HydroCAD^® Stormwater Modeling - Since 1986

Modeling Chamber Overflow

Underground storage systems are often designed to retain smaller rainfall events, such as a 2-year storm.  But larger events, such as a 100-year storm, may completely fill the chambers and generate a "storage exceeded" condition.  In order to get an accurate simulation of these conditions, it's essential that you define all auxiliary storage volumes and overflow controls that could become active during the larger event.  Without this information the model may report an excessive water surface elevation, representing the head required to force the excess flow through the defined outlets.

If you get an "overfilled storage" message or very high water surface elevations, make sure you have defined all the available storage and overflow capabilities of the system.  You don't necessarily need to make the system larger, but you must provide complete information for all storage and outlets below the peak WSE.

This page discusses a hypothetical offline chamber system.  In practice, every system is different, and will vary depending on the requirements of each project, and the specific chamber product being used.  This page is intended only to discuss one approach to modeling these systems in HydroCAD, and is not a specific recommendation for the design or construction of any system.  Always contact your chamber manufacturer for specific requirements and design recommendations for each product.

System Operation

A typical underground storage system is illustrated above.  This diagram shows not only the basic chamber system, but also the auxiliary storage provided by the inlet pipe, catch basin, and above-grade storage.  The system also has an overflow weir, in the event that all the available storage is utilized.  The basic system operation is as follows:

1) An inflow hydrograph is routed to the system.  This could be a single subcatchment, or the final flow from a complex watershed model.

2) The inflow enters the underground system through a catch basin.  Although there will typically be a grate at the inlet, this example assumes that the inlet will not significantly reduce or detain the inflow, and therefore the grate is not represented in the HydroCAD model.

3) An inlet pipe conducts water from the catch basin to the actual chamber system.  The pipe should be large enough so the catch basin and chamber system can equalize and maintain the same WSE at all times.

4) The chamber system provides the primary storage volume for the pond node.  This is typically modeled using the HydroCAD chamber wizard.  When modeling exfiltration into the surrounding ground, an exfiltration outlet is also included in the pond.

5) The final outlet control may be designed to drain the system over a certain time period, or it could be a complex multi-stage outlet optimized for multiple storms.  There may also be an exfiltration outlet (not shown).

6) If the chamber system fills completely, the WSE elevation will continue to rise in the catch basin, eventually reaching the rim and ponding in the above-grade storage volume.

7) If the above-grade storage fills completely, any excess will be handled by the overflow weir.

Setting up the HydroCAD Model

Based on the operation discussed above, the HydroCAD pond would include the following elements.

Outlet definitions:

Device#1=Outlet Culvert, Routing=Primary
Device#2=Overflow weir, Routing=Secondary
Device#3=Exfiltration, Routing=Discarded

Note that the devices may be appear in any order.  As long as the routing is correct the results will be the same.

Storage definitions:

Chamber Wizard - for the chambers and surrounding stone
Pipe Storage - for the inlet pipe
Vertical Cylinder or Prismatiod Storage - for the catch basin
Custom Storage - for above-grade storage

Note: When modeling exfiltration from the chamber system, be sure to de-select the "Allow exfiltration" box for the other storage definitions.  Otherwise the exfiltration area will include the connecting pipe, catch basin, and above-grade storage.

For further information read about chamber modeling and overfilled storage.

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