EFFECTS OF SPUR DIKES ON BRIDGE BACKWATER: LABORATORY REPORT

FHWA's current emphasis on bridge replacement conbined with the potential of spur dikes (elliptically shaped dikes projecting into the stream or river upstream from bridge abutments) to reduce pier and abutment scour and to improve the hydraulic performance of the opening, prompted a model study to clarify some of the conflicting ideas about the effects of spur dikes. The study evaluated the effect spur dikes have on bridge backwater and other hydraulic parameters. Experimental results were compared with the analytical procedures for predicting the backwater effects of spur dikes, found in HY-7/WSPRO (FHWA/RD-86/108 Bridge Waterways Analysis Model - Research Report). Model results for low gradient stream confirm that procedures in HY-7/WSPRO properly predict the backwater effects. In addition, it was found that spur dikes reduce backwater by up to 5 percent. More importantly, it was noted that spur dike length has a significant impact on the magnitude of the stream's energy gradient as it passes through the bridge constriction. This has implications with respect to channel stability in the vicinity of abutments and piers.