Like everything, there are dozens of ways to do stuff. It comes down to your personal choice and the results you want. What I'll discuss is mainly current best practices imo.
I am planning my 2nd pond and researching bottom drains, can anyone help me understand drains? I am building a 16x16 foot pond L shaped pond with 8 foot Legs.
Right off, for that configuration you should consider a "river flow" or "stream flow" type drain. Google "river flow pond bottom drain" for more info. Not a lot of info, but it isn't the complex. Basically in a round or square pond a single drain is placed in the center and water current is created in a circular motion to flush debris to the drain. In a rectangular pond 2 drains are used, 2 opposite direction currents are set up to flush debris to each drain. Basically divides the rectangular pond into 2 square sections. I'm not sure of your exact pond shape, each leg is 8' long but how wide is each leg? Posting a rough picture is best.
Drains are to be installed at the Lowest point in the pond right, I should slope all other areas to direct debris and water towards the drain right?
Nope. This is probably the biggest misconception. If the purpose of the drain was to remove marbles the sloped sides would be perfect. But it isn't. The stuff you want removed almost floats in water. It can settle on a 45 degree side and stick there just fine. Over time enough build up might avalanche down. Exactly the same deal as dust in your house.
So myth #1 is sloped sides. "Well they help a little"...no, they don't.
Are 2 inch drains common?
Probably the most common drain sold is 2" because it's sold into the mass market. Many of these are never installed, they're just impulse buys. Seemed like a good idea in the store. Those that are installed I'd say manyl are turned off shortly there after due to clogging and the pond bottom being no cleaner than before.
The most common working drains are 3 or 4". The mass market brands wouldn't sell as the bulk would scare most customers away.
Does a bottom drain Need a Pump?
There has to be a pump some place. Whether the pump is connected directly to the drain or not is a personal choice that should be based on your personal goals.
If so it is the drain, to a 2" flexible PVC pipe that connects to a pump inlet and the outlet goes to filtration and back in the waterfall?
Yes when connected directly to the drain. This would normally be an external pump with a strainer pot as part of the pump. The strainer keeps how large stuff that might damage the pump. You might have to clean out the pot often, even once a day, depending on time of year, stuff you have around your pond, string algae, etc. As the strainer clogs water flow is reduced. So the net is you'd spend more $$$ to pump less water than if the pump is not connected directly to the drain. Most of these pumps have auto shutoff because if the strainer clogs too much the pump burns out.
Finer stuff, or stuff that breaks down fast like fish poo go on thru the strainer basket and is ground up into a poo frappuccino most of which goes right on thru any filter. Kind of defeats the whole purpose of having a bottom drain imo.
You may have to be careful on the pump elevation. Pump mounted below the pond water level will flood out the strainer when you open the cap.
Best practice...
The other pump placement is where it isn't connected directly to the drain. Here's a crude picture of this basic configuration.
This is called a gravity fed system. A reservoir, or small pond, or bog holds the pump which can be a submerged pump or external, doesn't matter. As water is pumped into the pond the pond overflows thru the bottom drain. The drain pipe opening is positioned at or just below the desired pond water level.
Instead of the strainer pot a simple pre filter screen is placed under the drain pipe opening. Very easy DIY, just a screen. This would be called a sieve filter. Couple of benefits...way less cleaning and easier to clean than a strainer. Gets debris out of the water column right away as the debris is washed to the edges, so the stuff isn't rotting in you pond so you get better water quality. If clogged the screen will over flow so the pump won't be starved for water and burn out as fast. But it's easy to DIY screens that hold 100 times as much debris as a strainer pot.
In the picture the pump sends water back into the pond, but that outflow could be sent to filters, falls, stream, whatever.
The picture above is a DIY version of something like a Cetus Sieve filter. You can check those out to see if you'd prefer buying one.
There are many other choices...that was just my fav.
Tangential Pond Returns
We've covered 2 parts to a bottom drain system. A hole in the bottom pond connected to a pipe and a pump. If either one of these is missing you got nothin'. There is a third piece that is just as important as the other two. It is almost always forgotten in water gardens. It's the primary reason those mass market 2" drains are turned off.
There has to be a way to move debris to the drain. I already covered why sloped sides don't work...never have...never will.
Stuff is moved to the drain by water current. Think leaf blower. It takes very little current. For example, if you placed a regular garden hose into a pond so it was parallel to the pond side, even just below the surface and turned it on you would see the entire volume of the pond turning in just a couple of minutes. And on the bottom you would see debris moving to the center and spinning around like a tornado. That's a Tangential Pond Return or TPR. Super simple, super necessary for a good performing system.
A standard practice is to have the pump as shown above and have another pump for the skimmer. The drain pump output would be sent to filters and falls. The skimmer pump would go to the TPRs because the skimmer pump can generally be smaller and that all a skimmer and TPs need. TPRs are general located 18-24" above the bottom. The direction they point and number needed depend on the shape of the pond. There are some detailed diagrams on the web on different configurations.
Sloped Sides
Although sloped sides aren't needed there is one kind of sloped feature that is good. For a circular motion BD on a flat bottom you dig a 4-6" deeper area with the drain in the center. The deeper area is also flat and about 1-2' out from the drain. As the water current sweeps around the pond that little 4-6" drop will be a water break of no current and so debris will fall into that lower area and into the drain.
