Patching gap in drainage channel

Question

Our property includes a dam with a storm overflow. I've discovered that the drainage channel, 50 years old, has started to slide apart, leaving a 2-3 inch gap in two places. When water drains, it falls down the gap and runs underneath the channel, presumably carving out an underground cavern.
What's the best way to seal these cracks? It obviously doesn't have to 100% water-tight, so I'm thinking of dumping as much gravel as the cavern will take, topped with concrete? Is there a Sikaflex type product that will bind and allow for a little movement over time? Everything I've found is for smaller cracks.
Thanks!

Ecnerwal · Accepted Answer

Plain old mortar (or concrete with only small aggregate) would be my choice - it does not need to be a perfect seal. If it's dry when you are trying to do the work, pre-wet the area you are patching for a better bond. You'll also want to scrub off as much of that decaying organic material as possible. Use a fairly stiff mix and beat it into place.
It needs to be wetted out, not dry, but you want a mix you can place, not one you pour. Slapping or beating it once placed (equivalent to vibration but on a reasonable DIY budget) will help get the mix in good contact with the old concrete without having to have a very liquid mix. Apart from being hard to place, a too wet mix is weaker.

Patching gap in drainage channel

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