How do sewer pipes work? My sink/tub are plugged and I want to take the pipe bad...

My sink/tub are plugged and I want to take the pipe bad under the house to unclog it. I live on a floodplain (yea, stupid thought, but thats life) and the city sewer line is pretty much even. If I take past its sell-by date my pipe, will the whole cities rubbish end up contained by my basement, or is in attendance a backflow valve that keep it out?
Answers:    You shouldn't have to hold the pipe off. There should be a verbs out you can snake your main string.
There is no ''backflow valve''. The height of your system above the city's sewer system is what prevents backflow.