715 RIVERSIDE RD 2016-01-01 MF Import

, ; Pacific Topsoils Rhrerside site�torm Drainege Report GFK Consulting Inc. 4 DESIGN OVERVIEW AND STANDARDS The subject site contains two surface typas that have a great affect on rainfall as it falls onto the site. Approximately 4.0 acres is covered with an asphalt cap, the remaining 4.4 acres consists of an earthen cap originally designed to infiltrate rainfall and has been observed to do so. All runoff from the site drains into the Snohomish River which is listed as a receiving water body so no stream bank erosion control or detention is considered in this design. No material proceasing is being conducted on the earthen capped area and because it infiftrates adequately, should provide natural water quality treatment and will not be included in this design. The asphalt capped area is where the wood material processing is undertaken and open stockpiles are exposed to rainfall. This is the area subject to concentrated runoff and is the are� considered in this design. Runoff will be calculated using the SBUH methodology using Waterworks soflware and a water quality rainfall event of 1.12 ". This will be compared with runoff calculated using W1NHM3 with a correction factor applied. The two runoff values will be compared and the most conserva'ive will be used for design. The 1997 COE Stormwater Manual will seNs as the design guide for water quality treatment. Runoff from the paved area is cuRently collected within perimeter asphalt gutters along the south and east edges and is conveyed to the south east comer where it is then discharged directly into the Snohomish River. These gutters intercept 100% of the runoff generated from the asphalt yard area and have adequate capacity to convey 't. While it makes sense to intercept the untreated flow at the discharge, it is located within the buffer area associated with the Snohomish River and no new stortnwater facilities are allowed within this buffer. As a result, this design proposes the improvement of existing conveyance facilities to include water quality treatment. Most of the subject site has been contaminated by prior uses and the caps placed to contain these contaminates are not to be penetrated. Integrated into the downslope perimeter of the asphalt cap are two drainage swales that are outside of the underground cap and banier walls installed to isolate the contaminated material within. This design proposed modification of 100' of the lower sections of both ditches transforming them intc, i�nctioning bio-swales. 8/20/2010 5 / //, ` � C:\Project data\Project files\PTopsoils Rluerside site\Reports\Riverside4.doc l�/�j�