City of Morgan Hill Sewer System Master Plan 2017

2017 City of Morgan Hill

3.0 CHAPTER 3 - SYSTEM PERFORMANCE AND DESIGN CRITERIA This chapter presents the City’s performance and design criteria, which were used in this master plan for evaluating the adequacy of capacity for the existing sanitary sewer system and for sizing improvements required to mitigate deficiencies and to accommodate future growth. The design criteria include: capacity requirements for the sanitary sewer facilities, flow calculation methodologies for future users, flow peaking factors, and accounting for infiltration and inflows. 3.1 HYDRAULIC CAPACITY CRITERIA In addition to applying the City design standards for evaluating hydraulic capacities; this master plan included dynamic hydraulic modeling. The dynamic modeling was a critical and essential element in identifying surcharge conditions resulting from downstream bottlenecks in the gravity sewers. 3.1.1 Gravity Sewers Gravity sewer capacities depend on several factors including: material and roughness of the pipe, the limiting velocity and slope, and the maximum allowable depth of flow. The hydraulic modeling software used for evaluating the capacity adequacy of the City’s sewer system, InfoSWMM by Innovyze Inc., utilizes the fully dynamic St. Venant’s equation which has a more accurate engine for simulating backwater and surcharge, in addition to manifolded force mains. The software also incorporates the use of the Manning Equation in other calculations including upstream pipe flow conditions. Manning’s Equation for Pipe Capacity The Continuity equation and the Manning equation for steady-state flow are used for calculating pipe capacities in open channel flow. Open channel flow can consist of either open conduits or, in the case of gravity sewers, partially full closed conduits. Gravity full flow occurs when the conduit is flowing full but has not reached a pressure condition. x Continuity Equation: Q = V A Where: Q = peak flow, in cubic feet per second (cfs) V = velocity, in feet per second (fps) A = cross-sectional area of pipe, in square feet (sq. ft.)

V = (1.486 R 2/3 S 1/2 )/n Where: V = velocity, fps

x Manning Equation:

n = Manning’s roughness coefficient R = hydraulic radius (area divided by wetted perimeter), ft S = slope of pipe, in feet per foot

August 2017

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City of Morgan Hill Sewer System Master Plan