Hydraulics get complicated fast, the beaty of weirs (that's the function of your bulkhead hear) is that you simplify the hydraulics, as you separate the flow upstream of the weir from the flow conditions downstream the weir: As long as water level downstream is a bit below the crest, flow conditions upstream can be calculated backwards from the weir withour regarding downstream conditions. This is why you can build flow distributing structures as per your drawing: Flow into one compartment depends only on weir length (again, only so long as the water level downstream is below the weir crest). You "pay" for this with hydraulic losses.
Your guesses is correct, provided two things:
- the wall partinioning B & C is higher than the resulting water level in those compartments.
- There's enough distance between upstream of the bulkhead for the flow to evenly distribute, or the flow velocity is low (how much distance? How low? I don't know. You want even distribution along the bulkhead). Given the shape of your flume upstream, I'd assume that you are good.
If gate C1 is closed (effectively shutting off the water to that pipe), and the downstream gate C2 is closed, what percentage of the water goes into Pipe A and B? (My guess is A 33% and B 66%)
Reservoir C will fill up to the water level upstream of the bulkhead, then flow over the bulkhead into the compartments A and B will be proportional to bulkhead lengths. IF the bulkhead between B and C is lower than that level, you will have an additional flow into compartment B, to be calculated by the weir formula. You need to check the hydraulic losses for the pipes from reservoir A and B and calculate the water levels in A and B at the relevant flow rates.
If gate C1 is closed (effectively shutting off the water to that pipe), and the downstream gate C2 is open, what percentage of the water goes into Pipe A and B? (My guess is A 25% and B 75%)
IF the hydraulic losses from the pipes A or B are so high that the water level rises above weir crest, you'd need to account for a lower discharge coefficient and the math gets hariy fast.