1707 SCENIC DR SNO CO PUD 2025-07-28

BEVERLY PARK SUBSTATION EXPANSION—ADDENDUM SEPTEMBER 10,2010 SNOHOMISH COUNTY PUD PAGE 2 WWHM3 output is located in Appendix B. It shows that 93.7% of the total runoff infiltrates through the pad. The remainder flows into the interceptor trenches where 88.1% of that runoff is infiltrated. This gives a total of 99.2% infiltration between the two systems. While that is significant, it primarily deals with smaller storm events which make up a large portion of storm volume. Larger storm events will overflow these systems during the peak of the storm. These flows will be discharged over the level spreader. Expected flow rates are as follows: Return Rate Existing (forest) Mitigated 2 yr 0.09 cfs 0.37 cfs 5 yr 0.12 cfs 0.64 cfs 10 yr 0.15 cfs 0.74 cfs 25 yr 0.20 cfs 0.81 cfs 50 yr 0.23 cfs 0.84 cfs 100 yr 0.27 cfs 0.86 cfs Notes: I: For existing basin flow rates, the basin has been assumed to be forested regardless of current ground cover. 2: With so many of the smaller storm events infiltrated, the calculation of the return period for the mitigated flow begins to be skewed as it is only based on the uncontrolled overflow without the smaller storm events. Attached in Appendix B is a list of yearly maximums for the 48 year control period. Using the broad crested weir formula, Q = CLHA 1.5 with C = 3.3, L=40 feet and Q =0.86 cfs for the 100 year storm, H = 0.03 ft or 0.4 inches. Using that depth and the width of the level spreader gives an expected velocity over the weir of 0.6 fps. This is below standard erosive velocity in vegetated surface conditions. The geotechnical engineer has reviewed these flow rates and the downstream slope, soil and vegetation. His findings are included under separate cover. LOWER PAD (230 Kv Yard) This comprises an area of 2.15 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: 2.15 Pad infiltration area*: 22,500 sf Length of interceptor trench: 750 If Width of trench: 3 feet Depth below pipe: 2 feet Length of level spreader: 40 feet J:\SnoCoPUD\Beverly Park 09.158\CE\Drainage\DRAINAGE ADDENDUM.doc DW H/srm