Why does a vortex form where there is no manhole cover when it rains?

Because of the earth's rotation, all objects moving on the earth will be affected by geostrophic deflection, with the southern hemisphere leaning to the left and the northern hemisphere leaning to the right.

When it rains, where there is no manhole cover, water will flow into the well from all directions. Due to the existence of geostrophic deflection force, deflection will occur in the flow process, and these deflections in different directions jointly form a vortex.