Abstract:

In the aftermath of the Deepwater Horizon oil spill, thousands of ocean drifters were deployed in order to better understand the pathways and dispersion processes that drive oil slicks from the deep ocean to the coastline. Most of the drogued drifters released had a 0.60 m draft, hundreds of undrogued drifters had a 0.05 m draft, and thousands of drifting bamboo plates had a draft of 0.017 m. But typical slick thickness is 0.01 m or less, which raised an important question to interpret the ocean datasets correctly: how drifters move compared to oil slicks? The experimental study presented here addresses this issue by quantifying the differences, in terms of advection and diffusion, between the motion of oil slicks and the three groups of drifters mentioned above. Oil and drifters were released side-by-side in a vertically sheared flow controlled by waves produced in the National Oil Spill Response Research and Renewable Energy Test Facility (OHMSETT) wave-tank in April 2016. Trajectories were recorded by an aerial camera with spatial accuracy of 1.5 cm. The trajectories of each material patch were analyzed in details to determine how fast they were advected, but also how fast they spread and in which direction.

Suggested Citation:

Guillaume Novelli, Cedric Guigand, Michel Boufadel, Tamay Özgökmen. 2018. Comparison of oil slick and drifter motion at OHMSETT wave tank. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N7Z89B0D

Publications:

Novelli, G., Guigand, C. M., Boufadel, M. C., & Özgökmen, T. M. (2020). On the transport and landfall of marine oil spills, laboratory and field observations. Marine Pollution Bulletin, 150, 110805. doi:10.1016/j.marpolbul.2019.110805

Purpose:

Thousands of ocean drifters were deployed in order to better understand the pathways and dispersion processes that drive oil slicks from the deep ocean to the coastline.

Data Parameters and Units:

Dataset contains processed videos and positions of drifters and oil (in cm) over time (s). For T27, the wave characteristics were Hs = 0.48 m, T = 1.5 s, U_10 = 4 m/s at 45 degrees from the wave propagation direction (and against). For T26, the wave characteristics were Hs = 0.72 m, T = 2.2 s, U_10 = 5.5 m/s at 25 degress from the wave propagation direction (and against). Header and units for “Points” files Nr: The measurement number. Runs from 1 to the total number of points. TID: The ID number of the track to which the point belongs. PID: The ID number of the point. x [cm]: The calibrated x coordinate of the point y [cm]: The calibrated y coordinate of the point. t [s]: The calibrated t coordinate of the point. I [unit]: The calibrated image intensity value at the point. For RGB-images, the intensity is computed as I = 0.3R + 0.6G + 0.1B. Len [cm]: The length of the track from the start (first) point of the track to the current point (inclusive). Thus the value of this quantity for the end (last) point of a track equals the total length of the track. D2S [cm]: The distance from the start (first) point of the track to the current point. Thus the value of this quantity is always less than (or at most equal to) the value of Len. D2R [cm]: The distance from the current point to the reference point. NA if no reference point defined. D2P [cm]: The distance from the current to the previous point of the track. v [cm/s]: The speed, at the current point, of the object represented by the track. This quantity is computed as the magnitude of the most recent displacement vector (pointing from the previous point to the current point of the track), divided by the frame interval. α [degrees, column M]: The angle of the in-plane component of the most recent displacement vector (pointing from the previous point to the current point of the track) with respect to the x-y coordinate system of the image (with the origin taken in the previous point). Angle values range from -180 to +180 degrees, where 0 degrees means the vector component runs parallel to the positive x axis (pointing to the right), +90 degrees (or -90 degrees) means it runs parallel to the positive (or negative) y axis (pointing downward, or upward, respectively), and +180 degrees (which is the same as -180 degrees) means it runs parallel to the negative x axis (pointing to the left). Δα [degrees, column N]: The angular change between the in-plane components of the most recent displacement vector (pointing from the previous point to the current point of the track) and the preceding displacement vector. Header and units for the ellipses: Area [cm2]: Area of fitted ellipse in calibrated squared units. X [cm]: Center point of the ellipse. This the average of all the x coordinates of all of the pixels of the ellipse. Y [cm]: Center point of the ellipse. This the average of all the y coordinates of all of the pixels of the ellipse. Perim. [cm]: Length of the outside boundary of the ellipse. Major [cm]: Length of the primary axis of the best fitting ellipse. Minor [cm]: Length of the secondary axis of the best fitting ellipse. Angle [degrees]: Angle between the primary axis of the ellipse and the X-axis of the image. Circ. [unitless]: Circularity = 4 pi x [area] / [perimeter2]. 1 indicates a perfect circle. Slice [unitless]: Frame number in the stack. AR [unitless]: Aspect ratio = [major axis] / [minor axis]. Round [unitless]: Roundness is the inverse of aspect ratio. Solidity [unitless]: [area] / [convex area].

Methods:

Videos were filmed with a camera GoPro Hero 3 mounted on a crane and looking at nadir. In the first case the wave height HS1 = 0.48 m and wave period were T1 = 1.5 s. The mean wind speed at 10 m was U_10 = 4 m/s at 45˚ against the wave direction propagation. In the second case the wave height HS2 = 0.72 m and wave period were T2 = 2.2 s. The mean wind speed at 10 m was U_10 = 5.5 m/s at 25˚ against the wave direction propagation. Videos were corrected for lens distortion with standard barrel correction for that particular camera using Adobe Premiere. Frames were extracted every 5 seconds and imported as stacks in ImageJ (image analysis software). Drifters and oil were tracked using MtrackJ plugin for manual tracking of individual particles in sequence of frames.

Instruments:

Lens-corrected field of view of the camera is ~ 24 x 12 m^2 with a horizontal resolution of 1.5 cm / pixel. CARTHE drifter with and without drogue (draft of 60 cm and 5 cm respectively) Bamboo plates (28 cm diameter, 1.8 cm draft) 3L of oil

Individual Files: