1707 SCENIC DR SNO CO PUD 2025-07-28

BEVERLY PARK SUBSTATION EXPANSION—ADDENDUM SEPTEMBER 10,2010 SNOHOMISH COUNTY PUD PAGE 3 *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. WWHM3 output is located in Appendix B. It shows that 86.2% of the total runoff infiltrates through the pad. The remainder flows into the interceptor trenches where 78.3% of that runoff is infiltrated. This gives a total of 97.0% 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 this system 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 0.06 cfs 0.34 cfs 5 yr 0.09 cfs 0.45 cfs 10 yr 0.11 cfs 0.47 cfs 25 yr 0.15 cfs 0.47 cfs 50 yr 0.17 cfs 0.47 cfs 100 yr 0.20 cfs 0.47 cfs Notes: 1: 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. This can especially be seen here as the return rates of the larger storms reach a plateau of 0.47 cfs. 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 = 0.47 cfs for the 100 year storm, H = 0.02 ft or 0.3 inches. Using that depth and the width of the level spreader gives an expected velocity over the weir of 0.5 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. SCENIC/GRANT UPSTREAM FLOW The existing level spreader serving the Scenic Drive/Grant Drive basin is being relocated. The upstream area tributary to the level spreader is 3.75 acres. This is based on City of Everett storm utility mapping and Snohomish County Aerial and Lidar mapping. Figure 2: Upstream Basin shows the extent of the basin and the following indicates the basins characteristics: Pervious Area: 2.15 Impervious Area: 1.6 J:\SnoCoPUD\Beverly Park 09-158\CE\Drainage\DRAINAGE ADDENDUM.doc DWH/srm