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1 Introduction

1 Introduction

Granular filters are used for the protection of base material against scour and erosion. For a proper functioning it is necessary that the interfaces between the filter structure, the subsoil and the water flowing above the filter structure are stable. Stability means that there is no transport of base material through the filter to the water above the filter, and there is no filter material removed by the currents above the filter. This paper focuses on stable geometrically-open filters and particularly on a design formula.

Recently, various studies have been carried out to the interface stability as function of the thickness of the filter layer. Based on a theoretical approach it resulted in a new design formula for geometrically-open, but hydrodynamically sand tight granular filter structures subject to currents. The new formula relates the required filter layer thickness to a characteristic diameter of the filter material taking into account the influence of the grading of filter and base material, turbulence and the damping of the hydraulic load in the filter.

This paper summarizes the recent CUR report 233 published in 2010, recent studies at universities within the framework of Master theses, and a publication of Hoffmans (2012). Finally, a design formula is presented for uniform flow with values based on the relative turbulence intensity, accompanied by the range of validity.