This paper describes a 3D model that was constructed in response to the need for a simple model for use in coastal and estuarial applications where tidal range may be large in relation to water depth. The model was required to simulate density driven flows. The model configuration selected to meet these objectives is presented, together with the assumptions, formulation and solution techniques upon which the model is based. The validation of the model against analytical results, an empirical relationship and other model results are also described.