With the Arc Hydro output, they have a defensible, scientific model. They can show the developer that while the slope goes north, the hydraulic capacity forces a southward divergence during peak events.
Without Arc Hydro, the team had a static picture of the earth. With Arc Hydro, they had a dynamic simulation of a storm. It turned a subjective argument ("I've seen it flood") into an objective engineering fact ("The flow accumulation model confirms a breach at 50 cubic feet per second").
"But watch this," Sarah says. She adjusts the "Flow Accumulation" threshold. She simulates a heavy rainfall event—a "100-year storm."