Granular slopes with open filters under wave loading

• Physical model tests on the erosion and accretion of sand underneath rock slopes under wave loading.
• Influence of loading conditions and structural parameters on the response of the sand–rock interface.
• Prediction method and criteria for the amount of erosion and accretion of sand at the sand–rock interface of open filters.

Permeable hydraulic structures that consist of rock material typically contain granular filters in one or more layers. These filters are normally geometrically tight to prevent material washout. Geometrically tight filters are often difficult to realize in the field and expensive. An alternative is a geometrically open filter. A geometrically open filter has a large ratio of the size of toplayer material (rock) and underlayer material (e.g., sand) and is designed in such a way that only minimal base material loss or settlement occurs. Potential applications of open filters include bed protections, toe structures, and slope protections. Proper guidelines on the design of open filters under wave loading could lead to significant cost and material savings, and to more practical applications of filters in the field. Physical model tests were conducted in a wave flume of Deltares. The analysis of the tests with 1:4 and 1:7 slopes and fixed test durations of 3 h (< 10,000 waves) has led to a method of predicting the amount of erosion of the sand underneath granular filters and of the sand accretion within a granular filter. Ranges of applicability have been provided for the underlying formulae. The tested material ranges included filter rock sizes close to prototype scale. In addition, a criterion has been developed to define the amount of acceptable transport in open filters.