The hydraulic flow in storm and sanitary sewers is analyzed by using Manning's equation. Manning's equation may be easily solved with the Full Flow Curves, the Relative Velocity and Flow curves, and the minimum velocity table found at the end of this chapter. The design of concrete storm and sanitary sewers involves three steps:

         Determine required flow
         Select pipe size
         Calculate flow velocity

    DETERMINE REQUIRED FLOW: Flow in storm sewers is a result of storm water runoff. Runoff in small watersheds is usually determined by the Rational Method. The method directly relates runoff to the drainage area, the rainfall intensity, and the coefficient of runoff. Rainfall intensity is a function of storm frequency and duration. The coefficient of runoff represents the ratio of runoff to rainfall and considers ground cover, soil type and topography. The required flow in a storm sewer is the maximum flow resulting from the collection of runoff at any point in the system. The Rational Method is described in more detail in the American Concrete Pipe Association (ACPA) Concrete Pipe Design Manual. Local conditions can be obtained from the U.S. Weather Bureau, state transportation department's manuals, and other sources.

Flow in sanitary sewers consists of residential, commercial, and industrial sewage. The average, peak, and minimum flow must be considered in design. The average flow is estimated by considering future population growth and land use, water consumption, historic records, and code requirements. Infiltration and extraneous inflows also contribute to flow volume. Peak and minimum flows are determined by applying factors to the average flow. These factors are generally based on local experience. Peak flow is used to select pipe size and minimum velocity. The ACPA Concrete Pipe Design Manual, textbooks, and government manuals describe in detail the method for determining flow.

    SELECT PIPE SIZE: Storm and sanitary pipe sizes may be selected from the Full Flow Curves found on Figures 1 through 6 when the flow and slope are known. Concrete storm sewers are designed with a roughness coefficient of n=0.012. A roughness coefficient of n=0.013 should be used for concrete sanitary sewers.

    CALCULATE FLOW VELOCITY: Full flow velocity can be found on the Full Flow Curves, Figures 1 through 6. Partial flow velocity, cross-sectional area, and depth are proportional to full flow values and may be calculated using ratios found on the Relative Velocity and Flow curves, Figures 7 through 9.

    Minimum velocities for self-cleaning are usually specified at 3 feet per second in storm sewers and 2 feet per second in sanitary sewers. The pipe slopes required to maintain these velocities in circular concrete pipe can be found in Figure 14.

EXAMPLE: Storm Sewer Flow. Find the full flow capacity and velocity in a 24 inch equivalent circular size concrete arch pipe storm sewer. The sewer is installed on a 0.40 percent slope. The n value is 0.012 for concrete storm sewers.

   The full flow capacity and velocity can be found using the Full Flow Curves for Arch Pipe with n=0.012, Figure 4. From 0.40 percent slope on the horizontal axis, project a line vertically to the 24 inch arch pipe size curve. The full flow capacity is found by extending a line horizontally to the vertical axis. The velocity is found by interpolating between the 4 and 5 feet per second velocity curves. The full flow capacity is 13 cubic feet per second at a velocity of 4.6 feet per second.

EXAMPLE: Storm Sewer Size. Select the size of a circular concrete pipe required for a storm sewer installed on a 0.1 percent slope with a required flow of 40 cubic feet per second. The n value is 0.012 for storm sewers. The minimum velocity is 3 feet per second.

    The pipe size may be found using the Full Flow Curves for Circular Pipe with n=0.012, Figure 2. Locate the intersection of a horizontal line through Q=40 cubic feet per second and a vertical line through a slope of 0.10 percent. Select the next larger pipe size. The flow velocity is found by interpolating between the 3 and 4 feet per second velocity curves.

    A 48 inch diameter circular concrete pipe installed on a 0.1 percent slope will flow full at a rate of 50 cubic feet per second at a velocity of nearly four feet per second. Both the design flow and velocity exceed minimum requirements.

EXAMPLE: Sanitary Sewer Size. Select the size of a circular concrete pipe required for a sanitary sewer installed on a 0.5 percent slope. The average flow rate is 10 cubic feet per second. The peak flow is estimated at 2.5 times the average flow and the minimum flow is approximately one-third the average flow. The n value is 0.013 for sanitary sewers. The minimum required velocity is 2 feet per second.

            The design peak flow is:

                10 x 2.5 = 25 cubic feet per second

   The pipe size may be found using the Full Flow Curves for Circular Pipe with n=0.013, Figure 3. Locate the intersection of a horizontal line through Q=25 cubic feet per second and a vertical line through a slope of 0.50 percent. The minimum pipe size is 30 inches in diameter.

    To determine the minimum flow velocity, the Relative Velocity and Flow curves, Figure 7, must be used. To use this chart, the proportion of minimum flow to full flow must be calculated. The minimum flow is:

            10 x 0.33 = 3.3 cubic feet per second

    From Figure 3, the full flow rate is 29 cubic feet per second and the velocity is 5.9 feet per second for a 30 inch diameter pipe on a 0.50 percent slope. The proportion of minimum flow to full flow is:

           

   On Figure 7, for circular pipe, find the proportional value 0.11 on the horizontal axis. Project a vertical line from this point to the discharge curve. Extend a horizontal line to the velocity curve. From this point, project a vertical line to the proportional value on the horizontal line. The value is 0.64 and is the proportion of minimum flow velocity to full flow velocity. The minimum flow velocity is:

        5.9 x 0.64 = 3.8 feet per second

    This velocity exceeds the minimum required velocity of 2 feet per second.

    A 30 inch diameter concrete pipe meets the requirements.