A desk study has been carried out regarding interface stability of granular filter structures. Actually, the project focuses on two particular aspects: (1) interface stability as function of the thickness of the filter layer consisting of standard armourstone gradings, and (2) interface stability of gravel mixtures with a wide gradation The objective of the study is to present an overview of the existing design formulas for interface stability for the above mentioned aspects, including remarks which formulas need improvement or are even missing. On the basis of the inventory recommendations are presented for future research including a list of priorities and quick wins. First, an overview of the existing filter criteria has been made. Secondly, a new formula has been derived. The formula includes the influence of the grading width as well as interface stability as a function of filter layer thickness. It is recommended to validate the new filter design formula in a hydraulic model.