Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

stood hereafter as the rolling, sliding motion of cohesionless sediment particles. The fluid forces acting on grains forming the channel bottom of a watercourse may be resolved into two components, one being the tractive force of a direction parallel to the main direction of flow, while the other is lifting force which acts upward, in a direction perpendicular to the former.;' In cases, where the laminar boundary layer forms coherent envelope over the uneven channel bottom, the full tractive force is represented by the viscous shear, whereas if the boundary layer is disrupted and turbulent eddies are formed in the wake of individual particles, the differential pressure between the upstream- and downstream faces of each particle exposed to the flow becomes increasingly responsible for the tractive force. The uplift force is developed in a similar manner as the difference of momentary pressures acting on the top and underside of individual particles. In turbulent flow both components are subjected to continuous fluctuations, as regards both magnitude, direction and point of application. The submerged weight of the particles as well as the friction and supporting forces due to the surrounding particles tend to resist the mobilizing effect of the aforementioned forces. The magnitude and effectiveness of the resisting influences depend substantially on the grain-size distribution of the bed material. As soon as the fluid forces exceed the resisting ones the particle is displaced from its position of rest, thus further complicating the physical picture, since above the list of forces is completed by the impact force exerted by the moving particle on the others. Owing to the turbulent velocity — and pressure pulsation the characteristics of forces acting on the particles vary as already mentioned — from instant to instant, which variation can be described by statistical relationships only. The number of particles starting movement over unit surface area of the bottom, the frequency of displacement, the length of paths travelled, the length of periods spent in movement and at rest, etc., may also be regarded as random quantities. The statistical character of incipient sediment movement has already been observed by Shields in his fundamental series of experiments [11], while the familiar bed-load transportation relationships of Einstein were developed on this basis [12J. In view of the abovementioned difficulties it will be understandable that no accurately defined hydraulic conditions are available, which would describe the beginning of movement in a unique manner. The critical tractive force, bottom- or mean flow velocity are the parameters most frequently encountered in the relevant literature, but these are related to stages of movement more or less arbitrarily defined by various investigators and established by visual observation in the laboratory. For examples, three stages have been defined by Kramer, namely „weak", „medium" and „general" movement [13]. „Weak" movement is generally accepted as the critical condition, as was done by 44