Dancing manhole-cover basics

A manhole is a shaft that functions as an access point to the underground infrastructure and is covered with a very heavy lid, sometimes weighing more than 100 kg. Occasionally a strange phenomenon occurs in which such a manhole cover is lifted above its opening and sort of dances on or above its supporting ring without any human intervention. This usually happens when it is stormy with heavy rainfall, but it is not tied to one specific location. Videos from all over the world can be found on the internet showing such dancing manhole covers . Sometimes air seems to be the main driving force behind the behavior, sometimes water, and sometimes both. Although the videos are funny, the behavior can create a very dangerous situation for both traffic and pedestrians. In this report the cause of these dancing manhole covers is studied. The dancing is simplified into two different problems: one with an overflow of air and one with an overflow of water. For both problems a simple model consisting of differential equations is proposed and the numerical results are studied. The problem with an overflow of air is driven by an influx of air into the manhole from below, resulting in an increase in pressure, which lifts up the cover, until air is allowed to escape, and the pressure decreases again. Two different approaches for the escaping discharge of air are tried. The overflow of water is driven by a constant pressure that is exerted on a water column inside the manhole. Furthermore, a solution to the dancing problem is proposed: Attaching the manhole cover to the ground with a hinge. This solution is tested by using a similar model as the one used for the overflow of air.