Dam rectangular drop shaft spillway

[bean65]

I'm designing a dam spillway to be able to pass the 100 yr flood and eventually 0.5PMF for a class 1 dam. Unfortunately, there is not much storage upstream of the dam, so what comes into the dam, needs to exit. I would like to consider several pricipal spillways to give my client the best economical choice. I've seen a rectangular drop shaft spillway used and was not aware of any weir coefficients. I believe it may be a good idea due to the weir length (rectangle with ratio of 3:1), but I cannot find coefficients. I've searched my USBR Design of Small Dams and internet, but haven't found anything yet. Circular drop shaft spillways, the weir coefficient drops from say 3.3 to 1.5 as the head increases. Is there anything published for rectangular weirs??

 

[jartgo]

I'm not sure I understand your question, but I think your weir coefficient would be the same for the rectangular or circular, assuming you are referring to them that way as they are represented in the horizontal plane? Of course, the structure would eventually switch to orifice conditions after a certain head is acheived. Let me know if I didn't understand the question.

 

[cvg]

Effective weir coefficient is reduced at higher heads due to convergence. See following discussion published by Washington State Dam Safety Office:

“Convergence” occurs when weir flow from opposite sides of the riser intersects at the center of the riser to essentially provide a water seal near the riser crest. This phenomenon is described in more detail in USBR (1987) on pages 407 to 409. For weir flow convergence, the distance to intersection Lo is typically either 0.5 or 1.0 times the riser diameter or width, depending on whether flow occurs from opposite sides to intersect in the center, or flow occurs from one side and must cross the entire riser width to intersect the opposite wall.

I don't believe I have seen a published reference to rectangular drop spillways, possibly because it would need to be supported by testing and also due to the large variation in possible dimensions of the rectangle, would be difficult to come up with a generalized relationship.

I believe a ratio of 1:1 (square) would probably give the best hydraulic performance by allowing maximum flow over all 4 sides of the riser before convergence begins to occur. However, this would be highly dependant on the size of your structure and the flows involved. Keeping your head to a minimum will minimize convergence and give you the maximum discharge through your spillway.

 

[cvg]

the attached paper may be useful

 

[GeoPaveTraffic]

I agree with cvg, Convergence of flow will cause the coefficient to change. Also, the corners of non-circular spillway will not be as efficent and those effects may effect the flow a greater distance from the corner than an off the cuff thought give you.

My suggestion would be to use a circular design for now, then if a drop spillway appears to be the way to go, you could look at some model tests to determine the effectiveness of a non-circular design.