Rinse Joustra

Introduction 
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. 

  
Model tests
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. Three types of flow conditions are tested: (1)  Uniform flow conditions, (2) Additional turbulence conditions, generated by a sill and (3) Conditions with a cylindrical pier. The first focus of these tests is on the degree of filter and bed mobility for different grainsizes, layer thicknesses, flow velocities and degree of turbulence.The second focus of these tests is on turbulent energy damping inside the filter layer.

 
  
Thesis aim
The research aim contributes to how the minimum layer thickness required for a stable granular open filter is designed. The following two objectives are the main aim of this thesis:

• Testing the validity of the design formula of Hoffmans(2012) for uniform flow conditions, conditions with sill-induced additional turbulence and conditions with a cylindrical pier.
• Quantifying the effect of the granular open filter and flow characteristics on a winnowing-induced filter settlement for conditions with and without a cylindrical pier.

Research method
Sub-surface camera videos, stereo-photography and internal camera images from the tests will be processed and analyzed to achieve the two thesis aims.