Complex sentence: This portion of the project was undertaken to control potential flooding in an area along Napa Creek between Jefferson Street and the Napa River in downtown Napa an area which is more prone to flooding than the river itself.
Split into simpler sentences: This portion of the project was undertaken to control potential flooding in an area along Napa Creek between Jefferson Street and the Napa River in downtown Napa. This area is even more prone to flooding than the river itself.