Contributions_files/FULL 2

CHAPTER 2 Waves & Currents Shear Stress Antonio Tomás, Fernando J. Méndez & Ínigo J. Losada Environmental Hydraulics Institute “IH Cantabria”, Universidad de Cantabria Abstract The objective of this work is to obtain the wave -­‐and current-­‐ induced bed shear stresses throughout the Catalonia's coast (Spain) due to the 26th of December 2008 storm. We use the wave and surge reanalysis GOW 2.1 and GOS 2.1 (Global Ocean Waves 2.1 and Global Ocean Surges, IH Cantabria). From those hourly databases we downscale -­‐propagate to shallow waters-­‐ the sea states up to ~250 m spatial resolution, from December 26 th 7:00 to December 27th 19:00. High resolution waves and storm surge currents allow us to calculate the maximum bed shear stresses produced during the storm following Soulsby (1997), and therefore characterizing the storm impact to the different communities in each study site. Image: Maximum bed shear stress, results and methodology by IH Cantabria (UC) 45 A. TOMÁS, F.J. MÉNDEZ, AND I. LOSADA Tomás, A., Méndez F.J., Losada, I.J. (2012) Bed shear stress during Sant Esteve’s storm (26th December 2008) along the Catalonia’s coast (NW Mediterranean). In: Mateo, M.A. and Garcia-­‐Rubies, T. (Eds.), Assessment of the ecological impact of the extreme storm of Sant Esteve’s Day (26 December 2008) on the littoral ecosystems of the north Mediterranean Spanish coasts. Final Report (PIEC 200430E599). Centro de Estudios Avanzados de Blanes, Consejo Superior de Investigaciones Científicas, Blanes, pp. 45 – 54. 46 Proyecto Intramural Especial Cofinanciado CEAB-­‐CSIC, PIEC 200430E599 FINAL REPORT, p. 45 – 54, April 2012 Bed shear stress during Sant Esteve’s storm (26th December 2008) along the Catalonia’s coast (NW Mediterranean) By Antonio Tomás, Fernando J. Méndez, Ínigo J. Losada* Environmental Hydraulics Institute "IH Cantabria". Universidad de Cantabria. Parque Científico y Tecnológico de Cantabria. C/ Isabel Torres, 15. 39011, Santander, Spain. *[email protected] Resumen Abstract El objetivo de este estudio es la obtención de la tensión tangencial en los fondos costeros catalanes debidas al oleaje y a las corrientes producidas por el temporal del día 26 de diciembre del 2008. Para ello se han utilizado los reanálisis numéricos de oleaje y marea meteorológica denominadas GOW 2.1 y GOS 2.1 (Global Ocean Waves 2.1 y Global Ocean Surges 2.1, IH Cantabria). A partir de dichas bases de datos horarias se propaga el oleaje hasta la costa, desde las 7:00 del 26-­‐12-­‐2008 hasta las 19:00 d el 27-­‐12-­‐2008, llegando a una resolución espacial de ~250 m. La obtención del oleaje propagado a alta resolución y las corrientes de marea meteorológica nos permiten calcular las máximas tensiones tangenciales en el fondo, durante el temporal, siguiendo las formulaciones de Soulsby (1997), caracterizándose la incidencia del temporal en las diferentes comunidades de cada zona de estudio. The objective of this work is to obtain the wave -­‐and current-­‐ induced bed shear stresses throughout the Catalonia's coast (Spain) due to the 26th of December 2008 storm. We use the wave and surge reanalysis GOW 2.1 and GOS 2.1 (Global Ocean Waves 2.1 and Global Ocean Surges, IH Cantabria). From those hourly databases we downscale –propagate to shallow waters-­‐ the sea states up to ~250 m spatial resolution, from December 26th 7:00 to December 27th 19:00. High resolution waves and storm surge currents allow us to calculate the maximum bed shear stresses produced in the bottom during the storm following Soulsby (1997), and therefore characterizing the storm impact to the different communities in each study site. Introduction meteorological event on coastal natural resources. Once the shear stress applied at each sampling station has been estimated, the path is free to relate shear stress to the degree of impact on the various species. This contribution covers that first step. K nowing how wave and current action transforms into shear stress at the surface of the benthic organisms or at the substrate where they live is crucial to understanding, quantifiying and predicting the impact of an extreme 47 A. TOMÁS, F.J. MÉNDEZ, AND I. LOSADA Methods The general methodology to calculate the maximum bed shear stress in each study site along the Catalonian’s coast can be described in 4 general steps (see Figure 1): 1.-­‐ 10-­‐m wind and sea level pressure. 2.-­‐ Waves. 3.-­‐ Storm surge. 4.-­‐ Shear Stress. A summary of the explanation of each step is shown below. representation of orography) they cannot detect small scale phenomena associated to the Sant´s Esteve storm. To solve this problem we nested a Limited Area Model (LAM) to a global reanalysis to obtain proper representations of the atmospheric conditions in a regional mesoscale resolution. Then, we validate the winds obtained using instrumental data. We used the global ERA-­‐Interim reanalysis data from the European Centre for Medium-­‐Range Weather Forecast (1989-­‐present) with a spatial Figure 1. Diagram of the methodology.
1.-­‐ 10-­‐m wind and sea level pressure. First step of the methodology is the downscaling of global atmospheric reanalysis (wind and pressure); because their low resolution (coarse 48 resolution of ~0.7º (Dee & Uppala, 2009). Nested in ERA-­‐Interim we used the LAM model Weather Research and Forecasting model with the Advanced Research Dynamical solver (WRF-­‐
ARW, Skamarock et al., 2008). WAVES, CURRENTS, AND SHEAR STRESS The regional wind and pressure fields from this downscaling procedure (SeaWind 2.1) constitutes an hourly atmospheric reanalysis database over a 15 km spatial resolution grid for the 1989-­‐2009 period, which covers the South Atlantic-­‐European region and the Mediterranean basin. Then, a special effort is employed in the evaluation of the accuracy of this regional reanalysis by using both satellite and buoys data (Menéndez et al., 2011). 2.-­‐ Waves IH Cantabria has generated numerically an hourly 20-­‐year (1989-­‐
2009) database in the Mediterranean, with a spatial resolution of 0.125º. Such reanalysis, known as GOW 2.1 (Global Ocean Waves) has been run with model WaveWatch III (WWIII, Tolman, 2002) using as forcing the hourly winds of SeaWind 2.1 and boundary conditions from GOW 1.0 global wave reanalysis (Reguero et al., 2012) The GOW 2.1 database has been calibrated with instrumental information (all the data from 6 missions of the satellites from 1992 to 2008). We have used a non-­‐linear calibration technique based on directional quantiles (Mínguez et al., 2011). The wave generation model WWIII solves the equation of spectral density balance. The basic hypothesis assumed by this model in the numerical resolution is that the properties of the currents and bathymetry as well as the wave fields, vary in space and time at scales that are much longer than one wave length. Consequently, a limitation of the model is the inability to simulate effects of wave propagation to shallow waters. To propagate the calibrated GOW 2.1 database to shallow waters we have used SWAN model (Booij et al., 1999). This model is based in the equation of phase-­‐averaged wave action that allow correctly simulating the processes of refraction, shoaling, dissipation, diffraction around abrupt bathymetric features, dissipation near the bottom, non-­‐linear interactions, breaking waves and wind wave generation. The final propagated waves from this downscaling procedure were named DOW 2.1. (Downscaled Ocean Waves). As the objective of this work is the characterization of the coastal dynamics throughout the Catalonia’s coast during Sant Esteve’s storm (26th December 2008), we have propagated 37 sea states, from December 26th 7:00 to December 27th 19:00. By means of consecutive grids the calibrated 37 sea states of the storm of Sant Esteve are spectrally propagated with the SWAN model. In Figure 2 we present the 6 grids used to reproduce the waves along the entire Catalonian’s coast (GM17, GM18, GM19, GM20, GM21 y GM22). The blue dots show the location of the study sites. 49 A. TOMÁS, F.J. MÉNDEZ, AND I. LOSADA Figure 2. Grids used in the propagation with SWAN model (GM17, GM18, GM19, GM20, GM21 y GM22), to reproduce the sea state conditions throughout the entire Catalonia’s coast. The blue dots show the location of the study sites. Figure 3. Examples of time series validation in year 2008. In the upper and medium panel: Global Ocean Waves Reanalysis (GOW 2.1) vs Palamós buoy (OPPE). In the lower panel: Storm Surge Reanalysis (GOS 2 .1) vs Barcelona tide gauge (OPPE).
50 WAVES, CURRENTS, AND SHEAR STRESS We have validated the GOW 2.1 time series with the Palamós buoy of “Organismo Público Puertos del Estado” (OPPE, Spanish Ministry of Public Works), obtaining a good agreement between reanalysis and observations (see figure 3). 3.-­‐ Storm surge. Ocean currents can be defined as the summation of the current generated by the astronomical tide and the storm surge. As in the Catalonia’s coast the astronomical tide is practically negligible in relation to the storm surge, we have calculated only the storm surge currents. IH Cantabria has generated numerically an hourly 20-­‐year (1989-­‐
2009) database of storm surge in Southerm Europa using the circulation model ROMs (Regional Ocean Modelling System, Shchepetkin & McWilliams, 2005) with a spatial resolution of 0.125º. Such reanalysis, known as GOS 2.1 (Global Ocean Surges) is also run with the winds and pressures from the database SeaWind 2.1. We have validated the GOS 2.1 database with Barcelona tide gauges time series of storm surge (non-­‐tidal residual) of “Organismo Público Puertos del Estado” (OPPE, Spanish Ministry of Public Works). The validation analysis indicates a good agreement between reanalysis and observations (see figure 3). 4.-­‐ Shear stress. We have applied the formulation of Soulsby (1997) (see Equation 1) to determine the evolution of the maximum shear stress produced during the 37 hours of the storm. The formulation is based on the waves (DOW 2.1) and currents (GOS 2.1). To apply Soulsby (1997) formulation three wave parameters are required: the average quadratic orbital speed at the bottom (Urms), the peak period (Tp) and the mean wave direction (θw). Note that SWAN model allow obtaining the Urms, Tp and θw of the entire propagation grid. Besides, two current parameters are needed: the vertically averaged speed (Uc) and the mean direction (θc). Equation 1. Soulsby (1997) formulation used to calculate the maximum bed shear stress induced by both current (module and direction) and wave (orbital velocity, period and direction).
51 A. TOMÁS, F.J. MÉNDEZ, AND I. LOSADA Figure 4. Examples of the maximum bed shear stress in grid GM22 (up) and GM17 (down) for the beginning of the storm (left) and the most energetic instant d uring the storm (right).
In Figure 4 we show some examples of the maximum bed shear stress. Finally, for each of the study sites we know the evolution of the maximum bed shear stress during the 37 hours of the storm, as well as their maximum and mean. Therefore, we have obtained high resolution time series of bed shear stress (covering all the Catalonian coast at a spatial scale of 250 m) allowing a better characterization of the storm impacts throughout the 52 different bottom communities in each study site. Acknowledgments The authors are grateful to CSIC for funding the general framework project “Assessment of the ecological impact of the extreme storm of Sant Esteve (26 December 2008) on the littoral ecosystems of the north Mediterranean Spanish coasts” (PIEC 200430E599). WAVES, CURRENTS, AND SHEAR STRESS References Booij, N., Ris, R.C., Holthuijsen, L.H.; 1999. A third-­‐generation wave model for coastal regions, Part I: Model description and validation. Journal of Geophysical Research, 104(C4). pp. 7649-­‐7666. doi: 10.1029/98JC026222. Dee, D.P., Uppala, S; 2009. Variational bias correction of satellite radiance data in the ERA-­‐Interim reanalysis. Quart. J. R. Meteorol. Soc., 135, 1830-­‐1841. Menéndez, M., Tomás, A., Camús, P., García-­‐
Diez, M., Fita, L., Fernandez, J., Méndez, F.J., Losada, I.J; 2011. A methodology to evaluate regional-­‐scale offshore wind energy resources. Oceans 2011. IEEE. Spain. ISBN: 978-­‐1-­‐4577-­‐0086-­‐6 Mínguez, R., Espejo, A., Tomás, A., Méndez, F.J., Losada, I.J.; 2011. Directional Calibration of Wave Reanalysis Databases using Instrumental Data. Journal of Atmospheric and Oceanic Technology, 28. doi: 10.1175/JTECH-­‐D-­‐11-­‐00008.1. Reguero, B.G., Menéndez, M., Méndez, F.J., Mínguez, R., Losada, I.J.; 2011. A global ocean wave (GOW) calibrated reanalysis from 1948 onwards. Coastal Engineering (under review) Shchepetkin, A.F., McWilliams, J.C.; 2005. Regional Ocean Model System: a split-­‐
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docs/arw_v3.pdf) Soulsby, R; 1997. Dynamics of marine sands. Thomas Telford Publications. pp 272. ISBN: 0 7277 2584 X Tolman, H.L.; 2002. User manual and system documentation of WAVEWATCH-­‐III version 2.22. NOAA / NWS / NCEP / MMAB Technical Note 222, 139 pp. (Available at: http://polar.wwb.noaa.gov/mmab/pape
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