Comparison of Finite Difference and Finite Element (Telemac) Models of Dublin Bay

A number of hydrodynamic and solute mathematical models of Dublin Bay have been developed in the Centre for Water Resources Research in University College Dublin. One of these was an Eulerian-Langrangian finite difference Extended Dublin Bay model named SW2D which was developed by Hussey (1996). It was used in various studies such as the Dublin Bay Water Quality Management Plan and the Howth Outfall Study to predict the effect of the proposed upgrades at the sewage treatment plants on the water quality in these areas. The finite element TELEMAC modelling system developed by the French EDF group was used in later studies of Dublin Bay by Bedri (2007). The primary objective of this study was to assess the performance of the TELEMAC system in comparison with the original finite difference Extended Dublin Bay Model, SW2D. The comparison is on the basis of accuracy, stability and computational time. The ease of use is also an important factor in terms of the man hour costs involved in undertaking modelling studies. The finite difference Extended Dublin Bay Model coded SW2D is the Benchmark model against which the TELEMAC models were compared. The model grid which extends from -6° 15’ to -5° 50’ in the East-West direction and 53° 10’ to 53° 30’ in the North-South direction uses over 72000 grid boxes. An attempt was made to model a baseline model using a uniform mesh with a similar resolution but this proved impossible because of stability issues at the open boundaries in the TELEMAC model. A mesh convergence study was conducted which determined that the solution converged with a TELEMAC mesh of approximately 25,000 nodes in which the coarser mesh of 750m at the open boundaries was gradually refined down to a resolution of 50m at the coast.