What is a drop manhole and how do I model one?

Product(s):  SewerGEMS, SewerCAD, StormCAD, CivilStorm
Version(s): CONNECT Edition, V8i
Area:  General

Problem

What is the Difference Between a Drop Manhole (drop structure) and a Regular Manhole?

How do I create a drop structure?

Solution

A drop manhole is used in areas with a steep slope when one or more of the inlet pipes has an invert elevation significantly higher than the invert of the outlet pipe. Typically the invert elevation of the "stop" end of the inlet pipe is set to the invert elevation of the manhole. However, in the case of a drop manhole, the stop invert of the pipe is not set to the manhole invert elevation but is at a significantly higher elevation.

When using the GVF Rational Solver (StormCAD, CivilStorm or SewerGEMS) or the GVF Convex Solver (SewerCAD or SewerGEMS) you may want to design manholes to be drop manholes. To automatically create drop structures make sure you have the "Allow Drop Structure" checkbox checked in the node tab in the Default Design Constraints window.  Please note that checking this option to allow drop structures does not necessarily mean that it will always create one, rather that it will have the freedom to do so if it needs to in order to meet constraints.  For example, if the network is set up in such a way that a drop structure could be used to prevent the Maximum Slope constraint from failing, it may do so.  After this has been selected all you simply need is that pipes meet at different elevations at a junction. The constraint based design routine automatically creates the drop structure to accommodate the difference in elevations.

There is also a "Design structure elevation?" option that gives the constraint based design calculations the freedom to adjust the manhole invert if needed to meet your design constraints.



Reducing Plunging Velocity

The velocity of the water dropping into (plunging) a deep drop structure is outside of the scope of the software - this calculation is not done, as it only calculates the velocity of the flow through the pipes and through the structure itself (benching). Further above you can see two diagrams of possible considerations for connecting pipes to a drop structure to help avoid issues with high drop depth / plunging depth, such as odor control.

Using a baffle or bends in the backdrop (vertical pipe directing flow down into the drop structure) could be an option but it would be an engineering decision unrelated to the software. You might find further guidance on this topic in an engineering reference book such as our Stormwater Conveyance Modeling and Design.

Note: any resistance from options like bends, baffles and benching configuration can be modeled using the Structure loss method in the manhole properties.

See Also

Reporting drop depth of a manhole or catchbasin

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