Gongcheng Kexue Yu Jishu/Advanced Engineering Science

Mountain rivers often carry a large amount of sediment in the process of flash floods and cause severe deformation of the riverbed. In particular, the riverbed of the wide and narrow rivers is adjusted sharply and the water level rises sharply, which can easily increase the risk of flood inundation in the widened rivers. The study of water and sediment transport and riverbed deformation in the evolution of mountain torrents is of great significance for flood control and disaster mitigation. In this paper, two-dimensional numerical experiments of water and sediment dynamics are used to explore the process of water and sediment transport and river bed deformation in the wide and narrow reaches under the conditions of upstream sediment changes. The results show that: when there is sufficient sand from the upstream, the flood carries the sediment in the widening section and scours the narrowing section, and the river bed is lifted up in the process of sediment transport. The main reason is that the saturated sediment transport of the channel leads to the overall siltation of the river bed, which leads to the steep rise of the upstream water level, the increase of the water surface gradient, the increase of the shear stress of the bed surface, and the increase of the sediment transport rate of the river channel, and the increase of the bed surface fluctuation of the wide and narrow reaches. The morphological resistance of the bed surface causes the upstream water level to rise; when the amount of sediment is small, the river bed is mainly scoured, the water level drops, and the water surface gradient tends to slow down.

To effectively evaluate the loss of axial force for the bolts in cable clamp of suspension bridge, the influence factors analyses and application of ultrasonic identification method for cable clamp blot axial force in suspension bridge were carried out. Firstly, the axial force calculation formula was established based on the acoustic elastic effect, and the accuracy of this formula was verified by experiments. Then, effects of the non-stress acoustic time difference and stress coefficient difference on the recognition accuracy was analyzed. Finally, this method was utilized to identify the tension efficiency and the axial force loss during the lifting process. The results show that, 1) The acoustoelastic effect of bolts is obvious and the recognition error of screw axial force is less than 1.1%. 2) Identification errors caused by the coupling state of sensors and the blot geometry and material parameters are 47.7 kN and 43.1 kN, respectively. 3) The stress coefficients of different bolts are different. Identification deviation caused by the difference of stress coefficients between calibrated bolts and tested bolts is 4.75%. 4) With the increase of screw nut tightening degree, the tension efficiency can reach more than 94%, which is 95.93% higher than that before the tightening degree of nut is not controlled. The bolt axial force is seriously lost with the increase of beam weight. The measured minimum average axial force of cable clamps is 424.32 kN, which is only 56.58% of the designed axial force. In the process of beam section hoisting, the cable clamp bolts should be tensioned in time to ensure construction safety.

China is one of the countries with the most serious disasters of dammed lakes, where the dammed lakes pose a great threat to the safety of people’s lives and properties among the upstream inundated area and the downstream outburst flood routing area. A deep understanding of the scouring and breaching processes of landslide dam can provide important scientific and technological basis for the design of emergency discharge channel and the formulation of downstream emergency measures. According to the event of 2018 Baige landslide-dammed lake in the Jinsha River, the scouring and breaching processes of landslide dam were systematically studied by conducting the laboratory physical modeling tests. Test results showed that the erosion and breaching processes of landslide dam could be divided into four stages, i.e., flow incubation stage, retrogressive scouring stage, dam break development stage, and riverbed rebalancing stage. When the retrogressive erosion scarp was traced back to the upstream slope top and the inlet section of the discharge channel was eroded, the discharge channel would connect to form a ramp with the bottom slope i>0. Then the flow velocity and flow rate increase suddenly, and the dam break developed rapidly. The results also showed that when the excavation width of the chute remained constant and the depth increased, the peak flow would decrease, the peak time would be delayed and the process of the burst flow became smoother; when the depth of the chute was fixed, the peak time would be delayed as the width increased. Finally, according to the testing results, some suggestions were put forward for the optimal design of the discharge channel: The location of the discharge channel should be arranged in the pass with the lowest crest elevation to reduce the peak flow and shorten the breaching time; when excavating the spillway, priority should be given to increasing its depth to minimize the water level of the dammed lake during dam breaching.

Three kinds of magnesium-based foamed concrete with dry density grade A05 were prepared by chemical foaming with magnesium oxychloride cement, magnesium oxysulfide cement and magnesium phosphate cement as cementing materials respectively. By designing orthogonal tests, the influences of water/cement ratio, magnesium cement component ratio, retarder content, fly ash content and polypropylene fiber content on the compressive strength of three kinds of magnesia-based foamed concrete were determined, the action mechanisms of the important influencing factors were compared and analyzed, and the functional relationships between the specific strength of magnesium-based foamed concrete and the component ratio parameters of magnesium-based foamed concrete were established. The results showed that the primary and secondary factors affecting the compressive strength of magnesium oxychloride foamed concrete were the ratio of magnesium cement components>water/cement ratio>fly ash content>polypropylene fiber content>retarder content. The influence of various factors on compressive strength of magnesium oxysulfide foam concrete was the same as that of magnesium oxychloride foam concrete.The relationship between the factors influencing the compressive strength of magnesium phosphate foamed concrete was as follows: the ratio of magnesium cement components>retarder content>water/cement ratio>fly ash content>polypropylene fiber content. Different from magnesium oxychloride foam concrete and magnesium oxysulfide foam concrete, the content of retarder had a higher degree of influence. The component ratio of magnesium cement was an important index affecting the strength of magnesia-based foamed concrete. The compressive strength of magnesium oxychloride foamed concrete and magnesium sulfide foamed concrete had the same change trend with the increase of the component ratio of magnesium cement, both of which first decreased and then increased, while magnesium phosphate foamed concrete showed the trend of first increased and then decreased with the increase of the component ratio of magnesium cement. There was a power function relationship between the specific strength of three kinds of magnesium-based foamed concrete and the component ratio of magnesium-based foamed concrete.

Frequency diverse array (FDA) radar applies a small frequency offset between the adjacent elements to synthesize a range-angle-dependent beampattern. It is difficult to control the beam steering for the coupled time-variant beampattern. For this problem, the time-variant characteristic and point beam forming of FDA radar were mainly studied in the paper. Firstly, the characteristic of transmit beampattern of FDA using linearly increasing frequency increment was analyzed. Secondly, two signal models of FDA using logarithmically frequency increments (log–FDA) and multicarrier frequency increments (multi-FDA) were constructed, where the multi-FDA transmit beampattern has lower sidelobes. Then, the relationship between the time variable of the frequency increase term and the time variable of the propagation term in the two time modulated signal models was studied in detail. If both of them are the same, the time-varying characteristics of the transmit beampattern can be eliminated by the two time variables offset. However, they have different physical meanings. When the electromagnetic signal is generated and propagated in space, the time variable in the frequency increase term will not change, whereas the time variable of the propagation term in the propagation process is related to the propagation characteristics of the electromagnetic wave, which is a variable quantity, so the two time variables cannot offset each other. Finally, the simulation results verify the correctness of the transmit beampattern analysis of time modulated FDA radar, and demonstrate that the spot transmit beampattern with dynamic forward propagation can be synthesized using log–FDA and multi-FDA, where the beampattern of the multi-FDA has lower sidelobes.