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Modeling manhole structure with flows exiting top of structure

Modeling manhole structure with flows exiting top of structure

Modeling manhole structure with flows exiting top of structure

I am analyzing a proposed 84" culvert under an embankment that will feed into a proposed manhole structure DMH-1 that will connect to an existing 4'x4' stone box culvert. The existing box culvert sits on grade in a ravine that has become a dumping grounds for local residents and is not in good shape.

I am working with an existing HydroCAD model that has provided me with anticipated flows through the culvert. The existing HCAD model was created several years ago and anticipated free discharge out of the culvert. Since then, decisions were made that the culvert would feed into the existing box culvert as it has historical value and cannot be removed at this time. The existing box culvert will max out at around 100 cfs so suffice to say that it will not provide nearly enough capacity to carry the flows from the 84" culvert. The proposed manhole and existing culvert are on the lower side of the embankment, which is about 25' below existing grade on the upper side of the embankment. We anticipate some storage in the pipes and structures upstream of DMH-1, but not nearly enough to keep the system from flooding upstream.

We are proposing a grated opening in the top of DMH-1. As the storm progresses, DMH-1 will fill up very quickly as Qin exceeds Qout. Water will stage up to the top of the structure (EL=30.5) before spilling over into the ravine, where it will run alongside the stone box culvert along the length of the ravine. The ravine is about 450' long with a slope of 1.5%, with the low point on the downstream end at around EL=11. As the ravine fills up it will submerge the stone box culvert and weep into it. I have created a pond node with a culvert at the bottom for DMH-1 but don't know how to model the top opening. I've tried it as a weir, but the volume coming into the structure stages up well past the top of the structure. It isn't actually modeling the water flowing up and out of the structure. If anyone has modeled something like this previously or has any advice I would really appreciate it.

Thank you,

RE: Modeling manhole structure with flows exiting top of structure

You can use a weir or a horizontal orifice to model the overflow. You WILL get a peak WSE above the overflow, indicating the head that is operating on the weir. This is normal. I'm not sure why you say "It isn't actually modeling the water flowing up and out of the structure." Make sure you use a separate (i.e. secondary) routing for the overflow to allow separate reporting for that device.

Peter Smart
HydroCAD Software

RE: Modeling manhole structure with flows exiting top of structure

Hi Peter, thanks for responding. The issue I'm seeing is that HydroCAD is adding a large amount of storage above the rim of my structure.

I have my primary outflow exiting through the stone box culvert and my secondary outflow exiting through the opening (modeled as a weir) at the top of the structure. Primary outflow is routed downstream through the stone box culvert and secondary flow discharges into a pond representing the ravine. Our goal is to limit the amount of water staging up in the ravine as it is not in good shape, and enough flow and volume could cause erosion which would compromise surrounding structures. The upstream network has capacity to store some of the volume rather than dumping it all into the ravine.

As DMH-1 fills up (when Qin exceeds Qout), I would expect the structure to become pressurized and force the upstream pipes to fill up. Instead, the 84" culvert leading into DMH-1 doesn't actually fill up, it just keeps discharging and DMH-1 keeps staging up to store the volume. Is there a way to model this backup/tailwater effect back upstream instead of the DMH-1 structure staging up past its rim? I'm not sure how well I'm explaining this, I would be happy to send you the HydroCAD file in question if you think it might help.

Thanks again.

RE: Modeling manhole structure with flows exiting top of structure

HydroCAD will never add storage beyond the data you provide. But if the defined storage becomes pressurized the WSE will rise above the top of the defined storage, representing the pressure (HGL) at that point in the system. This is normal and correct.

I suspect you need to switch to a tailwater-sensitive routing procedure (DSI) in order to cause the WSE to rise in the structures upstream. Please check your warning messages. However, you will tend to get a more stable routing if you include the 84" pipe as part of the storage definition for DMH-1, rather than depending entirely on the tailwater procedures.

Peter Smart
HydroCAD Software

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