I think you are very wise to think about this carefully. I have spent many of my horse involved years keeping horses in various forms of flood plains, and it is not for the faint at heart.
So much of it depends on the type of flooding that you get.
When I boarded on a river with all the pastures in flood plains (it came over it’s banks between zero and three times a year, often the fencing would remain, but often the fencing would be torn off the posts. Jumps (including cross country would be gone). They used the cheapest lightest white electrical tape on plastic insulators and wood posts. The wood posts were pounded into the ground which helped but sometimes they got bent/removed in flooding based on what hit them. Usually the water level went over the fence posts by a lot. I think now they use high tensile wire (also on wood posts) for this same application. I don’t know how it has worked out, but the fencing always looks good when I see it. I do know I would prefer not to have my horses in high tensile wire though.
Second scenario, a smaller creek that overflowed it’s banks by up to 14 feet (5 to 7 more common) several times a year. Water would rise and fall within a 24 hour period taking everything that was not nailed down-mounting blocks, water troughs (more than 2 fences and a foot ball field away from the creek) away forever. This place was sort of at a bend of a creek and while the current could be very bad, I think this helped the fence to stay up better since it was on the side where it was more overflow than current. All the fencing was 4 board. It bent and it broke and boards needed to be replaced after logs went through it, but surprisingly this was rare. It absolutely would not have worked 1/2 a mile down the creek though. Gates often could not be opened as the posts would shift. Sometimes the footings around posts had to be shored up as the water would pull away at the foundations.
The current scenario that I know of (but do not have to experience thank goodness) is the kind of small creek that most people wouldn’t even think of as a creek and would never imagine flooding. The property has been through 5 different people in the last few years as I think it keeps getting sold without anyone realizing the severity of the issue as it seems so unbelievable. It can go from water you can splash through with short boots on to 20 feet up with one good rainfall. This property had the 4 x 4 squares which would get pushed down a bit and eventually had to be lined with an interior electrical fence. It had 4 board wood on the parts furthest from the creek and that went the same direction as the creek. This fencing also “lost it’s balance and was pushed one direction, but it still worked.” Anyway, a new owner thought 3 board would be great and spent a ton of money replacing all of the fence. 3 months later, one rain and most of it is gone, has been pushed flat, or has been steam rolled by debris. This property is dangerous and there is no where completely safe on the 7 acres although if horses have access they can get as close to the road as possible in one corner which would prevent drowning. The old owner used to trailer the horses away whenever the forecast looked bad. The house can be unreachable when it rains. We are in a fairly large city and no one would believe this unless you experienced it.
Because land is in such short supply here, much of the open space turns out to be flood plains. This is a big deal both for the quality of the pasture but also for the safety of the horses and other animals and for the cost of maintenance.
I am finally at a property with no flood pain and I breathe a sigh of relief every time that it rains.
In your situation, I might think about driven in wood posts with a coated high tensile wire but I say that without any knowledge of what the personality of your flooding is. Or, the cheapest electric that you can either take down or not loose your shirt over if it washes away. I would almost recommend living there for a few years and putting some stuff out that you are okay with floating away so that you can better understand what is likely to work.
Good luck!