Spillway Flow

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 (see, don't automatically start selling all your old references) 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.

You are going to model this by hand? God bless you

What routing program(s) do you use? If you dont use any computer software to do your modeling, I can do it for you for nothing if you wanna email me all pertinent data... for both pre and post development scenarios

Let me know

Sorry, nothing near that elaborate....just me and my Excel.

There's no way I'm going to try and do this 100% the way it should be done with that goofy spillway, just trying to get close enough to satisfy some pencil pushers.

Right now I've got a spreadsheet set up from El. 751.7 to 762 in 0.10 to 0.20 increments with three different columns (there's a road with a low elevation of 755 so there'll be some water going over that with our 762 HHW) that each figure their own flow and then there's a fourth column that totals them.

The first column figures the flow over a 39.77-ft weir from 751.7 to 762. The next column kicks in at 752.7 and figures the flow for a 45-foot weir from 752.7 to 762. The third column kicks in at 755 and figures the flow over the road up to 762. The last column just adds up the first three.

Ya I wouldnt think its anything more than using your basic discharge over a weir formula which you indicated before. I dont think you will have to worry about the weir being submerged either.

Is this an emergency spillway? If so, I can't imagine any justification is needed since there are typically sized for a minimum 100 year return period.