On the Behaviour of Open Filters Under Wave Loading

The design of granular open filters under wave and current loading has raised increasing interest in recent years, especially under marine contractors and consultants. Proper guidelines on the design of open filters, which allow an acceptable and predictable loss of base material under wave and current loading, could lead to significant cost and material savings, and to a more practical application of filters in the field. In order to improve the knowledge on the behaviour of granular open filters under wave loading, laboratory experiments have been conducted in the Scheldt flume of DeltaresG Delft Hydraulics. This paper summarizes the model set-up, test programme and test results. The results include erosion (transport) rates and filter settling for open filter materials on sand. LITERATURE REVIEW Granular filters typically employed in coastal engineering fulfill several functions. They prevent e.g. the erosion (washing out) of finer base material or sub- layers due to waves and currents, contribute to the energy dissipation by turbulent flow through void spaces and provide drainage. Granular filters can be designed as geometrically tight filters or geometrically open filters. The design of geometrically tight filters (no material washout) is relatively simple, but often an unnecessary high number of filter layers and material volume is required. Furthermore, geometrically tight filters are often difficult to realize in the field because of quarry material limitations and when the structure is constructed underwater. An alternative is a geometrically open filter. In this case the filter is designed in such a manner that the hydraulic loading is too low to initiate significant erosion of the base material. Limited settlement is often permitted in the field. Typical applications of open filters include e.g. offshore bed protections and toe & slope configurations of coastal structures. The allowed settlement depends on the structure type. For breakwaters and revetments even small amounts of toe settlement can endanger the stability of the armour layer by loosening the bonds between interlocked armour units or placed