1707 SCENIC DR SNO CO PUD 2025-07-28

BEVERLY PARK SUBSTATION EXPANSION—ADDENDUM SEPTEMBER 10,2010 SNOHOMISH COUNTY PUD PAGE 1 INTRODUCTION • This addendum to the original report titled Stormwater Site Plan, Beverly Park Substation Expansion by RW Beck dated 10/02/09 is to provide runoff calculations for the two level spreaders proposed for the expansion of the sub-station. There is a third level spreader near the southeast corner of the site which will handle the runoff from Scenic Drive and Grant Drive. WWHM3 was used for the calculations as this system provided for better evaluation of infiltration in systems like the sub-station. According to the geotechnical evaluation, the site is underlain by weathered and dense till. The sub-station pads will be formed by clean structural fill, overlain with a clear crushed surfacing base course (fines reduced), overlain by sub-station rock. Sieves for each of these materials are provided in Appendix A. The geotechnical engineer has provided design infiltration rates of 0.2 in/hr through the weathered material. Direct rainfall will flow through the sub-station rock and onto the modified CSBC. This will allow infiltration into the weathered portions of the underlying soils. In areas of structural fill, this infiltration will be more pronounced. The pads will be sloped at 2% towards drain rock filled interceptor drains that will collect any runoff in excess of that which infiltrates into the sub-soils. These trenches will also allow for further storage and infiltration of runoff. Any flow in excess of the infiltration capability of both systems will then be collected and directed to level spreaders to allow flow to disperse into the heavily vegetated slopes of the site. The geotechnical engineer has evaluated the slope stability as well as the stability of the areas downstream of the level spreaders. This is provided under separate cover. The site is divided into two basins, the upper pad and the lower pad each served by its own storm system. Figure 1: On-site Basin Map gives a graphic representation of the basins. The Scenic/Grant basin consists of the streets and upstream lots as indicated on Figure 2: Upstream Basin Map. UPPER PAD (115 Kv Yard) This comprises an area of 2.87 acres. It has been modeled in WWHM3 as impervious even though it will primarily be a gravel surface. This was done because infiltration through the pad itself has been modeled using a gravel trench facility. That facility will overflow into the trench facility with the discharge from the trench flowing to the level spreader. The system has the following characteristics: Impervious Area: 2.87 Pad infiltration area*: 42,000 sf Length of interceptor trench: 1,2001f Width of trench: 3 feet Depth below pipe: 2 feet Length of level spreader: 40 feet *This is the area of the pad that is underlain with weathered till and structural fill through which direct rainfall can infiltrate. It does not include areas cut into unweathered till. J:\SnoCoPUD\8everly Park 09-158\CE\Drainage\DRAINAGE ADDENDUM.doc DWH/srm