How does stormwater flooding cause my sanitary sewer to back up? Are there not two separate drainage systems?

Because sanitary sewers are not watertight, stormwater can get into the sanitary sewer in a number of ways:

  • at manholes, through manhole covers, and through cracks and voids around the manhole
  • from the surrounding ground, through cracks or separations in the sewers and from private service connections
  • from weeping tile (foundation drainage) systems at private homes and buildings, since poor lot grading contributes to weeping tile flows
  • from downspouts that have been incorrectly connected to the sanitary sewer system rather than the storm sewer system.

Also, during larger storm events, much more water pools on the surface than normal. This can cause an increased amount of water to enter the sanitary sewer.

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1. I have installed a backwater valve, have a sump pump, and good lot grading, but I still get flooded. Why did this happen?
2. The downspout from my eavestrough is connected to the sewer pipe. Should it be disconnected?
3. What is the importance of good lot grading?
4. How does stormwater flooding cause my sanitary sewer to back up? Are there not two separate drainage systems?
5. How can I stop flooding on my property (yard or basement) caused by discharge from my neighbour's downspout or sump pump?
6. I live in an older neighbourhood. My neighbour has built a new house with a higher surface elevation than surrounding properties. Are there set standards for drainage elevation, and what should I do?
7. My neighbour's re-graded lot is higher than mine. What should I do to prevent flooding on my property?
8. My neighbour's new sidewalk is at a higher elevation and water runs into my yard. What should I do?