Sand on the sea floor is rarely flat, particularly close to the shore. Currents and waves interact with bed sediment to produce bedforms: ripples, dunes, and more complex structures. The type and shape of bedforms strongly influence the processes that cause sediment to be resuspended, or lifted from the sea floor. For this reason, prediction of bedform morphology is an important component of modeling the transport of sediment by water currents.
Bedform morphology (size, type, and orientation) is influenced by grain size, current speed, wave energy, and the angle between waves and currents. Whereas bedforms produced by currents or waves alone are fairly well understood from field and laboratory observations, bedforms produced by a combination of waves and currents have proved more difficult to study. Laboratory studies of combined waves and currents have dealt almost exclusively with waves and currents traveling in the same direction, because of the difficulty of generating waves transverse to the current flowing down a laboratory flume. In contrast, a wide range of angles between waves and currents occurs in naturefor example, waves close to shore are typically almost perpendicular to the longshore current.
In January, U.S. Geological Survey (USGS) scientists Dave Rubin and Jessie Lacy traveled to Japan to investigate ripples produced by a combination of waves and currents at varying angles in a giant flume at the University of Tsukuba, about 60 km northeast of Tokyo. The research in Japan, which is part of the USGS Coastal Evolution Modeling project, is supported by a grant from the Office of Naval Research to Dan Hanes and Dave to investigate ripple morphology and evolution (see Sound Waves article "Ripples for Everyone! Investigating How Sediment Moves on the Sea Floor").
To produce combined flows of waves and currents at varying angles, we planned to propel a large (2-m diameter) sand-covered tray back and forth across the floor of a wide flume, under water. Movement of the tray back and forth produces flow relative to the sand bed that mimics wave motion. By varying the current speed, the period and length of the tray oscillations, and the angle of the oscillations relative to the direction of the current down the flume, we could create a wide range of wave-current combinations. We designed a frame attached to the tray to hold instruments to measure water velocities above the sand bed and to monitor the evolution of bedforms. This approach requires a flume that is wide enough for the tray to move side-to-side at realistic wave velocities. At 160 m long by 4 m wide, the giant flume in Tsukuba is one of the few flumes in the world large enough for our experiment.
Kevin O'Toole manufactured the oscillating tray at the USGS Marine Facility (MarFac) in Redwood City, CA. He mounted the 2-m-diameter circular tray on 20 rollerblade wheels and designed and assembled a motor system to drive it. Not only did the system have to move the tray loaded with 10 cm of sand through water over a rough bottom, but it had to do so at various angles to the flow. The motor speed was controlled by a laptop computer; our code allowed us to specify the period and length of the oscillations, so that we could simulate a range of wave conditions. We conducted initial testing and debugging at MarFac and then shipped the tray, motor, and drive system to Japan.
Our host in Japan was Dave's longtime collaborator Hiroshi Ikeda, professor of fluvial geomorphology at the University of Tsukuba. Ikeda, his research associate Kuniyasu Mokudai, and technicians Hideo Iijima and Kazuhiro Yuhora worked with us throughout our three weeks in Tsukuba; their participation was critical to the success of the project.
The first task in Tsukuba was to install the oscillating tray system in the flume and complete testing of the computer code. Since each experiment would consist of several hundred oscillations, the position of the tray had to be controlled quite precisely to prevent it from migrating toward (and crashing into) the side of the flume. We were dismayed to find that the floor of the flume was not smooth steel but was pitted with rust and embedded gravel, which made the tray motion quite bumpy. On the positive side, the tray and motor system worked beautifully. After a few initial crashes, more than a few code modifications, and a couple of sessions scraping the floor of the flume, we were ready to start the experiments.
We completed 18 experiments with different combinations of waves and currents at angles of 90°, 60°, and 45°. During the experiments, two rotating imaging sonars mounted on the instrument frame recorded bedform evolution. Acoustic Doppler velocimeters measured water speed and direction at two heights above the bed, and an acoustic backscatter sensor measured vertical profiles of suspended-sediment concentration. After each experiment, we drained the flume to measure the wavelength, height, and orientation of the bedforms and to take photographs. The experiments produced a wide range of bedforms, from linear wave ripples to more chaotic patterns, with ripple heights from 1 to 5 cm and wavelengths from 6 cm to more than 25 cm.
Fortunately, our long hours in the lab left us some time to appreciate Japan. In the evenings, Jessie, Dave, and Dave's wife Michelle Rubin sampled delicious and varied cuisine, after negotiating menus in Japanese with a combination of pointing and a smattering of phrasebook Japanese and the help of very patient waiters. Each Saturday, Ikeda took us on an excursion to see the lakes, rivers, coast, and geology of the region, as well as local villages and temples. Every day we enjoyed the company of our Japanese colleagues, who served us green tea, answered our questions about Japanese language and culture, and always made us feel welcome.
in this issue:
Giant Flume Used to Study Bedform Morphology