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Rain Gardens

Modeling Rain-Gardens

Rain gardens, or bioretention cells, are readily modeled in HydroCAD by using a "pond" node with the appropriate storage and outlet definitions.  This discussion also applies to most roof gardens, planters, and prefabricated structures such as a "Tree Box".

Note:  This is a complex topic.  Please read this page in it's entirety to determine the best modeling approach for specific situation.  If you're just looking for a quick answer, skip ahead to the general recommendations at the end.  Otherwise you will need to spend some time to understand the complexities of modeling these systems.

Storage Options

Typical Rain GardenThe storage characteristics may be described using any of the standard pond storage options, with the exact choice depending on the shape of the rain garden.  Remember that each pond can contain multiple storage definitions, which can be "stacked" one on top of the other to model compound shapes.

To allow for reduced storage in stone or sand, the "voids" parameter can be set to an appropriate value, such as 40% for clean stone.  But since only part of the "pond" will be stone-filled, you will generally need  a separate storage definition corresponding to each voids value.

Another option is to use a single custom-storage definition using the "Stage Voids" option.  This allows you to set different voids for each layer of the pond.  (Stone, sand, open water, etc.)

Outlet Controls

Most rain gardens will have some type of overflow device.  This can generally be modeled as a weir and/or culvert.  You may even have a compound outlet structure, such as riser.  Details here.

For rain gardens that are located in pervious soils, you can include an exfiltration outlet.  This will allow for percolation of water into the surrounding ground.  Details here.  Although exfiltration has many desirable effects, it should be noted that it often takes many hours for significant exfiltration to occur, so it may not be a significant factor in mitigating the short-term peak runoff from a site.

Modeling Under-drains

The correct modeling of an under-drain depends on the exact role it plays in the hydrograph routing.  In most cases the under-drain consists of a perforated pipe buried beneath a layer of sand, stone, or other media.  These situations can be divided into two basic categories:

A) If the growth media is able to freely pass the incoming water, and the pipe is the effective outlet control, then the pipe and/or perforations would be modeled as the outlet device, using a culvert and/or orifice array.  Furthermore, the voids in the material would be considered as part of the pond storage, since they lie upstream of the effective outlet control.  The easiest way to model this situation is to use a custom storage definition along with the "Stage Voids" option.  This lets you enter the voids for one or more layers of media, plus the 100% "voids" in the open water, all with a single storage definition.

B) If the growth media restricts the flow of water from the pond, then it becomes the effective outlet control, and the storage definition should include only the (open water) volume above this point.   If you know that the pipe and/or perforations are sufficient to handle the flow through the media, the exact characteristics of the under-drain will not effect the hydrograph routing.  In this case you can use exfiltration as the "outlet" device, and the pipe may not need to appear in the model at all.  On the other hand, if the pipe is a possible restriction, then you could use a compound outlet, with the exfiltration flow routed through the pipe like this:

    Device #1 = Culvert,      Routing = Primary
    Device #2 = Exfiltration, Routing = Device #1 

General Recommendations

Although rain gardens can be beneficial for the management of pollutants and low-runoff events, they often don't provide enough storage to cause significant peak-flow reductions for larger events.  The media infiltration capacity also tends to be small in comparison to the volume and flow rates of these larger events.  Therefore, modeling objectives should be clarified before undertaking the modeling of these systems.  Although there is no single "best way" to model a rain garden in HydroCAD, the following setup works well for many situations:

1) Use an exfiltration outlet to simulate the flow through the media.  If you are capturing this flow in an underdrain (perforated pipe) that is subject to further routing, change the exfiltration routing to something other than Discarded in order to retain the flow.  For further details read about exfiltration modeling.

2) When using an underdrain that might further restrict the flow, you can include this in the outlet setup.  However, if the underdrain is properly sized to handle the maximum media flow rate, there is no need to include this detail in the model.

3) Define the appropriate overflow devices.  This might include a weir and/or a horizontal orifice at the top of a riser structure.

4) Set the outlet device routing.  In the simplest case, the overflow and exfiltration might both be routed directly to Primary.  If you have an underdrain that is connected to an overflow riser, you can use the device routing to configure the outlet like this:

    Device#1=Culvert, Routing=Primary  (Pipe accepting flow from the riser and underdrain)
    Device#2=Horizontal Orifice, Routing=Device#1
(Overflow into top of riser, routed to culvert)
    Device#3=Exfiltration, Routing=Device#1
(Media flow, routed to culvert)

For further details see this pond example.

5) Define the storage volume above the media only.  Since the voids in the media or stone layers are "downstream" of the media flow control, they are not part of the same level pool and should not be included in the pond storage definition.  In order to avoid a storage exceeded condition, make sure the storage range extends to some point above the highest outlet.

For further reading

For a more comprehensive discussion of rain gardens, and the use of single-event models vs. continuous simulation, please see the following article.

Design of Integrated Bioinfiltration-Detention Urban Retrofits by William Lucas


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