Many times, the concrete ring that supports the storm drain from underneath the drain will crack and break up into pieces. Once this occurs, excessive water will wash away supporting gravel underneath the asphalt. When this happens, the area of asphalt around the storm drain will sink along with the drain. Typically, for something like this, the concrete ring will need to be replaced, along with the washed away gravel and sunken asphalt.
The first step in repairing a drain is to cut a square line in the pavement around the perimeter of the drain. Normally this would extend 2 feet beyond the edge of the drain in all directions. The material around the drain would be excavated and removed from the site. The casting would be removed so as to assess the damage to the structure.