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Pool–riffle sequences are common bed forms in mountain rivers that have a significant effect on hydraulic and hydro-environment characteristics. Relatively few studies exist on the comparison of two and three dimensional modeling for bed forms in gravel channels. In this paper, flow structure and habitat modeling are performed in an urban river, by a two-dimensional depth-averaged finite element and a three-dimensional control volume model. Comparison of results showed that predicted velocities by SSIIM are lower than measurements data, while River2D simulations are at the same of measured magnitude. By comparing River2D-simulated shear stress and field data, we observed that estimated data are representative of field data, while the magnitude may be over-predicted compared with three-dimensional modeling. Habitat modeling showed maximum used area for River2D velocity modeling. In contrast, SSIIM simulations overestimate the depth of used area values in comparison with River2D.
Pools and riffles in gravel-bed rivers have a major effect on the variables of the flow equations. Obtaining measurements of these variables requires comprehensive research conducted in rivers. Detailed measurements were taken from one reach of the Kaj River, Iran. The subsequent results showed a phase shift for: X-component of velocity, near bed velocity in X and Z directions, and bed shear stress versus bed elevation profiles. In the riffle section, vectors of the vertical velocity component were oriented towards the bed. However, in the pool section, vectors were oriented downward close to the bed, and upward at higher levels. Quadrant analysis for the pool illustrated the dominance of ejection and sweep interactions near to the bed and near to the water surface respectively. However, in the riffle, outward interactions were dominant near the bed, and sweep interactions were dominant near the water surface. The spectral analysis revealed that flow over pool-riffle does not follow the scaling regime of Kolmogorov, used to illustrate the slope of −5/3 in inertial sub-range.
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This paper presents results of the friction factor of cobble-bed and boulder-bed rivers based on 23 field measurements in three mountainous rivers in Iran. Results reveal that features of our measured velocity profiles in boulder-bed rivers in mountainous region do not present a specific shape but illustrate specific pattern in cobble-bed rivers. This difference is attributed to scale effect of relative submergence. It is found that the relative submergence in cobble and boulders-bed rivers is the only key parameter that influences friction factor estimation and other parameters such as particle Froude number and Froude number have few impacts on flow resistance of cobble-bed and boulder-bed rivers. The proposed relationship for estimation for the friction factor should be useful for large coarse-bed rivers.
In this experimental study, the turbulent flow in a channel with vegetation by using sprouts of wheat on channel bed was investigated. Two different aspect ratios of channel were used. An Acoustic Doppler Velocimetry was used to measure parameters of turbulent flow over submerged sprouts of wheat, such as velocity profiles. The log law and the Reynolds shear stress distribution were applied. Results indicate that the position of the maximum turbulence intensity superposes on the inflection point situated over the top of submerged vegetation cover. Quadrant analysis shows that near the vegetation bed, the sweeps and ejections appear to be the most dominant phenomenon, while far from the vegetated bed, the outward is dominant event. Results also show that the aspect ratio plays an important role on the contribution of the different bursting events for Reynolds stress determination.
The influence of a combined system of jet injection through the pier and bed suction on the reduction of local scour around a circular pier was investigated in a laboratory flume. Experiments were conducted for two water depths (10.5 and 28 cm) under 1-jet and 3-jet injection and different angles between the jets, with bed suction rate Q(s)/Q(0) = 2%, in turbulent flow and clear-water scouring conditions. The results show that the bed suction modifies the effect of jet scouring and reduces the local scour and sediment transport at the pier (36% for scour depth, 25 and 42% for the area and volume of scour hole). The maximum reduction of the scour depth was about 50% (about 20, 39 and 39% for the area, volume and the upstream slope of the scour hole respectively) in the best configurations for a combined countermeasure.