The vertical distribution pattern of the fine suspended sediment concentration (SSC) plays an important role in the aquatic ecosystem of shallow water. To investigate the SSC profile in submerged vegetated river flow, a series of indoor flume experiments was conducted. Glass spherical balls of three different diameters (40, 77, and 90 μm) with a uniform specific gravity of 2.5 were used for the model sediments. A set of sampling devices was designed for the simultaneous measurements of SSCs at multipoints along a vertical line. Experimental results show that the SSC profile was re-distributed in the vegetated region, gradually forming a new equilibrium pattern in the overcanopy flow layer with its maximum occurring near the canopy top. From this maximum point near the canopy top, the suspended sediment formed a dynamic balance between upward turbulent diffusion and downward gravitational settling. Meanwhile, the momentum diffusivity was observed to linearly decrease upward from the maximum-SSC elevation toward the water surface, thereby allowing us to deduce an empirical negative linear formula. Based on the convection-diffusion equilibrium mechanism and with assistance of the empirical formula of flow momentum diffusivity, a new form of the Rouse formula was derived for the overcanopy flow layer. In general, this equation agrees well with the measured data. Some undercalculated deviations appear in the well-above canopy zone due to the upskewed turbulence and the weak upward secondary flow. This deviation decreases with increasing sediment diameter. The bed deposition mainly comes from the deep part of canopy-occupied flow layer.

Runoff of some rivers in the world has decreased significantly due to climate change and enhanced human activities, resulting in severe eco-environmental problems. As a large Asian river with a channel length of 2309 km and a basin area of 5.568 × 10 5 km 2 , the Songhua River basin is an important bread basket in China. Although its runoff has experienced dramatic changes over the last decades, the relative importance of climate and human activity on the runoff changes has not been assessed heretofore. The purpose of this study is to decouple the relative impacts of precipitation, evapotranspiration (ET), and human activity on the runoff changes in the river basin using a SCRCQ method. Based on annual runoff dataset at 3 gauging stations on the main stem channel during the period 1955-2010, three turning years of runoff changes were detected that divided the entire time period into four parts for the above Dalai (1963Dalai ( , 1982Dalai ( , and 1998 and Haerbin-Jiamusi (1966, 1980, and 1998 sub-basins. While for the Dalai-Haerbin sub-basin, only one turning year of 1988 was detected. The first period was taken as the reference baseline period while the others regarded as measure periods. Based on mean annual precipitation and mean annual reference crop ET observed at 62 meteorological stations during the period 1955-2010, the relative impacts of precipitation, evapotranspiration and human activities on runoff changes were assessed. For the runoff changes in the above Dalai and Haerbin-Jiamusi sub-basins, the results show that the precipitation impact ranged from 3.3% to 24.7% and from 15.4% to 33.9%, respectively, and the impact of ET ranged from 0.6% to 10.8% and from −7.3% to 9.8%, respectively. In comparison, the anthropogenic impact ranged from 64.8% to 96.1% and from 56.3% to 91.9%, respectively. For the Dalai-Haerbin sub-basin, the impacts of precipitation, evapotranspiration and human activities on the runoff changes were 29.7%, −15.6% and 85.8%, respectively. Human activities have become the most important factor in controlling runoff changes in the river basin. The magnitude of human impact reached its maximum in the 1990s, and then decreased remarkably as a result of the implementation of some new measures, such as water-saving irrigation, wetland conservation and restoration, and reforestation, in agriculture production and environment protection. Given the severe water stress, anthropogenic impact on runoff changes in the Songhua River basin must be given more attention in the future.

Abstract:The impact of reservoirs on downstream river channel change has been a scientific issue in fluvial geomorphology during the last few decades. However, it is still a difficult issue as to how to express quantitatively the channel adjustment in the Inner Mongolian reach of the Yellow River induced by the joint operation of upstream reservoirs. Based on the shape parameters of channel cross-sections at four gauging stations in this river reach over a flooding season in two periods, 1978-1982 and 2008-2012, the present work investigated the channel changes in terms of shape parameter change rate under the same controlling water level in each flooding season at the channel cross-sections. Results showed that most of the change rates of the parameters evidently increased over a flooding season in both periods. However, the change rate of each parameter at the cross-sections decreased evidently in the latter period, compared with the former period. At the same time, the distribution pattern of the change rate of the shape parameters along the cross-sections thus changed from a convex curve in the former period to an S-shaped curve in the latter period. The obvious decrease of the change rates is related to the joint operation of the Liujiaxia and Longyangxia reservoirs. The reservoirs stored a large volume of water and decreased the peak discharge and maximum velocity in the flooding season; as a result, the erosion ability of the flood decreased accordingly. With the joint operation of the large reservoirs, the Inner Mongolian channel shrunk markedly. Therefore, the channel will present the possibility of an extreme flood in the future. Consequently, it is reasonable to adjust the function of the reservoirs in future. The total water and sediment discharges and the peak discharge in flooding seasons should be effectively controlled. Continuous shrinkage of the channel can thus be avoided and it can be ready for a potential extreme flood.