20-0915 Use of WT-001 Site-Constrained Stormwater Discharge Exception

QUESTION

As an existing building, our project requests an adjustment to the stormwater management scope such that we would not be required to treat and manage the water that falls on a portion (approx. 1/3) of our existing roof.
As-built information available at the time that we took possession of the building in 2009 was unclear as to the disposition of primary roof drainage piping beyond the building footprint. The remodel that took place at that time included the development of a new parking lot on the lower east side of the site. This parking area has a large stormwater management system installed beneath it and all of the rooftop water from the east half of the lower roof and all of the upper roof is directed to this system as underground piping was discovered during construction activities that made a connection possible.
No such discoveries were made for the west half of the lower roof, and field investigation conducted in August 2020 similarly failed to discern the path of stormwater that enters the primary roof drain system once it leaves the building footprint. We presume that there is an existing underground connection to the municipal storm sewer system in the street as a result.
Notably, the existing building is proximate to the property line on the south (18') and west (22') sides. Our Civil Engineer has advised that this amount of space is not sufficient for surface solutions (no evaporation/evapotranspiration), nor is it advisable to infiltrate large amounts of water close to existing building foundations lest we induce structural settlement (no infiltration). What's more, the path of the existing buried pipe is not discernible without excavation which would destroy the existing established landscape plantings (including a mature conifer that provides shade to windows on the southwest corner of the building) that we hope will survive after the irrigation system is shut off at the commencement of our Performance Period (no beneficial reuse).
An exhibit that describes the watershed aspects of the existing roof is included in this Post. Additional exhibits were provided to ILFI as part of our DD Review and are available upon request or can simply be accessed by speaking with your internal teammates.
ANSWER

Additional information provided from the team clarifies that the project is already providing, for beneficial re-use, the maximum quantity of rainwater collection permitted by the state. This point, in addition to the site constraints making infiltration and evaporation/evapotranspiration infeasible downstream of that section of the roof, justify use of WT-001 Site-Constrained Stormwater Discharge for stormwater draining from the west portion of the lower roof, as illustrated in the attached diagram.

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