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You don't have the amount of attempts to cross the border in the hills, mountains, desert, etc. that you do in the city. So a wall is not necessary there and you might be able to get by with something that is just as effective at much less the cost.
I'm not talking about dams. There are tall walls like 15 feet high lining the river for miles on both sides. In other areas there are rock rip raps (hills of rocks) and in other areas the water just overflows the banks and spreads out on land that is not used for anything. So they looked at what made sense for each location and didn't treat a downtown area the same as a rural area.
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