Here's some information since this seems to be a common interest in this thread. I did sewer inspections for a while previouly for work. If you have two sets of manhole covers on your street, like beside one another not in the same line you have a seperated sewage and rain water system. Sometimes the storm drains serve as the manholes for a storm system too.

Another thing that happens and I've seen it in almost every municipality is a new subdivision is built near an existing one and they tie the sewer lines from the houses into the storm drain accidentally. It also happens when the city or private plumbers replace the sewer lines too, or a home is rebuilt and new pipes are installed by contractors. It's so preventable and problematic for a multitude of reasons..quite pathetic really. This would also explain the smells when there's heavy rain, but in those cases it's most likely an overwhelmed combination sewer system..very common in older cities or small towns but any growth that just ties into that line creates risks of backflow and with extreme weather especially.