Material type for abutments

I have a question about the way HEC-RAS models scouring for abutments. How does the abutment scour calc account for what type of material you're bridge is built in. I see there are entries for contraction and pier scour, D50 and D90, but not for the abutments. ????

Glen Parker
DOKKEN ENGINEERING

Dear Glen :

The equations used to estimate the magnitude of abutment scour were developed in a laboratory under ideal conditions and for the most part lack field verification. Because conditions in the field are different from those in the laboratory, these equations tend to over predict the magnitude of scour that may be expected to develop. They do not include particle size.

Recognizing this, it is recommended that the abutment scour equations be used to develop insight as to the scour potential at an abutment. Engineering judgment must be used to determine if the abutment foundation should be designed to resist the computed local scour. As an alternate, abutment foundations should be designed for the estimated long-term degradation and contraction scour. Riprap and/or guide banks should be used to protect the abutment for this alternative. In summary, riprap or some other protection should always be used to protect the
abutment from erosion.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com