Drying–wetting approach for 3D finite element sigma coordinate model for estuaries with large tidal flats

An approach to represent drying and wetting processes in a three-dimensional finite element sigma coordinate model is described. This approach makes use of capillaries in dry areas, which can connect to the nearby wet areas. The time marching of the mass conservation equation is modified by introducing a “size factor” coefficient and a water level diffusion term. Therefore, the fictitious water level of the dry nodes can fluctuate with the adjacent wet nodes. This eliminates the artificial pressure gradient appearing in some drying and wetting approaches in the partially wet (transition) elements. This approach results in a null momentum computation at the dry areas, which can guarantee numerical stability and satisfy the mass and momentum conservation. The approach has been applied in a hypothetical case and a real case in Xiamen Estuary, China, with satisfactory results.