An improved simulator for the transient behavior of horizontal depth filters, based on the one-dimensional simulator published recently by the authors, is developed. The need for a different approach in designing horizontal filters arises from the inherent two-dimensional character of horizontal filtration, since gravity causes a drift of particle motion in a direction normal to the main flow, as opposed to the one-dimensional character of upward or downward filtration. Calculation of horizontal and vertical filtration coefficients is based on a new set of phenomenological equations which take into consideration the two-dimensional character of the particle motion. This approach also considers the motion of particle clusters that become detached from grain surfaces under the effect of local shear stresses, become reentrained, and usually clog downstream and/or downhill pores. Results are given in terms of the evolution of the specific deposit, the directional filter efficiencies, and the directional losses of filter permeability. (C) 2007 Elsevier B.V. All rights reserved.