Spillway Flow

[jeb6294]

Here's the situation...got a lake adjacent to the property were designing with an existing spillway structure. I am trying to come up with some calculations showing that the spillway and outlet pipes will be fine with our additional development.

For simplicity sake, the spillway is basically shaped like a capital "D". The curve of the "D" is at water level and is 39.77-ft in length. The straight portions ("[" this part of the "D") are 1-ft. above water level and total 45-ft in length. The whole thing dumps into a pair of 90-in pipes. We're doing a stage-discharge up to 10.3-ft above water level.

The big question is how to go about figuring the flow these outlet pipes are going to see.

1) use the standard Q=cLH^1.5 with L=39.77 for the first foot and then L=45+39.77 for the rest?

2) I found something sorta similar in my Gupta book about a shaft spillway that's based on a vertical pipe with a flared end. It says for a shaft spillway to treat the first part of the flow as weir flow (Q=cLH^1.5), then switch to orifice flow (Q=CA(2gH)^0.5), and finally to pipe flow (Q=A(2gh/K)^0.5) once the whole thing is submerged.

3) Something else I haven't thought of.

 

[blueoak]

For weir flow run 2 cLh^1.5 equations, one for each base elevation since H will be different for the [ compared to the curve.

A good reference is page 407 and on from Design of Small Dams:

http://www.usbr.gov/pmts/hydraulics_lab/pubs/manuals/SmallDams.pdf

c is also shape dependent and changes with h, normally you can show min and maxs for the variables and show that regardless you don't have problems.

Depending on what your development is adding there may also be some stability issues. I think it is addressed, but it is about if you have a lot of shifting from weir to pipe you can have problems with pressure fluctuations.

 

[bltseattle]

Like blueoak says....

You also should check the inlet headwater on the outlet pipes to make sure HW elev is beneath the weir elevations, otherwise you have submerged weirs with lower capacity.