9424 18TH AVE W 2018-01-02 MF Import

Redundant Arienuation Provided by Compost Filter Berms Compost filter berms are proposed as a redundant attenuation and treatment facility for runoff generated within the Central Core and the Northern Tier. Originally, the site had been designed ta empioy only compost filter berms for treatment and attenuation (following discussions with City of Everett staff, the rain garden system was added to provide a"front line" treatment and attenuation facility outside the buffers). Consisting of a sand core overlain with compost, these berms create a condition similar to the hummocks and undul2tions of a mature forest floor. Both the sand and the compost provide nutrient removal treatment, and serve to decrease rur�off. 8ecause of the invert elevation of the underdrains beneath RG-3 and RG-4 is too low to Fnter RG-7, they are directed to a discharge point above the c�mpost filter berms. As shown in Table 1 above, the peak discharge from these two rain garden underdrains is less than 0.02 cfs. STORAGE STRUCTURE LIST The "storage structures" ior this site con�+st of a system of rain gardens followed by a system of three compost filter berms that provide redundant storage capacit}�. Rain Garden Storage The rain gardens serve as the initial st�rage from tiie pollutant generaiing surfaces of the site. Engineered, compost-amended soils provide treatment by intercepting and absorbing runoff. Because the size and tributary area are sim;lar, the rain gardens were combined and modeled as a single rain garden containing the to:al volume and area of all seven. As shown in Table 1, no runoff will discharge to the compost filter berms during normal operating conditions. For many hours following the event, runoff retained within the rain gardens will continue to enter the underdrain system at a rate of approximately 0.02 cfs, alihough significant volumes will be retained in the soil. Compost Filter Bgrm Storage The compost filter berrns provide additional storage in the event runoff exceeds the capacity of the sever� rain gardens. The storage capacity of 1590 cubic feet provided Cy the berms exceeds the maximum storage volume for the 100-year storm of 1370 cu.ft. The design inter;t of the compost fiiter berms is to mimic the water quality benefits and runoff attenuation found in mature forests. The compost berms imita;e the undulating hummocks created by failen decayed trees that serve to impede surface runoff, allowing time for infiliration. HEARTHSTONE LANE 7 � DRAINAGE REPORT APRIL 2006