erosion around storm drain in backyard

I have a large storm drain in the back corner of my lot and the grass and soil around it is eroding. The drain sits flush with the ground, but the erosion has left a large hole and a dangerous situation for children, as it is a concrete storm drain.
My question is, who is responsible for the storm drain? Me, the property owner, the HOA, or the water and sewer company? I need to address this erosion and sinkhole, or have whoever is responsible to fix it.
Thanks...

 

http://www.loudoun.gov/news/erosion.htm

maybe you can ask them! =)