Structuring Of Local Space-time Flow Field And Precise Estimation Of Real-time Discharge In The Yangtze Estuary

The Yangtze River Delta is one of the most developed and the most populated areas in China with abundant water resource that plays an important role in its economy development. Acquiring the accurate space-time flow field and real-time discharge information of this area is of great significance for the planning and construction of water conservancy project, flood control, disaster prevention, scientific management and reasonable allocation of water resource. Under the mutual influence of wide channel, complex interactions of runoff and tide and rapidly changing velocity, the difficulty of velocity directly obtaining, complex component of flow field and tough real-time discharge estimation in Yangtze River Estuary negatively affect the accurate obtaining and application of the flow field and discharge information. Accordingly, this paper solves the above problems by carrying out the study on methods of structuring the space-time flow field and accurate estimation of real-time discharge. The primary work and contribution of this paper is as follows.1. Accurate measurement of ADCP velocity(1) This paper introduces the principle of ADCP measurement, and analyzes the impacts on traditional ADCP velocity measurement which caused by moving bed, inaccurate magnetic compass and misalignment angle. On this basis, this paper then deduces the absolute velocity error model under the impact of speed error and azimuth error, and presents the error curve. It provides guidance for ADCP velocity measurement. When azimuth error is 1° and the ratio of vessel speed and flow velocity is 0.8, the maximum velocity error is less than 2% and the maximum direction error of the flow is less than 1°, which can meet the requirements of high precision velocity measurement.(2) In order to overcome the impact on traditional ADCP velocity measurement caused by moving bed and magnetic orientation disturbing, this paper proposes a method of ADCP velocity flow accurate measurement and data processing based on external transducers and multi-source information fusion. The experimental result indicates that this method enhances the applicability of ADCP significantly, and doubles the precision of velocity measurement.(3) In allusion with the deficiency of high quality of detecting environment, sophisticated implementation, and the result apt to be affected by the positioning gross error on the course method based misalignment angle detection of traditional ADCP transducer installation, a method of multi-epoch underway method is presented which presents a precision better than ±0.10°, improving the detection precision significantly and simplifying the operational detection procedure.(4) Summarizes the four common velocity distribution models, and simplifies each model to cover the shortage of many unknown parameters to be calculated and that cannot be directly applied to ADCP unmeasured area calculation, deducing a unmeasured area velocity extrapolation model applicable to ADCP measured vertical velocity data. On this basis, to remedy the limitation of hypothesis that vertical velocity variation with monotonous change, this paper presents a self-adaption method of ADCP optimal velocity calculation model of unmeasured area. Experiments show that this method can obtain a precision better than ±2.0 cm/s, which is much higher than that of the traditional whole method with a precision of ±5.8 cm/s. Thus, the method proposed in this paper remarkably improves the estimation accuracy of unmeasured area.2. High precision structuring of space-time flow field of tidal river based on boat-mounted ADCP data(1) Analyzes the localization of traditional current harmonic analysis method applied to moving vessel ADCP data, The analysis of the features of the 3 fitting functions of Candela tidal separation methods by experiment shows that, as polynomial function has the character of "global", it is difficult to get the difference of the flow field details, and with a high polynomial order number, the edge of interpolation region will yield turbulent "Runge" phenomenon. However, Gauss function is more sensitive to node location than polynomial function, and has the strong "local" property. Thus, this function is resulting in that subtidal current and tidal values are zero in region far away from the node. Apparently, it is not in conformity with the reality. However, Green function not only has the stronger "local", but also reflects the details difference of flow field relative to polynomial function. Therefore, Green function is the optimal.(2) Focally studies the existing problems in the parameter calculation of basis function by least squares method. It considers that the precision of tidal current separation primarily depends on the selection of the number and the location of nodes.Considering the current status of lacking of basis to choose the nodes, the inconsideration of flow trend change on the current node position selection and the fitting results apt to be affected by morbid matrix, based on all these presents, and the thought of fitting error with node numbers increases to stabilize and seeking partial derivative for overall fitting error, this paper proposes a method of optimal nodes selection, which is using threshold and gradient algorithm to determine the number and location of nodes. The experimental results indicate that the algorithm adapted by this paper improves the efficiency of tidal current separation, and obtains the fitting accuracy of ?0.23 m/s. In summary, this algorithm is better than traditional algorithm.(3) Adopts the optimal tidal current separation method,3 periods of moving vessel data included spring tide, moderate tide and neap tide separated by optimal separation method. After, the subtidal current field and tidal current field are obtained and then constructed to flow field. The results indicate that the external prediction precision of three periods of data is in accordance with the inner precision of the flow field fitting. It indicates that the flow field structured in this paper has a better short-term extrapolating function.(4) Through analyzing the flow field evolution, the rules of section location and topography difference impacting the phase and amplitude of flow field are acquired. The rules are shown as follows. For phase, it predates the north section in the cross section of the south and the downstream, and is ahead in the flood tide or ebb. For the same section, the phase in the shallow waters is earlier than in the deep waters. For velocity, compared to the shallow waters, it is greater in the deep waters. Affected by jacking of runoff, the maximum value of flood tide is much smaller than the ebb. And in a semidiurnal tidal cycle, the duration of the ebb is about 8 hours, much larger than the flood tide lasted for 4 hours.3. Real-time discharge estimation in tidal river(1) Introduces stage-discharge method, and analyzes the limitation of this traditional method applied to real-time discharge estimation in the tidal reach. Also, this paper studies single vertical index velocity method. It considers that the main reasons of the low estimation precision in Yangtze River estuary discharge are that the inferior representativeness of single vertical, without considering transverse slop of water surface caused by river regime and Coriolis force and other factors, and did not take into account seasonal variation of flow velocity and section area, etc.(2) Studies the relationship between depth-average velocity and cross-section averaged velocity changes with seasons and positions, and sectional area estimation impacted by curved shape of bending river and riverbed erosion, and on which basis, real-time discharge estimation method based on multi-vertical is raised. In Xuliujing section of the Yangtze estuary, the discharge estimation shows that relative to single vertical method, the relative accuracy of multi-vertical method increased by at least 5% and expands the application of index velocity method in tidal rivers.(3) Filtering the tidal current signal of real-time discharge data in three different seasons of Xuliujing section, then a daily average discharges are gained and compared with that of Datong station. The results show that discharge differences between the two sations varies with seasons. Because Xuliujing station is closer from Yangtze river, the discharge of Xuliujing station could be considered to represent the discharge of the Yangtze into the sea.This paper proposes a high precision of flow velocity measurement method, flow field structuring method based on underway observation data, and multi-vertical discharge estimation methods, which solve the difficulty of obtaining of high precision of tidal flow field and the flow information in the Yangtze estuary. It has an important supporting role in related scientific research and engineering application and could be a reference for flow field structuring and discharge estimation in similar rivers.