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# Angled riser outlet

## Angled riser outlet

(OP)
Hello, I am modeling a concrete riser and grate in a detention pond. The 30 inch diameter grate on top of the riser will be angled to match the 3:1 slope it is embeded in. I am not sure the best way to model this angled outlet.
Replies continue below

### RE: Angled riser outlet

You can model this using a trapezoidal weir, where the two sides are essentially bent back at a 90 degree angle.

Peter Smart

### RE: Angled riser outlet

(OP)
So, using a vee/trap weir outlet, the invert should set to the elevation of the spillway, the notch left blank, and the width set to 2.5'- the  diameter of the riser.  Is this the configuration you would recommend for modeling this scenario? thanks for clarifying.

### RE: Angled riser outlet

You also need to set the notch angle to twice the slope of the grate.  (The notch angle is the total included angle, so it would be the sum of the two sides)

Basically, you're flattening out three sides of the grate, which will form a trapezoid.

Peter Smart

### RE: Angled riser outlet

(OP)
Peter, I'm having some difficulty visualizing your recommendation. Just to make sure I provided a clear description of the configuration- the grate is 30" diameter, round.  One side will be tipped up from horizontal to match the 3:1 slope of the basin. The result will be a round grate that is flush to the surrounding 3:1 slope.

### RE: Angled riser outlet

Sorry, I was providing details for a rectangular grate.

For a circular grate on a slope you will probably need to use a custom weir/orifice.  As before, you will be "flattening out" the perimeter so you can model it as a curved weir in 2-dimensions.

Peter Smart

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