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So I have a townhouse in New York City and I've been getting sewage backup from storm water. I added a sewage backflow prevention valve at the front of the house at the sewer, but the shared storm water sewage system in my house still overflows in the basement because of the roof and yard water that goes into my backyard storm drain.

If I wanted to mitigate the flooding by stormwater from the roof, can I just simply run the storm downspout away from my storm drain and 20 feet away from the house foundation over the concrete pavers and into my small garden patch in the back yard? I'm not sure what effect dumping the water into the yard would be on the house and foundation - even if it's 20 feet away. Is there enough drainage at that distance away to not affect my foundation? Most all of the backyards in the neighborhood are all concrete including mine except for small patches of soil, and the backyards face inwards into each other each block, with the rowhouses pointing outwards.

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and the downspout from the roof leading into my storm drain in the backyard. This and another drain exists in the yard. enter image description here

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  • If your backyard it pitched away from your house it shouldn't be an issue, if it's pitched towards your house it probably will cause you problems. If it causes your neighbor(s) problems you could be held liable. Sep 23 at 12:08
  • This is a solution. How porous/permeable is your soil? When you have enough water to cause a backup in the sewer lines, do you get flooding in your backyard? If so, then this likely won't help any. If your backyard is damp but not soggy, then it's likely to work. A couple of sections of drain pipe to run the water there shouldn't be overly expensive to use as a test. TBH, it sounds like it may be time to talk to the neighbors to see if they're having similar problems and if so, there may be a larger issue in the sewer line on your block, or perhaps in your feed to the main line.
    – FreeMan
    Sep 23 at 12:09
  • It might be time to call in a pro or the city water dept to have the line from your sewer exit to the main line checked for blockage, collapse, or other issues.
    – FreeMan
    Sep 23 at 12:09
  • The water in the backyard doesn't usually pool - we are on the higher ground so the ground water is usually ok. This is a known problem with New York City sewers and there is no immediate solution. Sewage backup will be the intermittent new normal with the increased flash flood rains - this only happens with high volumes of water in short periods of time... the sewer systems can't handle surge levels... When my system disconnects from the main sewer line during a backup, I then only have to deal with my own storm sewer overflow. Sep 23 at 20:26

2 Answers 2

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No assuming your soil isn't sandy loam and quickly draining and that your water table isn't super low.

Your city connection has a Y before the sewer and it takes both your sewer and your raindrains? Do you have different systems for front and back yard?

Generally the modern practice is for cities to separate the storm and the sewer. You then have sewer into sewer and storm/perimeter drain into storm.

What is your setup? Do you have a powered sump pump? Are your rain drains separate from your foundation perimeter drain? What year was your town house built?

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    Most of this answer is just questions for clarification and should be asked in comments on the OP, not in the answer box.
    – FreeMan
    Sep 23 at 12:06
  • New York City has a combined storm and sewer and so does the house. The storm line from the back yard combines with the house sanitary at the front of the house right before it goes into the single combined sewer line of the city. I'll add blueprints to the question. Sep 23 at 20:17
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It's cheap and easy to try. Set up the pipe, wait for a storm, go out back and observe. If it's "working" you'll see your storm water rolling into the street or your neighbor's yard.

Doubling your neighbor's flooding may not be your intent, so that's another reason to try it and observe. If the water starts pooling or vanishing into the soil you need to figure out the impact.

Note: your basement is acting as a dry well. If everyone diverts their roof water into their yards without building actual dry wells, the street flooding will be worse and might end up coming back in through your basement windows anyway.

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