Abstract: Aerial Photograph Interpretation of High-Velocity Wind-Driven Currents

R. A. Morton

High-velocity wind-driven currents and attendant coastal-storm deposits formed during hurricanes are recognized best with the aid of aerial photographs that document (1) the extremely large-scale but low-relief bed forms and sedimentary features, (2) the diverse relations of flow directions with respect to normal directions of gravity flow, and (3) the unique morphology and characteristics of sedimentary features resulting from high-wind shear stress.

The combined erosional and depositional features commonly preserved on sandy coastal barriers that serve as criteria for inferring wind-driven currents include: (1) counterclockwise curvature (in northern hemisphere) of washover channels; (2) closely spaced, slightly curved striations; (3) transverse bars along bay shorelines; (4) individual flame-shaped fans emerging from deeply incised channels; and, locally, (5) rhomboid bed forms. Other large-scale features of wind-driven currents are striated fans and tongues composed of sand and shell fragments. These fans coalesce to form sediment wedges with crenulated landward margins that terminate at an avalanche face nearly parallel with the barrier trend; sediment bypass areas separate the wedges from storm berms.

According to field measurements and experimental studies in engineering publications the ratio of surface-current velocity to wind velocity is between 3 and 5%. These data represent velocities far below those expected in hurricanes. If this ratio is invariant or if it increases with increasing wind velocity, as some data suggest, then average wind-driven current velocities of 2 to 4 m sec^-1 are minimum estimates of actual field conditions observed on aerial photographs.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC