Yes, ponds overflow in heavy rain. I set mine unlevel purposefully to deal with this. It has a high side and a low side. The low side passes out to a gravel area with a mild slope away from the pond. The hostas love the extra water at that location. Here's a couple pics. In the first one you can see the high side on the left and the low side on the right. On the second pic, you're looking over the pond across to the low side and the gravel overflow area.