HEC-RAS Scour
HEC-RAS Scour
(OP)
I am analyzing several bridge crossings for scour using HEC-RAS. I am getting contraction scour values of around 30 ft., but my water depth is only about 10 ft. during the 500 year event. I believe the 30 ft. of contraction scour is too high, but I can't figure out what is wrong. I have delineated the flood plain using ineffective flows for contraction and expansion. I have use the HEC-18 equations to determine the lengths of the expansion and contraction reaches.
One issue I noticed with HEC-RAS is that the Width Values used (W1 & W2) are top widths, which does not match the HEC-18 guidance which implies using the Bottom Width of the channel...
Does anyone have any other ideas of what might be going wrong for me??
One issue I noticed with HEC-RAS is that the Width Values used (W1 & W2) are top widths, which does not match the HEC-18 guidance which implies using the Bottom Width of the channel...
Does anyone have any other ideas of what might be going wrong for me??





RE: HEC-RAS Scour
RE: HEC-RAS Scour
RE: HEC-RAS Scour
RE: HEC-RAS Scour
RE: HEC-RAS Scour
One other question, when determining your Expansion and Contraction Lengths, do you use the equations from HEC RD-42 (equations 17-20)? For wide basins, the contraction and expansion reaches become fairly long. In our state, DNR is the jurisdictional reviewer for submittals, and their guidance is to assume the effective flow contracts at a 1:1 making the Contraction Length = 3*(Effective Flow Width), and water expands at a 2:1 making the Expansion Length = 6*(Effective Flow Width). Is this similar to your methodology, or would you use the HEC RD-42 Regression Equations? Sometimes I do get strange results using the RD-42 Equations...
RE: HEC-RAS Scour
I place the most upstream bridge xsec at the furthest point of contraction intection with floodplain. Hopefully that makes sense. I will then remodel the existing condition with that xsec in place and then compare results.
Have some fun with this; I really enjoy hydraulic modeling.