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HydroCAD Stormwater Modeling - Since 1986

Offline Chambers

Offline Chamber Modeling

A common approach for implementing underground storage is to use a control structure that diverts flows to the chamber system, and then allows the water to return and leave the system later in the storm.  If the chamber system fills completely, the control structure can also divert the excess to an appropriate overflow, rather than passing the entire volume through the chamber system.  This is often referred to as "offline storage", because the inflow does not enter the chamber system directly.

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 offline storage system is illustrated above. The key to modeling these systems in HydroCAD is to use a single pond to model the entire system.  Although other solutions are possible (such as several interconnected ponds) the single-pond solution is much easier to implement and is suitable for most applications.  The basic operation and modeling of this system is easily understood by following the flow of water through the system:

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

2) The inflow enters the control structure through a drop inlet.  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) The control structure can contain a number of flow control devices.  In this example, a weir wall causes the initial inflow to pass through the equalizing pipe into the chamber system.

4) The equalizing pipe maintains a relatively constant water surface elevation between the interior of the control structure and the chamber system.  Early in the storm, water flows from the control structure to the chamber system.

5) 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.

6) When the water in the chambers and control structure rises to the top of the weir wall, water will overtop the weir, allowing any additional inflow to bypass the chamber system.  The weir wall may also contain a small orifice at the bottom, which allows the system to empty completely after the storm.

7) After passing over the weir wall, water leaves the control structure through a suitable pipe, modeled as a culvert outlet.  In some situations, the culvert may be deliberately under-sized in order to restrict the flow and cause water to be detained in the above-grade storage.

8) If the chamber system has completely filled, and the outlet pipe cannot handle the additional inflow, water will begin to pond above-grade.  This volume can be modeled as an additional storage definition in the same pond.

9) An overflow weir can be used to handle system overflow during more extreme storms.  This is modeled as a HydroCAD weir outlet.  The discharge from the weir can be reported and routed separately by setting the device routing to "secondary".

10) As the water level in the system drops, overflow will cease, and the system will gradually be emptied by exfiltration and/or the low-flow orifice in the weir wall.

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=Weir wall, Routing=Device#1
Device#3=Weir wall orifice, Routing=Device#1 (optional)
Device#4=Overflow weir, Routing=Secondary (optional)
Device#5=Exfiltration, Routing=Discarded

Storage definitions:

Chamber Wizard - for the chambers and surrounding stone
Prismatoid storage - for the interior of the control structure (optional)
Custom storage - for the above-grade volume (optional)

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 above-grade storage and the interior of the control structure.

For further information read about chamber modeling.

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