and you think that in the absence of any of those restraints, the cows or floods, or whatever will move in the exactly the same way as if they were in place. And also that it is just as easy to escape an enclosure by tunneling as is getting out if there were no barrier at all.
How interesting.
Of course not, you're really stretching to miss my point there.
A cow may or may not have any desire to leave the field. If its needs are being met in the field the chances are it's not going to wander very far, and hence a simple fence is probably all that is required to keep it in place. Imagine the cow having a specific desire to leave the field and a clear benefit in leaving the field. Now you need something stronger than a simple fence with a wooden post every 10 feet or so.
Now imagine that the cow has time and resources to dig under the fence or fly over the top of it. Or imagine that the cow could simply approach the gate and say it wanted to visit its friend in the next field, knowing the farmer would allow it out with few if any checks on exactly where it went once it was released.
As with most other things, where there's a will there's a way. Perhaps it would make more sense to figure why these people are seeking to come to the US and address that. If people want to come as law-abiding guest workers so they can pick tobacco or something, perhaps the answer is to let them do it, or to enforce the law rigidly and impose heavy fines on companies found to be hiring illegal labor. If they want to come to take advantage of a welfare system, cut the welfare and let it be well known in their host nations that there is no welfare for them.
The most problematic are the organized criminals and the wall will make little to no difference to their ability to come and go. The trouble is they are the ones with very deep pockets so even if you did have a wall that was adequately staffed, maintained and monitored, the big-time criminals can easily make it worth a border patrol cop's while to be distracted just as they cross the border. And hence the problem. The coyotes will continue to fly small aircraft over the wall. People running drugs and guns and stuff will continue to either fly over or tunnel under the wall - maybe they'll need to dig a new tunnel if the new wall goes up but it's not as if they don't have the budget or the motivation to do it. The wall may help keep out the casual border crosser but it makes more sense to make crossing the border less attractive than to build a physical monstrosity and hope it doesn't fall into disrepair.