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Being influenced by the factors of water level difference, tide, shortening closure gap, surrounding terrain and riverbed topography, etc., both the variations of flow velocity and water level in the closure gap are markedly larger than those in the general river. The phenomenon will bring potential safety hazards to the reservoir dam under construction, and thus the measurements of flow velocity and water level in the closure gap become very important in guaranteeing the safety of the reservoir dike project. Because of the safety, the measuring accuracy and the instrument performance, the traditional measuring methods of water level and flow velocity are less than ideal. In order to solve the problem, a buoy-based synthetic measuring system, which includes buoy body, balance weight, GPS (Global Positioning System), MRU (Motion Reference Unit), compass and computer, is developed. Besides, a complete data processing method, which includes the data synchronism, attitude correction and quality control of the measuring data, the self-adaptive extraction of water level and the calculation of flow velocity, is put forward in this paper. The system and the method were used for the project of Qingcaosha reservoir closure, and achieved the water level with a bias of less than 7 cm and the standard deviation of ±3.7 cm, and the flow velocity with a bias of less than 7 cm/s and the standard deviation of ±3.8 cm/s, satisfied the accuracy requirements of monitoring water level and flow velocity, and solved the measurement problem in the project. [ABSTRACT FROM AUTHOR]

The integration of GPS compass, GPS differential technique and ADCP effectively eliminates the impact of magnetic interference and bottom tracking bias due to bedload movement, well improves the accuracy and widens the applications of ADCP measurement. However, an accompanying problem, the heading misalignment angle α between the ADCP's Y axis and external compass will be introduced into ADCP measurement and brings a significant impact on the calculation of flow velocity. Therefore it becomes very necessary to determine and compensate α. Although the conventional method, the Course Method, has been widely used in the determination of α, it still needs to be improved in the operability, efficiency, robustness, self-adapting detection and accuracy. Thus, a robust method, Multi-epoch Method, is proposed in this paper. In this method, with the help of GPS velocity and bottom-tracking velocity, a α can be got at each epoch, then a series of αs can be obtained at a course. Then, the final α can be achieved through filtering and averaging the series of αs. The new method efficiently overcomes the drawbacks of the conventional method, and can conveniently achieve an accurate α. A synthetic experiment has implemented to testify the new method. A α of 6.22° with internal-verification accuracy of ±0.04° and external-verification accuracy of ±0.04° is determined by the new method, which causes an absolute flow velocity error of −0.31~0.29m/s and a relative flow velocity error of nearly 56% in 4 transects, while after the correction of α, both of them decrease to less than 0.02m/s and 3.3% respectively, which are meet with the requirement of ADCP measurement. [ABSTRACT FROM AUTHOR]