21-0512 Responsible Water Use for Muhlenberg College

Question

Imperative 3: Responsible Water Use, Requirement 3: Treat all stormwater on site, through natural or mechanical means and without chemicals, and manage all stormwater based on both pre-development hydrology and current ecological conditions, as determined by a qualified professional. Post-development rates shall not exceed historic peak rates for 2-, 10-, 15- 25- and 100- year events. Retain 98% of the average annual precipitation volume on site. Exception Narrative: For this project, the team has completed a single-event hydrologic analysis to estimate runoff from the site. It has been confirmed by these calculations that runoff is being managed to not exceed the peak rates for the 2-, 10-, 15- 25- and 100- year events. In order to demonstrate compliance with retaining 98% of the average annual precipitation volume on site (per Table 5-4), the team plans to calculate volume storage by calculating infiltration rates, analyzing the size of each BMP, and using the Pennsylvania Stormwater Best Management Practices Manual with at least 40 years of historical data. Several strategies are being incorporated on site, including removing existing parking and roadways, rain gardens, retention basins, pervious hardscape, regrading previously manmade slopes to decrease slope, erosion and rainwater runoff. We have also added 10,000 gallons of rainwater capture and reuse from the 7,000 SF roof. We feel that the project is easily achieving Imperative intent through the restoration of natural site conditions to the point that karst soils will allow. We are concerned that our stormwater strategies are taxing the karst limestone soils and may lead to sink holes. We have pushed as far as we should and not infiltrate more than we are. We feel that the project is easily achieving the Imperative by our efforts and would like to complete the runoff and volume calculations without performing a continuous simulation model to demonstrate compliance. In order to optimize efforts on all Living Building Challenge items, the team plans to highlight all of the good stormwater design being incorporated via volume and runoff calculations based on the abovementioned and 40 years of historical data. Please confirm that this approach is acceptable.

Answer

The project team is meeting the intent of this Imperative through the hydrologic analysis they have performed on their site, despite not using the continuous simulation model method. The project team has include the mandatory 40 years of data and infiltration rates, along with the best management practices that they are planning on installing, such as rain gardens, retention basins, and pervious hardscapes. Note that if the team cannot meet the 98% infiltration rate because of the karst geography and potential for sinkholes, the project team will need to submit a letter from a qualified professional (e.g. local geologist, landscape architect, or government official) stating that.

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