9606 19TH AVE SE McDONALDS 2018-01-02 MF Import

The existing conditions are to model the entire site area which is equal to 0.95 acres. The post -developed drainage area will remain the same with 0.18 acres being pervious (landscaped) and the remaining area 0.77 acres beir;; impervious (roofs and pavement). This project will incorporate the use of a constructed wetland which is designed in accordance with the City of Ea erett Standards to provide the required water quality treatment prior to release downstream. The post -developed conditions of the site model all pervious areas as being lawn or park -type vegetation. The CN number for this runoff will be modeled under Type "C" soils and given a CN=86. The impervious areas will be modeled using a CN value of 98. After performing the volume calculations for the site, a total volume of 3,770 cubic feet is required for detention storag,;. In order to configure the detention requirements and obtain a minimum 1-inch diameter orifice size, the amount of storage was reduced. The constructed wetland treatment facility for this project was constructed in accordance with the City of Everett design standards. The City of Everett design standards require providing a total Wet Pond Storage Volume (WPSV) equal to half the 6-month storm volume.. The constructed wetland has fore -bay in 3 feet deep and is incorporated into the storm vault dead storage. The wetland treatment cell which lies downstream of the detention vault is 30 percent of the total wet pond volume with an -verage depth of 1.33 feet. The length -to -width ratio of this facility is approximately 1:3. The constructed wetland system will be subject to higher flows, i.e., those flows exceeding the 25-year event in extreme cases. Note, however, all flows collected in the stormwater system for the proposed site will be routed westerly to the existing seasonal drainage course (head waters of Wood Creek). DOWNSTREAM ANALYSIS All runoff from the proposed site will be discharged westerly into the existing seasonal drainage course. This seasonal drainage course, which is not visually flowing during the summer months, is the actual head waters to Wood Creek. Once the drainage leaves the proposed site, it will be released westerly along the south line of the adjacent parcel into the constructed wetlands which will be lying in a storm drainage easement. The runoff from the constructed wetland will overflow westerly down a 12-inch storm pipe proposed for this project. This 12-inch storm line will run westerly down a 25 percent slope to the bottom of the ravine which is the headwaters of Wood Creek. The storm drainage will flow northerly through this existing ravine which is also along the east-,',, toe of the embankment for Interstate-5. All drainage proceeds northerly where it enters an existn�g . wi n drainage pipe which travels northerly under the northbound freeway off ramp to Interstate-5. From this point, the storm drainage proceeds northeasterly within the median grassy areas of the freeway interchange where it will travel underneath 19th Avenue S.E. From this point, all drainage within the Wood Creek Basin flows through a natural stream channel to the east. This concludes our quarter mile analysis downstream from our project site. STORM DRAINAGE CONVEYANCE The on -site storm drainage conveyance system will be designed per the City of Everett standards to convey a 25-year design storm. The collection system will also be checked to verify that a minimum 2 feet per second flow velocity will occur within the collection system during the design storm. 5449.002 IDTC/ca/krl