Bioretention Area Calculation

Dead storage = LxWxH Storage in Media = LxWxHx(void ratio of media/100) Your provided volume should meet or exceed the required volume to treat (and you also need to ensure that volume actually gets there).

@Michael, I think that your response is not accurate and you CAN'T calculate bioretention area using just simple volume calculation. As Christy wrote, it is also not described in details in the Reference Guide. The methodology of Direct Determination is partially described in  EPA 'Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act’. Using this Guidance (p.30-35 and Appendix A) you can see Step-by-Step how to calculate the Total Imperviousness, 95-100th Percentile and Expected runoff from each type of land cover using Direct Determination, but  it is not described how those results are then used to determine the area of stormwater management practices (bioretention, permeable pavement, cistern and green roof) to manage expected runoff from Total Area. Analyzing 9 scenarios given in Technical Guidance also gives no explanation. Most engineers outside of the US use different methodologies, mainly 10-30y design storms or other local methodologies, to calculate just the maximum runoff rate and prepare the infrastructure for the heaviest 15min storm events, but they don't feel comfortable with calculating LID for the purpose of LEED documentation.  Michael, as you are the Water Engineer, could you please describe (e.g. based on Scenario #8) how exactly the Stormwater management areas are estimated by Direct Determination (what equations used step by step to get the same results as in Technical Guidance), using 24-hr assumptions given on p.30-35 and in Appendix A for one specific Soil Group? I think this would help all LEED consultants and architects to implement green infrastructure in their projects (make initial assumptions) and support water engineers.

My response is correct as to how to calculate the size of a BMP. If your question is how to do the routing, which in turn tells you the volume you need to use in your design, my response will be that I went to school for a long time to learn how to do this, and it isn't something you're going to learn in a forum post. You can try to follow along with that Direct Determination Method, but I would highly recommend you consult with a CE and route the site using any number of methods (I prefer NRCS).  

Not exactly. Your answer suggests that you probably have not read this document yet. It is clearly stated that the case study scenarios described in mentioned EPA Technical Guidance do not include site specific design features such as runoff routing, specific site infiltration rates, the structural loading capacity of buildings, etc. in terms of stormwater practice selection. The slope of the pervious area was assumed to be an average of 2%. Many other assumptions has been done to just make initial stormwater management approach available for everyone and show that Direct Determination Method gives very comparable results to other more complicated methods (including computer simulation) with many variables. The same method was used in the LEED v4 Reference Guide and is sufficient to comply with credit requirements and to calculate the overall required area of bioretention. Of course you are right that final design and location of each GI should be approved by CE, but that's not the point of that discussion. I just asked you, if you can and have time, to explain the equations used to get the same results of bioretention areas as in Technical Guidance. E.g. Scenario #1:
Total Area (acres) - 0.7
Estimated Imperviousness- 73%
95th Percentile Rainfall Event (inches)- 1.23
Expected Runoff for the 95th Percentile Rainfall Event (inches)-0.82
Bioretention estimated by Direct Determination method (acres)- 0.03 for Soil Group B and 0.06 for Soil Group C
The effective design storage depth within the designated bioretention area was 8 inches.
24-hr infiltration losses for pervious areas and bioretention areas:
Soil Group B: 9.743 inches
Soil Group C: 4.430 inches
depression storage: 0.2 in

Has anyone come up with an explanation to the area calculation? Thanks

Your question is valid.  The document is detailed on some topics (like how to calculate the 95th percentile rainfall event) but the examples just show unexplained results for the bioretention areas.  I tried to find a reference on how to calculate the bioretention area with the Direct Determination method but got nothing.  I suppose it's an empirical method?  An example would be nice at least for the Rational Method.  We will try to model this in SWMM but it would be easier with a quick check with an empirical method.