The bi-directional flow often happens in the downdraft reach due to the influence of main channel, so that its velocity calculation is obviously different from normal river in practical engineering.

• 倒灌河段受干流影响往往存在双向流动，其流速的确定与单一河道明显不同，对缺乏必要水文和地形资料的大量中小倒灌河段，使用数学模型或物理模型常常遇到困难。