Standpipe riser calcs 1

[jdienst]

I am working with standpipe risers that have a low flow orifice and a top opening with grate. In running the calculations I want to account for the fact that the orifice flow will be affected by the changing water elevation inside of the riser. I need to calculate the relative head for the orifice. The problem is that the Q on the culvert is not known, so I can't just back calculate the orifice Q. I have stage/discharge data for the culvert and for the orifice individually, but not as a combined structure. Is there a common algorithm to calculate the correct discharge through the culvert or is this going to be an iterative process?

 

[nealt]

jdienst:

In modeling riser/barrel type outlet structures, you must calculate flows for the barrel and the riser separately using the head produced on each element by the water surface elevation in the reservoir. The riser should be calculated for both weir and orifice flow conditions. Weir flow will normally govern at the riser, however orifice flow can govern if there is sufficient head on the riser. Once these three flows are calculated, the lowest value flow is taken as the governing discharge rate through the structure.

If the riser flow (weir or orifice) governs, back calculate the head required to produce this flowrate in the culvert and this is the water depth inside the riser. Compare this depth with the elevation of your low-flow orifice and determine if a tail water condition exists on that orifice.

If barrel flow governs, the riser is flowing full and low-flow orifice flow is zero. Remember to account for any tailwater condition that may exist at the barrel outlet. "Free outfall" normally does not exist when discharging to a natural channel under design storm conditions.

A rule of thumb:
1. Typically it is desirable to size the riser diameter about 1.5 times the barrel diameter. This forces the barrel to control the flow. This is important to make the structure as economical as possible (smallest barrel), and to help minimize vortex formation at the top of the riser. Be careful with optimization of the barrel size. If you are dealing with a small watershed area compared with the size of your reservoir, a very small barrel may result which could be succeptible to clogging. I would recommend no smaller than a 15" diameter barrel, and be sure to design a trash rack at the top of the riser. Once the barrel size is chosen, the riser size can also be optimized if desired, but 1.5 times is usually a good place to start.

Hope this helps.