1707 SCENIC DR SNO CO PUD 2025-07-28

BEVERLY PARK SUBSTATION EXPANSION—ADDENDUM SEPTEMBER I0,20I0 SNOHOMISH COUNTY PUD PAGE 4 Length of level spreader: 50 feet W W HM3 was used to calculate expected runoff from the basin with results as follows: (Note: only Developed results are shown as the improvements are existing.) Return Rate Mitigated 2 yr 0.62 cfs 5 yr 0.89 cfs 10 yr 1.08 cfs 25 yr 1.35 cfs 50 yr 1.57 cfs 100 yr 1.80 cfs Using the broad crested weir formula, Q = CLH^1.5 with C = 3.3, L= 50 feet and Q = 1.80 cfs for the 100 year storm, H =0.05 ft or 0.6 inches. Using that depth and the width of the level spreader gives an expected velocity over the weir of 0.7 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. CONCLUSION The construction of the yard lends itself to support infiltration. Much of the direct rainfall will not leave the yard as surface runoff. It is expected that a portion of the runoff infiltrated through the yard will eventually emerge as surface water near the toe of slope as it follows the contact line of the unweathered till. The geotechnical engineer has reviewed overall slope stability. Even if flow does emerge from the slope, its time of concentration will be greatly increased and it will not be concentrated in a single location. Of the runoff that will flow to the level spreaders, the dispersion of the flow through native vegetation will disperse concentrated flows, attenuate the discharge, and allow for infiltration into the upper surface of the soils. Ultimately, the wetland to the northeast will serve to further reduce flows through its natural storage ability. The relocation of the existing level spreader for the Scenic/Grant upstream basin will be to an area with established vegetation downslope, away from neighboring properties and with a flow path that is entirely on the PUD property. J:\SnoCoPUD\Beverly Park 09-158\CE\Drainage\DRAINAGE ADDENDUM.doc DWH/srm