Variations in the Characteristics of Water and Sediment in Response to Extreme Weather Conditions in the Qinjiang River of the Beibu Gulf

Small and medium-sized mountain rivers have characteristics of short processes, rapid flow rates, and quick responses to extreme weather, thereby playing an important role in the global marine geochemical cycle. Understanding the changes in water and sediment characteristics of small and medium-sized rivers in response to extreme weather can provide scientific support for disaster prevention and mitigation and channel management of river basins. This study analyzed data from the past 60 years to understand the variations in the characteristics of water and sediment in the Qinjiang River of the Beibu Gulf under extreme weather conditions by estimating their variation coefficients and contribution rates. The results of the analyses show the following: (1) The Qinjiang River has an average discharge of 32 and 290 m3/s in normal and tropical cyclone weather conditions, respectively. During a tropical cyclone year, the average flow rate is 9.06 times that during normal weather. The average sediment transport rates during normal weather and tropical cyclones are 0.05 × 104 and 1.15 × 104 t, respectively, wherein the latter is 23 times higher. (2) The average flow rates during normal weather and flood periods are 375 and 2,725 m3/s, respectively; the latter is 7.27 times higher. The average sediment transport rates in normal weather and flood period are 0.07 × 104 and 1.14 × 104 t, respectively, wherein the latter is 16.28 times higher. (3) The average annual contribution rates of tropical cyclones and floods to the runoff and sediment discharge of the Qinjiang River are 10.75% and 20.95% and 16.75% and 30.07%, respectively. Extreme weather contributes significantly to the variations in the water and sediment characteristics of the Qinjiang River. During extreme weather, the Qinjiang River exhibits a surge of water and sediment with inter-annual variations. The peaks of runoff and sediment transport during the transit of tropical cyclones tend to decrease gradually over the years, probably because of the reduction in the impact of their activities. The Qinjiang River is located in the South China Sea. The under construction western land-sea new passage grand project, namely the Pinglu Canal, will provide access to the Beibu Gulf along the Qinjiang River. The results of this study will provide scientific support for the disaster prevention and mitigation of the Pinglu Canal and the construction of the port