Why was an underwater bridge built?

There's a bridge in the middle of Seoul that gets submerged every time it rains. Instead of building it high to escape floods, they chose to build it low, letting it go underwater. An underwater bridge has no target for the floodwaters to hit. However, the problem is that you can't cross it when it's submerged, so they built another bridge on top of it. That's how Banpo Bridge, Korea's first two-tiered bridge, was born. Even now, if the Han River's water level reaches 6.2 meters, the Jam수gyo (underwater bridge) quietly hides beneath the water.