Understanding Bottom Drains

In the last five years Bottom Drain Technology has come a long, long way!  Much of the educational material available for bottom drain installation is either swimming pool technology or dated "Koi Pond Technology".  That combined with some clever marketing by a water garden pond equipment manufacturer to discourage pond drain installations, it's no wonder that bottom drains are still so sadly misunderstood by contractors and homeowners trying to figure them out.  Only in the last five to seven years have gravity fed settlement units become commercially available as a proprietary product from a manufacturer.

Why would you want a bottom drain?  The simple reason is to REDUCE MAINTENANCE.   Pond drains done right, remove organics from the pond and create a unique dynamic in the pond's circulation system.  Water Gardens/Eco-System ponds rely on beneficial bacteria colonies to breakdown fish waste, and other organics such as leaves, pine needles, pollens, dust and all other forms of water logged debris that typically settles to the bottom of a pond. 

Water Gardens/Eco-System ponds usually rely on a shallow bed of gravel on the bottom of the pond to help manage organics because it serves as a place for beneficial bacteria to colonize. (See Fig. 1) The gravel needs to be managed regularly to avoid undesirable conditions for beneficial bacteria colonies.  As a rule, this requires draining the pond each spring to wash and clean the gravel bed.  A dedicated Koi pond is completely opposite and traditionally will not have gravel in the pond.  (See Fig. 2) Dedicated Koi ponds rely on drains, circulation jets, aeration and pond vacuums to keep the pond free of debris.

Only within the last five years has a bottom drain been commercially available with an air diffuser built in to the unit to optimize efficiency of the bottom drain's ability to capture solids.  You can visit just about any Specialty Koi Dealer across America and witness the change in technology first hand!  Any display pond at a Koi Specialty Facility that is older than 2003 will likely not have an aerated bottom drain!  If you find a display pond at a Specialty Koi Dealers place five years or older WITH an aerated bottom drain, I would be very surprised and say you are at a facility that has helped pioneer aerated drains from the very beginning.  Please spend money with them!
  They deserve your financial support!
This is extremely exciting to me and it is only because of these advancements in Drain Technology with in the last few years that we even offer pond drain installations today.  Our construction team has only offered these installations for 18 months and only in the last 12 months have we made it a point to really begin promoting bottom drains in our pond design consultations.
It is my goal to help promote the latest drain technologies that we apply in our basic Koi pond designs each and every day; help you to determine what size bottom drain is best for your pond, show you how to determine optimal flow rates for your drains, & explain how proper use of aeration complements a drain's ability to remove solids from a pond and explain the fundamentals we use for designing bottom drains into Koi pond systems.

We only recommend Gravity Flow Bottom Drain Installations as opposed to direct suction drain installations.  Gravity flow means we are plumbing the bottom drain to a pre-filtration unit and from the pre-filtration unit we are draining water with a pond pump and sending the water to filtration.  For example, the drain can be plumbed to a settling chamber, a settling tank, a skimmer, or sieve filter.   Where ever you are draining your GFBD to will be at the same water level as your pond and by removing water from this pre-filter,  the law of gravity is pulling water from the pond through the bottom drain line to fill the pre-filter back up.
There are many different options where you can GRAVITY FLOW your bottom drain too. Some options are more difficult to install than others, some installations are easier to maintain than others and of course, the cost of gravity flowing your BD to a determined location varies greatly!
Which ever the case I am not a fan of plumbing bottom drains directly to the pump and I will NOT assume the reasons are obvious.  If you use direct suction drain technology in a koi pond, here is what you set yourself up for.  Fish poop, among other things, makes its way to the direct suction drain (typically 2" or 2 1/2" PVC line) and is rapidly sent to a strainer basket in front of a pond pump. 

Fish poop is blended up "Margarita style", without the ice, yummy flavor or intoxicating effect, and is sent to a pond filter in a very messy way.  For you pond keepers below the age of 21 we can refer to it as a Fish Poop Smoothie.  The strainer basket in front of the pond pump likely needs to be opened and cleaned once a week but I have seen extreme cases where the homeowner was cleaning their strainer basket several times a week!  Because of a well designed Gravity Flow Pre-Filtration System, I've only cleaned the strainer basket on my pump once in the last 11 months and it didn't even really need to be cleaned!

Now that we all accept gravity flow technology as the superior construction method over direct suction technology, we have speed limits we need to recognize.  Most ponds will require 3" or 4" bottom drains so we will only cover these pipe sizes in this exercise.

    3" G.F.B.D. should optimally have a max flow rate of approximately 1800 gallons per hour and will service an 8' diameter nicely.
    4" G.F.B.D. should optimally have a max flow rate of approximately 3600 gallons per hour and will service a 10' diameter nicely.

Please do not ignore these speed limits like you do on the freeways!  If you flow water faster than these speed limits the velocity of the water will begin to break up solids such as fish poop defeating one of our main purposes and on the flip side if you flow too little water, you aren't removing solids from the pond in the most effective manner possible. The service diameter of the G.F.B.D. can be enhanced greatly with a bowled excavation with the drain installed at the lowest point of the pond, with current jets flowing water to the drain and even more so with the addition of air to the top of the drain's dome.  Current jets can be designed in the pond to create a slow vortex, spinning solids to the bottom and heavy aeration from the top of the drain actually pulls neutrally buoyant solids, like fish poop directly into the suction of the G.F.B,D.  I would say with these additions to a drain installation, IF YOU WANTED TO PUSH THE LIMITS, you could extend the service diameter of a 3" G.F.B.D. to 12' and of a 4" G.F.B.D. to 14'.

Use these Bottom Drain Fundamentals that we use to professionally design Koi ponds each day.  Keep in mind the length and width of your pond will determine the recommended size & quantity of bottom drains.  The number of drains (combined with total pond gallons and number of skimmers) will help us determine what size pond pump is necessary for the life support system of the pond.  I was never a wiz in History back in school but we sure can learn a lot about pond construction by keeping a watchful eye on techniques and philosophies used over the years.  The Bottom Drain philosophies of today are certainly on the rise and I predict will soon become main stream in the industry as a standard item in all new pond construction, water gardens, eco-system and dedicated Koi ponds alike.

If you are undecided on whether a bottom drain is right for your pond design, if you need more help wrapping your arms around the concept or you simply want to discuss options for your pond, please feel free to contact our office toll free (800)-522-5043 or visit our TalkPonds Message Board and post your questions. 

(Fig. 1) This is a typical construction detail on an eco-system pond or water garden with aquatic plant shelves and a thin layer of ornamental gravel across the bottom of the pond. The ornamental gravel serves as a place for benefical bacteria to colonize to help manage sludge and debris that typcially settles to the bottom of a pond. Bottom drains can be installed into rock and gravel ponds too, with just a little extra work.

(Fig. 2) Here is an example of a cross section detail of a deticated Koi pond. Typically, a deticated Koi pond will have a deeper excavation and the bottom of the pond sloped towards a low point in the pond where a bottom drain would be stategically placed to capture heavy solids. Instead of using gravel to help manage the pond's eco-system, koi ponds rely on technology with U.V. Filters, settlement tanks and large biological filters.