Stability of open granular filters under wave loading

Abstract

A filter can be applied to protect a bed against scour. Normally, these filters are geometrically closed, to prevent winnowing of the base material through the filter. However, geometrically closed filters are expensive and difficult to realize in the field. Geometrically open filters can serve as an alternative. Within an open filter, the pores of the filter material are large enough for the base material to move trough. The philosophy behind this concept is to reduce the hydraulic loading within the filter, in order to avoid initiation of the base material. This research focusses on open filters. A design formula for open filters was established by Hoffmans (2012). This formula is recently validated for uniform flow (Van de Sande, 2012) and for flow with additional turbulence by a sill and a pier (Joustra, 2013). The applicability for wave loading - although suggested by Hoffmans- is not yet confirmed. No validations study was carried out in which Hoffmans formula was validated for wave loading. In this research, the use of Hoffmans formula for horizontal filters under wave loading was evaluated. Laboratory experiments are carried out in a wave flume. For several configurations, the critical wave height was determined for a wave period of 2.0s, 2.5s and 3.0s. The critical wave height is defined as the height for which incipient motion of the base material occurs. The filter material was stable during all tests. The depth above the filter was held constant for the tests. Together with an older dataset from Halter (1999), the results were analyzed. With the introduction of several assumptions, insight was gained in the damping of the hydraulic load inside the filter. The analysis showed that the Hoffmans formula cannot be applied in the same way as for uniform flow, since no constant value for the load damp coefficient alpha is found under wave loading. Two hypotheses which were stated for the applicability of Hoffmans formula under wave loading by Hoffmans & Verheij (2013). For the experimentally obtained results, these hypothesis did not give accurate results. It is speculated that the hydraulic gradient should be included in Hoffmans formula. In the original formula as proposed by Hoffmans (2012), this term was neglected.