1410 SEINER DR FISHERMAN JACKS 2025-08-21

WATERFRONT PLACE CENTRAL PORT OF EVERETT - PSA 1-2015-005 HYDROLOGIC AND HYDRAULIC REPORT PND No. 154016 - 005.01 8.6. CONVEYANCE ANALYSIS FOR PIPE SIZING The WPC drainage system shall be designed for a 25-year recurrence interval peak flow from the contributing area for conveyance systems per City of Everett Design and Construction Standards and Specifications. For this site, the Rational Method, Q=CiA, has been utilized to determine design flow rates and volumes for the conveyance system pipe diameter sizing. The system has been modeled assuming the conveyance pipes flow directly to the outfall excluding the head requirement of the design treatment system and bypass structure. The system analysis has been calibrated setting the minimum allowable freeboard to 0.5 feet in the most hydraulic sensitive inlets (CB 112 for FHA and CB 102 FHB) by adjusting the tailwater elevations at the outfall. The intent of this analysis is to provide appropriate pipe sizes while not being overly conservative. The following coefficients will be used from the City of Everett Stormwater Management Manual: Q=CiA, where: C = Runoff Coefficient (unitless) = 0.25 (lawns), 0.9 (pavement and roofs) i = m/Tn, where: m = Dimensionless Coefficient =7.83 (25 year), 10.07 (100 year) n = Dimensionless Coefficient = 0.582 (25 year), 0.586 (100 year) T = Time of Concentration=<60 Minutes Detailed analysis and results are for the conveyance design is provided in Appendix C of this report. 8.7. HYDRAULIC BACKWATER ANALYSIS The WPC drainage system have evaluated for backwater conditions based upon predictive tailwater elevations within the Central Marina assuming sealevel rise conditions over the next 100 years. The design backwater analysis has been performed in two separate steps: • Step 1 — Verify Backwater Conditions from system outfall to the downstream side of the flow control weir. This analysis was performed to verify the control weir elevation with regards to tailwater conditions. • Step 2 — Verify tightlined backwater conditions of peak flowrates upstream of the flow control weir. Specific performance criteria for these analyses include: • Peak Design flows shall be applied from the MGSFlood model for the 25 year and 100 year return periods. • Tailwater conditions for this analysis have been set within Central Marina to an elevation of MHHW (11.11ft) plus 2 feet of predictive sea level rise per the City of Everett Comprehensive Plan Climate Change and Sustainability Element Background Report. • Structures for proposed pipe systems shall have a minimum of 0.5 freeboard between headwater surface and the top of the structures for the flow from a 25-year storm. • Structures may overtop for flow resulting from a 100 year storm. A"V