Modeling surcharging of an enclosed storage

 Applies To 
 Product(s):CivilStorm, SewerGEMS, PondPack, WaterGEMS, WaterCAD
 Version(s):CONNECT Edition, V8i
 Area: Modeling
 Original Author:Jesse Dringoli, Bentley Technical Support Group



I need to model an enclosed storage such as an underground storage chamber or storage pipe (or a tank for water models), but I need to be able to model a surcharged / pressurized condition, where the hydraulic grade is above the top. However, (for the storm and sewer products) I get a message that the pond has overtopped or that overflow is occurring. Or, the proper headwater/tailwater is not being communicated to the adjacent pond via ICPM when the HGL is above the storage top. Or for WaterCAD or WaterGEMS, the built-in altitude valve closes when the tank becomes full.

Solution - Storm and Sewer products

When the hydraulic grade exceeds the top elevation that you specify in the pond's elevation-area / elevation-volume curve (or constructed elevation-volume curve based on entered pipe or storage chamber dimensions) the program assumes that overflow will occur. There are two options to consider if you need to model surcharging:

Option 1 - "Chimney"

One solution is to add a “chimney” to the top of your pond’s elevation-volume curve, to allow surcharging. You’d essentially add additional elevations to the curve, covering up to the maximum expected surcharge hydraulic grade, with a corresponding tiny increase in volume. This is similar to the Preissman slot method used in the calculations for surcharged pipes with SewerGEMS and CivilStorm (see the Help file for more on that). The pond will then be allowed to reach a higher hydraulic grade, without accounting for any additional storage or any overflow problem.

Note: if you're using PondPack with the "Pipe" or "storage chamber" pond option, you can right click on the pond, choose "Pond volume results table", click the data table tab, copy out the elevation and volume columns, change the pond type to elevation-volume, then add the "chimney" elevations. You may need to use a smaller calculation timestep or ICPM timestep to achieve stable results.

Note: if you're using SewerGEMS or CivilStorm, you may notice numerical instability and unexpected results in some cases when water reaches the "chimney". If this happens, try using a smaller calculation timestep (even as low as 0.002 hours if needed), as the instability can be due to rapid changes that occur in this condition. If that does not help, the Explicit (SWMM) numerical solver tends to work better with pond modeling, with the Routing Step calculation option set to 1 second (or in some extreme cases, 0.1 seconds).

The below example shows a pipe pond with an upstream crown elevation of 7.67 ft. Surcharging is expected, to a maximum elevation of 15 ft. So, an additional entry of 15 feet is added to the elevation-volume table, with only a tiny increase in volume.

Option 2 - Conduit (SewerGEMS/CivilStorm)

If using SewerGEMS or CivilStorm, model this as a control structure on a conduit. If you need to model a multi-stage outlet with more than one component (such as an orifice plus a weir), set up your composite outlet structure with multiple outlet components in the Composite Outlets managers, click the compute button in there to compute the rating table, then click the Data Table tab, highlight the elevation vs flow columns and use CTRL+C to copy into memory. Then, Add your stop control structure (or start control structure on the next downstream pipe – one method may be more stable than the other), set it up as a “depth-flow” type and paste in the data from the composite rating table. This way you can have the pipe modeled as a conduit, with the multi-stage outlet modeled as the single rating table on the in-line control structure.

Solution - Water products

In WaterCAD and WaterGEMS when a tank becomes full, it is currently assumed that an altitude valve closes the inlet pipe to prevent overflow. If you have a tank that needs to become pressurized/surcharged, you can use the Variable Area tank type (selected in the field called "Section") and configure the curve to essentially model a tiny "straw" or "chimney" above the actual top. This enables the HGL to rise above the actual top (to model the surcharging) without introducing any extra storage.

Note: you may need to use a much smaller hydraulic timestep to prevent numerical instability that could occur when transitioning to the "chimney" (surcharge state).