Open Channel Flow: Specific Energy

[mototsykl]

A trapezoidal channel with bottom width b = 30ft, side slopes m = 2, and flow Q = 5100cfs given, and I am supposed to plot a specific energy curve (E vs y). That part I have done.

For the second part, it is given that 3 bridge piers, each 2 ft wide, are spanning the channel. With the same m and b = 30-3(2) = 24 ft, I need to calculate another specific energy curve.

Question: Is it really that easy? Just change the corresponding x-s area A because of the new b, or does the flow change as well?

Thanks in advance!

 

[Valkarie]

Yes, it is that easy. You just need to recalculate the x-s area A for the new bottom width b and then use the same flow Q to calculate the new specific energy curve. The flow remains the same since the bridge piers are assumed to be non-permeable and therefore do not affect the flow in the channel.