10602 16TH AVE SE 2025-07-28 - Laserfiche WebLink

8. RUN-OFF TREATMENT This project is required to provide runoff treatment due to the amount of PGIS proposed on the site. All runoff from the proposed paved driveways and access roads excluding the fire lane which will not be used regularly and a short portion of the entrance road that could not be collected due to grade constraints, will be treated within one of four Perk Filters. These filters were sized by analyzing each tributary sub-basin to the individual filter to determine the peak treatment flow rate for each. Then the sizing table provided by Filtera the Perk Filter developer/supplier was used to determine the height and number of filters required. The following table summarizes the flow rates to each filter. Catch Basin WQ Flow Rate Cartridge Height No. Cartridges (15 min) #2 0.0228 cfs 18" 1 #3 0.0028 cfs 12" 1 #4 0.0225 cfs 18" 1 #5 0.0446 cfs 18" 2 #6 0.0152 cfs 12" 1 #8 0.0523 cfs 12" 4 Details for each of the different design scenarios are provided on the engineering plans. The WQ flow rates were calculated using the WWMH2O12 which is the most current version as required by code. 9. FLOW CONTROL For this project the total area to be disturbed is 3.11 acres. As noted previously in this report the site has been broken into three on-site sub-basins. The NW draining to the northern wetland west of the development, the SW draining to the southern wetland west of the development and the east basin draining to 16th Ave SE. In addition, for the eastern basin there is a small area of bypass (0.17 ac) including 0.09 acres of pavement and 0.08 acres of landscaping. These areas could not be collected due to the need to connect the project via road/access to 16th Ave SE which is much lower then the site. In addition, along the southern property line there is approximately 0.18 acres of off-site area tributary to the final development. This area is going to be captured and detained to off-set the bypass area listed above. The use of dispersion along the west side of the project will both provide the necessary flow control for the NW and SW while maintaining the hydrology of the two on-site wetlands. The project proposes to disperse flows from the SW basin via two level spreaders. One will serve the runoff from the pavement and some landscaping while the second will be for roof runoff. In addition, portions of the future landscaping that does not get collected by the pavement system will be allowed to sheet flow west without being collected or concentrated. The NW basin design is similar in that there is one level spreader for the pavement and some landscaping runoff and then individual spreaders for each of the two buildings. The use of multiple level spreaders will aid in spreading the sheet flow out along the length of the western edge of the project and allow for greater absorption of the flows in the buffer area prior to the flows entering the wetlands. The level spreaders have been sized to provide a minimum of 10-feet of trench per 0.1 cfs of Northlake Court Townhomes 13-0609 May,2014 Page 12