The Alternative Plan does not require additional lands off-site to handle an excess of water volume that the levee/by-pass channel design cannot handle.

You see, the Trinity Uptown Plan shortens the length of the river by a little more than a mile, and therefore it loses that much storage for flood waters in its valley shape, so they need the mitigation areas shown (a few hundred acres) to hold water during a  flood - for its volume or its velocity.