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Speed of Water Through a Culvert

04/19/2019 2:02 PM

A.) How fast is water coming inside the Culvert, from the LAKE end, in meters/sec , If the water level is 6' high entering the 9'x6' ingress to the culvert?

B.) Is there any difference in water velocity at the River end of the culvert?

I am concerned about the speed, approx. 24" inside the lake end of the culvert.... and at the immediate outside of the River end.

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#1

Re: Speed of Water Through a Culvert

04/19/2019 2:55 PM

I get a velocity of about 23 m/s both coming and going...actual results could vary...

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#2

Re: Speed of Water Through a Culvert

04/19/2019 4:20 PM

excellent. thank you

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#4

Re: Speed of Water Through a Culvert

04/20/2019 1:36 AM

Oh sorry I read that wrong....~30 m/s... 10% drop 10' vs 10° drop = 17.6'

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#3

Re: Speed of Water Through a Culvert

04/19/2019 11:51 PM

Dear netmaker,

What solar eagle informed velocity of 23 feet/sec., is correct, as it is theoritical based on the equation,

Velocity = (Sq. Root of (2g) x H)

and you have told height as 9 feet. It is also mentioned in his posting, the actual will vary and I want to supplement as additional points to his posting.

1. The above formula will give theoretical velocity.

2. Generally the co.Efficient of Velocity is considered as 0.8 which means (the theoritcal velocity) x (0.8) will be the actual velocity and in this case 23 X 0.8 = 18.4 Feet per second. This is valid for smooth surface. If the surface is rough the effective frictional loss will be high and resistance to flow will increase.

3. Since you have mentioned it is from a lake, there is a possibility that small small stones or pebbles etc. if found - it will totally resist the smooth flow.

4. If the exit of water is having an upward slope the effective head of 9 feet will come down.

Thanks.

DHAYANANDHAN.S

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#6

Re: Speed of Water Through a Culvert

04/20/2019 11:25 AM

The 100' culvert is rough concrete

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#5

Re: Speed of Water Through a Culvert

04/20/2019 8:00 AM

There's a calculator here, where you can enter more detailed information:

http://onlinecalc.sdsu.edu/onlineculvert.php

I looked at this last night and came up with a figure of 23.5 ft depth at the end of the culvert for a drop of 17.5 ft over the culvert length, because the equations were simplified to discount velocity. The relationship between flow velocity and depth is interesting! That is, depth increases more than velocity... discussed on page 8 of this pdf, which also gives a handy table of Manning coefficients for different materials on page 10.

http://www.dartmouth.edu/~cushman/books/EFM/chap15.pdf

Manning's equation is used to calculate discharge velocity for supercritical flow, but another equation is given for subcritical flow calculation here:

https://wisconsindot.gov/rdwy/fdm/fd-13-15.pdf

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#10

Re: Speed of Water Through a Culvert

04/20/2019 5:08 PM

Thank you for the Table and chart

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#7

Re: Speed of Water Through a Culvert

04/20/2019 11:28 AM

What does g and H stand for in the velocity formula?

As usual, I have come to the right place for technical answers. Thank all of you.

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#8

Re: Speed of Water Through a Culvert

04/20/2019 2:19 PM

I think that means gravity and height....

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#9

Re: Speed of Water Through a Culvert

04/20/2019 5:07 PM

ok

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#11

Re: Speed of Water Through a Culvert

04/23/2019 7:33 AM

It depends upon whether or not the <...lake...> is infinite or not.

• If it is not infinite, then the level will fall with time, which means the velocity will fall with time also.

It depends upon whether the <...river...> is infinite or not.

• If it is not infinite, then its level will rise with time. Once the level in the river exceeds the level of the bottom of the culvert, the flow will reduce with time.

It depends upon the roughness of the <...culvert...>.

• The rougher it is, the more resistance there will be to flow.

It depends upon whether or not the <...culvert...> is straight.

• The straighter it is, the less resistance there will be to flow.

The <...culvert...> as depicted is not the full story. There has to be some device to reduce or even stop the flow at some point in it, otherwise a non-infinite lake will drain to the bottom of the culvert and flow will stop.

• The further open the device is, the less resistance there will be to flow.
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#12

Re: Speed of Water Through a Culvert

04/23/2019 8:38 AM

It will also depend upon temperature:

• The viscosity and density of water vary with temperature, viscosity strongly and density less so. In particular, below 0degC, the features in the illustration can be expected to have ice in them, which will affect the flow.
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#13

Re: Speed of Water Through a Culvert

04/27/2019 10:34 AM

If you’re still interested in this, I’ve had a look at it, but I can’t agree with some of the earlier posters.

I assume the level in the lake is 6’ above the channel bottom at the lake end. There is a fall in surface level from lake to culvert, about 0.5*V2/2/g for entry loss, and V2/2/g for Bernoulli’s equation. That results in a depth just inside the culvert 4’, flow 6.3m3/s and velocity at that point 2.8m/s. Critical depth at that flow = 3.5’, and critical velocity 3.23m/s.

Due to the steep slope (10° is a lot for a channel) the velocity then goes above critical, depth below critical depth, ending up at 1.2’ deep and 9.3m/s about 4’ from the end of the culvert. Followed by a drop-down curve to depth about 1’ and 11.3m/s at the end of the culvert (the immediate outside of the River end). This assumes free outlet from the culvert i.e. bottom of culvert above river water level (as your pic seems to indicate).

It’s an interesting one, if others disagree with my figures I’m happy to discuss.

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#14

Re: Speed of Water Through a Culvert

04/28/2019 1:22 PM

Kudos for working through the problem with all its changes. If I read you right, there is just a trickle coming out of the pipe (or gushing out of the pipe) on the other end ?

That seemed surprising, since the comment about depth increasing instead of velocity seemed to suggest otherwise. But I may be mistaken. I also completely missed the loss of depth just inside the culvert, but definitely got the impression that these problems are so complex given real world situations that they are usually measured and adjusted in the field in order to address site specifics.

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#15

Re: Speed of Water Through a Culvert

04/28/2019 5:35 PM

The way I look at it it's the entrance to the culvert from the lake that limits the flow, and everything flows (pun intended!) from there. Obviously the flow is constant at any section of the culvert (not pipe) so there is 6.3m3/s coming out the end (if my calcs are correct).

The very steep slope accounts for the increase in velocity/decrease in depth going down the culvert. I integrated the gradually-varied conditions equation (using Mathcad) and trying various depths at or near the end of the culvert till the calculated length came to about the 100' specified.

BTW, the mixed units is because the OP wanted it, not my choice!

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#16

Re: Speed of Water Through a Culvert

04/29/2019 10:17 AM

That makes sense to me. Bearing in mind I have no practical experience in this area, no reason I wouldn't expect to be surprised.

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#18

Re: Speed of Water Through a Culvert

04/29/2019 4:57 PM

Flow in open channels can have a wide range of characteristics, depending on eg flow rate, bottom slope, length, inlet and outlet arrangements etc etc.

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#19

Re: Speed of Water Through a Culvert

04/30/2019 8:50 AM

Drainage design is increasingly interesting, due to the risk of higher precip events. There are a lot of open storm drains in my area, with culverts passing under people's driveways. It is hilly so the slopes vary but those ditches have been quite filled by the heavier rain we've had - and we've been lucky so far. A few minor road washouts or shoulder damage at worst. We just haven't had the extreme amount that is happening on the continent.. yet.

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#17

Re: Speed of Water Through a Culvert

04/29/2019 1:31 PM

As the velocity increases, the depth decreases. In this video example, the depth goes from ~6' deep at the start and is ~2' deep at the end.

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