Results of an experimental study on sediment transport by channel flows with tall rigid stems are presented. The experiments were conducted for various stem areal densities, channel slopes and sediment sizes with uniformly distributed unsubmerged cylindrical rigid stems forming a regular square staggering pattern in plan. The rates of sediment transport in the presence of simulated vegetated surfaces were observed to be significantly smaller than those without the vegetation. A method is proposed to determine the effective (grain) shear stress in presence of vegetation under the condition that the apparent shear stress is available from easily measurable flow parameters. The present data along with literature data are used for quantification of the reduction in the rate of sediment transport by the rigid tall stems. The grain shear stress rather than the apparent shear stress is observed to better describe the variations in the rate of sediment transport by vegetated flows.
RÉSUMÉLes résultats d'une étude expérimentale sur le transport de sédiments en canal avec de hautes tiges rigides sont présentés. Les expériences ont été menées pour différentes densités spatiales de tiges, pentes de canal et tailles de sédiments avec des tiges émergentes cylindriques rigides uniformément distribuées formant en plan un réseau de carrés réguliers en quinconce. Les taux de transport de sédiments que l'on observe en présence des surfaces de végétation simulée sont considérablement plus faibles que ceux sans la végétation. Une méthode est proposée afin de déterminer le cisaillement efficace (de grain) en présence de végétation à condition que le cisaillement apparent soit disponible à partir des paramètres de l'écoulement facilement mesurables. Les données présentes ainsi que les données de la littérature sont utilisées pour quantifier la réduction du transport de sédiments par les tiges hautes rigides. On observe que le cisaillement de grain est plus adapté que le cisaillement apparent pour décrire les variations du transport de sédiments en écoulements avec végétation.
Short-time heavy rains occurred in Fukuoka City, Japan on June 29, 1999. As a result, the Mikasa and Sanno-Channel rivers overflowed their banks and the flooding water moved down roads to the railway station known as 'Hakata-eki'. This area is a dense city centre with many houses and office buildings.
A nondimensional parameter governing debris and mud flows and dry-grain flow is derived from the comparison between intergranular-stress and inertia terms in the momentumu equations. From the result nondimensional distance from the bed is introduced for velocity profile of these flows. Granular and inertial sublayers are found to exist near the bed in the flows. The former sublayer has linear velocity profile due to the major role of intergranular stresses, while the latter sublayer has logarithmic velocity profile due to the inertial forces.Nondimensional surface velocity us fu * and average velocity u /u* are calculated and the results are compared with the experimental ones.
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