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1 Introduction

1 Introduction

The majority of rubble mound coastal structures contain granular filters, often with one or more layers, where the larger rock grading is positioned on top of the smaller grading. These filters are normally geometrically tight filters (no material washout). Underneath the filter layers quarry run m aterial is present, or a combination of a geotextile with sand as core material. These geometrically tight filters can be difficult to realize in some circumstances. An alternative is a geometrically open filter (i.e. a large ratio of the size of the top layer material to the underlayer material). Applying an open filter may lead to a reduction in the number of filter layers. However, when applying an open filter the layer thickness may have to be increased compared to a layer thickness in a geometrically tight filter. The determination of the required layer thickness is essential for applications with granular open filters.

For open filters, the required layer thickness is designed in such a way that the hydraulic loading at the interface is too low to initiate erosion of base material (or settlement) outside an acceptable range. Limited settlement can sometimes be accepted. Potential applications of open filters include bed protections, toe and slope configurations of coastal structures.

Previous physical model studies on granular filters focused primarily on steady flow and the initiation of base material transport through the filter. The studies therefore do not specifically address unsteady flow processes with high turbulence, material transport or filter settling effects.

In order to improve the knowledge on the behaviour of open granular filters under wave loading, laboratory experiments have been conducted in the Scheldt Flume of Deltares, Delft, as part of the JIP TOPFILTER project, see Wolters (2013). Partners of this project are Deltares, Van Oord, Boskalis, Witteveen+Bos, Rijkswaterstaat and SBRCURnet. The physical model tests focussed on a sloped granular filter construction (slope 1:7), placed directly on sand with a wide filter gradation (D85/D15 = 6.3), see figure 1-1.

Figure 1-1 Model set-up in the Scheldt Flume of Deltares.

The physical model tests showed that a granular filter layer on sand can be an effective way to minimize base material erosion (see also horizontal filter tests, Wolters & Van Gent, 2012). Based on the physical model tests formulae were proposed for the sand erosion under wave loading, depending on filter thickness. The formulae are so far based on a limited number of experiments. Additional validation at (near) full scale, and for 3D effects like oblique wave attack is strongly recommended. The incorporation of data with other material gradings for both base and filter layer, multiple filter layers, and other slope angles is also recommended.

The development of a design guideline for open filters was not part of the report on physical model testing. Part of that guideline should be a recommendation on how to use the erosion/accretion parameter and a discussion of the reliability of the design approach and possible safety factors. This document provides guidance for applications of open filters.