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Technical Information Report July 2015 <br /> Everett Public Storage <br /> 4.3 Conveyance System Analysis & Design <br /> This section verifies the capacity of the stormwater conveyance system conveying site runoff to the <br /> proposed stormwater vault shown in Figure 3. Typically, design and analysis of the conveyance <br /> system is per the City of Everett's criteria for conveyance capacity (using Uniform Flow and <br /> Backwater Analysis Methods and the Modified Rational Runoff method). As described below several <br /> conservative assumptions have been made for this analysis. The results of the analysis indicate that <br /> the system meets the required design criteria and has adequate capacity are shown on the enclosed <br /> spreadsheets. <br /> The capacity analysis of the storm conveyance system is based on the following assumptions. <br /> Assumptions: <br /> 1. The stormwater conveyance pipes are sized for events up to the 100-year, 24-hour storm using <br /> the Rational method per the City of Everett SW MM. The downstream system has been assessed <br /> using the 25 and 100-year storm events. <br /> 2. The soils on the site are assumed as Alderwood series soils with a type C classification. For the <br /> Rational method, impervious areas have a C=0.9, and C=0.25 is used for grassed areas; <br /> however for assessment of the conveyance system the site has been assumed to be 100% <br /> impervious. <br /> 3. For the analysis a minimum Time of Concentration Tc, used with the Rational Formula is 5 <br /> minutes. <br /> 4. Mannings roughness coefficients used was: n = 0.014 for N-12 or concrete pipe. A Mannings <br /> roughness coefficient of n=0.012 was used for the backwater analysis. <br /> 5. Using The City of Everett (COESWMM) Table 5-4 the following precipitation and values were <br /> used: <br /> 25-Year= 2.6 in, and 100-Year = 3.25 in <br /> 6. Per Table 5.2 (COESWMM) the following rainfall coefficients were used: <br /> 25-Year: M = 7.83, n = 0.582 <br /> 100-Year: M = 10.07, n = 0.586 <br /> 7. Drainage areas and the storm sewer layout are shown on the engineering plans and on the <br /> Developed Conditions Map, Figure 3. <br /> 8. Tail water elevations: The tail water condition used for the conveyance analysis is set at the <br /> water surface elevation in the proposed vault. <br /> 25-Yr= 426.92 ft <br /> 100-Yr= 427.57 ft <br /> 9. The vault emergency overflow analysis has been completed using the 100-Year rational method <br /> peak site runoff rate of 6.51 f . <br /> p cs <br /> 14114 Everett Storage TIR Revised 0715.doc 25 is <br />