222 W CASINO RD 2020-05-06 - Laserfiche WebLink

IN engineering p c 4. The swale flows around the northeast corner of the existing fenced wetland and outflows northwest though a private 24-inch concrete pipe. 5. The flow is then conveyed,along with flows from the wetland and Merril and Ring Creek, northerly through another private 24-inch concrete pipe. 6. From there,the flow continues northwest via a public 24-inch concrete pipe to a structure located just south of the W Casino Road ROW. 7. From there,the flow is conveyed northerly via a public 36-inch concrete pipe that crosses underneath W Casino Road to a structure located just north of the Casino Road ROW. 8. From there,the flow is conveyed northerly via another public 36-inch concrete pipe which discharges into Merrill and Ring Creek at approximately 0.21 miles from the site. Refer to Figure 5—Downstream Drainage Map for layout of the downstream path from the site. During the field investigation,there were no signs of erosion or overtopping of the downstream drainage system. At this time,the existing downstream drainage course appears to have adequate capacity to convey the proposed flows from the project. Also,based on correspondence with the City of Everett,there are no known stormwater issues or complaints on record for the downstream stormwater system in the immediate vicinity of the site. However,the City has acknowledged that there are some known capacity and flooding issues within the Merrill and Ring Creek drainage basin. The project proposes limited increases in the overall stormwater flows from the project area. PERMANENT STORMWATER CONTROL PLAN As noted above,the project is required to address Minimum Requirements#1 through#5 for the new and replaced hard surfaces and the converted vegetation areas for the project area. Water quality treatment and flow control are not required for this project. Pre-developed Site Hydrology Refer to Figure 4—Existing Impervious Coverage for a layout of the existing surfacing of the field. In addition, see the Offsite Analysis section of this report for more on the existing drainage system. Proposed Site Hydrology As previously mentioned, a double classroom portable structure will be added in the southeast portion of the school property, along with perforated metal deck,ramps, and stairs, and new asphalt pathways approaching the portables to enable pedestrian access to the structures. The project areas are summarized in Table 1. LPD Engineering, PLLC Page 5 Horizon Elementary School — Classroom Portable Installation Storm Drainage Report,June 5, 2019