Chen, Margaret

Gourgue, Olivier

Komijani, Homayoon

Sishah, Biniyam

Vanlede, Joris

English

Hydraulic Engineering Repository

Conference Paper, Published VersionGourgue, Olivier; Sishah, Biniyam; Vanlede, Joris; Komijani, Homayoon;Chen, MargaretModelling tides and storm surges on the Europeancontinental shelfZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:TELEMAC-MASCARET Core GroupVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/104326Vorgeschlagene Zitierweise/Suggested citation:Gourgue, Olivier; Sishah, Biniyam; Vanlede, Joris; Komijani, Homayoon; Chen, Margaret(2015): Modelling tides and storm surges on the European continental shelf. In: Moulinec,Charles; Emerson, David (Hg.): Proceedings of the XXII TELEMAC-MASCARET TechnicalUser Conference October 15-16, 2022. Warrington: STFC Daresbury Laboratory. S.154-159.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Modelling tides and storm surges on the European continental shelf  Olivier Gourgue1,2, Biniyam B. Sishah1, Joris Vanlede2, Homayoon Komijani3 & Margaret Chen1 1 Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Brussels, Belgium 2 Flanders Hydraulics Research, Flemish Government, Antwerp, Belgium3 Hydraulics Section, Katholieke Universiteit Leuven, Leuven, Belgium ogourgue@gmail.com  Abstract—This paper presents an effort to simulate the propagation of tides and storm surges through the Northwestern European continental shelf using a slightly modified version of TELEMAC-2D v7p0r1. The area of interest is the Belgian Continental Shelf and the model is calibrated accordingly. I. INTRODUTION A storm surge is an offshore rise of water, which is primarily caused by winds pushing on the sea surface. In shallow water areas, storm surges can be particularly damaging when they occur at the time of a high tide, potentially causing devastating coastal flooding. This phenomenon is one of the major natural threats for the Belgian coastal area and the region surrounding the tidal part of the Scheldt basin. It is therefore a subject of high interest in terms of long-term coastal protection and sustainable development in Belgium.  Studying local impacts of storm surges is a rather multi-scale problem as the latter are formed at the scale of the entire North Sea. The use of an unstructured grid model seems therefore to be a natural choice, thanks to their ability to simulate various scale processes in a rather flexible way. Storm surges are especially damaging when occurring during a very high tide. It is therefore essential to represent accurately the tides when evaluating the impact of storm surges. In a first step, the present paper focuses on the set up of a tidal model (no influence of wind, nor waves) based on TELEMAC-2D and its calibration for best representation of tides in the area of interest. The influence of meteorological conditions (wind velocity and air pressure) to form storm surges is discussed in a second step. II. MATERIALS AND METHODS A.  Grids Two grid configurations are considered in this study, both covering the entire Northwestern European continental shelf. In the first configuration (Fig. 1), the open boundary offshore is located at the shelf break, which is defined as the 200 m isobath. This strategy already proved its efficiency to simulate the tides in Belgian and Dutch coastal waters with the finite element model SLIM [1]. In the second configuration (Fig. 2), the computational domain is extended to deeper parts in the Atlantic Ocean, so that the formation and propagation of very long waves can be simulated explicitly without nesting with a larger scale model or resorting to external data. The coastlines are generated using the PUG1 Matlab toolbox and the grids are generated using the open source application Gmsh [2,3]. In both configurations, the mesh density varies as a function of  1. the distance to the coastlines (the mesh density increases when getting closer to the coastlines, to have a good representation of them without increasing the computer cost offshore), and  2. the distance to the area of interest (the mesh density increases when getting closer to the Belgian coast and the Scheldt estuary). B. Model setup TELEMAC-2D v7p0r1 is used in this study, but it is slightly modified so that  1. the Coriolis parameter is evaluated as a function of the latitude, even on a Cartesian grid,  2. the tidal force is taken into account in the momentum equation, even on a Cartesian grid, 3. the wind velocity and the air pressure vary in space and time and are extracted from NetCDF files, and  4. the wind drag coefficient is a function of the wind velocity and is parameterized using Flather’s formulation [4]: 𝑐! =0.565 ⋅ 10!! if   𝒘 ≤ 5  ,−0.12 + 0.137 𝒘 ⋅ 10!! if  5 ≤ 𝒘 ≤ 19.222.513 ⋅ 10!! if   𝒘 ≥ 19.22  ,  , where 𝒘 is the wind velocity vector 10 m above the water surface and is expressed in m/s in the above equation. Manning’s formulation is chosen to parameterize the bottom friction. A constant viscosity of 10-6 m2/s is chosen for the turbulence model, assuming that the numerical diffusion of the model is sufficient to account for subgrid scale phenomena. The generalized wave continuity equation reformulation is                                                             1 http://www.oliviergourgue.net/pug 22nd Telemac & Mascaret User Club STFC Daresbury Laboratory, UK, 13-16 October, 2015   Table 2: Coordinates of the Flemish Banks Monitoring Network stations where observation data have been collected. Abbreviation Official name Latitude Longitude App Appelzak 51°21'46"N 3°17'24"E BvH Bol van Heist 51°23'22"N 3°11'55"E BvK Bol van Knokke4 51°25'6"N 3°17'54"E Wan Wandelaar 51°23'40"N 3°2'44"E   III. RESULTS AND DISCUSSION A. Influence of the offshore boundary location As mentioned previously, two configurations are considered for the location of the offshore open boundary. In the first one, the offshore boundary is located at the shelf break (Fig. 1). In the second one, the computational domain is extended further in the Atlantic Ocean (Fig. 2). The first configuration already proved appropriate to simulate the tidal dynamics in the area of interest [1]. The second configuration could be interesting to simulate explicitly the formation of wind waves and storm surges in deep areas, together with their propagation towards the continental shore, using a single multi-scale model. The first objective of this paper is to verify that the quality of the tidal simulations is not altered when extending the computational domain. To do so, the model is roughly calibrated over July 2013 using different values of the Manning coefficient, in both grid configurations. The influence of the meteorology is not taken into account here (see simulations S1a to S2f in Table 3). Taylor diagrams of the water elevation at Wandelaar for simulation sets S1 and S2 are displayed on the top panels of Figs. 4 and 5, respectively. The results at the other stations are very similar and are therefore not shown. Those diagrams show that the Manning coefficient has a clear effect on the standard deviation of the water surface elevation. This is due to the damping role that bottom friction has on tides: the higher the bottom friction, the lower the amplitude of the tidal variation. Also, using the second grid configuration (extended domain) instead of the first one (offshore boundary at the shelf break) seems to move the simulation dots towards the right, i.e. towards higher standard deviations of the water level. As a consequence, the optimal value of the Manning coefficient is not the same for both configurations. It is 0.022s/m1/3 for the offshore boundary at the shelf break (simulation S1b), and 0.024s/m1/3 in the case of the extended domain (simulation S2c).                                                                  4 Also know as Scheur Wielingen. Table 3: Main parameters for each simulation.  Grid configuration Manning coefficient [s/m1/3] Tidal forcing shift [m] Meteo Period S1a Shelf break (Fig. 1) 0.020 No No Jul. 2013 S1b 0.022 S1c 0.024 S1d 0.026 S1e 0.028 S1f 0.030 S2a Extended (Fig. 2) 0.020 S2b 0.022 S2c 0.024 S2d 0.026 S2e 0.028 S2f 0.030 S3a 0.024 -0.05 S3b -0.10 S3c -0.15 S4 -0.10 Yes S5a No Dec. 2013 S5b Yes  B. Influence of the mean sea level at the open boundary When compared to observations, the results of simulation S2c present a negative bias of the order of 0.1 m. One explanation could be that the bathymetry is defined with respect to the vertical datum NAP (Normaal Amsterdams Peil, or Amsterdam Ordnance Datum) while the tidal boundary conditions are defined with respect to the ocean mean sea level, which could be slightly different. In order to neutralize this bias, a uniform vertical shift is applied at the open boundary to the mean sea level, around which the water elevation tidal forcing oscillates. A set of three simulations is run with different values of this tidal forcing shift. The rest of the setup is the same as the best extended grid simulation S2c (see Table 3). The evolution of the bias at the different measurement stations is presented on Fig. 6. The best model results are obtained with a tidal forcing shift of -0.1 m (simulation S3b), but the optimal value seems to be somewhere between -0.15 and -0.1 m. C. Influence of the meteorological conditions Simulation S4 is run to investigate the influence of meteorological conditions during calm weather. The setup is 

Modelling tides and storm surges on the European continental shelf

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Modelling tides and storm surges on the European continental shelf

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