text: Hydraulic Engineering Circular Manual No. 18 (HEC-18) was published by the FHWA, and includes several techniques of estimating scour depth. The empirical scour equations for live-bed scour, clear-water scour, and local scour at piers and abutments are shown in the Chapter 5General Scour section. The total scour depth is determined by adding three scour components which includes the long-term aggradation and degradation of the river bed, general scour at the bridge and local scour at the piers or abutment. However, research has shown that the standard equations in HEC-18 over-predict scour depth for a number of hydraulic and geologic conditions.  Most of the HEC-18 relationships are based on laboratory flume studies conducted with sand-sized sediments increased with factors of safety that are not easily recognizable or adjustable. Sand and fine gravel are the most easily eroded bed materials, but streams frequently contain much more scour resistant materials such as compact till, stiff clay, and shale. The consequences of using design methods based on a single soil type are especially significant for many major physiographic provinces with distinctly different geologic conditions and foundation materials. This can lead to overly conservative design values for scour in low risk or non-critical hydrologic conditions. Thus, equation improvements are continued to be made in an effort to minimize the underestimation and overestimation of scour.