Study on the up-downstream water level correlation and the extreme water levels under flood-tide encounters of the Feiyun River main stream

The tidal section of the Feiyun River is affected by the dual effects of the upstream flood discharge and downstream tidal uplift, resulting in complex water level changes. To determine the extreme water levels of the tidal section, the methods of the statistical analysis, harmonic calculation and joint probability analysis are utilized to study the correlation between the water level changes and the downstream estuarine tides and upstream floods based on the observation data and explore the extreme water level characteristics caused by flood-tide encounters. The study results indicate that the interannual water level correlation between Rui'an Hydrological Station and the up-downstream is not significant, but the correlation of their monthly water levels is enhanced. During the rainy season, there is a positive correlation between the daily average water levels of the upstream and Rui'an Hydrological Station. The water levels at Feiyun River (Rui'an section) are more affected by the combined influence of the astronomical tides and storm surges than by the upstream runoffs and water levels. The main contributing factor to the extreme skew surges at Rui'an Hydrological Station is the offshore storm surges. There is a significant time difference between the upstream flood peaks and the skew surge peaks at Rui'an Hydrological Station. The skew surge peaks at Rui'an Hydrological Station are at the forefront of the flood peaks superimposed by the extreme skew surges of the storm surges. The combined maximum total water levels for 1 year, 5 years, 10 years and 50 years calculated by the joint probability analysis method are 4.16 m, 4.44 m, 4.59 m and 4.90 m, respectively, which are basically consistent with the analysis results of measured data in the past 50 years. According to the joint probability distribution of the storm surges and astronomical tides, the reliability of short period calculation results is relatively high.