Stability of open granular filters under wave loading

Delta marine Consultants (DMC) Tradename of BAM Infraconsult bv. IV Summary 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 main question is defined as follows: " Is the design formula of Hoffmans (2012) valid for wave loading on horizontal filters? " In the original formula by Hoffmans (2012), it is considered that the penetration of turbulence energy dominates the instability of the base material. Two different hypotheses are stated for the application of the Hoffmans formula under wave loading. Both hypotheses give a relation between the load damping length and the wave period. The load damping length is a parameter which determines the efficiency of the load damping of the turbulence energy inside the filter. Laboratory experiments are carried out in a wave flume, with the purpose to obtain values for the load damping length. These experimentally obtained values are tested against the hypotheses. For the model tests, a horizontal filter structure was built and loaded by waves. Multiple configurations of the horizontal filter were tested, in which different layer thicknesses and different filter materials were used. For each configuration, 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. To express the loading on the base and filter material, Hoffmans uses …