Headwater: 2" drop in a 40 foot length culvert

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Discussion Overview

The discussion revolves around calculating the headwater for an 18-inch culvert with specific water levels at the inlet and outlet, as well as a drop over a 40-foot length. Participants explore the implications of these measurements on flow rate and culvert design, focusing on hydraulic principles and calculations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant seeks to determine the flow rate (fps) and headwater for the specified culvert configuration, noting the water levels at both ends.
  • Another participant questions the meaning of "fps" and suggests that the flow rate for a 2-inch drop over 40 feet would be very small, implying limited energy generation potential.
  • A third participant outlines that calculating flow through a culvert involves multiple factors, including culvert dimensions, shape, roughness, slope, and water velocities, and emphasizes that there is no single equation for this calculation.
  • This participant also references a standard hydraulic design document and warns about potential errors in nomograms used for such calculations.
  • A final post appears to introduce an unrelated analogy involving a stick in water, which may not directly contribute to the discussion on culvert flow.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the specific calculations or implications of the culvert design. There are differing interpretations of the initial question and the relevance of various factors in determining flow rates.

Contextual Notes

Participants highlight the complexity of hydraulic calculations, including the need to consider various physical parameters and the potential for errors in existing resources. The discussion remains open-ended regarding specific methodologies and outcomes.

John1397
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TL;DR
How to figure headwater
I am trying to figure out the fps of 18 inch culvert with 13.5 inches water over the top on inlet and about 2 inches lower on the outlet and 2 inch drop on the outlet end over 40 foot length, what I need to know what would be the headwater on this type of configuration?
 

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John1397 said:
Summary: How to figure headwater

I am trying to figure out the fps of 18 inch culvert with 13.5 inches water over the top on inlet and about 2 inches lower on the outlet and 2 inch drop on the outlet end over 40 foot length, what I need to know what would be the headwater on this type of configuration?
Sorry, what is the "fps" that you are asking about? Are you asking what the flow rate would be for a water channel that has a drop of 2" over a 40 foot length? If so, the answer is very small -- hopefully you are not trying to generate any useful energy out of this flow...
 
It sounds like you want to calculate the flow through a culvert, given some dimensions and depths. There is no one equation to calculate this. There is a procedure that takes into account the culvert dimensions, culvert shape (round vs arch bottom), roughness (CSP vs concrete vs plastic vs ...), length, slope, headwater level, tailwater level, water approach velocity and direction, tailwater velocity, entrance configuration, etc. That procedure starts by determining whether the flow is under inlet control, outlet control, or in the transition region between. Here is a link to the standard reference, HDS5 Hydraulic Design of Highway Culverts, 3rd Edition: https://www.fhwa.dot.gov/engineering/hydraulics/pubs/12026/hif12026.pdf.

Be advised that at least one of the nomograms is in error by more than an order of magnitude, so use the equations.
 
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Seen that throwing stick in water time how many seconds from into out works to.
 

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