Jim Beachy

Graham, oops, I see that the link does not display as it get displayed when viewed directly on YouTube. So for the 12 minute complete video, including the overflow details, go to https://youtu.be/6hUhESbJU7Y .

Graham, yes there is a 5/8” drilled hole at the bottom of the PVC elbow, high in the tank. When the siphoning action kicks in, water gushes out of the overflow, sucking debris from the bottom.  This lasts for about 50 gallons before the siphon is broken, and is how I keep the bottom of the tank as clean as possible.  At the end of this 90 second video is a link to another video which shows the install steps I took.  The overflow plumbing is detailed there.  

We installed a 2000 gallon rainwater harvesting system in the Shenandoah Valley of Virginia.  As part of simplification, we eliminated any first flush, but I took three steps to keep my tank as clean as possible.  This 90 second video explains what we did.