Abstract

Streamlined islands in rivers and comparable landforms in the Channeled Scabland of Washington and in the outflow channels on Mars have shapes that are closely similar to symmetrical airfoils with length:width ratios averaging 3 to 4. A review of measurements of drag on airfoils demonstrates that a minimum total drag is achieved at such length:width ratios, supporting the hypothesis that these islands acquired shapes that minimized the drag or resistance to the flowing fluid when they were formed. This minimum drag is achieved by the reduction of the wake while maintaining a small skin friction. Flume experiments were undertaken with model islands to examine how they achieve their streamlined form. When the island is not fully submerged, a combined bow wave and hydraulic drop play a major role in their erosion, but most of the streamlining is brought about by deposition filling the wake region. When the island is submerged, the flow over the top can become supercritical, producing a hydraulic jump in the island lee and focusing the erosion in that region, rapidly eroding the island to a streamlined shape. These experiments are consistent with the prototype streamlined islands, duplicating many of their features.

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