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 Dickinson
 Code of Ordinances
 Appendix B. DRAINAGE CRITERIA

  V. - HYDRAULICS  

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  • Hydraulics is the study of fluid flow behavior. Policy makers and engineers must study and understand hydraulics because they are responsible for designing and constructing conveyance and storage facilities capable of managing storm water runoff in a safe and effective manner while reducing the potential for flooding. The following sections discuss specific methods and parameters to be used in analyzing proposed developments in the City's service area.

    Open Channel Flow

    The vast majority of conveyance capacity within the City's service area is located in the network of open channels that the City builds and maintains. The Chezy-Manning equation will be used to estimate a ditch's conveyance capacity. This equation is in the following form:

    Q = 1.486/n * A * R 2/3 * S 1/2

    Where:

    n = Manning's Roughness Coefficient (unitless)

    A = Flow Cross-sectional area (sf)

    R = Hydraulic Radius (ft)

    S = Slope of the Hydraulic Grade Line (ft/ft)

    Typical values for Manning's 'n' are included in ATTACHMENT B. The flow area (A) is estimated from the ditch cross-section, and is the area that will be conveying water (also called the wet area). The hydraulic radius is calculated as the wetted area divided by the wetted perimeter. The wetted perimeter is defined as the length of water/surface interface around the perimeter of the wetted area (does not include the water/air interface length). For open channels, the slope of the hydraulic grade line is estimated to be the same as the ditch slope.

    Closed Conduit (Pipe) Flow

    The Chezy-Manning equation presented earlier is also applicable for estimating flow capacity for closed conduits (i.e., pipes). There are some important distinctions to remember, including:

    • Manning's 'n' for pipe materials are significantly different (i.e., smaller) than those for bare earth or vegetative surfaces. See ATTACHMENT B for appropriate 'n' values.

    • The assumption of hydraulic grade line slope being approximately equal to the pipe slope is only valid under free flow conditions. Once the pipe is full and experiences surcharge conditions, the hydraulic grade line slope will increase as flow increases.