Water level process of water conservancy projects (e.g., Oroville dam, USA; Three Gorges Dam, China, etc.), overcurrent structures (e.g., spillway, lock approach, and plunge pool, etc.), and rivers is one of the important hydraulic indexes. In particular, for newly constructed dams and river engineering works, it is necessary to conduct accurate field measurements of water level and its surface fluctuations in order to analyze their operation behaviors, test engineering design, and provide operation guidance. Therefore, the high-precision measurement of water level is significant for the safe operation of water conservancy projects [1
Instruments for field measurement of water level should be advanced and automatic with the premise of reliability, economy, durability, and practicality [1
]. Common instruments include water level measuring rule, float-type water level gauge, pressure-type water level gauge, ultrasonic water level gauge, and radar water level gauge. However, these instruments have some disadvantages, especially in the complex natural environment, details are as follows.
Water level measuring rule is a traditional contacting measurement method, which needs manual measurement, suggesting that it is difficult to continuously measure water level for its poor automaticity. In addition, it is easily damaged under high-speed flow, and also difficult to accurately obtain the water level when water surface fluctuates intensely.
Float-type water level gauge is suitable for low sandy flow, and the water level measuring well needs to be built for its operation [2
]. Regularly adjusting the water level and cleaning up siltation in the measuring well and pipeline often results in high operating cost.
Pressure-type water level gauge is installed underwater, thus its accuracy is affected by water impurities and wave, and its calibration process is complex, resulting in a high failure rate [3
Ultrasonic water level gauge and radar water level gauge are vulnerable to external interferences, and they have relatively high operating costs for regular calibration [4
Moreover, for some overcurrent buildings, such as spillways, in order to measure water level fluctuations, plurality of instruments along the flow need be installed, which increases instrument installation workload and costs. In some particular cases, such as short-term field measurements during the flood season of a new water conservancy project, the workload and cost of using above-mentioned instruments are higher, indicating their low applicability and efficiency.
Image recognition technology, an important automatic, informative, and intelligent method, has been applied to many water level monitoring systems [6
]. These systems improved measurement efficiency. However, these systems need to be equipped with a measuring rule and a fixed image acquisition device, which limits its applicability in complex conditions, such as high flow velocity and wide channel. Recently, with rapidly improving performance of unmanned aerial vehicle (UAV), captures of objective imagery by airborne camera, identification of dynamic morphological characteristics in imagery based on image recognition technology has become a research hotspot in the field of water conservancy management [9
]. Wang [10
] introduced the characteristics of UAV photogrammetry technology and prospected its application in water conservancy domain. Lin [11
] applied the UAV and image recognition technology to identify and analyze river ice, as introduced its implementation process, and applied this technology in the ice section of Yellow River, China. Photogrammetry and image recognition technology were combined by Ahmad and Room to classify riverway and floodplain area and obtain their scale in physical river model [12
]. Stephen [13
] introduced a river boundary recognition system based on UAV images, and provided, in detail, the image recognition and planning algorithms. Tammingac [14
] assessed the capabilities of UAV to characterize the river channel morphology and hydraulic habitat, including bathymetry, grain sizes, undercut banks, forested channel margins, and large wood, and discussed the advantages and challenges of this technology for river research and management. Woodget [15
] exploited a novel approach for characterizing river physical habitat, which consists of a small unmanned aerial system (sUAS) and Structure-from-Motion photogrammetry (SfM). Their results showed that the sUAS-SfM approach provided high-resolution and spatially continuous, and explicit measurements of water depth and point cloud roughness at the microscale. Thumser [16
] developed a real-time measurement system for natural river surface flow velocity fields that are based on drone images and applied it in the study of Brigach River, German.
In order to overcome the disadvantages of current image-based water level monitoring systems, this study utilized the technical advantages of both UAV (e.g., flexibility and mobility) [17
] and image recognition technology (e.g., high automaticity and efficiency) [18
], and developed a new UAV imagery based technology for the field measurement of water level (UAVi-fmwl). The objectives of this study are to: (1) introduce the research idea of the measuring technology and components of the measuring system; (2) introduce its implementation processes in detail, including coefficient calibration, image preprocessing, offset correction, and water level identification and calculation; and, (3) examine its applicability and reliability by applying the new optical measuring technology in a hydraulic prototype measurement task.
Aiming at solving the limitations of current measuring instruments and methods for water level measurements in the field, we integrated UAV photogrammetry and image recognition technology, and developed a newly optical measuring technology to measure water level in the field, and constructed the corresponding measurement system, including UAV, airborne camera, plane wall, baffle or straight line, calibration points, correction point, and image processing software.
The UAVi-fmwl theory and implementation processes are as follows: (1) capture the video of water surface fluctuation processes, (2) dedicate image processing software orderly preprocess images separated from video captured by airborne camera, (3) segment the water body and background, (4) use the traversal algorithm to calculate the pixel coordinates of measurement section, and (5) obtain the actual water level at that time according to the conversion coefficient and the conversion relationship. Particularly, the UAVi-fmwl considers the offset error of UAV and then provides a UAV offset correction method.
The UAVi-fmwl was applied to water level measurement in the plunge pool downstream of the spillway of a hydropower station, and the results showed that the newly developed technology has good reliability, good progressiveness, and strong potential to monitor water level in the field environment. In addition, an operation and scheduling reference of the hydropower station is acquired based on the water level analysis results.
The UAVi-fmwl proposed in this study has the advantages of maneuverable and flexible UAV, high image recognition and analysis automation, high recognition accuracy, non-contact, low cost, and no requirement for water quality. It is suitable to water level and water surface fluctuation measurement in complex field environment (e.g., high and steep mountain slope, wide river, high-speed flow), especially for short-term water surface fluctuation and urgent water level change process measurement (e.g., dammed lake). This technology can also be applied to hydraulic model experiments and fluid surface fluctuation measurement of the oil industry and metallurgical industry.