Contraction Scour - Approach Section Covered with Vegetation or Armored
Laursen's contraction scour equation, as mentioned above, assumes the bed materials and the shear stresses in the approach and the contracted
sections are the same. Where the bed material of the approach section is not the same as the contracted section, the following equation is used.
Where the upstream overbank section is covered with vegetation and no sediment is transported (clear water scour), either Neill's or Laursen's
clear water scour equations may be used in determining contraction scour. The bed material in the contracted section will be eroded until (1) the bed
shear is reduced to its critical value, or (2) the flow depth increases until it reaches the depth where the shear stress is reduced to the value of the
critical shear stress. The clear water contraction scour equation, then may be reduced to the following form:
y2 = yc
or
y2/yc = 1
where yc = flow depth in contracted overbank or channel section when bed shear is at the critical value.