9909 2ND AVE W 2018-08-01 - Laserfiche WebLink

In addition to filtration a sediment chamber located prior to filtration will provide 3.5' of dead storage to act as sedimentation control. The chambers have been sized to equal at least 30% of the total volume of the 6 month storm as specified in the 2001 DOE. The sedimentation control will occur immediately after the inlet to the vault. Frontage Improvements Frontage improvements will consist of widening 1s`Ave. SW and installing curb,gutter and sidewalk. A drainage system consisting of catchbasins and underground pipes will convey stormwater from the project side of ls`Ave. SW to Vault 2 where it will be detained, treated and released at pre-developed runoff rates of 50% of the 2-year and 100%of the 10-year and 100- year storm events. UPSTREAM ANALYSIS Based on a site visit and survey data there is offsite flow onto the site via a 12"culvert under 2nd Ave. SW near the northwest corner of the site. The culvert currently flows into a drainage ditch along 2nd Ave. and flows south to an existing 24"culvert. We propose to tight line the ditch between the two culverts via a 24"pipe within the 2nd Ave Right-of-Way. This 24"pipe was sized based on upstream flow potentials to through the existing north culvert. Exploration in the area of the north culvert suggested that stormwater does not overtop the road. Therefore maximum flow caused by full pipe flow and the additional pressure due to the head of water over the elevation of the crown of the culvert was compared to full pipe flow of a proposed 24"pipe in the location of the existing drainage ditch. We concluded that the 24"pipe was more than capable of handling the level of flow that might cross the road via the 12"culvert. Calculations were done with Flow Master 2005 (See attached Calculations in Appendix). DOWNSTREAM ANALYSIS The downstream relative to this project is to the east of the project site and consists of runoff that is collected in a wetland that flows to a tributary stream of North Creek along the sites east boundary. The tributary stream flows south east until it reaches North Creek. Stormwater on site will be detained and released at pre-developed rates and there will be no changed to the down stream drainage courses or patterns,therefore no impact is anticipated to the downstream drainage.