Friday, December 11, 2009

Flood & Drain

There are two systems I will describe, "Normal" flood and drain operation and a "Continuous Flow + Flood and Drain" system. What is meant by "F&D"(Flood and Drain) is that your grow bed will fill to a level that you decide, drain itself, and repeat continuously. This is in contrast to a "Continuous Flow" operation, where the water level remains constant. The advantage of F&D is that the plant roots are supplied with oxygen when the water is low and then supplied with nutrients when it is up. With a continuous flow operation the roots have problems either getting oxygen or getting to the nutrients. F&D is the preferred method for almost all aquaponic systems. But, as I will show you, you can combine the two for better filtering with a large pond.


The continuous flow can be integrated with F&D by adding a drain hole that allows water to freely drain, but at a rate that is lower than the input. This allows the grow bed to slowly fill. The auto-siphon will kick in once the desired water level is achieved. Once the water level gets low enough, the auto-siphon will stop and the bed will fill up again. The advantage here is that the water will never stop flowing to your pond. This is great for water features such as a waterfall and for giving your pond a strong current for better aeration. I initially designed this to keep a constant supply of filtered water to my waterfall, but water flow it created was a great side benefit. I highly recommend this method regardless of you having a waterfall or not. But if you want simplicity, go with normal F&D. Getting the flow correct won't be as tricky.

The normal flood and drain will not supply water to the pond on a continuous basis. While the bed is filling and before the auto-siphon kicks in, there is no water flowing to the pond. This might not be a problem if you do not have a water feature and/or more than one grow bed. While this may be OK for a smaller system, I still recommend the "Continuous Flow + F&D" simply for the aeration that the water flow will help provide.


When you drill your holes for the drains you will be using a 1 3/8" drill bit for EPDM liners and 1 1/4" bit for construction plastic. For continuous flow you will want a three hole setup. For normal operation you will drill two holes, one for the drain(auto-siphon) and one for overflow.

Type of Liner
Drill Bit Size
1 3/8”
1 1/4”

Type of Drain
Number of Holes
Normal F&D

The location of your holes is up to you. They should be wherever it is convenient and close to your return pipe. The auto-siphon hole will be at the very bottom of a side wall. The overflow hole is near the top, below the edge of your liner. The continuous flow hole will go on the bottom of the grow bed, up through the plywood.

Example of hole locations:

IMPORTANT NOTE - Make sure you drill the holes before you put the liner in.


  1. I'm planning my first Pond Water Feature and found your write up exceptionally insightful and useful. Although I can see how you have combined the two; I feel I would still be best served by the Flood and Drain System.

  2. Flood waters may contain pesticides, herbicides and lead/heavy metals—all of which you may be forced to wade through. In addition, you run the risk of cutting yourself on sharp debris or getting your feet caught in unstable pockets or holes.