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pond outlet culvert vs. pipe reach question

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ctbailey

ctbailey

Civil/Environmental
Joined
May 11, 2005
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Hi. I know, this topic has been beaten badly, but i still have a question.

I have a simple driveway culvert that I have modeled as a reach, and also as a catchbasin pipe outlet.

The pond outlet model indicates the culvert is undersized, and the flood elevation is breached. (I set the flood elevation as the centerline of the road.)

The reach model indicates the culvert has excessive capacity, based on the expected peak flow, and the culverts' manning capacity.

So I set the auto-resize to on, performed the calculation, and both culverts are resized, to the top of pipe crown.

However, the pond still overtops the road. Why doesn't the pipe get resized to NOT allow overtopping?

Second question: (Theory) why does the "pond" method produce such a different capacity of the culvert? The manning capacity of the pipe is far greater than the peak flow, and the inlet condition setting in the pond outlet screen effects the pipe very little. So I would expect my "no storage" pond would pass all the flow into the culvert, and no headwater at all would occur. However, in my parralell analysis my "no storage" pond is overtopping the road.

Very curious.

Thank you very much.



___
Craig T. Bailey, PE
 
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1) Automatic resizing will set the culvert size to achieve the desired headwater condition. Please check the resulting culvert size - It's possible that the (higher) crown is above the centerline of the road, which could cause a constant overflow warning.

2)The inlet energy loss will frequently restrict the flow to less than the Manning's capacity. If you set a very low Ke value the culvert capacity will approach the Manning's capacity of the pipe.
 
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Where can I find the information on automatic sizing?
 
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In the calculations settings, there is a tab labeled "resize."

In the resize tab, you can choose to have the program automattically resize all, or certain nodes. Everytime you change rainfall, and "calculate," those items will get resized. Or - "if your feeling lucky, punk," you can select "ongoing resize." That will certainly keep you on your toes.

Good luck,

Craig Bailey

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Craig T. Bailey, PE
 
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I have 500 cfs and want to divert the flow into a detention basin, then divert the flow into 2 separate channels. What is the approach to create a solution automatically?
Do I estimate the basing size and elevation data, also the culvert sizes and slopes?
Thx,

 
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500 cfs! Wow!

What are you trying to detain? The entire cities' 500 year storm? :)

The approach I would take would be to determine the storage required at the basin first, manually. Hydrocad does not have pond sizing capabilities, other than the estiamted graph feature on the reports window.

Then set up your assumed culvert outlet parameters, (using 2 as your multiplier.

set the auto-resize settings, and 'let er rip!

You won't be able to auto-resize the channels.

good luck, and I'm curious as to what you are analyzingto give such big numbers.

ctbailey

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Craig T. Bailey, PE
 
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In Arizona, major watersheds can generate theses flows and impact proposed developments. The County does not want to encourage any home builders
to develop these lands without a rigorous analyses of the site. The site in question is in a floodway. The only way to develop such a site is to re-direct the flows via a channel and relese the flows such that the 2 current natural outlet locations (split flows) are maintained.
A detention basin can capture the volume and reduce the peaks if properly sized and outlets are also sized to handle the releases.

 
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