Formation Antidune

water flowing on bedforms in sand under unidirectional flow right. numbers correspond broadly increasing flow regime, i.e., increasing water flow velocity. dunes, water surface low on dune , high on interdune. antidunes, flow depth same everywhere, means water surface high on antidune , low on interdune.

antidunes typically found in fluvial environments in shallow areas high flow rate. unlike ripples , dunes in lower flow regime, antidunes symmetric , migrate counter flow direction. antidunes evolve rapidly, growing in amplitude migrate against current. when surface wave above them becomes unstable , breaks (when surface wave amplitude reaches 1/7 wavelength) of antidune bedform destroyed , sediment carried down stream.

antidunes commonly observed in small streams flow across beaches ocean. flume studies have shown can occur in submarine environments beneath density flows turbidity currents. antidunes produce sedimentary structures characteristic of flow regime, allow sedimentary geologists understand past flow conditions. unlike low flow regime bedforms dunes , ripples produce downstream dipping cross stratification, antidunes produce mixture of low-angle downstream , upstream dipping strata. while antidunes migrate upstream, upstream dipping cross-stratification not indicative of antidunes or upper flow regime conditions.

antidunes migrate upstream because stream flow shallow , fast in trough , slows , deepens on crest. result, shear stress on bed decreases trough crest, allowing sedimentation, , increases crest tough, causing erosion. inertia of flow moves shear stress maximum , minimum downstream of trough , crest. allows bedform amplify time erosion occurs in trough , deposition occurs @ crest.

christopher r. fielding observed link between formation , climate. climates have extreme rainy seasons resulting in runoff create higher flow velocity within streams , rivers, increasing ability of upper flow regime structures form. here video showing formation , destruction of modern antidune.