If the sewer is not clogged, a whirlpool can form. If the sewer is clogged, there will be no whirlpool and it will not be easy to find. This can prevent the manhole cover from loosening after rain and water and cause life-threatening hazards. The most important thing is to avoid stagnant water. Because where water accumulates, not only the manhole cover may shift, but there may also be leakage. Once water accumulates after heavy rain, try to avoid the water and find other ways to pass through. However, if you choose a manhole cover with a lock, it will not cause the manhole cover to shift even if it rains. You can choose a ductile iron five-proof manhole cover, or a stainless steel manhole cover, or a cement manhole cover, which can be locked during installation, making it safer and more stable.