Subsidence of riprap protection without filters for different installation types of riprap around a pier in sands

Synthetic and/or stone filters should be applied to prevent riprap subsidence due to sediment winnowing or leaching between the riprap and sand bed layers when riprap protection is installed around the pier to prevent scour. However, riprap protection without filters is commonly constructed in the field due to the technical and economical difficulties of installing the filter layer. Riprap is also sometimes placed on the bed rather than flush with the bed, especially in the case of deep water and tidal conditions at marine structures. Hydraulic experiments were conducted in this study to quantify and analyze riprap subsidence without filters placed on sand beds for various riprap layer thicknesses, riprap and bed material sizes, riprap installation types, and hydraulic conditions. The experiments showed that, as the flow velocity was increased and the size of bed materials and thickness of the riprap layer were decreased, the subsidence of the riprap protection was increased. A dimensionless riprap subsidence equation was developed using the experimental results for two types of riprap installations (on and below the bed surface). The results of this study can be used to evaluate the subsidence stability of riprap protection without filters.

► We examined the stone filter role to prevent riprap subsidence due to sediment winnowing through hydraulic experiments.
► Hydraulic experiments were conducted to quantify riprap subsidence without filters on sand beds for various conditions.
► The deeper riprap subsidence was observed with thinner riprap thicknesses and smaller diameters of bed materials.
► The riprap layer thickness was the main parameter to reduce winnowing failure.
► A dimensionless riprap subsidence equation was developed using the experimental results for two types of riprap installation.