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River Islands Development at Lathrop November 2020 Final Phase 2 Sewer Technical Rei3ort Rev 1 #6520 4. Sewer Collection System Design and Performance Requirements The City Standards establish the minimum design and performance requirements for any sewer facility or collection system in the City of Lathrop that will be installed within the public right of way or easement. The following requirements taken from the City Standards provided the evaluation criteria for the sewer collection model. 4.1 Pipe Capacity 4.1.1 Pipe Capacity- Depth/Diameter Ratio Table 4-1 lists the maximum depth to diameter ratio allowed for the design of sewers under PWWF conditions. In no case should any sewer line be less than 8-inches except in cul-de-sacs or alleys servicing six or fewer homes. In addition, in no case should a smaller diameter pipe be used in any downstream location of a larger pipe. Table 4-1: Pipe Depth/Diameter Ratio Pipe Size Depth/Diameter Ratio (inches) (d/D) < 15 0.50 18 > 0.75 Notes: 1. Pipe Depth/Diameter ratios taken from the City of Lathrop's Design and Construction Standards January 2019. To determine the flow capacity of each pipe, manning equation was used at the maximum allowable d/D ratio along with the following sewer design parameters provided by Odell Engineering. 1.49 Manning Equation Q = 0.013(A) \Rh2/3l\S1/2l Where, Q—Volumetric flow, cubic feet per second A—Cross sectional area of flow, square feet. Per City Standards, the cross sectional area is based on the maximum allowable depth Rh— Hydraulic radius of the pipe (Area/Wetted Perimeter), ft S—Slope of the Pipe, ft/ft. If improvement plans or as built drawings are not available(see Section 5.2 for further details), then the minimum required pipe slopes are used. 4.1.2 Pipe Capacity— Minimum and Maximum Velocities and Slope Per Section 5-4.1 in the City's Standards,the minimum velocity for any gravity sewer lines, should be 2 feet per second for pipes flowing half full. The maximum velocity should not exceed 10 feet per second under any operating condition. Sewer velocities are a function of the pipes slope and size and maintaining a minimum slope is necessary to ensure self-cleaning and oxidizing velocities are present. With the ongoing trends of lower sewer generation rates, all future lines, specifically trunk lines, shall be designed with sufficient scouring velocities at peak flows. This reduces the generation of hazardous, odorous, and corrosive sulfur compounds. Table 5-1 in the City's standards (see Table 4-3 below), lists the minimum pipe slopes for lines 6-18 inches in diameter. Pipe sizes larger than 18 inches were analyzed with a minimum slope ratio of 0.0012 ft/ft. In locations were substandard slope and velocities are present, corrosion resistant pipe and materials are required to be installed per the City Standards. It is also recommended that corrosion resistant coatings such as 2-part epoxies be providing in manholes downstream of substandard slope or velocities. 14