Methods/Prodecures

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    Methods and Procedures:
    Precipitation data was obtained from a study of the watershed and was used along with geographic information to determine the runoff from the area. The runoff was calculated using the rational equation: q = 1.008CiA
    q = Runoff (ft3/s)
    C = Runoff Coefficient (Based on Soil Type and Slope)
    i = Rainfall Intensity in the Area in (in/hr)
    A =Drainage Area in Acres.
    1.008 = Conversion Factor (Usually Taken as 1)

     

    Time of Concentration

    Sheet flowof less than 300 feet, use Manning’s kinematic

    solution to compute Tt:

    Tt = (0.0007(nL)^(0.8))/(P2^(0.5)*s^(0.4))

    Where:

    Tt = travel time (hr),

    n = Manning’s roughness coefficient, Table 11.2 (Appendix),

    L = flow length (ft),

    P2 = Two-year, 24-hour rainfall (in) = 2.7 inch for our area, and

    s = slope of hydraulic grade line (land slope, ft/ft).

     

    Shallow concentrated Flow  of distances over 300ft in length Tt:

    Tt = L/(3600V)

    Where:

    Tt = travel time (hr),

    L = flow length (ft),

    V = average velocity (ft./sec.), and

    3600 = conversion factor from seconds to hours.

     

    Manning’s equation is:

    V =(1.49(r^(2/3))s^(1/2))/n

    Where:

    V = average velocity (ft/s),

    r = hydraulic radius (ft) and is equal to a/pw,

    a = cross sectional flow area (ft2),

    pw = wetted perimeter (ft),

    s = slope of the hydraulic grade line (channel  slope, ft/ft), and

    n = Manning’s roughness coefficient for open channel flow

     

    The erosion within the Turtle watershed area is a major concern with the amount of farming that takes place within the watershed. The soil erosion for the area was assessed using soil quality and “K” factors that vary with different types of soil. Erosion factors were provided in tables along with other information on the land such as vegetation and soil types in the Soil Survey of Spink County, South Dakota study. These “K” factors may be used to determine the amount of erosion that will occur by using the Universal Soil Loss Equation:
     A = RKLSCP1
    Where: A= Average Annual Soil Loss in tons/ac
                   R = Rainfall and Erosivity Index
                   LS = Slope Steepness and Length Factor
                   C = Cover Management Factor
                   P = Conservation Practice Factor.

    The actual amount of erosion was not found in the study.

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