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.