Transport of Sand and Partly Cohesive Sediments in a Circular Pipe Run Partially Full

Many sediment deposits contain not only a granular but also a mineral and/or organic cohesive fraction, which has the potential to change significantly the erosion response when compared to noncohesive sediment deposits. Sediment deposits containing a low proportion of cohesive sediment display a specific, transitional behavior between cohesive and noncohesive sediments. In the past, several studies have examined partly cohesive sediment, however, the quantification of its erosion behavior is still unsatisfactory. In an attempt to advance the understanding and qualitative description of erosion and transport of mixed sediments in pipes, a comprehensive experimental investigation on the erosion behavior was designed. Tests were conducted in a flume with semicircular cross section with a diameter of $0.39\mathrm{m}$ using fine sand with a $d_{50}=0.19\mathrm{mm}$ and partly cohesive sediment deposits being a mixture of sand and clay with proportion of 3–10% by weight of clay. Tests covered a range of flow conditions with Froude numbers up to 0.6. Data on bed form geometries, friction characteristics, and sediment transport rates were collected. In steady sediment transport conditions the eroded partly cohesive bed produces an active granular top layer over an intact substrate, the thickness of which is up to ten times smaller than in the case of a granular bed. The cohesion of the substrate reduces the height and length of bed forms, bed form friction, and sediment transport rates. A comparison of the mobility of noncohesive sediments in circular pipes obtained in other studies is presented to indicate the generality of these results.