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1
- #1
fel3
Civil/Environmental
- Jul 9, 2001
- 915
Greetings...
I was brought in late to a project to perform QA/QC reviews. The centerpiece of the project is a new oil-water separator (OWS) at an airfield. As part of this work, the project engineer built a model of the storm drainage system upstream of the proposed OWS using Autodesk Storm and Sanitary Analysis (2014). The watershed covers 22.5 acres and is divided up into 22 subbasins ranging from 0.32 ac to 4.93 ac. It's not a very big model.
The original model was based on the 2-yr event because that is the smallest "standard" event that exceeds the regulatory requirement for this particular installation and is what the client wanted us to design to. I reviewed the results from the original model and they looked good. The model matched the survey information, the rainfall data matched county requirements, etc. Using a couple programs I wrote years ago for my HP-42S calculator, I verified the peak runoff flow rate for each subbasin and verified the hydraulics for each pipe segment (all pipes were flowing partly full). I am fully satisfied it is a good model.
Then the client decided to change the design requirements and asked us to model the 10-yr event and design a bypass around the OWS to handle the excess flow. So, the project engineer replaced the 2-yr rainfall parameters with 10-yr rainfall parameters and re-ran the model. Herein lies the problem: some (but not all) of the results from the 10-yr run are downright weird. I spot checked subbasin peak flow rates and they look good. The total peak flow rate looks good. Some of the pipes in the system are now surcharged, which is to expected since it's a pretty flat site. The problem lies in the reported HGLs at inlets and manholes that are connected to the surcharged pipes. At more than a dozen inlets and manholes, HGLs are reported that exceed the elevation of the inlet grate or manhole lid by more than one foot to an astounding 619.82 feet. I'm not kidding. Somehow the model thinks I can stack water that high over a catch basin.
So now to my question: has anyone else encountered anything like our weird results when SD pipes get surcharged and, is there a solution? We are supposed to include these results in an updated tech memo on the subject, but I told the project manager it was a no-go until we resolve the weird results.
I did an internet search for bugs and problems with Autodesk Storm and Sanitary Analysis (2014) and came up empty. Maybe there is something out there and I didn't find it, but who knows. I have not run this particular software myself, but the project engineer has used it quite a bit and he had never seen anything like these weird results. If anyone is interested in helping me figure out what went wrong with the 10-yr model, I have attached an annotated copy of the printout for the 10-yr run and it includes a diagram of the system. On page 2 I have highlighted the highest inlet grate (elev. 16.50'), the lowest manhole rim (elev. 7.36'), and noted that the highest ground elevation on the rim of the watershed is about 17.6'. On page 14, I have highlighted the weird results. On page 16 I have highlighted the surcharged pipes.
Any help y'all can provide will be greatly appreciated. Thanks in advanve.
============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
I was brought in late to a project to perform QA/QC reviews. The centerpiece of the project is a new oil-water separator (OWS) at an airfield. As part of this work, the project engineer built a model of the storm drainage system upstream of the proposed OWS using Autodesk Storm and Sanitary Analysis (2014). The watershed covers 22.5 acres and is divided up into 22 subbasins ranging from 0.32 ac to 4.93 ac. It's not a very big model.
The original model was based on the 2-yr event because that is the smallest "standard" event that exceeds the regulatory requirement for this particular installation and is what the client wanted us to design to. I reviewed the results from the original model and they looked good. The model matched the survey information, the rainfall data matched county requirements, etc. Using a couple programs I wrote years ago for my HP-42S calculator, I verified the peak runoff flow rate for each subbasin and verified the hydraulics for each pipe segment (all pipes were flowing partly full). I am fully satisfied it is a good model.
Then the client decided to change the design requirements and asked us to model the 10-yr event and design a bypass around the OWS to handle the excess flow. So, the project engineer replaced the 2-yr rainfall parameters with 10-yr rainfall parameters and re-ran the model. Herein lies the problem: some (but not all) of the results from the 10-yr run are downright weird. I spot checked subbasin peak flow rates and they look good. The total peak flow rate looks good. Some of the pipes in the system are now surcharged, which is to expected since it's a pretty flat site. The problem lies in the reported HGLs at inlets and manholes that are connected to the surcharged pipes. At more than a dozen inlets and manholes, HGLs are reported that exceed the elevation of the inlet grate or manhole lid by more than one foot to an astounding 619.82 feet. I'm not kidding. Somehow the model thinks I can stack water that high over a catch basin.
So now to my question: has anyone else encountered anything like our weird results when SD pipes get surcharged and, is there a solution? We are supposed to include these results in an updated tech memo on the subject, but I told the project manager it was a no-go until we resolve the weird results.
I did an internet search for bugs and problems with Autodesk Storm and Sanitary Analysis (2014) and came up empty. Maybe there is something out there and I didn't find it, but who knows. I have not run this particular software myself, but the project engineer has used it quite a bit and he had never seen anything like these weird results. If anyone is interested in helping me figure out what went wrong with the 10-yr model, I have attached an annotated copy of the printout for the 10-yr run and it includes a diagram of the system. On page 2 I have highlighted the highest inlet grate (elev. 16.50'), the lowest manhole rim (elev. 7.36'), and noted that the highest ground elevation on the rim of the watershed is about 17.6'. On page 14, I have highlighted the weird results. On page 16 I have highlighted the surcharged pipes.
Any help y'all can provide will be greatly appreciated. Thanks in advanve.
============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill