The instability of the river channels has increased in response to the combined effects of global warming and human activity. This instability threatens the lives and property of people who live along river courses. This study takes the Pingluo section of the upper Yellow River, which is ~28 km long and ~400 m wide, as its research focus. We studied 11 periods of Landsat remote sensing images from 1973 to 2023 and analyzed the evolutionary characteristics of the Pingluo section over the past 50 years. The channel morphology indices included the channel braiding index (BIT), the bar braiding index (BIB), the average area of the bar (Ab), and the width of the wet channel area (BW). The results showed that there was an overall shrinking trend in this section of the Yellow River; more particularly, fluctuations in indicators such as the river width and the braiding index highlighted an increasing–decreasing–increasing pattern of change. During the 1973–1986 period, the river showed a widening trend, with narrow anabranches cutting through the floodplain and both the river width and the braiding index increasing dramatically over a short period. Over the 1986–2018 period, the area of the wet channel continued to shrink to its lowest level for the past 50 years, the river’s branches were diverted and abandoned, and the channel tended to straighten out. Between 2018 and 2023, the river showed a slightly expanding trend. The evolution of the river channel appears to be related to regional human activity and climate change. For example, after the joint filling of the Longyangxia and Liujiaxia reservoirs in the upper reaches of the Pingluo section of the Yellow River in 1986, runoff and sediment load along this section decreased, flood events became less frequent, and the channel tended to contract. In addition, the increase in extreme precipitation events over the last five years has led to an increase in the magnitude and frequency of peak discharge values in the region, which is the main reason for the increase in the river braiding index and area.
This article experimentally validates a high-sensitivity vector magnetic field (MF) sensor based on two parallel Fabry-Perot interferometers (FPIs). Firstly, two standard single-mode fibers are interposed into a capillary tube to constitute a cantilever beam structure FPI1. FPI1 achieved a high axial strain sensitivity of 15.0 pm/µε. Then, FPI1 was bonded with the magnetostrictive material Terfenol-D, and the sensitivity of its MF intensity and direction reached 91.43 pm/mT and -12.75 pm/°, respectively. Finally, a reference interferometer FPI2 was matched to FPI1 to constitute the harmonic Vernier effect sensor S1. The average sensitivity of the MF intensity of S1 reaches -4.308 nm/mT, and the MF direction sensitivity of S1 reaches 892.3 pm/°. It amplifies the MF intensity and direction sensitivity of FPI1 by 47 times and 33.0 times, respectively. Therefore, the proposed MF sensor can measure MF intensity and direction with high sensitivity. Additionally, the proposed sensor only involves fiber optic cutting, splicing, and cold bonding during the manufacturing process, without damaging the structure of the fiber optic. Therefore, it is easy to manufacture, highly reproducible, cost-effective, structurally robust, and easy to operate. It is one of the preferred choices for vector MF sensors in practical applications.
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