Pipe to Open Channel Modeling

[tsoukup]

I'm dealing with a complex urban drainage system and thus using StormNet to model the situation.

My questions are: How do you model the junction nodes between circular pipe links and open-channel links? Junctions assume a storage but in reality there is none. Also, all of my junctions are flooded --- do I set the max depth equal to the pipe diameter? Or to the depth of the open channel? Should a surcharge depth be used?

Please see below for a more thorough description of the situation. Thank you.

I have a detention pond, discharging into an inverted siphon, which then discharges into an open channel. This open channel flows until it encounters 4, 4 ft culverts under a road. The open channel resumes on the other side of the roadway and continues to carry the flow to a lift station. The stormwater is then pumped to a final stormwater pond.

My understanding of StormNet is that I need a junction node between different conduit links (inverted siphon, culverts and open-channels).

From upstream node to downstream I have: detention pond, orifice, junction, inverted siphon (circular pipe), junction, open-channel, junction, culverts (circular pipe), junction, open-channel, pumps (yet to be analyzed)

[Chris Maeder]

Note that there is an example model that illustrates this situation. Please look in the StormNET Lessons folder labeled "Lesson Siphon".

Also, we have added additional material to the next StormNET User Manual regarding modeling of pressurized flow and flooding flow. And, the next StormNET shows open channel links that are flooding on the plan view. This new release should be available in a couple of weeks. Contact our technical support team if you want to try out a beta of this next release. (This also includes the new User Manual.)

The "Max/Rim Elevation" entry defines the elevation of the junction manhole rim (or height of the junction above the junction invert) in ft or m. If you are modeling a pipe bend without a manhole, this should be the height of the pipe. If you are modeling a channel, then this should be the height of the ditch.

The "Surcharge Elevation" entry is used to denote the elevation value (or depth above the junction invert) when pressurized (surcharged) flow changes to flooding overflow (ft or m).

To simulate bolted (sealed) manhole covers and force main connections, then this value should be the specified high enough above the MAX/RIM ELEVATION value so that the computed hydraulic gradeline is less than specified value. When the computed hydraulic gradeline is greater than this specified value, flooding is assumed to occur at the node.

Note that if the manhole is to be allowed to overflow and flood, then the node cannot become pressurized and this value should be set equal to the junction invert or 0. Then, when the computed hydraulic gradeline is above the MAX/RIM ELEVATION, flooding will occur. Similarly, to simulate a blowout of a manhole cover, this value can be set equal to the specified MAX/RIM ELEVATION value.

[tsoukup]

Thank you for your reply.

One further question and one further verification:

Scenario: one 100' long, 4' diameter pipe discharging into a 5' deep, 200' long open channel. Bottom of the channel elevation = 100 ft

Question: what would the following input parameters be to model this situation, taking into account there is no ponding, surcharging or storage at the junction?

Pipe Link:

Shape = Pipe
Barrels = 1
Diameter = 4
Length = 100
Inlet invert offset = 0
Outlet invert offset = ?

Junction

Invert elevation = 100
Max depth = ?
Initial depth = 0 (assume channel and pipe are empty)
Surcharge depth = ?
Ponded area = ?

Channel Link

Shape = Open Channel -- Rectangular
Height = 5
Width = 10
Length = 200'
Inlet invert elevation = 100
Outlet invert elevation = 95


Verification: if a max depth of 0 is used in the junction dialog box, then StormNet will default and use the depth of the connecting links.

For example: a 4' pipe connecting to a junction with a max depth of 0 entered, would (and shows in profile plot) use 4' as the depth in the behind the scenes calculations of HGL.

[Chris Maeder]

Note that you might find it easier to define your data in "Elevation Mode" rather than "Offset Mode". Select Input | Project Options and select "Elevation" for the field ELEVATION TYPE. All of your offset data will automatically be converted to the appropriate elevation.

Note that your pipe link outlet can be at whatever elevation (or offset) you want. Generally, however, it is at the junction invert.

If the hydraulic gradeline can rise above the ground surface at an access hole site, such as in a sanitary force main or siphon, special consideration must be given to the design of the access hole frame and cover. The cover must be secured so that it remains in place during peak flooding periods, avoiding an access hole "blowout." A "blowout" is caused when the hydraulic gradeline rises in elevation higher than the access hole cover and forces the lid to explode off. The difference between specified SURCHARGE ELEVATION and the MAX/RIM ELEVATION should correspond to the maximum or design pressure for the access hole frame and cover. If "blowout" conditions are possible, access hole covers should be bolted or secured in place with a locking mechanism.

A foot of surcharge on a 3 ft diameter manhole cover exerts an upward force of about 441 pounds. It would not take much surcharge to cause the manhole cover securing bolts to fail. In addition, generally a weaker part of the manhole is the connection between the frame and the manhole, between manhole rings, and between manhole riser sections. At some point, the surcharge pressure may even cause unrestrained pipe joints to fail. In practice, it could be easy to envision any of these joints separating enough to relieve the surcharge pressure and then settling back down, leaving little evidence that the surcharge event occurred.

If you continue to have questions, I recommend that you call into our technical support team and they can remotely jump on to your computer and review your data and provide you some suggestions in defining the data to meet what you are attempting to model.