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Trouble Modeling Subsurface Facility

Trouble Modeling Subsurface Facility

Trouble Modeling Subsurface Facility

I am trying to model a subsurface facility that works in conjunction with a detention pond. The goal was to utilize as small of a bed as possible to receive runoff from about 2 acres, infiltrate the 2-yr, and then let the rest of the water flow out to Basin 1, which provides rate control. The bed is provided with an outlet manhole, with a weir plate set at an elevation such that the 2-yr storm is kept within the bed. Everything else will be above the elevation of the plate and ideally SHOULD flow freely out of the bed. In order to model this, the bed has a starting elevation (under the Advanced tab)set at the elevation of the weir plate. This initially didn't work and I received errors about the max WSEL exceeding the top of the bed. To fix this a prismatoid was added as storage to model the conditions in the outlet manhole (storage above the weir plate).

This seemed to fix it though now I am having two issues: 1.) peak outflow slightly exceeds peak inflow for the bed for 25, 50, and 100-yr storms and 2.) if I re-run the critical duration analysis at the POI for the storms smaller than 100-yr, the bed will again not work due to the varying duration for these other storms (i.e. 100-yr duration is 60 min, 25-yr is 42. model will show bed top being exceeded if I use a duration of 42 for 25-yr, but works fine with 60 min). The model I have set up is attached, the bed in question is BMP 4. Ideally I'd like to figure out how to make the bed work at the size it is at since the size is optimized for the 2-yr storm, while providing enough of an orifice in the OS to discharge the 100-yr storm inflow rate. If it is not possible to make it work as it is currently sized, I would like to have a good explanation to satisfy my boss. Hopefully someone here can help because I am stumped. Thanks!

RE: Trouble Modeling Subsurface Facility

I'm not sure why you've set the starting elevation at 441.50. This will prefill the pond to that elevation, thus making that volume unavailable for retention of the smaller events. Unless this is a prefilled "wet" pond, I think you want to leave the starting elevation blank.

The oscillations are occurring due to the abrupt change in the stage-storage curve as the WSE starts to rise above the main storage at 442.15' Make sure you are defining the full, actual storage as the WSE rises. Otherwise you may need to use a smaller time step (not finer routing) in order to get a stable result.

PS: Since you didn't post your custom IDF file, I was only able to examine the preset rainfall of 60 minute duration, intensity=2.75 in/hr, depth=2.75"

Peter Smart
HydroCAD Software

RE: Trouble Modeling Subsurface Facility

Thanks for the reply. The starting elevation was recommended by a coworker to model the fact that the storage was "unavailable", though as I think about it in light of what you're saying, it really doesn't make sense to do it that way for what I am trying to accomplish.

Per your second point, what is shown in there is technically the full, actual storage. Bed design is 2.65' with the pipes embedded as shown. Those were modeled with storage multipliers to account for the # of pipes (as opposed to factoring the number of pipes into the length #). Then the manhole "storage" is included via the prismatoid as previously mentioned. I haven't done this yet but I am considering revising the pipe storage portion to provide more storage with the same footprint.

When I modeled a "weir" with a height from my weir elevation (441.5) to the top of my bed (442.15) it is able to pass the 100-yr inflow so I guess I am confused why HydroCAD seems to need additional storage when it could just pass through.

I attached the IDF file.

RE: Trouble Modeling Subsurface Facility

Reducing the dt to 0.001 hours allows a stable routing with the actual storage. You don't need additional storage.

To reduce the dt you will need to customize the units:
Open the Settings | Units screen
Click Custom
Scroll down and select "Time Inc"
Increase the number of decimal places from 2 to 3
Click Ok a couple of times.

Now you will be able to reduce the dt down to 0.001

Peter Smart
HydroCAD Software

RE: Trouble Modeling Subsurface Facility

Thank you very much! Worked like a charm. Would you mind giving me a brief explanation for why this fixed the issue?

RE: Trouble Modeling Subsurface Facility

You need a smaller time step in order to handle the sharp inflection point in the stage-storage curve where the WSE starts to rise in the manhole above the main storage.

If you look at the stage-storage graph the inflection is clearly visible - it's the abrupt vertical rise in the curve at the far right.

Peter Smart
HydroCAD Software

RE: Trouble Modeling Subsurface Facility

Also, make sure you set the Finer Routing back to the default value of 1. In this case, reducing the actual time step is the preferred solution.

Peter Smart
HydroCAD Software

RE: Trouble Modeling Subsurface Facility

Thank you. I will be sure to set that back to the default value. I really appreciate the help as this issue had me and a couple of coworkers rather stumped as to what to do.

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