In view of the abnormal phenomenon that a flood peak increased in August 2004 and July 2005 along the lower Yellow River, the mechanism of this abnormal phenomenon is studied. It is found that the flood increase was due to the decrease the channel roughness in the propagation of high concentrated flood carrying the extra fine sediment which was discharged from xiaolangdi reservoir. In view of this, the equation of drag reduction rate is derivated under variable k pattern, and is applied to the reality. The results show that the equation is reliable and has important role in flood control and calculation in water level with hyperconcentration flow. The study of this paper only is a try and discuusion, and need to be improved.