In recent years, hydraulic structures crossing rivers have been widely used in channels to process the water resource management. Although the upstream channels can be stabilized, the gravel deposition in the hydraulic structures can increase the slope of the upstream channel bed and consequently lead to a sudden vertical change of the channel slope and induce a free overfall flow. As a result, the large impact forces of a free-falling from the free overfall flow also weaken the hydraulic structures. The scouring phenomenon of free overfall flows, constructed by a series of non-horizontal free overfall flow channel bed slopes unit width discharges heigh of the overfall, acting on a different particle size of bed material was studied. Analyze impact force from free overfall flow affect for scour hole volume. The focus of the research is to investigate the upstream channel bed slope effects on the scour hole volume per unit width, based on the theoretical derivation and hydraulic experiments. The approach suggestions can be applied to design of protection works for hydraulic structures.