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.
Storage Options
The 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:
1) If the material 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.
2) If the material 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