Determination of Live-Bed or Clear-Water Contraction Scour

To determine if the flow upstream is transporting bed material (i.e., live-bed contraction scour), the program calculates the critical velocity for beginning of motion Vc (for the D50 size of bed material) and compares it with the mean velocity V of the flow in the main channel or overbank area upstream of the bridge at the approach section. If the critical velocity of the bed material is greater than the mean velocity at the approach section (Vc > V), then clear-water contraction scour is assumed. If the critical velocity of the bed material is less than the mean velocity at the approach section (Vc < V), then live-bed contraction scour is assumed. The user has the option of forcing the program to calculate contraction scour by the live-bed or clear-water contraction scour equation, regardless of the results from the comparison. To calculate the critical velocity, the following equation by Laursen (1963) is used:

1)	$\begin{array}{l}\displaystyle V_c = K_u y^{1/6}_1 D^{1/3}_{50}\end{array}$

Symbol	Description	Units
$\begin{array}{l}V_c\end{array}$	Critical velocity above which material of size D50 and smaller will be transported	ft/s (m/s)
$\begin{array}{l}y_1\end{array}$	Average depth of flow in the main channel or overbank area at the approach section	ft (m)
$\begin{array}{l}D_{50}\end{array}$	Bed material particle size in a mixture of which 50% are smaller	ft (m)
$\begin{array}{l}K_u\end{array}$	11.17 (English Units), 6.19 (S.I. Units)