Flexible bed sills are extensively used in rivers in several applications: to protect pipeline crossings, for slope control and to enhance environmental aspects. Use of riprap provides the flexibility required to adjust to small changes in river bed level and makes this material one of the most commonly used. Shear failure of riprap has been extensively studied, but little knowledge is available on edge failure of flexible bed sills and on the design of the required extent of sills to protect assets. Experimental work in a large test facility was carried out to gain improved understanding of flat sills and sills protruding above the bed. Significantly lower depth and length of the scour hole were observed at the downstream edge of flat riprap sills than at rigid sills. Existing equations for rigid bed sills were found not to be appropriate for estimating scour at the flexible bed sills tested. As expected, protrusion enhances scour potential significantly: it was found that scour depths were of the order of 100% or greater than those associated with flat sills. The research has provided useful evidence for avoiding placement of pipelines straight on to river beds and instead to adopt trenches whenever feasible.