Scour represents one of the most critical threats to water infrastructure throughout the world. Therefore, protection against scour is a crucial aspect in the design of any hydraulic structure. Traditionally, scour protection consists of a combination of an armour and filter layer consisting of loose rock. The function of the armour layer is to provide resistance against external hydraulic forces, while the filter layer prevents the finer underlying sediment from eroding through the armour layer (maintain internal stability).
At particular sites, e.g. in areas with strong currents, the construction of a fine-grained filter layer poses practical challenges. At those sites, the two-grading system would preferably be replaced by a single grading, which has a sufficient size to resist external forces but simultaneously prevents suction removal of sand. This requires the design of a geometrically open filter. In an open filter, the pores are large enough to allow passing the underlying sediment, but the hydraulic gradient at the interface is too small to transport the underlying sediment. Recently, design equations have been developed for open filters.
There are still uncertainties which prevent the application of the design formulas. Therefore, the design formula's will be validated with physical model tests. Physical model test are performed in the period between 23rd of July and 24th of August. These tests will focus on (1) the failure mechanism 'winnowing' relative to the filter layer thickness and grain size, (with and without a hydraulic structure) (2) Turbulent damping within the filter layer, (3) the effect of high turbulent conditions on the degree of 'winnowing'.
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