check dams/rip rap channels

[murdock]

I am evaluating the velocity of flow through a rip rap channel with stone check dams. Modeling just the reach with the rip rap (n=0.04) gave a velocity of about 3 ft/s. When I used two ponds (modeling the two check dams), the velocity was much higher (6.2 ft/s).

I recognize this as the pond not accounting for the roughness of the channel. How do I account for both the roughness of the channel and the storage of the pond? I considered using a reach (with the full length of the swale up to the check dam) followed by a pond (to account for the storage). However, is this making the actual structure longer than it really is? IOW, breaking up the swale into 2 nodes gives it the appearance of flow traveling from the swale into a storage structure, instead of the swale *being* the storage structure?

 

[LHA]

Use the Reach between two ponds, that is hydraulically what is happening. The flow will be at design velocity per its characteristics, until it hits the check dam. In that "Pond", it will - for all intents and purpose - be zero. When the dam/pond overflows, flow will return to design velocity.

Engineering is the practice of the art of science - Steve

 

[psmart]

Murdock: You didn't specify exactly where you got the higher velocity. I suspect this is the weir discharge velocity, which could be higher than the Manning's velocity for the reach.

 

[DDaugherty]

What is the purpose of your analysis? To size the riprap stone? If so, I would just look at the channel without the check dams because before water fills behind the check dams, it will be flowing over the stones. If you wish to know the instantaneous velocity within the ponding area, you would use Q=AV, where you know Q (your flow), and A= the cross-section of water at that location. If you are looking for the time it would take water to travel through the system, I would break it into open channel segments and ponding segments. I'd take the volume of storage behind the check dam and divide by the flow rate to get the time to travel through the "pond" and add that to the channel times.