2300 KASCH PARK RD PAVEMENT MAINTENANCE AND RENOVATION 2022-02-17

The on -site runoff is collected within the paved area of the project site by a combination of curbs, catch basins, and storm drainage pipes. The total paved area being collected is approximately 13.5 acres. Collected runoff is routed to oil/water separators, located at the southerly portion of the site. After being treated by the oil/water separators, the runoff is routed south to an existing detention vault. The vault provides 8 feet of live storage depth, with three chambers. The chambers are used to further filter out sediments. They range in size from 205 feet x 21.67 feet, 200 feet x 21.67 feet and 200 feet x 21.67 feet. This detention vault has a total live storage volume of 104,883 cubic feet. Runoff then leaves the detention vault to the south and enters into a structure that splits the flow. The water quality storm event that is to be the 2-year, 24 hour storm event is sent south to a pump station. Any water volume above the water quality treatment volume is then routed into a 30- inch HDPE diversion pipe. This pipe routes runoff to the northeast and outfalls to northwest corner of Wetland 3 (see Existing Site Plans in Appendix A). DOWNSTREAM ANALYSIS Runoff from the project site enters the existing bioswale after being treated for flow control in the vault and then is pumped via a 4-inch force main to the bioswale. This bioswale is located immediately to the south of the pavement area. The bioswale provides basic water quality treatment, and routes the runoff approximately 200 feet to the east. At this point, runoff is routed into a grated Type 1 catch basin via an 8-inch HDPE pipe. Runoff exits this existing structure to the northeast via another 8-inch HDPE pipe and runs for approximately 50 feet to the northeast. At the pipe outlet, flow is discharged to Wetland 3, located to the east of the site. Also discharging into this wetland to the north of the 8-inch pipe outfall is the 24-inch HDPE pipe that routes runoff from the west side of the site. Finally, to the north of the 24-inch HDPE outfall is the outfall for the 30-inch HDPE diversion pipe from the detention vault. This pipe outfall is located at the northwest corner of Wetland 3. This pipe routes flows greater than the water quality treatment volume (2-year 24 hour stormwater event). The topography of Wetland 3, located to the east of the site, routes runoff in a generally east pattern in a permanent pond area. It winds for approximately 425 feet through this wetland before reaching a 30-inch concrete culvert that conveys runoff east beneath Kasch Park Road. There have been some problems with beavers in the area clogging up this important culvert outfall, which in the past caused some flooding. To address this issue, a large "Beaver Deceiver' has been constructed upstream of this culvert. This 30 inch concrete culvert routes runoff to the east beneath Kasch Park Road for approximately 70 feet to a drainage structure in a landsape island. From there, runoff is routed via another 30-inch concrete pipe for approximately 135 feet beneath a paved parking lot for Kasch Park. At that point, it discharges into a grass lined, trapezoidal channel. Runoff travels along the grass lined, trapezoidal channel to the east for approximately 180 feet. At the end of the channel, flow runs into a 12 inch HDPE pipe. This pipe routes to the east for approximately 5 feet into a large drainage structure. Flows leave the structure to the north in a 30 inch concrete pipe. This 30 inch pipe discharges flow approximately 5 feet to the north of the large drainage structure into an open concrete channel. The open concrete channel winds to the north for approximately 250 feet within a wooded area. At the end of the channel, flows are directed over the east side of the channel into a wooded, wetland area. This area is located to the east of Kasch Park and is known as Loganberry Lane Park. It is highly wooded and mostly inaccessible. Runoff winds through this area in a permanent wetland pool in a southeasterly direction for approximately 1000 feet. Downstream of this wetland, runoff flows to the east into an un-named creek that flows from north to south. This un-named creek eventually becomes Swamp Creek. However, this is beyond the extents required for the downstream analysis. Based on natural topography and outfall locations, the site is contained within a single threshold discharge area (TDA). See Figure 4-1 for a schematic of on -site and off -site drainage patterns. See Figures 4-2 through 4-22 for photos. DOWNSTREAM DRAINAGE COMPLAINTS There are no documented downstream drainage complaints associated with the flowpath from this property. Temporary Erosion/Sedimentation and Drainage Report — Community Transit 7