Waterfall height value calculation

[drbballwater]

For the purpose of calculating the total head of a waterfall (ie. for sizing my pump), is the ‘height’ considered the highest apex of the pipe run to where the pipe run ends or the height of the where the water finally leaves the closed system of the plumbing?

For example due to the topography of my yard, on the way to the bog filter that begins the cascading stream and waterfall back to the pond my plumbing has to go up and behind some boulders at about 8’ above pond surface before reaching the end of the plumbing run in the bog which is only about 3’ above pond surface. So is the ‘height’ value I plug into the total head equation 3’ or 8’?

It seems relevant that whether the water is still in the closed system or not at a given height would matter but I don’t remember my fluid dynamics from high school very well - was a while ago.

 

[Lisak1]

Gooood question! My guess would be the maximum height the water has to flow... but that's just a somewhat educated guess!

 

[drbballwater]

Gooood question! My guess would be the maximum height the water has to flow... but that's just a somewhat educated guess!

Ya I really don’t know... intuitively to me - when the pipes are totally empty I would think it would initially need to hit the 8’ but once the entire piping system is full of water I would think the 5’ drop in plumbing after the apex would exert some form of ‘pull’ on the water in the direction of flow via gravity/siphon and make life easier for the pump. Only a guess though. Do any hydrologists lurk around on here?

 

[combatwombat]

Do I understand correctly that you're essentially describing a hump that your piping goes over before exiting to your waterfall?

I just read about this, actually. Wish I could remember where so I could point you to the source. You can omit humps from your calculation for total dynamic head because yes, the pump has to overcome the hight of the hump when the pipe is empty. But once it's full, the water at the top of the hump is pushing down on the water in front of it, making up for the head loss on the other side. So, the net effect is 0.

I guess where you could run into issues is if the top of the hump takes your head over the shutoff height of the pump. That obviously wouldn't work as you have to be able to get over the hump on startup.

 

[drbballwater]

Do I understand correctly that you're essentially describing a hump that your piping goes over before exiting to your waterfall?

I just read about this, actually. Wish I could remember where so I could point you to the source. You can omit humps from your calculation for total dynamic head because yes, the pump has to overcome the hight of the hump when the pipe is empty. But once it's full, the water at the top of the hump is pushing down on the water in front of it, making up for the head loss on the other side. So, the net effect is 0.

I guess where you could run into issues is if the top of the hump takes your head over the shutoff height of the pump. That obviously wouldn't work as you have to be able to get over the hump on startup.

Your understanding is indeed correct. And thanks for confirming my intuition, I don’t need the resource as I trust your recollection.

I will oversize my pump slightly anyway and control with ball valves if I need to so the startup situation shouldn’t be an issue.

Thanks again!