Estimating fluid flow from a gravity-fed pipe?


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I made a few design changes to how I was planning my garden pond, and I'm trying to estimate the viability of moving the pump until after I filter the solids out of the water, in a sump. The idea behind this is to reduce the amount of waste that might be forced through it, and make accessing it easier...

I've been trying to estimate the flowrate by gravity using Bernoulli's Equation, mostly from memory of a Fluid Dynamics class from many years ago, and came up with this:
1674452874036.png

Where:
desc1905975377109864412.png



For a 1 foot (~0.3m) difference between the drain and top water-level, with a 2-inch diameter pipe (~0.05m), the flow equals.... 1 Liter per hour. That's pretty abysmal, and increasing the height doesn't really do much to increase the flow rate either.
Is this a sensible prediction? It doesn't seem reasonable to me.

Say I was draining a 500-gallon pond through that same pipe, at 1L/hr draining it would take more than 2000hrs (since height becomes a function of time). That's just not sensible and I feel like I'm doing something wrong here.

Has anyone here managed to do any estimation of the sort of water flow a gravity-fed pipe might be able produce (assuming a constant water level)?
 
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Small correction:
1674454364125.png


Was being stupid with the units, corrected it and now I'm calculating ~10,000L/hr, which means the 500 Gallon pond I mentioned would take a bit over 10min to drain at that flow rate.
That feels a lot more reasonable, this might actually be a viable way to arrange the pond filters.
 
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