The role of particles heavier than the fluid (glass spheres in water) in a turbulent open channel flow over a smooth bed is examined at volume concentration about $10^{-3}$. The present work focuses on the dynamical interaction between the solid (particles) and the fluid phases in the near-wall region. Experimental measurements have been performed by means of phase Doppler anemometry to acquire two velocity components, particle size and concentration data simultaneously; the Reynolds number of the flow was close to 15 000. It is observed that in the particle-laden flow, the vertical profiles of the streamwise mean velocity (for both fluid and solid phases) are reduced in the outer layer ($y^{ + }\,{ >}\, 20$), but increased in the viscous sublayer ($y^{ + }\,{<}\,5$) in comparison to the clear-water conditions, leading to an apparent slip kinematic boundary condition close to the wall ($y^{ + } \,{\approx}\,2$). Moreover, in the presence of solid particles, the flow exhibits a velocity close to the wall ($y^{ + }\,{ <}\, 15$) which is smaller than that of the particles, while in the outer layer the opposite takes place. In particle-laden flow, turbulence intensities of the streamwise and especially of the vertical velocity are damped for $y^{ + }\,{>}\,10$–20 (depending on particle inertia) but enhanced in the very near-wall region ($y^{ + }\,{ <}\, 5$), as is the Reynolds stress. These findings can be explained if they are referred to the mechanism of particle entrainment and deposition, which takes place close to the wall. This mechanism is related to particle inertia and to the dynamic of the structure of near-wall turbulence, which connects the buffer and outer regions with the very near-wall region. A significant momentum exchange between the two phases, which is particularly effective in the buffer region, is revealed by the quadrant analysis of the Reynolds stresses.

A threshold criterion for incipient motion of cohesive‐adhesive sediments, based on moment balance and dimensional considerations, is here developed. The criterion discriminates between single particles and flocs critical conditions and induces a modification to the traditional Shields curve, commonly adopted for noncohesive particles. This modification is particularly effective for the smaller size particles but tending to vanish for the larger ones. The proposed approach is validated on the basis of experiments performed on sediment cores, sampled at different depths, from seven lakes with different trophic conditions and organic matter content. The incipient motion conditions are evaluated by means of an original procedure based on image analysis techniques, which enables the entrainment between single particles and flocs to be distinguished. The adhesion force, which is mainly due to the biological activity at the sediment water interface, is estimated following the proposed criterion and it is found to depend on the organic matter content. In particular the cores sampled in the littoral photic zone are characterized by a bell shaped dependence between adhesion coefficient and organic matter content, which is typical for indicators of biological activity of the littoral ecosystems.

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The accumulation of fine sediments in rivers is a pernicious problem with wide-ranging consequences for the healthy functioning of rivers throughout the world. It is linked to a range of landuse changes and human activities that have increased sediment inputs leading to elevated fine sediment loads that exceed the sediment transport capacities of rivers. Surficial deposits of fine material can also create the conditions for fine sediment to move into and accumulate within the coarser bed substrate, a process known as colmation and the focus of this review. Colmation, also referred to as clogging, fine sediment infiltration, fine sediment deposition, ingress, infilling, intrusion of fines, siltation, and the surface-subsurface exchange of particles, is particularly damaging to river habitats and ecosystems. It causes degradation through the physical effects of reduced porosity and flow connectivity and the biogeochemical changes arising from the hydraulic and hydrological impacts and the effects of sediment-bound contaminants, all of which can impact on river ecology. Different aspects of the phenomenon of colmation have been studied across a number of disciplines and over several decades and this paper synthesizes this wide literature to provide a multidisciplinary perspective on the mechanisms, causes and impacts of colmation and discusses some key management challenges.

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