Circulation for ponds is an aspect of design that needs to be addressed early on. Many of us have had to assess a poorly performing pond only to determine that, even with good equipment, one of the main problems is poor circulation. This can be in the form of debris that collects in some areas (either at the surface or on the bottom), sections of the pond that have poor water quality, or just basic water clarity.
I have written several articles about the importance of removing the water from both the top surface and the bottom, where the worst water in a pond calls its home. Good circulation from top to bottom and back to top is an important fundamental. This is accomplished with skimmers and bottom drains to remove the water from the pond, and aeration to move the water from the bottom back to the top. In some instances, a return from the center of a vertical pond return drain is used to move water from the bottom to the top in real time. My pond designs that incorporate vertical pond return drains are some of the best performing projects I’ve ever been a part of.
Tangential pond returns are used to return water inward from the sides at an angle or tangent to slowly rotate the body of water. Gap returns are used to split water between two drains and form opposite rotating sections of a pond around each drain. Some returns are used to circulate an odd or unusually shaped area. In-floor returns can spread out the incoming water over the floor when side returns are not possible, like when the walls are part of another structure. Occasionally, I have used a small airlift-operated current jet in a corner or other area that pulls water from the bottom to the top, circulating the water and adding aeration at the same time. This works well when you don’t have enough water flow in the design for another current jet, or if you don’t need more water flowing through the filters but need more circulation. It’s easy to run a small ½-inch airline around the pond perimeter to a spot on the edge for an airlift-operated current jet.
Rock Bottom Ponds
As designers, we have many options when it comes to designing a circulation pattern other than just a waterfall or spill return. These all work well on a dedicated koi pond that is free from decorative rocks on the bottom; however, rock bottom ponds pose more of a circulation issue than clean surface ponds. It’s no secret that I’m not a big fan of rock bottom ponds, but I have dealt with some over the years as rebuilds. Sometimes the client isn’t willing to give up the rock look, or there isn’t enough biofiltration to eliminate the rocks without a serious filter upgrade. Whether you’re a fan of rock bottom ponds or not, the rocks do provide a lot of surface area for bioconversion.
The issue for me with rock bottom ponds has always been debris entrapment and circulation. I have used screen-domed bottom drains to keep the rocks out of the drains, and for very small rock bottom ponds, this works well. Over the years, I have worked with industry professionals like Eric Triplett (The Ponddigger) and others on some under-gravel grid systems.
Triplett has had a lot of success with this type of bottom water removal. He and other contractors have operated under-gravel systems with both mechanical pumps and airlifts, which makes me very happy. The key is doing the math with the right-sized main line, with the correct number and placement of properly sized holes in the grid lines. These systems, when coupled with under-gravel aeration and backflush capability, can produce amazing results for clients who want the rock-bottom look.
What if we thought about pond circulation a little differently? When I started incorporating adjustable in-floor returns, I started thinking about a completely different flow dynamic. What about an up-flow pond to help with the circulation of rock bottom ponds? If the concept is circulating water from the bottom of the pond to the top or eliminating dead zones on the pond floor, why wouldn’t it be a good idea to bring in most of the incoming water through the floor below the rocks and gravel instead of trying to evenly suck the water from all points on the floor through the rock and gravel? Even suction distribution is one of the fundamental issues with an under-gravel suction grid. The water will always take the path of least resistance, favoring the holes closest to the end of the outlet on the main connecting line. This is the reason the math is so important when building an under-gravel grid system.
There are two ways to create an up-flow pond. In the first way, the water exits the pond through the skimmers and mid-water drains placed 1 to 2 feet above the floor around the perimeter of the pond. The water returns through the under-gravel grid along with a waterfall filter or spill if necessary. Without a waterfall or spill, the pond maintains a still-pond look while circulating itself more than one time per hour. A two-speed pump could be used, with the higher speed coming on occasionally, increasing the flow through the grid at times to keep the buildup of debris from forming over time. In real time, the pond would always look clear, and the water would always circulate in an up-flow pattern. Aeration tubing could be placed in strategic positions under the gravel with the air pump on a timer for better circulation and aeration. Restricting the inlet side of a pump is not a good idea. Intake restrictions can cause cavitation and overheating. Slightly restricting the outlet side of a pump poses no problem at all. This means that slightly smaller calculated hole sizes in an up-flow grid can balance out the flow, creating a more even distribution of the return water throughout the floor area. The rock and gravel bottom would have tremendous bioconversion capacity and be self-cleaning in real time.
Alternatively, you can use a double-grid system. The hole spacing and size would have to be calculated in the same manner as the original under-gravel grid, but there would be two alternating grids lying beside each other, about 1 foot apart. The water would exit the pond through skimmers and one of the under-gravel grids. Water would return to the pond through a waterfall filter or spill as well as through the other grid. The swimming pool industry makes motorized three-way valves that can be placed on timers in sizes as large as 3 inches. With one of these on the inlet side of the pump and the other on the outlet side, the bottom grids can be switched back and forth every couple of hours, keeping the bottom rock and gravel cleaned and flushed at all times. The valves on each side of the pump would switch at the same time, alternating the incoming and outflowing water from one grid manifold to the other. Aeration under the gravel would have to be placed so that the grid system would not be allowed to pull in any bubbles cavitating the pump.
In both of these systems, good prefiltration for the removal of solids is still important. This may sound complicated, but it’s worth the effort.
Several years ago, I developed the vertical pond return drain after a discussion with others about the concept on some koi boards. I haven’t done construction in years now; I just design systems and manufacture equipment.
I hope some of you out there will take under consideration the up-flow concept and create a system like I’ve described. If anyone has interest in collaborating on these concepts, please feel free to contact me. I’d love to work with someone on one or both of these designs.