In areas prone to flooding, storm surges can occasionally reverse drainage systems, sending dirty water spewing up through storm drains. Additionally, stagnant water beneath a storm drain can become a breeding ground for mosquitoes.
The FloaX concept, designed in Taiwan by Yu-Chun Chang & Joseph Kah-Rong Wong, is intended to protect against both problems. The solution consists of a buoyant basin captured in a channel below the drain cover. This basin--the designers, for whom English is presumably a second language, refer to it as a "valve plank"--has a drain hole and contains a buoyant ball larger than the hole.
With this arrangement, water running into the storm drain will displace the ball but not the floating basin:
In the event of flooding from below, the basin itself will rise, plugging itself against the drain cover: