Please note that this information is of particular importance in the event that clear water scour will occur for the worst case scour event. Clear water scour estimates are very sensitive to particle size and vegetative cover. For this case, determine the particle size of the bed material at the estimated depth of the contraction scour. If the subsurface material varies with depth, divide the soil data into layers, utilizing the information from the soil borings. Refer to Chapter 14 for additional discussion on the estimation of the critical velocity for particle movement. We recommend that you independently check the field conditions and make your own determination of critical velocity; then use the over-ride feature to enter your value if it is different from the value computed by the program.
A complicating factor in selecting a representative particle size for clear water scour is the potential for armoring of the channel bed. A comprehensive treatment of the armoring of channel beds is beyond the scope of this guide, and the user is referenced to the FHWA publication Highways in the River Environment or similar texts on river mechanics to evaluate this condition. In general, great reliance should not be placed on the expectation that armoring of the bed will limit the extent of contraction scour.
The ABSCOUR clear water scour equations compute scour by comparing the average flow velocity with the critical velocity (competent velocity) of the D50 particle in the channel bed or overbank area at the estimated depth of scour
Soils with a D50 less than 0.0007 ft. are likely to have cohesive properties. For such cases, use Neill's method for cohesive soils (as presented in the ABSCOUR Users Manual - Chapter 11 Appendices) to select the appropriate critical velocity. Then use the override feature on the project information card to insert this selected critical velocity in the spaces provided on the downstream data card.