Problems of wave disturbance in harbour basins. Study cases: Zarzis, Tunisia and Chipiona, Cadiz, Spain. V. Negro-Valdecantos & O. Varela-Carnero. Port and Coastal Engineering Department. E.T.S. Ingenieros de Caminos, Canales y Puertos. Madrid Politechnical University. Ciudad Universitaria, s/n. 28.040 Madrid, Spain Abstract - Summary The on - going development and evolution of numerical hydrodynamic modelling has allowed for and facilitated the propagation phenomena of wave movement, refraction, diffraction, shoaling, reflection, friction, percolation, transmission,...to be studied whilst the sensitivity of specialists and designers to them has increased. A large part of outer protection constructions in Spain are located in deep depths and do not call for wide approach channels and dock access manoeuvring circles to fulfill the cargo transfer ruction which the port system affords. The purpose of this paper is to analyze the effects of shipping channels on quay operativeness. Two study cases, one in the Mediterranean area and another in the Atlantic Ocean are therefore presented; Zarzis on the Coast of Tunisia and Chipiona on the Spanish South West Coast, Cadiz. 1 Introduction. Numerical model method The advance made in numerical techniques for differential equation resolution in partial derivatives has enabled a wide range of models to be developed over the last few years, which analyze and study wave propagation. Most of them provide the numerical solution integrated into the vertical with the Bousinesq terms of the mass conservation and momentum equation in the two direction. The equations are:
Mass equation, continuity Computer Modelling of Seas HI t ~dx ~dy x - Momentum equation dx ( ). _ \ = E y - Momentum equation -~2 y. 3 / PQ[ dt dx dx dx*-dt dx dy dt h* (2) -_ - J2/2 d*p 3 [ d*v dt dx _ in which, (3) E, Water surface level, m, (x,y,t) p Volume flux density in the x - direction, mvs/m, (x,y,t) q Volume flux density in the y - direction, mvs/m, (x,y,t) h Water depth, m D Still water depth, m, (x,y) g Gravity, m/s^ C Chezy resistance, m^/s E Eddy viscosity, mvs, turbulent coefficient, (x,y) x, y Space coordinates, m t Time coordinate, s H Wave height, m L Wave length, m LQ Deep water wave length, m U, Ursell number, - Q Courant number, - A Time step Ax Distance step, x-direction, m Ay Distance step, y-direction, m
Computer Modelling of Seas HI 245 Ax and Ay represent the distance between information points, gridspacing scheme. The Courant number is: (4) The common value is 1 in short wave applications, the relative weight of Bousinesq term to the errors terms is proportional to the square of the ratio of the water depth to the grid size. The method is resolved in rectangular grids by finite difference techniques in an implicit scheme and double swept. The normal ADI, Alternating Direction Implicit, algorith, with side-feeding is invoked for solving the system. The gravity term is traced with nonlinearity, to resolve the error given in set-up during the shoaling tests. This specialized software enables those phenomena occurring in propagation, as well as its modifications by natural and/or artificial obstacles, structural porosities, sharp changes in depth, dredging works... to be combined. Discretization into spatial grids definig the hydrodynamic and wave variables at each node and each time interval enables results to be obtained assuming two-dimensional flows and single or multilayer elements. This tool, together with exhaustive data taking and suitable calibration with in situ oceanographic measurements contrasted with historical series presents a 5 to 10% margin of error, including the definition of the lay - out, bathymetry, oceanographical parameters, boundaries, surface and water elevation, porosity, tide, currents and so on. The solution method is described in reference ACCURACY OF SHORT WAVE NUMERICAL MODELS, M. Abbot et al, 1.984. Reference 1. 3 Study cases Both examples submitted respond to a very similar characteristic layout. A breakwater, completed with a manoeuvring circle and shipping channel providing entry to the harbour facility. Zarzis is a commercial terminal with arigid,vertical caissons defence structure with total reflection; whilst Chipiona is a marina with a rubblemound breakwater formed by loose concrete bloks placed and concerted, with a low absorbing level. Local effects are pronounced with winds developed in small fetches with low wave height and short period waves.
246 Computer Modelling of Seas III Notables changes in wave celerity when changing from depths to 12.00 to 6.00 metres and from 5.00 to 2.00 metres according to the cases in scarce transition give rise to energy fluxes which enter the inner basins, causing a lack operativeness in certain directional situations during the year. Tests were carried out by varying the shipping channel, without dredging area, and extending it fanshaped, in order to diagnose and solve the aforementioned problem. See drawings below of the two cases with entrance channel and without and the problems of wave disturbance into the basins, using Lagrange law of celerity. The differences between different kind of structures do not give a lot of changes in magnitude. The comparison was made it with vertical breakwaters and rubblemound protections. c = \/g-d, C^ = 1.00, reflection vertical walls C^ =0.60-0.80, absorbing rubblemound coefficient coefficient 4 Figures index The two cases study are presented in the following figures: Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Lay - out of Zarzis Commercial Harbour, Tunisia Cross section of the vertical breakwater Bathymetry with the entrance channel and the manoevring circle Porosity map Wave disturbance coefficients Bathymetry without entrance channel Wave disturbance coefficients Chipiona Harbour. Lay - out and bathymetry Wave penetration into the harbour Finite element analysis Surface elevation. Chipiona Harbour The drawings are presented right below. They try to explain the behaviour of the entrance channel and the manoeuvring circle in the lay - out of a conventional harbour with restricted conditions of wave penetration and wave disturbance coefficients. Problems of long waves and harbour resonance have not be included in this work.
Computer Modelling of Seas HI 247 Figure 1 Lay - out of Zarzis, Commercial Harbour, Tunisia 'Zt-&#'28 Figure 2 Cross section of the vertical breakwater (Gridspocmg 8 m) Figure 3 Bathymetry with the entrance chann, and the manoeuvring circle
Computer Modelling of Seas III Figure 4 Porosity map so 200 (Gfkfepocioq «m) 100 120 MO Figure 5 Wave disturbance coefficients Figure 6 Bathymetry without entrance channel
Computer Modelling of Seas III 249 (Grtdspocing «m) Figure 7 Wave disturbance coefficients without entrance channel 50 50 70 80 90 00 Itt 120 0 80 100 120 HO 160 180 200 220 Figure 8 Chipiona Harbour. Lay - out and bathymetry 150 200 250 Figure 9 Wave penetration into the harbour
Computer Modelling of Seas HI Figure 10 Finite element analysis (Gridspocing 2.5 m) e 50 100 150 200 250 _j TJT "" Figure 11 Surface elevation. Chipiona Harbour
Computer Modelling of Seas HI 251 5 Conclusion Numerical results obtained in both cases provide a very high energy rates at the mouth, with greater celerity flows in deep water, contrasting with the proximity of the least depth at the quay area. This causes progressive waves and reflections which periodically put the basin out of service. It calls for an overwidth and clearance in radius both in turning circles and approach channels. The main breakwater must keep the basins in the dark zone to maintain the level of non-disturbance and do not create a small zone to pass through the energy flux between the structure and the navigation channel and the manoeuvring circle. The two cases presented above represent the examples of the design of the entrance quite close to the mouth with a wrong funtionality of the harbour. 6 References 1. Abbot, MB., McCowan, A.D. and Warren, I.R. Accuracy of Short Wave Numerical Models. Journal of Hydraulic Engineering, Vol. 110, No. 10, 1.984. 2. Abbot, M.B., Petersen, H.M. and Skovgaard, O. On the Numerical Modelling of Short Waves in Shallow Water. Journal of Hydraulic Research, Vol. 16, No. 3, 1.978. 3. Danish Hydraulics Institute. Wave disturbance in Zarzis Harbour, Tunisia. Internal Report. 1.992 4. Martin, J. and Negro-Valdecantos, V. Efecto de laproximidadde un canal de navegacion en la agitacion interior de unpuerto. Aplicacion al Puerto Comercial de Zarzis, Tunez. I Jornadas de Ingenieria de Costas y Puertos. Santander, 1.992. 5. Negro- Valdecantos, V and Garcia Palacios, J. Estudio de Agitacion Interior del Puerto de Chipiona, Cadiz. Interim Report for the Andalucia Port Authority, Seville, 1.994.
252 Computer Modelling of Seas III 7 Acknowledgement The major part of the work reported here was carried out in Intecsa, Consulting Company in Madrid, Spain. It was jointly funded by Dragados y Construcciones, S.A. and Ente Publico Puertos de Andalucia, S.A. The authors would like to thank both these bodies for their support. Mr. Garcia Palacios, Civil Engineer, contributed in the development of the work.