Page 18 - planning-committee-packet

Stormwater Quality Treatment

Based on the proposed impervious cover of approximately 43,503 square feet within the LOD area, the percentage of
proposed imperviousness is 48%. Based on type D soils and the percent of impervious, the required rainfall depth is
1.8” (Pe = 1.8 inch). The computed target Environmental Site Design Volume (ESDv) is 6,550 cf. The proposed
impervious cover includes the hotel, pool, parking lot area, and entrance drive.

The objective of stormwater quality treatment is to provide the target ESDv using Environmental Site Design (ESD)
features/practices to the Maximum Extent Practicable (MEP) before utilizing structural stormwater practices from
Chapter 3 of the Manual.

ESD Practices Evaluation

Alternative Surfaces
• Green Rooftops – Not applicable
• Permeable Pavements – Permeable pavements are not ideal in D soils making the not practical for this site.
• Reinforced Turf - Reinforced turf areas are commonly used for emergency equipment access or for areas
that receive minimal vehicular use. Not applicable.
Nonstructural practices
• Disconnection of Rooftop Runoff – There is not adequate room on-site to meet disconnect requirements.
• Disconnection of Non-Rooftop Runoff – There is not adequate room on-site to meet disconnect
requirements.
• Sheetflow to Conservation Area – The configuration of the improvements do not allow for sheetflow
discharge to the existing non-tidal wetlands on-site.

Micro-scale practices
• Rainwater Harvesting – Opportunities for on-site reuse of harvested rainwater are limited. Operations and
maintenance, especially during cold-weather periods, is also a concern.
• Submerged Gravel Wetlands – This practice is best suited for sites with a high groundwater table and poor
soils, making this an ideal practice for the site.
• Landscape Infiltration – Not applicable on-site due to soils.
• Infiltration Berms – The layout of the site is not ideal for this practice.
• Dry Wells –Dry wells are typically reserved for the treatment of building rooftops in areas of well drained
soils. Due to the large roof areas and high groundwater potential, this is not an ideal practice for this site.
• Micro-Bioretention – This practice is ideal for smaller drainage areas with well drained soils. High
groundwater and D soils make this practice not applicable.
• Rain Gardens – This practice is better suited for residential environments.
• Swales – Due to poor soils and limited fall on-site grass swales and bio-swales are not ideal for the site. Wet
swales to not provide enough treatment for practical use on-site.

The Submerged Gravel Wetlands proposed to address the ESDv requirements will provide 10,055 cf of treatment which
is greater than the 6,550 cf required. Therefore, ESD has been achieved to the MEP. Refer to Appendix C for calculations.

Conclusion

This stormwater management analysis indicates that the proposed design will successfully meet stormwater quality
and quantity requirements. It is respectfully requested that the stormwater analysis and report and project design be
approved as presented.

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