2007-09-26 CC FYI Meeting - Laserfiche WebLink

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"' · restrict the discharge for larger storm events to the overflow location and therefore minimize erosion As the rain garden is currently constructed the overflow is southwest of the swale through the center of the park. The ground elevation should be raised to force the overflow to the swale. If the existing outlet structure remains as 893.06 elevation, the overflow elevation should be set at 893.6 to contain the 5-year and smaller events in the rain garden. Therefore the ground elevation everywhere else in the rain garden should be approximately 894 and above in order to force the overflow to the swale. Extreme events would not stay localized in the swale, but the majority of storm events would be contained in the rain garden and swale system. If the primary outlet elevation is lowered to 892.9 (the minimum elevation to meet the RCWD proposed rules), then the minimum overflow elevation is 893.5. Draintile As noted above, draintile was provided due to the hydrologic soil group D soils. The rain garden functions more as a biofiltration area then an infiltration area. This method is also allowed in the RCWD Rules. Filtration of larger particles in the engineered soils at the base of the rain garden, uptake in the root structure of the plants and evapotranspiration are the primary means of water quality treatment. There is nominal volume infiltrated, but it was not considered since the soils are dominated by clays. The as-built survey shows that a minimum of one foot of separation is provided between the bottom of the rain garden and the draintile invert. This sand layer acts as a filter system. The compost in this layer allows for growth of the plants. The plants then remove phosphorus, nitrogen and other compounds from the stormwater runoff. The draintile was re-routed to the lower depression, bypassing the swale. The original intent of the design was to route the draintile to the swale. Although the water quality treatment achieved in the swale was never directly accounted for since it is insignificant. Therefore routing the draintile to the swale first would not significantly add to the water quality treatment. A longer, very gradual sloping, vegetated swale functions better for water quality purposes, but conversely can lead to pooling and stagnant water if not graded correctly.