Barchan dunes are widely distributed in swarms on Earth and other planetary bodies. Collisions among barchan dunes have been extensively investigated because of the homogeneous dune size distribution through mass exchange. Double-dune collision modes can be divided into coalescence and ejection according to the morphologies after the collision. Different thresholds of transition between coalescence and ejection on the sizes of two dunes have been widely studied. However, the impact of dune spacing in flow direction on collision modes and the cases that the upstream dune is larger than the downstream one have seldom been mentioned. We use a 3D cellular automaton model to study the effects of dune size and spacing on dune modes after collision. A transition surface distinguishing coalescence and ejection is found in the parameter space of dune size and spacing. We confirm this finding through field observations and experiments. Finally, a generalized function is established to quantify the factors affecting the strength of a special short-distance attraction mode in the extreme conditions for the double-dune collision. For the first time, we give a comprehensive depiction of dune collisions that may be used to bring new constraints on the morphodynamics of dune fields.
 

Barchan dune,Cellular automaton simulation,Collision mode,Transition surface,Coalescence,Ejection