Residual currents and transport in connection with two-dimensional model computations

In calculations of horizontally two-demensional (2 DH) advective transports in tidal regions on the basis of given velocity fields, no fundamental problems are met as long as residual velovity concepts are avoided and the complete time dependent velocity field is utilized. In certain cases an approach based upon tidal averages is desired e.g. for computing a stationary concentration distribution by means of the diffusion-advection equation in time independent form or just to save computing time. In such a case a sufficiently accurate approximation of residual displacements is needed which is generally not acquired by applying so called Eulerian residual current velocities or residual transport velocities. This is illustrated by computations for the southern part of the North Sea. A procedure is described for obtaining better results in certain cases.