Incipient motion of coarse gravel-sized sediment was investigated under a range of oscillatory flows. This article examines the relative significance of shear stresses and pressure gradients in triggering motion, which was directly measured with electronic Smart Sediment Grains (SSGs). The data suggest that incipient motion was induced by the pressure gradients in flows with large accelerations, by the shear stresses in flows with low accelerations and greater shear, and by the combined effects in intermediate flows. A modified incipient motion criterion was evaluated accounting for the combined effects of the shear stresses and pressure gradients, which may be more widely applicable in the marine environment.

Incipient motion experiments were conducted with natural gravel, acetate beads, and coarsegravel-sized electronic grains called Smart Sediment Grains in a Small-Oscillatory Flow Tunnel. Measurements of fluid velocity were made using Particle Image Velocimetry. The strength of the fluid shear stresses and the pressure gradients were examined for a range of oscillatory flow conditions at the onset of motion of the sediment particles to determine which mechanism had induced particle motion. The three sediment types utilized in these experiments facilitated an assessment of the effects of sediment grain size diameter, shape, and density on incipient motion. Results suggested that the onset of sediment motion was dominated by the pressure gradients for flows with small orbital excursion amplitudes, by the shear stresses for flows with large orbital excursion amplitudes and by the combined effects for intermediate flows. The denser, angular gravel required greater free-stream accelerations to trigger sediment motion than the spherical, less dense acetate beads, and Smart Sediment Grains. A combined parameter for incipient motion that accounts for the simultaneous effects of both shear stresses and pressure gradients while depending on the static coefficient of friction and the packing concentration of the mobile bed layer was evaluated for accuracy using a range of sediment types. The results suggested that the combined parameter may be a better indicator of sediment mobilization under oscillatory flows than the typically assumed shear stress criterion.

Measurements within the mobile bed layer have been limited by previous Eulerian-based technologies. A microelectromechanical system device, called a smart sediment grain (SSG), that can measure and record Lagrangian observations of coastal sediments at incipient motion has been developed. These sensors have the potential to resolve fundamental hypotheses regarding the incipient motion of coastal sediments. Angle of repose experiments verified that the sensor enclosure has mobility characteristics similar to coarse gravel. Experiments conducted in a small oscillating flow tunnel verified that the sensors detect incipient motion under various hydrodynamic conditions. Evidence suggests the influence of pressure-gradient-induced sediment motion, contrary to the more commonly assumed bed shear stress criterion. Lagrangian measurements of rotation measured with the newly developed SSG agreed to within 5% of the rotation estimates made simultaneously with high-speed video cameras.

Surface thennal features occur i n an a h a of 9700 m2 a t M t . Hood, on t h e basis of an aerial line-scan survey made April 26, 1973. The d i s t r i b u t i o n of the thermal areas below the summit of M t . Hood,,shown on planimetrically corrected maps a t 1:12,000, suggests s t r u c t u r a l control by a f r a c t u r e system and brecciated zone peripheral t o a hornblende-dacite plug dome (Crater Rock), and by a concentric f r a c t u r e system that may have been associated with development of the present crater.

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