Grundulis,
We installed an agri-drain system on one of our ponds. They are plastic, but much like the concrete one you constructed. We placed the unit on a concrete pad and anchored it down with rebar (otherwise it would float).
We are in South Dakota, and so we will get up to 1 meter of ice in the winter. This has wreaked havoc with the unit over time, pulling the unit away from the tube that is attached at the base. We've also had the ice distort the shape of the unit. Thus I am going to take it out and replace with a vertical tube, and then fill around the tube with clay, essentially making an addition to the front side of the dam.
With your system I would be cautious about having ice moving in the spring. If your pond is very big, you might have ice slabs moving with the wind during spring melt. This could easily bend over or even destroy the structure. If you are in this situation, I would be tempted to fill around the lower part of your structure with clay or rocks. You'd give up some ability to drop water levels to the very bottom, but if it's either that or having the entire structure destroyed, I'd pick the former.
Bill
