Supporting information for Transition between dune collision patterns — a numerical simulation study

Barchan dunes often occur in swarms on Earth and extraterrestrial bodies. Collisions among barchan dunes have been extensively investigated since it has been one of the main causes for the dune swarm to achieve size and migration speed selection. As a localized simplification of dune collision, the double-dune collision can be divided into coalescence and ejection. Effect of the relative sizes of two dunes on the collision patterns have been widely studied, whereas the influence of inter-dune space has been less mentioned. Here, the Real-Space Cellular Automaton Laboratory (ReSCAL) model is applied to comprehensively investigate the effects of inter-dune space and relative dune size on the dune collision patterns. A parameter space is established to include the abovementioned effects, where a transition surface distinguishing coalescence and ejection is determined. As confirmed by field observations and subaquatic experiments, a comprehensive depiction of transition between coalescence and ejection is provided that may be used to bring new constraints on the morphodynamics of barchan dune fields. The supporting information contains four texts: physical scaling in ReSCAL (T1), distinguishing upstream and downstream dunes (T2), data processing of the double–dune collision cases in the field (T3) and of subaquatic experiments (T4), as well as Video 1 that illustrates the lateral offset between two dunes caused by the periodic calving along the horns of large barchans.

新月形沙丘（barchan dune）常以沙丘群的形式分布于地球及地外天体之上。由于沙丘碰撞是沙丘群实现尺寸分选与迁移速度选择的核心机制之一，学界已对新月形沙丘间的碰撞开展了大量研究。作为沙丘碰撞的局域简化场景，双沙丘碰撞可分为合并（coalescence）与抛射（ejection）两种模式。学界已广泛探讨了两沙丘的相对尺寸对碰撞模式的影响，但针对沙丘间距的影响则鲜有提及。本研究采用实空间元胞自动机实验室（ReSCAL）模型，全面探讨了沙丘间距与相对尺寸对沙丘碰撞模式的影响。本研究构建了涵盖上述两类影响因素的参数空间，并确定了区分合并与抛射模式的过渡界面。经野外观测与水下实验验证，本研究给出了合并与抛射模式间过渡过程的完整刻画，可为新月形沙丘场的地貌动力学研究提供新的约束条件。本研究的补充材料包含四份文档：ReSCAL模型中的物理尺度律（T1）、上下游沙丘的区分方法（T2）、野外观测与水下实验中双沙丘碰撞案例的数据分析流程（T3、T4），以及一段演示大型新月形沙丘翼缘周期性崩解引发两沙丘横向偏移的视频（Video 1）。