online_scour_hec18: Calculation of bridge scour using the HEC-18 formula

Formula:

Fr_{1} = v_{1}/(g y_{1})^{1/2}

y_{s} /y_{1} = 2 K_{1}K_{2}K_{3} (a / y_{1})^{0.65}Fr_{1}^{0.43}

[ SI Units (metric): y_{s}, y_{1}, a, and L in m ]
[ U.S. Customary Units: y_{s}, y_{1}, a, and L in ft ]
[ Fr = Froude number; g = gravitational acceleration ]