Energy Gradeline Procedure

  1. Determine the EGL
    i
     and HGL
    i
     downstream of the access hole. The EGL and HGL will most likely need to be followed all the way from the outfall. If the system is being connected to an existing storm drain, the EGL and HGL will be that of the existing storm drain.
  2. Verify flow conditions at the outflow pipe.
    1. If HGL
      i
       is greater or equal to the soffit of the outflow pipe, the pipe is in full flow.
    2. If HGL
      i
       is less than the soffit of the outflow pipe but greater than critical depth, the pipe is not in full flow but downstream conditions still control.
    3. If HGL
      i
       is less than the soffit of the outflow pipe but greater than critical depth and less than or equal to normal depth, the pipe is in subcritical partial flow. EGL
      i
       becomes the flowline elevation plus normal depth plus the velocity head.
    4. If HGL
      i
       is less than critical depth, the pipe is in supercritical partial flow conditions. Pipe losses in a supercritical pipe section are not carried upstream.
  3. Estimate E
    i
     (outflow pipe energy head) by subtracting Z
    i
    (pipe flowline elevation) from the EGL
    i
     using Equation 10-38. Calculate
    γ
     + P/
    γ
     using Equation 10-41. Compute DI using Equation 10-45.
  4. Calculate E
    ai
     as maximum of E
    aio
    , E
    ais
    , and E
    aiu
     as below:
    1. If (
      γ
       + P/
      γ
      )>D, then the pipe is in full flow and E
      aio
       = E
      i
       + H
      i
       (Equation 10-42). If (
      γ
       + P/
      γ
      ) < D, then the pipe is in partial flow and E
      aio
       = 0.
    2. E
      ais
       = D
      o
      (DI)
      2
       (Equation 10-44)
    3. E
      aiu
       = 1.6 D
      o
      (DI)
      0.67
       (Equation 10-46)
    If E
    ai
     < E
    i
    , the head loss through the access hole will be zero, and E
    ai
     = E
    i
    . Go to Step 10.
  5. Determine the benching coefficient (C
    B
    ) using Table 10-4. Department standard sheets do not show any benching practices other than depressed (a) or flat (b). The values are the same whether the bench is submerged or unsubmerged.
  6. Determine the energy loss coefficient for angle flow (C
    θ
    ) by determining θ
    W
     for every pipe into the access hole.
    1. Is E
      i
       < inflow pipe flowline? If so, then the flow is plunging and θ
      W
       for that pipe is 180 degrees.
    2. If the pipe angle is straight, then θ
      W
       for that pipe is 180 degrees.
    3. Otherwise, θ
      W
       is the angle of the inflow pipe relevant to the outflow pipe. Maximum angle is 180 degrees (straight).
    Use Equation 10-49 and Equation 10-50 to calculate θ
    W
     and C
    θ
    .
  7. Determine the plunging flow coefficient (C
    P
    ) for every pipe into the access hole using Equation 10-52. The relative plunge height (h
    k
    ) is calculated using Equation 10-51. Z
    k
     is the difference between the access hole flowline elevation and the inflow pipe flowline elevation. If Z
    k
     > 10D
    o
    , Z
    k
     should be set to 10D
    o
    .
  8. If the initial estimate of the access hole energy level is greater than the outflow pipe energy head (E
    ai
     > E
    i
    ), then E
    a
     = E
    i
    . If E
    ai
    ≤ E
    i
    , then H
    a
     = (E
    ai
     - E
    i
    )(C
    B
     + C
    θ
     + C
    P
    ). If H
    a
     < 0, set H
    a
     = 0.
  9. Calculate the revised access hole energy level (E
    a
    ) using Equation 10-47. If E
    a
     < E
    i
    , set E
    a
     = E
    i
    .
  10. Compute EGL
    a
     by adding E
    a
     to the outflow pipe flowline elevation. Assume HGL
    a
     at the access hole structure is equal to EGL
    a
    .
  11. Compare EGL
    a
     with the critical elevation (ground surface, top of grate, gutter elevation, or other limits). If EGL
    a
     exceeds the critical elevation, modifications must be made to the design.