Bottom DrainsStandard equipment on high end Koi ponds but used much less in Water Gardens. Work best in ponds with sloped bottoms and without plants or other structures which can block muck from heading toward a drain. Drains are most often connected to settling tanks where muck falls to the bottom and can be flushed out by opening a valve at the bottom of the tank.
Settling Tank SetupSome ponders also place brushes and other materials in the tank to remove debris.
Note the flush valve is just a little below the waterline. It should be at least 6" below waterline and the further below it is the more forceful the water will drain out. Opening the valve every day or every few days for a few seconds is all the cleaning that's needed.
Simple system, so why don't all ponds have them? Space is one consideration. Settling tanks have to be large to work well. Pond shape can really affect performance and so can lower pump rates. To work best you want water to get into the settling tank before suspended debris has a chance to float to the pond's bottom and that can mean a pump that can pump the pond's total volume every hour or two. For Koi ponds these issues normally aren't a problem. In Water Gardens these can be issues that greatly decrease the value of a settling tank and bottom drain.
Bottom drains can clog with dead string algae. Again, not normally a problem in Koi ponds but can be a problem in Water Gardens.
Bottom Drain Only SetupIt is possible to skip the settling tank and instead use the pond as a settling tank.
In this case the pond is only flushed every week or every few weeks. Debris is brushed toward the drain and stirred up silt allowed to settle for a day before the drain is opened.
Adding a bottom drain to an existing pond.A bottom drain can go over the side of a pond using a siphon. The two lower valves are closed and the top valve is used to fill the pipe section with water removing all water. The top valve is then closed and the 2 side valves opened and water will start flowing. This is called priming the siphon. Bonnie Hill's web site (link no longer valid) has detailed info on building a slick over the side bottom drain along with the most complete source of pond info we've found on the net.
It's possible for the over the side drain to lose its prime over time. If you're on vacation that can be a problem since the pump depends on getting water from the drain and could burn out if run dry. To fix this you can put the pipe through the liner side near the water surface. This requires less digging and if a leak should happen between liner and pipe the pond would only lose a small amount of water. No priming is needed. The place where the pipe goes through the liner must be completely underwater and maybe a little lower than the normal waterline to account for evaporation.