# Presentation

## TELEMAC-3D - 3D Hydrodynamics

TELEMAC-3D is a three-dimensional (3D) model that uses the same horizontally unstructured mesh as TELEMAC-2D. The wave formulation for the updating of the free surface is used for efficiency. The model mesh is developed as a series of model planes between the bed and the surface planes. Flexibility in the placement of these planes permits the use of a sigma grid (each plane at a given proportion of the spacing between bed and surface) or a number of other strategies for intermediate plane location. One useful example is to include some planes which are at a fixed distance below the water surface, or above the bed. In the presence of a near surface thermocline or halocline this is advantageous in so far as mixing water between the near surface planes, where the greatest density gradients are located, can be avoided.

When drying occurs the water depth falls to zero exactly and the planes collapse to a zero inter-layer spacing. Finite volume style numerical techniques are used to ensure that both water and tracer can be well conserved in the presence of drying and subsequent rewetting.

The model was written primarily to solve the shallow water equations in 3D format but an option is also available to solve the governing equations including dynamic pressure so allowing shorter waves than those in a shallow water context (where wavelengths are required to be at least twenty times the water depth). This non-hydrostatic model formulation may also be important when modelling flows over trenches or on steep slopes.

Representation of velocity vectors of a TELEMAC-3D model near Torr Head (Northern Ireland)