Given that almost 485,000 bridges in the United States are over waterways and may be susceptible to scour to some degree, a detailed scour analysis at all sites may not be feasible. To ensure that the rapid-estimation method would be applicable to a wide range of geographic and hydrologic conditions, data from ten States were used to develop the method. Although scour depths can be explicitly calculated using the Level 2 equations, complex hydraulic variables in some equations cannot be easily measured or estimated in the field. Surrogate variables that could be easily measured or estimated were substituted for the more complex variables in the Level 2 scour equations to arrive at simpler forms of the scour equations. These simpler equations were used to develop relations between scour depths and the surrogate variables.
Froude number is a hydraulic term related to flow depth and velocity.
Of these variables, pier width was the most significant. Data from Level 2 analyses were thus plotted and an envelope curve was used to encompass the data. In a similar manner, envelope curves for abutment scour and contraction scour were also developed. Hydraulic variables needed to apply the envelope curves range from as few as two for abutment scour to ten or more for typical conditions of contraction scour. Variables used for estimation of pier scour.