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How to best model outlet structure...

How to best model outlet structure...

How to best model outlet structure...

(OP)
I have a pond with an outlet structure as shown in this link.. [IMG]http://i2.photobucket.com/albums/y3/itchynackers/outletjunk.jpg[/IMG]

I've been able to model it as shown here... [IMG]http://i2.photobucket.com/albums/y3/itchynackers/outletjunk2.jpg[/IMG]

but my model assumes the weir has vertical sides, when in fact it is sloped.  I've also tried to model it as a horizontal orifice.  Is there a more accurate way that I'm not thinking of?

Thanks,  Adam

RE: How to best model outlet structure...

You could approximate this by using a trapezoidal weir.  Basically "flatten out" the sloped sides of the box so they form the sloped sides of a trapezoidal opening.

This will overestimate the flow in the corners to some extent, so you may want to reduce the discharge multiplier to allow for this.  But the culvert will start to control before you get too much head, so any error will be limited.
If in doubt, do a simple sensitivity analysis:  Reduce the discharge multipler and see how much difference it makes to your peak storage and discharge.

RE: How to best model outlet structure...

As a side note, when using the sharp crested rectangular weir, the crest height is the height of the crest above the approach channel.  In your detail, this dimension appears to be 3.0ft rather than 1.0ft.  This can influence the discharge calculations.

Of course, as Peter mentions, you whould probably use a trapezoidal weir from your discription (which does not have crest height input).

RE: How to best model outlet structure...

(OP)
I'm actually going to use a crest height of 0, since the earth will match the leading edge.  I may have to switch to a broad crested.
Also, I did a sensitivity analysis on the structure.  Minor variances with different properties.

Thanks everyone.

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