H., Recio, J. (2010) Geotextile Sand Containers for shore protection, Invited Chapter in Handbook of Coastal and Ocean Engineering, World Scientific Publishing, Singapore, PIANC, (2011) The Application of geosynthetics in waterfront areas-pianc Report No. 113 by MarCom Working Group 113. Pilarczyk, K. W. (2000) Geosynthetics and geosystems in hydraulic and coastal engineering. A.A. Balkema, Rotterdam, the Netherlands. Recio, J. (2007) Hydraulic stability of geotextile sand containers for coastal structures- effect of deformations and stability formulae, PhD Thesis, 185

7 Hydraulic Stability Formulae And Nomograms For Coastal Structures Made Of Geotextile Sand Containers Leichweiss-Institute for Hydraulic Engineering and Water Resources, Braunschweig, Germany. Restall, S., Hornsey, W., Oumeraci, H., Hinz, M., Saathoff, F., and Werth, K. (2004), Australian and German Experiences with geotextile Containers for Coastal Protection, proceedings Eurogeo 2004 Vidal, C., Losada, M.A., Medina, R., Mansard, E.P.D., Gomez-Pina, G., (1992) A universal analysis for the stability of both low-crested and submerged breakwaters. Proceedings of 23rd International Conference on Coastal Engineering, ASCE,

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