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One of the common methods for scour protection around bridge piers is riprap layer. In previous studies, sizing riprap layer was used to ensure 100% protection against scouring. However, in many cases, limited scour depth around a pier may be accepted if only smaller riprap sizes are available. In the present work, the e ects of the smaller size of riprap stones compared to their stable size on the scour depth around a bridge pier were studied. Circular and oval shapes for riprap extent and both round and angular stone shapes were also tested. All tests were conducted at the threshold of bed sediment motion, and the maximum scour depth was measured. The results of these experiments showed that with stone sizes closer to stable riprap material, the e ciency of both round and angular stone shapes was identical. As the size of riprap reduced, deeper scour holes were observed with both round and angular shape materials. The results also indicated that increasing the extent of the riprap layer from circular to oval with 5 times more riprap volume had insigni cant e ects on scour hole for angular shape riprap and, also, reduced the scour hole depth with round shape material. Based on experimental data, a method was developed to calculate a smaller riprap size based on an accepted limited scour hole.