Bioretention Design

[Steven Reed P.E.]

I am for the first time looking at using a bioretention pond on one of my designs. I typically use proprietary systems or standard ponds for detention and treatment, but I would like to start utilizing LID's more for cost saving and potential utilization of the landscape areas. I have generated pre, post, and post-bypass hydrographs in the Autodesk Hydrographs extension, and exported them to excel to start the calculations. I would like to use the bioretention pond for both flow control and treatment, but I noticed that bioretention ponds typically only allow up to 6" of depth; presumably for the health of the plants. If this is the case, how are these ponds typically used for flow control? That's not nearly enough volume. Would it make sense to put an overflow weir at 6" above the treatment soil, and divert overflow to a separate detention pond? Are control structures ever designed with the bioretention ponds, with orifices, etc.? Also, are there any tools for designing these systems, or am I better off just continuing to build a custom spreadsheet for the design?

Thanks for any advice or thoughts!

 

[Ekenny_G]

Bioretention are more for providing water quality, not quantity control. There are usually a limit to the drainage area or volume they can handle, like you said something like 6-inches. For quantity control - you would have to consider diverting to a detention pond using a weir or Riser structure. So you could have a treatment train, where a bioretention is upstream of a detention basin. Essentially, a bioretention function as a water quality "forebay", and any overflow will discharge into the downstream detention basin.

 

[swazimatt]

as mentioned, normally bioretention is good for frequent low flow events (eg 2yr storm) If you intend using it to retain the 10 and 100yr storm your plant selection will be important as it will need to survive being inundated for a day or two without dieing and you will also need to ensure velocities do not damage plants and or resuspend all the solids you have trapped during small events. Often the design will allow the 2yr events to fill to 150mm depth (6 in) and soak through the planted media, you will allow enough volume to accommodate the larger events with controlled outlets (if detention is your aim)but position the high flow outlets near the inlet if possible that will create a "bypass" within your pond rather than having the outlet at the opposite end and the full event flows over your plants

 

[Martin Finch Jr EIT]

Bioretention ponds can be used for both quality and quantity control if they are designed correctly. A bioretention system allows 12" of ponding, not 6". Thus, a system of underdrain pipes must be used to control this ponding level. This will help, but in addition, spec a weir wall as the outlet structure and use multiple weir openings at diff elevations to accomodate low flows and higher flows. I have found that this can help mitigate your post development flows to be under your Pre development rates.
Another tip is model the underdrains in your bioretention cell as pipe storage with min. allowable exfiltration (modeled as discarded). The easiest way to model this scenario is by using HydroCAD.
Please feel free to look at the example in the link below. GL