‘Prevention’ hole cover / drain cover

Due to a large amount of water during the rainy season, the manhole cover on the ground may be huge pressure washed away.While on the water surface for pedestrians
not see the potential danger,so each year due to straying
hole cover countless pedestrians injured.
The design of the hole cover structure transformation,
when the hole cover by the upward force becomes free to open and close, which does not make displacement in order to protection pedestrian safety.