10330 4TH AVE W 2016-01-01 MF Import

[NTRODUCTION AND EXISTIN(> ��ONDITIOIVS: This project is the proposed expansion of the South Pointe Assisted Living Residence onto the adjacent lot to the west. The expansion will include a new building, parking, landscaping, and storm water controls. There are three existing single family� residences (SFR) with access Prom 5'h Place via gravel driveways. Vegetation around the SFR's is landscaped, surrounded by brush and trees. The property gently slopes ai approximately tw�� percent from north to south. Soils on the site are an Alderwood Urban Land'Complex (Hydrologic Group 'C'). These soils have the characteristics of being moderately deep with moderate permeability above the hardpan and very slow through it. Runoff is slow and the hazard of water erosion is slight. DEVELOPED CONDITIONS: As the site is surrounded by private tracts, drainage issues have prevented this particular parcel from being developed in the past. To circumvent these issues, storm discharge from the new development will be connected to the existins detention system that was constructed for the South Pointe Assisted Lirnng Residence (see Summary of Stortn Drainage Analysis for the South Pointe Assisted Living Facility dated December l2, 1995). This was accomplished by modeling the property to be developed and the existing South Pointe Assisted Living Residence property as one basin. The proposed design takes advantage of the existing underground detention pipe, and provides additional detention and water quality in a wetpond in the southwest comer of the new development. The proposed development will include removing the existing SFR's to construct the new building and parking area. Access to the site will be from 5�' Place to the parking area with an emergency vehicle turn around. Runoff from the asphalted area will sheet flow to the south, be intercepted by catch basins, and conveyed to a wetpond in the southwest corner of the property via 12" storm pipe. The wetpond provides water quality treatment for the six month developed stortn dead storage, and detention for the two, 10, and 100 year storms. The detention, or "live" storage, aspect ot' the pond will by hydraulically connected to the existing 48" underground detention pipe via approximately 300 ft of 12" storm pipe. Initially, as storm runoff begins to fill the detention system, only the existing 48" diameter pipe (outlet elevation of 557.12 ft) will be used. However, as stortn runoti' continues, once a stage of 559 ft is reached, the proposed detention pond will provide the additional storage required to account for the enlarged drainage basin area and new development. Using the As-Builts for the South Pointe Assisted Living facility, a stage vs. storage table was created that combines the storage from both the existing underground detention pipe, and the proposed detention pond. Included in the calculations section is a breakdown of the storage provided by each component of the detention facility with increasing stage. The storage capacity of the storm pipe and catch basins that connect the proposed pond to the existing underground pipe were not included in the stage vs storage table. 1: RI:1'ORTS—�y9278 Y')278U.Jnc U/� �