1 Abstract A one-way nested version of the hydrobiological model DIVAST (Depth Integrated Velocity and Solute Transport) is introduced. The nested model allows flooding and drying of intertidal areas, a process often present in areas where nesting is required but usually excluded from nested models due to the added complexity and instabilities. An overview of the nesting procedure used in the model is presented. The model was tested in Cork Harbour where extensive flooding and drying occurs and results show that the model is capable of reproducing hydrodynamic activity to a high degree of accuracy. Substantial computational savings are also achieved. It has been found that the location of nested open boundaries can significantly affect model accuracy; the placement of nested boundaries must therefore be given careful consideration. Fine and coarse grid models were used to investigate the relationship between temporal and spatial resolution and model accuracy. The results demonstrate the applicability and benefits of nesting. Keywords: one-way nested model, hydrodynamics, flooding and drying, DIVAST 1 Introduction One of the most common problems in hydrobiological modelling is the location of open boundaries; they must be located such that their conditions will not adversely affect model predictions in the region of interest. This problem often leads to a situation which requires a large computational domain, of which the region of interest (ROI) comprises only a small percentage. If a finite difference model is applied, the associated orthogonal finite difference grid can become very large, particularly if a high spatial resolution is required in the area of interest. In addition, a higher spatial resolution requires a higher temporal resolution. This may result in an excessively high computational cost. One common solution to this problem is the use of a nesting method. This method allows one to increase spatial resolution in a A Nested Hydrodynamic Model Incorporating Flooding and Drying S. Nash and M. Hartnett Cite as: Nash S and Hartnett M (2009) A nested hydrodynamic model incorporating flooding and drying. In: Topping BHV, Costa-Neves LF and Barros RC (eds.), Proceedings of the Twelfth International Conference on Civil, Structural and Environmental Engineering Computing, Civil-Comp Press, Stirlingshire, UK, paper 253. doi:10.4203/ccp.91.253