Top of Riser Pipe in a Pond Representation in SSA

[robalan88]

I am designing a stormwater bio-retention basin and I'm trying to compute total outflow from it in SSA.  To represent outflow from it that leaves the basin through the top of the riser box, do I need to create both a weir and an orifice link in SSA going from the storage node to a junction?  The junction represents the invert of the outflow pipe leaving the basin.   When pool reaches crest elevation of the riser, it "plunges" into riser pipe which acts like a weir.  It goes "down" the riser pipe and exists it at the outflow pipe invert.  I have that represented in SSA by using a weir with a crest length equivalent to the perimeter of the riser pipe.  If my riser pipe say is 36 inches in diameter then I have equivalent crest length of 12 feet defined (correct me if I am wrong).   It's a side flow type of weir in SSA. 

For cases when the pool elevation submerges the top of riser completely, do I need to define an orifice link to represent that case too?  Say I'd set the orifice in SSA to a bottom type, circular with a diameter of 36-in.  Set its crest elevation to the point where flow transitions from weir flow to orifice flow.  Hard to say when that is but maybe a rule of thumb I am aware of used for that is 0.2 * Dia or in my case 0.6 ft. above the crest of the riser. 

Am I creating an unnecessary link and over estimating outflow (double counting)  cfs by having 2 (1 weir & 1 orifice) representing the 1 physical element in the basin (the top of the riser pipe)?  Does SSA automatically switch from weir flow to orifice flow for a top of a riser pipe when it gets submerged?  It's unclear to me by searching other posts on here.  If it does switch automatically, how does it turn a crest length into a orifice diameter? 

Or maybe. I just represent the top of the riser pipe in the basin as an orifice outlet and get rid of the weir outlet since weir flow only happens for a very small case of when the pool elevation is right at its crest elevation and maybe a few inches above it

Any help would be appreciated. 

Thanks

Rob 

[SoaresASBCAD]

https://forums.autodesk.com/t5/civil-3d-forum/riser-in-a-pond-storm-and-sanitary-analysis/td-p/59311...