Air-discharge pits on the Yellow River delta plain

A large number of air-discharge pits, varying in size and shape, are developed on the Yellow River delta. Their planar cross-sections are circular, sub-circular, elongated and frequently irregular and their spatial configurations may be in form of a shallow dish to root like or other, extremely irregular, shapes. They range in diameter from less than 1 cm to over 60 cm and their depth varies from less than 1 mm to more than 20 cm. They most commonly occur individually or in groups on the point bar.Our investigation indicates that the occurrence of the air-discharge pits is closely associated with the unique hydrological, channel and hydrodynamic conditions caused by the intermittent flow interruptions of the Yellow River and the fine sediments typical of the river course. During a Yellow River flow interruption, an underground air-saturation zone in the dry layer of the riverbed may be formed due to the lowering of the ground water level. When flooding originates from the up-stream section of the Yellow River, the water quickly inundates the channel and rapidly submerges the extremely dry river bed and point bar. As a result, due to the blocking effect caused by the silty sand and mud, preferential percolation occurs along the outer edge of the point bar and levee, trapping air in the process.The resulting air pocket and pressure capsule are enclosed in the air saturation zone, providing the source material required for the subsequent formation of air-discharge pits. The trapped air may also form a high pressure capsule caused by the slow downward percolation of the river water. The differential, relatively lower pressure on the point bar permits the trapped air to escape. Provided the air is discharged in a shallow body of water (<10 cm deep), the force of the agitated water caused by the bubble explosion may liquify the surface of sedimentary deposits and thus facilitate the transportation of the sediment by the current flow which eventually results in the formation of an air-discharge pit. [Copyright &y& Elsevier]