08/12/2020 P&Z Packet - Laserfiche WebLink

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• Stormwater management ponds, if used, should be designed to be a site amenity, including appropriate landscape elements. Per the revised Storm Water Management Report prepared by Anderson -Johnson Associates dated Mar -eh July 13, 2020, rate control is being accomplished using a combination of a dry pond and a storm water treatment pond. The dry pond is now located where the original community gardens were proposed. The proposed rate is less than 80% of the existing rate of runoff. Water quality treatment is accomplished at the single stormwater treatment pond. True infiltration was considered during the design phase, but several factors contributed to not pursuing infiltration practices. First, shallow groundwater exists on site. According to the soil borings, groundwater is in the 895 to 897 elevation range. A majority of the low points are around elevation 905 with one parking lot having a low point of about 902.5. This would mean the surface of an infiltration area would need to be at elevation 900 - 901 for effective drainage to occur. Additionally, the site has been maximized and there is not adequate space for effective infiltration practices. Instead, a stormwater pond has been constructed. The stormwater pond, in addition to providing rate control, has been sized to provide adequate dead storage below the normal water level to meet water quality requirements. Finally, an irrigation pumping station will be constructed at the stormwater pond to draw water from the pond to irrigate the surrounding areas. As detailed in the May 11, 2020 Council staff report and Resolution No. 22-44 approving PUD Development Stage Plan Amendment #1 to the site plan and grading & drainage plan, the clubhouse was omitted in order to enlarge to the stormwater pond to provide more capacity. Pond Size Comparison Original Pond 3P Enlarged Pond 3P Surface Area 0.56 acres 1 0.82 acres Volume 2.810 acre ft 3.869 acre ft RCWD issued an amended Conditional Approval Pending Receipt of Changes (CAPROC) on March 13, 2020. The District Engineer finds the revised grading and stormwater management plans compliant with RCWD rules. The City's stormwater management ordinance requires 4 ft of separation between low floor freeboard and groundwater. In order to minimize the site being built up to meet this requirement, this separation distance may be reduced upon submittal of a geotechnical report and design by a licensed engineer. This report shall define ground water level elevations for the senior living complex and include plans demonstrating that 19