An Investigation on Application of Filter Layer in Reshaping Berm Breakwaters

Reshaping berm breakwaters has a large berm above still water level (SWL) and their seaward profile is reshaped under wave attack. When a quarry is available near the construction site but it is not possible to produce a sufficient quantity of large armore stones for a conventional rubble mound breakwater, a berm breakwater is a feasible solution. According to the conditions of the rock quarries in the coastal regions of Iran, it is difficult to produce large armor stone from them. Thus, the concept of berm breakwater has attracted engineers to develop design and construction of this type breakwater in many projects in Iran.Berm breakwater are normally constructed with only two layers. Due quarry yield of rock quarries, not only producing large armor stone is difficult, but also quarry runs - which are used for core layer - have very small size. So, application of a secondary or filter layer is necessary for using such small size materials as core layer. Thus, most of the Iranian berm breakwaters have been designed and constructed with three layers. The present study aims to investigate the necessity for of filter layer to be applied in reshaping berm breakwater by considering the influence of sea states and structural parameters on the erosion of the core layer materials that includes the number of waves, waves height and period waves, water depth, the gradation coefficient of filter layer and the berm width. In this regard, experimental studies were carried out on two layers and three layers breakwaters under irregular waves with JONSWAP wave energy spectrum. Results showed that a wide berm width can preclude erosion of core material under design wave condition. Hence, berm breakwaters can be constructed without filter layer by using an optimized berm width to protect core materials against erosion. A dimensionless parameter is proposed to evaluate the threshold for eliminating filter layer of core materials in berm breakwaters. Considering the aforementioned conditions for optimum berm width and sea state and structural conditions of the present study, required berm width of a two-layered berm breakwater can be calculated using formula of Shekari and Shafieefar (2013) for recession plus at least 4 times of stone diameter. Due to difficulties of implementing a filter layer, selecting the optimized berm width and removing the filter layer can be very effective in reducing the cost and execution time of this type of structure. Filter is a layer in penetrable structures (mostly in coastal and slope protection structures) that precludes erosion of materials due to waves and currents without increasing pore water pressure in materials of lower layers. Filter, in a sense, may contain granular materials, geotextiles, or a combination of geotextiles and granular materials. Further, filters have different applications including prevention of erosion in core's materials which is due to the negative pressure fluctuations and movement of fluid through the structure, reduction of hydrodynamics loadings on outer stone layers, and drainage. Filters are categorized into three types based on their resistance to erosion of bed materials in shore or slope protection structures: Geometrically tight filters, Stable with geometrically open filters and Unstable open filters.