The Central Valley in CA has some neat bypasses. When the Sacramento River reaches a certain height the water is diverted into the floodway (bypass). Essentially the dig a new and miles wide channel that rejoins the river down in the delta. The bypasses are protected by levees so the water stays in. The land is used for farming with lots of crops grown. Don't know who owns the land itself but I think a lot of it is privately owned. Can't have permanent structures in the bypass. Problem up here is that trees would likely want to grow in all that low-lying ground.

But, you could rather easily protect I-5 and and other current structures by setting the diversion wier at the right level. Probably would also mess up fish migration.