Site Runoff to City Sewer
Site Runoff to City Sewer
(OP)
I was recently asked to solve a flooding problem for a commercial area approximately 300’x200’. The bldg has a flat roof. After investigations, I found out that the storm runoff combines with an 8” dia. waste pipe before draining into the City sewer line. It seems like during heavy rainfall, the sewer might be backing up thus not allowing the water in the drain pipes to rise and flood the basement/lower level. There is no way for me to find out if the city sewer backs up or not. I am thinking of suggesting a check valve and some leaching basins as a overflow system. Based on the general method used in this area, the site drainage is designed for 2” of rain. Doing the math I end up with a large volume of for the leaching basins which I don’t believe the client will go for. Any thoughts as to how to make a determination what the sewer line capacity is. Are there any other options? Thanks.
RE: Site Runoff to City Sewer
Designing for 2" of rain will help with frequent rainfall events (multiple storms a year), but won't help with infrequent events. When designing for volume, you need to design for a sustained rainfall event (24 hours is typical) and if your detention system takes longer than 24 hours to drain, you might fall very short on a 2" design storm. Your client will not be happy to spend all this money and still get flooded out every year.
I see you're listed as a structural engineer. If possible, find a drainage engineer to look at the plans with you, including contours and near-by storm sewers. It shouldn't take long to see if it's viable to separate the storm water runoff from the sanitary sewer.
RE: Site Runoff to City Sewer
But be careful in doing this. If your storm piping is cast iron and using standard no hub couplings, these only hold about 10' of head. If the water in the storm backs up past this - which it could if the backwater valve is installed, then your storm downspouts might leak - which is probably worse than getting back up on the floor.
RE: Site Runoff to City Sewer
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Site Runoff to City Sewer
RE: Site Runoff to City Sewer
RE: Site Runoff to City Sewer
RE: Site Runoff to City Sewer
RE: Site Runoff to City Sewer
RE: Site Runoff to City Sewer