The mechanism of bridge pier scour becomes more complex in the presence of debris accumulation upstream of the pier.While using countermeasures may be effective in reducing scour,their efficacy could be undermined in such a situation.The current study investigates the effectiveness of using a collar in the presence of different types of floating debris accumulation in reducing scour around a cylindrical bridge pier with non-cohesive bed sediment.The experimental results reveal that using a collar can reduce the scour depth by up to 39％ when compared to that without the collar in the absence of debris accumulation under the same flow conditions.Also,the collar\'s efficiency is reduced by up to 25％ when debris accumulation is present.However,depending on the relative size of the accumulated debris and collar,the effectiveness of collars may be increased as a pier-scour protection device.A new parameter,which is defined as the Debris Protrusion Ratio (DPR),is used to quantify the effect of debris layout size on scour depth.Moreover,a semi-empirical equation that may be used to predict the depth of the scour hole that forms around cylindrical collar-fitted piers with floating debris accumulation is proposed.The equation\'s performance is evaluated using the available data in the literature.Based on the analysis,the current study shows that the accuracy of the proposed equation is satisfactory in predicting the maximum scour depth.