An experimental investigation of riprap stability at spill-through abutments has been carried out in two similar horiontal-bed flumes, avoiding riprap failure modes other than shear failure and erosion failure. Tests were conducted for two common abutment side slopes, subcritical flow regime, short to intermediate abutment lenghts and high bed roughness. The aim of the experiments was to determine the size of stone riprap necessary to resist (a) shear failure in aprons placed at the base of spill-through abutments, and (b) erosion failure over their side slopes. Results are compared with predictions of stone riprap size given by expressions found in the literature. The predictor of Pagan-Ortiz is confirmed as being appropriate for shear failure at abutment aprons and can be adapted to account for erosion failure over abutment side slopes. Alternatively, two news expressions, written in terms of the (critical) approach flow intensity required to make the riprap stones move, are suggested.