Changes to the calculation spreadsheet and plan template are coming later this spring.

Please read the following carefully and reach out to your plan reviewer if you have questions. 

Effective immediately for any Alternative Compliance Option plans under review, applicants: 

• Can count detention credit for urban planter box soil storage volume.  
• Can count detention credit for 50% of urban planter box dry well storage volume. 
• Can size and locate dry wells with more flexible guidelines.  
• Cannot connect pre-manufactured tanks in series, except for modular systems explicitly designed for in-series connection. 

Applicants will find that these adjustments will make compliance more feasible, while eliminating a practice (connecting tanks in series) that has proven to be difficult to construct.   

Detention Credits 

In the meantime, you can do the following to update your plan: 

• Calculate these additional detention credits in a narrative text box on the VRRM plan sheet if there's room, or a new page if there's not. 
• Subtract other detention tools from the DDA/NDDA tabs until the detention credit deficit for the project matches or is as close as possible to the new detention credits added.  For example, you could subtract a detention tank and/or detention-only dry well installation. 
• Applicants are still required to comply with the 50% tank detention storage requirement in the DDA. 

Dry Well Sizing 

In addition, we are providing more flexible dry well excavation sizing requirements as follows: 

• The excavation footprint for a dry well excavation for four (4) 100% void dry wells can be seven (7) feet by seven (7) feet.  Otherwise, dry well excavations cannot exceed four (4) feet wide but may be more than four (4) feet long.  
• There must be at least two (2) dry well installation locations on the site, unless otherwise approved during plan review. 

These more flexible parameters will allow you to combine dry wells for multiple purposes (e.g., planter box release and uncontrolled roof area) in a single excavation and reduce the total number of dry well facilities on the site. 

Please note that with the upcoming plan updates later this spring, we will also be adding a requirement that all dry well installations must, at a minimum, have one (1) 100% void dry well at the inflow connected to a second 100% void dry well at the outflow (pop up).   

Please also note that the 100% void dry well detail on the plan template is drawn such that users might conclude that a ‘double flo-well’ is required, rather than a standard single flo-well.  The deeper version is not required and in fact is generally not desirable given site and soil constraints.  The plan template detail will be updated.  In the meantime, please refer to this detail as a guide. 

Detention Tanks 

Pre-manufactured tanks can no longer be connected in series, except for modular systems designed for in-series connection.  This practice has proven to be difficult to construct.   

You are also strongly encouraged to consider concrete detention tanks first, since they are more durable and easier to maintain for the property owner.   

• A 3’ wide by 6’ long by 4’ tall concrete tank is equivalent to a 500 gallon plastic tank that stands more than 6’ tall and 7’ long.   
• Many concrete tanks have been approved so far, and the review and as built process has been smooth, and the tanks also look great. 

Before specifying individual tank volume and the total number of tanks, consider: 

• Maximizing the use and size of planter boxes.  Three (3) typical planter boxes sized at 100% WQV could replace a 300+ gallon tank, or sized at 200% WQV could replace a 500+ gallon tank.  
• Choosing permeable paving for driveways.  A typical permeable paver driveway could replace a 500+ gallon tank. 

Please be on the lookout for an announcement with the updated Stormwater Manual, spreadsheet, and plan template later this spring.