| I'm going to throw some dirt on tour plans. Don't let it stop you. A pond that size can be a dream most of us will envy. Not because of the size but because of it's potential to provide the healthy environment JR & folks talk about.
First consider the flow you want. Looks like you may have already done that.
Next lay out the filter components you hope to use. I suggest you look a little further before you go beyond this point.
Then compute the Total Dynamic Head you're pump will be dealing with at the flow you chose.
Then select and buy the pump.
You can design backwards, but it becomes a bigger task in that the pump will limit your choices of layout and components.
Bead filters are usually rated at the best flow for mechanical filtration. A bead as a Bio-converter should be operated at about 1/2 that. Don't build a 15000+ gallon and cut corners on filtration & other components. A well designed pond that size will stand well above one just put together with a minimum of components. To (sorta) answer one of your questions, bead filters don't have enough drop for them to dictate anything. As you determine the drops in the system you design, The bead filter will be less than 10% of the drop, if the system is properly designed.
If you want to design a really low head pond, you should drop the bead thinking altogether. You will also have to look for alternative pumping sources such as airlifts but you'll be walking a path without much history. Very few ponds have been designed using airlifts. But those that have, use between 25 & 30% of the power of our best centrifugal pumps.
Even a bead filter used as a mechanical filter should have good mechanical filtration before it. A well designed sump or microscreen. Additionally, the bead filter should be dumped daily. The fish S... that makes it passed the 1st mechanical will do a great job of feeding heterophic bacteria. Better to leave that room for nitrifiers and other beneficial bacteria.
Leave room for two or three bio-filters so you can experiment with the different stiles.
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