Department of Civil Engineering

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Browsing Department of Civil Engineering by Author "Abrey, Ryan Phillip Shaw"

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    Analysis of nearshore current patterns using aerial drone measurements at monwabisi
    (Stellenbosch : Stellenbosch University, 2019-12) Abrey, Ryan Phillip Shaw; Schoonees, K.; Croukamp, L.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: This thesis explores the development of a methodology for using quadcopter aerial drones to measure currents in the nearshore zone, along with assessing other possible applications within the marine environment. Current velocities were measured at Monwabisi beach in False Bay, Cape Town to enable the results to be compared with similar studies already conducted in the area. This provided a means of result validation, as the scope of the study did not include budget for measurement of sea currents with electronic instruments during the testing for comparison. The primary drone used during testing was a DJI Mavic Air due to its affordable price and ease of use, making it more accessible to most users. Calibration tests were conducted on land to measure a fixed known length in order to adequately scale the photos taken by the drone to the altitude recorded by the on-board altimeter. During calibration the drone was flown at different heights with the camera facing directly down and a set of photos were taken of a tape measure which could then be scaled in AutoCAD. This was crucial as the typical land-based method of photogrammetry, to determine height and distance from the drone, would not be reliable over water as its surface is constantly shifting. A set of coordinates of a static point was also measured in order to determine the horizontal accuracy of the reported coordinates which rely on the on-board GPS measurements. It was found that the measured coordinates required smoothing to compensate for GPS errors encountered by the drone. The drone used has a reported horizontal hovering accuracy of ±1.5 m but was found to produce a measured mean current precision below 2 cm.s⁻¹ once averaged over 10 seconds. This information filtered into a method for tracking drifters as they floated on the sea surface behind the surf zone. This is similar to the existing method of GPS current drifters; however, biodegradable oranges could now be used in their place with the drone above measuring their coordinates as they drifted on the currents. Several tests were then carried out during different environmental conditions at Monwabisi and a range of currents were measured for comparison with the previous studies. Measurements taken by the drone resulted in similar current patterns to those seen in the previous studies model results. Furthermore, the current velocities were of the same order of magnitude. It was also found that the drone could observe other aspects from above such as: effluent plumes at diffusers, marine life and even sediment movements. Each of these aspects could prove valuable for coastal studies in providing data at a reasonable cost. Overall, the findings proved that a quadcopter aerial drone is a versatile engineering tool which was able to measure currents in the nearshore zone while finding similar flow patterns to previous model results.