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Emergency Spillway: Length of Level Section

Emergency Spillway: Length of Level Section

Emergency Spillway: Length of Level Section

Hello All.
I'm getting some static from a new reviewer at our local Conservation District about this.

He wants ONLY a 4' long level section.  This, btw, is less tha what's required (6' min.) for embankment width.
We refuse to go any shorter than 25' (site permitting) as the USSCS requires, before sloping down to meet the ditch below the basin.

Anyone have a link to send me that would help me to show this guy what's right?

Thank you.

RE: Emergency Spillway: Length of Level Section

It sounds like you might be talking width (across embankment) and the reviewer might be talking length (along embankment).  I would first clarify this.

The SCS is currently called the NRCS.  I would go to their website and look around, if you think they require a certain width or length.

I've never heard of a minimum width or length of spillway.  
Width is dictated by the width of embankment; length by flow and design headwater + freeboard.  I can't see how a minimum of either could be accomodated, or what benefit it could be.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”

RE: Emergency Spillway: Length of Level Section


I believe Thurston is speaking of what is called the "inlet" or "approach" channel of the spillway in profile (along direction of flow).

NRCS National Engineering Handbook, Part 628 Dams, Chapter 50 Earth Spillway Design states:

"If a control section is used, the inlet channel is to be level for a minimum distance of 30 feet upstream from the control section."

It is curious that the document states "if a control section is used".  I found no discussion regarding when one would not used a control section.

TR60 states the same minimum length.

Agriculture Handbook 590 Ponds-Planning, Design, Construction refers to a 25 foot minimum length for the inlet section.

My understanding is that the flat inlet channel exists to establish a uniform flow in the spillway channel before the control section (where flow will transition from subcritical flow in the inlet channel to supercritical in the exit channel).

RE: Emergency Spillway: Length of Level Section

Maybe, but I have never proposed, nor ever reviewed a plan with an "inlet", "approach" or "control" section on an emergency spillway.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”

RE: Emergency Spillway: Length of Level Section


What I have found (and been taught in school and various seminars) is a 25' minimum.  30 is even better.

FYI:  The flat inlet channel also exists to reduce the velocity & therefore, the energy of the flow.

Thanks, men.

RE: Emergency Spillway: Length of Level Section

the inlet channel reduces the effects of energy loss which occurs due to the change in flow direction (momentum) as flow approaches the control section.  An inlet channel isn't essential, however, without it your spillway will be less efficient.  You could evaluate the spillway flow with a reduced weir coefficient to account for the additional head loss due to the short approach section.

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