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Breaching of a barrier under extreme events. The role of morphodynamic simulations

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Breaching of a barrier under extreme events. The role of morphodynamic simulations Vicente Gracia†, Manuel García†, Manel Grifoll† and Agustín Sánchez-Arcilla† †Maritime Engineering Laboratory (LIM) at Universitat Politècnica de Catalunya, Barcelona, 08034, Spain [email protected], [email protected], [email protected], [email protected]

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ABSTRACT

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Gracia, V., García, M., Grifoll, M. and Sánchez-Arcilla, A., 2013. Breaching of a barrier beach under extreme events. The role of morphodynamic simulations In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 951-956, ISSN 0749-0208. A study of three breaching episodes of the Trabucador barrier beach (Spanish Mediterranean coast) is presented. The analysis is done from a modeling perspective using XBEACH and SWAN. The morphodynamic model (XBEACH) has been validated with the available information. The so obtained Brier Skill Score index has been 0.44 which can be considered as acceptable. Results indicate that in all cases breaching was taking place after 8 hours of wave action (storm) or less. Besides, four main parameters control the final morphodynamic response of the barrier: the storm intensity, the water level and the initial emerged topography and the storm duration. Major breaching occurs when swell waves are in coincidence with high water levels whereas if they are uncoupled the barrier tends to be breached by a diversity of smaller channels. ADDITIONAL INDEX WORDS: Morphodynamics, barrier beach, breaching, numerical simulations, XBEACH.

INTRODUCTION Barrier beaches are one of the most vulnerable coastal systems in which morphological changes can take place very rapidly (Hayes 1979, Leatherman, 1988). Due to its nature, low lying and relatively narrow, these environments are typically flooded within a year and under extreme events can be breached. The understanding of the processes involved during the breaching is a fundamental issue for an adequate management of these systems. Sallenger (2000) proposed a scale to describe the impact of a storm on a barrier island. The author defined four states as a function of the runup elevation (including mean sea level variations) and the dimensions of the barrier identifying the most relevant topographic changes and forcing processes. An extensive review of the overwash processes can be found in Donelly et al. (2006). Breaching episodes have been widely reported (Sánchez-Arcilla and Jiménez, 1994; Terchunian and Merkert 1995; Kraus and Wamsley, 2003; Wamsley and Hathaway 2004; Wutkowski 2004; Freeman et al. 2004 among others) although detailed information of the hydrodynamics and morphodynamics of such events remains scarce. The development of analytical and numerical models have provided managers with a tool in the decision making process. Kraus and Hayashi (2005) developed a process-based model to reproduce the morphological evolution of a barrier beach. Connell et al. (2007) extended the model for multiple breaches. Cañizares and Irish (2008) used a combination of ____________________ DOI: 10.2112/SI65-161.1 received 07 December 2012; accepted 06 March 2013. © Coastal Education & Research Foundation 2013

models to simulate storm induced morphodynamics at Long Island barrier beach and compared the so obtained results with historically documented changes. Roelvink et al. (2009) presented XBEACH (an open source code to evaluate eXtreme BEACH behavior) a two-dimensional model for wave propagation, long waves and mean flow, sediment transport and morphological changes specifically created to simulate the impact of extreme storms over dune and barrier beaches. The model has been used by Lindemer et al. (2010) to simulate the effect of tropical storms in a barrier island in the Gulf of Mexico whereas Ciavola et al. (2010a 2010b) have applied it to assess the evolution of different barrier beaches along Europe. O’Shea et al. (2011) also use this strategy combining monitoring and modeling (MIKE21) in order to evaluate the evolution of a breached barrier (Rossbeigh, Ireland). In this work we analyze the morphodynamic response of the Trabucador beach (Spanish Northeastern Mediterranean coast) under extreme events from a modeling perspective and evaluate the role of the main driving forces, waves and water levels, controlling the barrier. We are using a sequence of models, SWAN and XBEACH, already tested for the Catalan coast (Bolaños et al. 2009; Sánchez-Arcilla et al. 2012) to determine the barrier response for the three main reported breaching episodes in the last years. The objective is to obtain a simpler conceptual model to be used as a tool for the barrier beach management. Besides, the validation of the models has also been considered in order to include them in a more general morphodynamic operational forecasting system.

Journal of Coastal Research, Special Issue No. 65, 2013

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Figure 1. Aerial photo of the Trabucador barrier beach (under permission of D. Bouyssi) and breaching event of October 1990.

STUDY AREA The Trabucador beach is a narrow sandy barrier (mean sediment diameter of about 210 µm) located at southern lobe of the Ebro delta (Figure 1) of about 5.5 km long and variable width, between 150 m and 200 m. Originally, the barrier was formed by the redistribution of the sediments delivered by the Ebro river until the 14th century (Maldonado, 1986; Somoza et al., 1998). At present the evolution of the barrier is strongly determined by the impact of severe storms since no additional sediments are supplied due to actual location of the river mouth towards the North. During the last 20 years the barrier has experienced 8 important breaching episodes. At the beginnings of the 90’s an artificial sand dune of about 3 m high was constructed to protect a road that connects a salt industry (located at the south) with the mainland; however, after its construction the only maintenance of the barrier has consisted in the infilling of the breaches with the sediment deposited at the bayside. The astronomical tide in the area is about 0.3 m, however, the main sea level variations are due to meteorological surges and can reach values of up to 1 m Bolaños et al. (2009). The mean significant wave height is about 0.8 m with an associated period of 5 s. The wave field is highly heterogeneous with main components from E, NW (due to the existence of strong land winds channeled through the Ebro valley) and S being the former the most important in terms of energy and the responsible of the breaching of the barrier. The maximum offshore wave height recorded by the existing network for oceanographic and coastal meteorological measurements (XIOM)) has been close to 10 m and maximum peak periods of about 14 s. Different studies have been conducted to analyze the morphological evolution of the barrier. Sánchez-Arcilla and

Jiménez (1994) studied the breaching of the barrier that took place in October 1990 by comparing beach profiles obtained just after the episode, reporting a breach of about 800 m at the center and an associated sand transport of about 70.000 m3 mostly transported into the bay. Guillén et al. (1994) studied the effect of the overwash processes at the Trabucador bar over a longer period by considering a reduced set of beach profiles obtained in a year. Jiménez and Sánchez-Arcilla (2004) modeled the spit-barrier unit at a decadal scale. Larson et al. (2009) applied an analytical model of beach erosion and overwash for assessing the quantity of eroded sediment and overwash duration in terms of empirical probability distribution functions. However, the authors simplified the complexity of the system by assuming a uniform longitudinal shape.

METHODS Three main extreme eastern events have been selected to study the morphodynamic response of the barrier: the October 1990, the November 2001 and the October 2003 storms. In all cases, a severe breaching was reported also affecting the existing power line that supplies energy to the salt industry. All events had the same meteorological origin characterized by the existence of a low pressure centre located over the W Mediterranean in what has been named a Mediterranean tropical-like storm (Fita, 2007). Maximum values of offshore Hm0 recorded by the XIOM go from 3.5 m to 5.95 m with peak periods comprised between 10 s and 14 s. All events showed a similar two peak temporal distribution although had different durations. According to Mendoza et al. (2011) the storms can be ranked between moderate (mean values of 4.4 m for Hs max and 77 h of duration) to extreme (with Hs max mean values of about 6 m or higher and 99 h of duration). Mean water level variations were obtained from the reported



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Table 1. Hydrodynamic and meteorological characteristics of the selected storms (categories from I to V according to Mendoza et al., 2011). (* considering the recorded data). Date Duration (h) Hs (m) Tp (s) Energy Max storm surge Local Storm (H2 hours) (m) category October 1990

89

4.51

10.5

578

0.47

November 2001

179

5.95

14.3

1413

0.39

V

October 2003

101

3.55

12.5

768

0.57

IV*

literature (Jiménez et al., 1991) and XIOM. A summary of the main hydrodynamic and meteorological characteristics is shown in Table 1. The selected morphological model has been XBEACH (version 18) which solves the hydrodynamic nearshore processes solving coupled 2DH equations for wave propagation, flow and sediment transport for varying wave and flow boundary conditions which has been successfully applied to reproduce erosion, overwash and breaching for severe storm and hurricane conditions. The hydrodynamic module consists of formulations for short wave envelope propagation, non stationary shallow water equations using a time dependent wave action balance solver which allows calculating wave refraction and dissipation of wave groups travelling in different directions. Wave-current interaction in the short wave propagation is also included and different wave breaking dissipation models can be selected. Depth-averaged

III

undertow is obtained using a Generalized Lagrangian Mean (Walstra et al., 2000) approach was implemented to represent the depth-averaged undertow is also evaluated. The sediment transport module includes Soulsby – Van Rijn formulations which solve the 2DH advection-diffusion equation and produces total transport vectors used to update the bathymetry. The wet-dry beach regions include an avalanching mechanism (Van Thiel de Vries, 2009). Two numerical simulation strategies have been considered: feeding XBEACH with the hydrodynamic recorded data and by forecasting the hydrodynamics from meteorological predictions. The first approach is used for all storms whereas the second is only used to refine the initial forcings of the October 2003 episode because of the intermittent functioning of the XIOM network. To do that wind and atmospheric pressure field forecasts of the area (provided by the Catalan Meteorological Office and obtained from

Figure 2. Measured and modelled beach profiles for the storm of October 1990 and spatial distribution of the bottom differences along the barrier between the December 1990 and the modelled results.


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a

b

c

d

Figure 3. Storm impact scale evolution according to Sallenger (2000) for the event of October 1990 (a); November 2001 (b); October 2003 (d) and by feeding the model with predicted meteorological and hydrodynamic conditions (c) for the same event. the MM5 model) were used as initial forcing conditions in SWAN model (Booij et al., 1996) which finally provided the relevant hydrodynamic parameters to XBEACH. Unfortunately only the October 1990 had detailed enough information of the bottom morphology which mainly consists in 9 topo-bathymetric beach profiles (covering the bay side and the open sea up to 15 m depth) surveyed just prior and after the pass of the storm. This data set has been used to validate XBEACH model. For the November 2001 and October 2003 storms the emerged topography of the barrier has been obtained from a LIDAR-based digital elevation model of 1 m by 1 m provided by the Institut Cartogràfic de Catalunya whereas the submerged part is constructed by applying a principal component analysis to a beach profile data set of 4 years of the area. In all three cases a curvilinear grid is constructed with variable node density, finer (5 m by 5 m) for the central part of the barrier which is more prone to breaching and with a lower resolution (an average 20 m by 20 m grid) for the rest. The so obtained computational domain has a length of 5700 m in the longshore dimension and 6370 m in the cross-shore (118755 nodes) although in the case of October 1990, the dimensions are reduced to 4460 m by 6070 m (79,632 nodes).

RESULTS AND DISCUSSION Model validation The comparison between measured (December1990) and modeled profiles using different XBEACH setup configurations is shown in figure 2. The goodness of the results is calculated using the Brier Skill Score index (BSS) proposed by van Rijn et al. (2003) for the most active part of the computational domain, between the contour -3,5 m for the open sea and the 3 m depth line at the bay derived from the existing alongshore transects. The best parameter combination (morfac=10; s=100; wetslp=0.6; smax=1,3; Smax=1,1) gave a BBS of 0.44 which according to van Rijn et al. (2003) could be considered as reasonable. It has to be

noticed that the results of the emerged part of the profiles fits more accurately than submerged active cell on both offshore and bay sides. If the distribution of the differences between modeled and observed is considered it can be seen how the model is underestimating the erosion in the sea side of the barrier (with mean differences of about 0.5 m) whereas at the bayside the model tends to overestimate the accumulation of the over washed sediment being the differences of about 0.5 m as an average. The central part of the barrier (where smaller numerical boundary effects are expected) shows the best agreement. Nevertheless, the relevant fact is that the model is able to reproduce the breaching.

Barrier response to storms XBEACH has been able to reproduce the breaching of the barrier for the three events such as it happened in Nature. However, its evolution is different depending on the observed combination of Hs, mean water level and initial topography. Figure 3 shows the predicted response according to the storm impact scale of Sallenger (2000). A common feature is the almost absence of the swash regime (all events started with approximately the same hydrodynamic conditions) along the barrier, which reflects the very low initial topography in conjunction with relatively high wave heights and mean water level, and a rapid transition towards a collision regime. The October 1990 event moved immediately towards an overwash state evolving entirely towards inundation in about 4 hours. This situation is maintained for about 8 hours at the extremes of the barrier and about 16 hours in its central part indicating the formation of an important breach at the center. After a small return towards overwash, in coincidence as expected with the registered decrease in the wave height, the whole barrier evolved to inundation again. The final morphological response was the development of a wide channel at the centre of the barrier of a length of about 2000 m and an important accumulation of sand at the bayside along the entire barrier. This mechanism, known as



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Figure 4. Breaching of the Trabucador bar after the November 2001 event and simulated evolution (at the beginning of the event). rollover, allows the barrier to migrate towards the bay while approximately maintaining its sand volume and it has been considered as the natural process that preserves the barrier by keeping its width. Results of the 2001 storm show a larger transition from collision to overwash reflecting a more gradual increase of the hydrodynamic conditions. Although the recorded wave height was significantly higher (see Table 1), the peak of the storm was not in coincidence with the highest mean water level of the event. This jointly with a relatively higher emerged topography (due to its reconstruction after previous storms) has allowed the barrier to resist wave impacts for about 36 hours. After that, waves and mean water level were kept at a high level producing the breach of the barrier at different locations, which was traduced in the development of a number of channels crossing the barrier and the consequent deposition of sediment at the bayside. The modeled morphology indicates the formation of a main channel at the centre and other small channels along it, however the barrier evolved towards a completely inundation status at the end of the event. Figure 4 shows an aerial photograph of the barrier after the pass of the November 2001 storm. As it can be seen the storm produced small breaches at the south and an important channel at the central part. The modeled evolution for the 2003 event showed how the barrier is in the overwash regime almost all the time without the development of a significant breaching. This morphological response is better reproduced if the model is fed by hydrodynamic and meteorological forecast conditions. In that case, although the barrier is maintained within the overwash regime a development of small channels crossing the barrier is produced in a relatively short time (less than 8 hours). In that case the final morphological response is the creation of small washover fan deposits. The morphological evolution of the Trabucador barrier beach depends on the combination of three main parameters: the initial topography (dune protected or low-lying), the incident wave height and the associated storm surge. Its response depends not only on the intensity of the process (high or low) but also on the moment when they take place (phase or lag between waves and mean water level). In case of having a double peak storm in coincidence with high water levels impacting on a low-lying barrier (as the October 1990 event) the response of the barrier is to rollover entirely creating an important sand accumulation at the bayside. On the other hand, high wave heights do not necessarily ensure the same response. If waves and mean water level are not coincident in time the capability of the event to breach the barrier is lower, this effectiveness is also reduced if the emerged part of the barrier is high (as the November 2001 event) and producing

important channels along the barrier. As expected, when less energetic events appear lagged with the mean water level their impact over a high emerged barrier (October 2003) triggers the development of multiple channels across it and the formation of small washover fans. The morphodynamic response within the event is as follows: at the beginning of the episode the eastern waves impact the barrier with a relatively large angle developing an important longshore current, as these swell waves increase the cross-shore component of the current also increases (due to refraction) and the barrier is overwashed. This mechanism is enhanced by the coexistence of waves with a high water level. The final state of this developed cross-shore current is the breaching of the barrier. The barrier breach creates a gradient in the longshore current being lower downwards. Furthermore, sediment arriving at that part is directly transported towards the bay and as a result, the breach acts as an attractor of sediment not only during the peak of the event but also when the forcing conditions diminish.

CONCLUSIONS The modelling approach used in this work has been able to reproduce the breaching of the Trabucador Barrier beach for different energetic conditions. In all cases the breach or breaches of the barrier takes place in a relatively short time (less than 8 hours). The morphological simulations have revealed a different response depending on the combined action between waves, mean level and initial topography. Major breaching of the barrier is reached, as expected, when high waves are simultaneous with high water levels and the emerged part is relatively low. Under these conditions the rollover of the barrier takes place. On the other hand, the development of channels across the barrier is obtained when waves are less intense and not simultaneous with the mean sea level surges. The refinement of the hydrodynamic conditions has permitted to identify a more detailed evolution of the breaching of the barrier as reported. Finally, the existence of a breach generates a gradient in the longshore current which, in conjunction with the developed crossshore current, acts as a sediment attractor during the whole episode.

ACKNOWLEDGEMENT This work has been partially financed by the European Union research project from the 7th Framework Program FIELD_AC (contract num. 242284). The authors also want to acknowledge the information supplied by the Servei Meteorològic de Catalunya and the Institut Cartogràfic de Catalunya. Part of the research was also


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supported by the Spanish research project Covariance (CTM201219709).

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