I am going to guess you are barking up the wrong tree on this idea. I grew up irrigating out of reservoirs that we often ran the level up and down in. So I do have
experience with that. But other experiences are probably more important here.
Clear back in jr high we had a corner of a field that we couldn't get dried up. Even if we plowed and disked and leveled it by mid way through the summer it was a cattail bog again. So my father had us
fence it with electric
fence and we put about 100 head of feeder hogs on it.(they had ground grain in a feeder here too so they were not just eating cattails) The area fenced was probably about 3 to 5 acres in size. Inside of about 2 days there was more mud than cattails showing and inside a week there were only a few stalks of cat tail left in what had been cattails 6 feet high. Inside a month the ground was dry and mostly staying dry except when irrigated. The hogs were held there a couple of years. It took nearly 25 years for that patch to reestablish itself afterward. For the first decade or so the patch didn't even get boggy. The hogs had completely changed the character of the dirt in the time they were there.
The second goes back to a reservoir from my post college years. The cat tails were taking it over. I tried pulling them and mostly they regrew before the next year was out. This was cat tails some of which were growing in water 3 and 4 feet deep. Then by accident being lazy I learned something. I could keep the deep water cat tails from reestablishing quickly if I chopped them off right next to the bottom instead of pulling them. If I pulled them they could be back by the next year but if I cut them off it took 3 or 4 years to regrow. Best guess is that the deep water roots were not getting enough light to regrow naturally and they were using so much
energy the plants too longer to reestablish because the only energy source was the shallow water cat tails. It also meant the water stayed muddier so the light didn't get as deep. I think also the existing roots kept the healthy plants from suckering out as fast too.
The third lesson is trying to transplant cattails. We had built a new reservoir and wanted a filter strip going in to it to keep the mud from the canal from settling out in it. We seeded the area with probably 50 or 100 cattail heads each fall and tried transplanting plants besides. It took about 4 or 5 years to get the cattails really established and healthy. A transplanted cattail just barely grows the plant the first year and by the second puts up one or 2 plants from suckers. It isn't till the 3rd year it really begins to spread. Germination from seed was really poor. It seems only to happen right near the water line and it takes a lot of seed to get a few plants. This slow regrowth without established roots is why I suspect hogs won't work the way you want them to here.
Given those things in combination is why I am guessing this yours is the wrong
answer. If I were going to harvest cattails for pig food I would get one of the gas powered sickle bar aquatic weed removal outfits and then I would go harvest narrow strips of cattail so the roots had lots of growing growth to support regrowth. I would float these strips as bundles over to the side of the reservior where the pigs could get to it to eat and just do that daily or ever few days.
If you try let us know as I would love to find out I am wrong. Now there are other reason to have a drainable pond. Fish and crayfish harvesting would be on that list. Simply drain the water away and catch the stuff as the water comes out the pipe. Irrigation supply is another reason.