Headwater: 2" drop in a 40 foot length culvert

[John1397]

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?

 

[berkeman]

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...

 

[jrmichler]

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.

 

[John1397]

Seen that throwing stick in water time how many seconds from into out works to.