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I want to understand the meaning of headwater, and I stumble across this page, and this diagram:

enter image description here

This is where it got me. Headwater is defined as

measured from the flow line (invert) of the culvert inlet, to the water surface elevation

But I have no idea what does water surface elevation mean. Is it the actual water surface ( doesn't look like it as there is another normal water surface elevation)?

And if it's not, what it is, and how it is being defined?

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The picture you've provided includes the answer that you require, the water surface elevation is the elevation of the water as depicted on the upstream portion of the culvert.

enter image description here

Therefore, the headwater elevation above the culvert pipe is providing a certain amount of pressure head that is contributing to pushing water through the pipe (in addition to gravity and velocity).

To clarify, the situation depicted in that image is a pipe in a pressure flow situation (as opposed to an open channel flow situation). The Normal Water Surface Elevation is what calculations would indicate the surface elevation should be when the system is analyzed without considering the downstream impacts.

What's not depicted is what those downstream impacts are. For example, they could be a blockage or other constriction in the pipe or perhaps a tailwater elevation, which is when the elevation of the water surface at the point of discharge. Were the pipe to discharge partially or fully below the surface of a wet pond, that tailwater induces a back pressure on the system which affects the hydraulic grade line profile across the pipe network.

If you are dealing with a pipe network, it is extremely important to be cognizant about when and what causes your system to go into pressure flow conditions. Generally, though, if your observations or calculations indicate a headwater at some point, you've entered a pressure flow scenario for that pipe and it is very important to review the hydraulic grade line profile through the network to ensure you don't have water coming out of your inlets.

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It means normal water surface elevation across the flood plain in the absence of roadway or bridge or culvert.

The following is a quote from the same site document.

normal water elev.

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  • $\begingroup$ See the image I posted above. water surface elevation and normal water surface elevation are two different thing. I'm asking about water surface elevation and not normal water surface elevation $\endgroup$
    – Graviton
    Feb 27 '19 at 5:18

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