Analysis of nearshore current patterns using aerial drone measurements at monwabisi

Abrey, Ryan Phillip Shaw (2019-12)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

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.

AFRIKAANSE OPSOMMING: Hierdie tesis ondersoek die ontwikkeling van 'n metodologie vir die gebruik van hommeltuie om strome in die nabystrandse sone te meet, asook die evaluering van ander moontlike toepassings in die mariene omgewing. Stroomsnelhede is by die Monwabisi-strand in Valsbaai, Kaapstad gemeet sodat die resultate vergelyk kon word met dié van soortgelyke toetse wat reeds in die gebied uitgevoer is. Hierdie toetse het 'n geleentheid geskep vir die bevestiging van resultate aangesien die meting en vergelyking van seestrome met elektroniese instrumente nie deel van die projekbegroting was nie. Die primêre hommeltuig wat tydens die toetse gebruik is, was 'n DJI Mavic Air as gevolg van die bekostigbare prys en gemak van gebruik wat dit toeganklik vir die meeste gebruikers maak. Ykingstoetse is op land uitgevoer om 'n vasgestelde voorafbepaalde lengte te meet sodat die foto’s wat deur die hommeltuig geneem word, se skaal bepaal kon word met betrekking tot die hoogte wat deur die aanboordse hoogtemeter aangeteken word. Tydens yking is die hommeltuig teen verskillende hoogtes gevlieg met die kamera direk na onder gemik om 'n stel foto’s te neem wat dan in Autocad geskaal kon word. Die metode is noodsaaklik aangesien die tipiese landgebaseerde metode van fotogrammetrie om die hoogte en afstand van die hommeltuie te bepaal, nie betroubaar oor water is nie aangesien daar konstante beweging is. 'n Stel koördinate van 'n statiese punt is ook gemeet om die horisontale akkuraatheid van die aangemelde koördinate wat van die aanboordse GPS-metings afhang, te bepaal. Dit is gevind dat die gemete koördinate gladstryking nodig het om vir foute wat deur die hommeltuig gemaak word, te kompenseer. Die hommeltuig het 'n gerapporteerde stilsweef-akkuraatheid van ±1.5 m, maar daar is gevind dat die gemete gemiddelde akkuraatheid onder 2 cm.s⁻¹ was nadat ‘n gemiddelde oor ‘n 10-sekonde-interval bepaal is. Hierdie inligting is gefilter deur 'n metode om boeie te volg terwyl hulle op die see- oppervlakte agter die nabystrandse sone dryf. Hierdie metode is soortgelyk aan die metode wat deur huidige GPS-stroomboeie gebruik word. Bioafbreekbare lemoene kan nou in hulle plek gebruik word met die hommeltuig wat van bo hulle koördinate meet terwyl hulle in seestrome dryf. Verskeie toetse is daarna uitgevoer tydens verskillende omgewingstoestande by Monwabisi en 'n verskeidenheid strome is gemeet vir vergelyking met vorige studies. Metings deur die hommeltuig het soortgelyke stroompatrone as vorige studiemodelle getoon. Verder, was die stroomsnelhede van dieselfde grootte-orde. Daar is ook gevind dat die hommeltuig ander aspekte van bo kon meet, soos uitvloeiselwolke by verspreiders, mariene lewe en sedimentbewegings. Elkeen van hierdie aspekte kan waardevol in kusstudies wees om data teen redelike koste te verkry. In die algemeen is daar gevind dat hommeltuie veelsydige ingenieursinstrumente is wat strome in die nabystrandse sone kan meet, terwyl die resultate soortgelyk aan vorige modelresultate is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107098
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