Evaluation of the measurement capabilities of an autonomous surface vessel in coastal regions

Reliable monitoring data underpins many coastal management decisions; such as decisions associated with development of defence strategies for coastal hazard protection or ecosystem-based estuarine management. For coastal monitoring, recent new technologies are providing opportunities to widen the scope of data collection. We assessed use of an Autonomous Surface Vessel (ASV), known as the JetYak, for taking a variety of measurements in two coastal environments. We examined the quality and reliability of data acquired by the JetYak and performance of steering for surveying in automated driving mode. The JetYak can be programmed to run in autonomous mode and follow a predefined path. However, the turning angle of the JetYak was quite large and hence, the vessel can easily overshoot or undershoot the target path. A series of optimisation tests were run at Lake Ngaroto, New Zealand to optimise the of steering and throttle parameters for the JetYak. The driving parameters FF gain, P gain, navigation period, P throttle and cruise speed were systematically varied to examine the effect of each parameter on the straightness of the path to each waypoint. With the right parameters, this deviation was minimised, and an optimal set of parameters were saved for future research using the JetYak. The JetYak was also tested for measuring flows around seagrass in the Tauranga Estuary, to investigate the possibility of resolving small-scale changes in velocity with a downward-mounted ADCP and independent GPS system. However, in this case, the performance of the instrumentation set-up proved inadequate. The JetYak was travelling too fast during the survey, and the influence of the vessel’s motion was not entirely removed from velocity measurements. For future studies of this nature, we would recommend using the JetYak at a slower speed or using a purpose-built boat integrated boat-mounted system for better results. Measurements of salinity, temperature and turbidity were taken in the Waihou River plume in the Firth of Thames, from low tide to high tide, using the JetYak and a research vessel. The JetYak successfully captured data inside and outside the surface plume, which revealed that as the flood stage of the tidal cycle progressed, the plume was pushed towards the west of the Firth and then towards the southern end into the mangroves with the incoming tide. Measurements attained from the JetYak and vessel were in good agreement for salinity, although some of the turbidity measurements did not appear to be correct, likely owing to air around the sensor. Overall the JetYak was successful at resolving the how plume progressed over spatial and temporal scales. The JetYak was proven to be a very useful tool for taking measurements in shallow coastal environments. However, it requires the appropriate set-up and the scientists and researchers need to be trained in both operational (piloting) skills and data processing skills. One advantage of the JetYak over a manned vessel is the precision of positioning, generally exceeding that of manual craft, thus allowing driving of near identical transects and to decrease errors in the data. Such data would suit monitoring over large temporal scales such as months to years to understand coastal dynamics and validate numerical models. Another key use of the JetYak would be in the completion of missions alongside a research vessel. Such an approach allows for the collection of data over multiple transects simultaneously, thus greatly increasing spatial resolution of measurements, without significantly increasing personnel requirements