Head Water Over Depth Ratio 1

[evett]

I was wondering if anyone out there knows why we use a headwater over depth ratio of 1.5 as a check in the design of cross drains? Why 1.5? Is it a pressure thing?

 

[LHA]

I do not know who the "we" referred to is, so the answer may be just an internal policy thing. As far as the science goes...

Inlet flow control in horizontally-aligned orifices changes from weir-type to orifice-type flow at around HW=0.4D. Since orifice flow is more restrictive, maybe it is an arbitrary height at which to rerun the numbers in case the analysis to this point has only considered weir flow.

This would be quite arbitrary indeed for verticle inlet alignments, which are usually considered to be limited to orifice flow at HW=D.

 

[bltseattle]

This is one of those issues that should be engineered for the situation at hand. If the agency says not to exceed HW/D = 1.5 then follow that, but it is often possible to go higher if the embankment can handle the impounded water. Likewise, a "weak" embankment not engineered to withstand ponded water on the upstream face should use less than 1.5. I suggest considering the various failure modes for your application and tailoring your design to what is needed.

 

[Rookie2]

That's an interesting question, something I'd never thought about. We have the same requirement here. From what I've read in Hec 10, it's basically becuase the HW/D charts that usually accompany those requirements are not accurate
for HW/d ratios over 1.5. It would be great to get someone elses interpretation on this. You can download Hec 10 at the link below. The section I am referring to begins on page 87.

http://www.fhwa.dot.gov/bridge/hec10.pdf