October 20 2021 Regular Board Meeting Packet

HISTORY & CONSIDERATIONS:

None.

RULE C: STORMWATER MANAGEMENT Standards  Proposed discharge rates for the 2‐, 10‐, and 100‐year events shall not exceed existing rates.  Developments and redevelopments must reduce runoff volumes in the amount equivalent to an inch of runoff from the impervious areas of the site.  Stormwater must be pretreated before discharging to infiltration areas to maintain the long‐term viability of the infiltration area.  Developments and redevelopments must incorporate effective non‐point source pollution reduction BMPs to achieve 90% total suspended solid removal.

Findings 1. A hydrograph method based on sound hydrologic theory is used to analyze runoff for the design or analysis of flows and water levels.

2. Runoff rates for the proposed activity do not exceed existing runoff rates for the 2‐, 10‐, and 100‐year critical storm events. Stormwater leaving the project area is discharged into a well‐defined receiving channel or pipe and routed to a public drainage system.

3. Stormwater runoff volume retention is not achieved onsite in the amount equivalent to the runoff generated from 1.1‐inch of rainfall over the impervious surfaces of the development. a. The amount of proposed impervious is 58,961 ft2. b. Volume retention required: 58,961 ft2 x 1.1 inches x 1 ft/12 inches = 5,405 ft3

Table 1. Proposed volume retention through abstraction (i.e. infiltration, reuse).

Volume Volume Retention 1.1‐inch 2.5‐inch Retention Runoff Runoff BMP Provided below Required (ft3) (ft3) (ft3) outlet (ft3) 5,405 None, filtration is proposed

4. Alternative compliance has been requested due to contaminated soils. A Phase II Investigation by Braun Intertec identified both petroleum and non‐petroleum related soil contamination at the site including PAHs, metals, and DRO. Groundwater was also found to be impacted with DRO. For these reasons, it was determined that infiltration is not feasible. a. The applicant did not partially comply with the volume retention standard. b. Filtration using Contech StormFilter with Phosphosorb Media (68% credit) is proposed; filtration required: 5,405 ft3 x 1.47 credit factor = 7,945 ft3 c. A variance has been requested from Rule C section 3.c.2.i to use an alternative flow through treatment calculation method when determining volume control application.

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