According to the locals, the worst flood was spring 1993. It followed a massive blizzard that dropped up to 5' of snow. It would have put another foot of water over that pic of the pasture from the road.
OK, maybe I am wrong, but I do not see the rip rap moving around from flooding. The stairwell will be low profile and tucked inside and anchored to said rip rap. They will also be behind(from the direct current) and in voids created by the pigs and the weir. This is about a thousand dollar investment and some time, so I think I am going to throw the dice.
Maybe I screwd up with the rip rap, and should have done what NOLA did in the 30's on the shore of Lake Pontchartrain:
Lately, I am not impressed with ANY pressure treated lumber. I swear, I think they spray it with iced tea and ship it out. The West deck of the house is 11 years old, & 10' off the ground....it is now rotting and will require a complete re-do(with composite). My walk bridge over the creek that is 12 years old...rotten boards. I have 6x6 PT posts in ground for less than 10 years that are rotted off at the ground....I am done with PT wood for deck/stairs/bridges.
Another drawback is PT decks and steps are slick as Owl snot. I did consider a swing up staircase, but really want a 'once and forget it' job. We can get weather moving so fast and unexpected, there is a good chance no one would be able to raise them in time.