[dragonwriter]

> As a non-American, can someone explain to me why the army was responsible for this work?

Because...path dependency. The Army Corps of Engineers has long been responsible, largely, as I understand it, from mission following capacity rather than vice versa.

> When does the normal tender/contractor process not occur?

It can, but that process requires a government agency over the top, and the ACE is the responsible agency. They contract out work.