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I am modelling a natural channel with a gated spillway. I am using the rectangular orifice option to install the gate .

In SWMM, orifice is added as a link between two nodes and according to the manual "located either at the bottom or along the side of the upstream node."

My question is does SWMM use the geometry ( depth + width) of the orifice to consider it as an usual  conduit for the flow from upstream node to downstram node as it is connected between two junctions  just like the conduis?

or Its just a gate acting at the upstream node though added as a link?

Thanking you in anticipation.


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  • Thank you Mr. Dickinson.

  • SWMM 5 treats the orifice like a link between the upstream and downstream nodes.  The orifice separates the nodes but is not part of the nodes.

    Note: Orifice and Weir Flow Computations

    The orifice flow calculation proceeds as follows:

    1. Initially and whenever the setting (i.e., the fraction opened) changes, flow coefficients for both orifice and weir behavior are computed as follows:

    a. For side orifices:

    Define Hcrit = h/2 where h is the opening height.

    b. For bottom orifices:

    i. For a circular orifice, compute area over length (i.e., circumference) as AL = h /4.

    ii. For a rectangular orifice compute AL = h*w/(2*(h+w)) where w is the opening width.

    iii. Compute Hcrit = Cd*AL/0.414 where Cd is the orifice discharge coefficient.

    At step 1b, the critical head for the bottom orifice, where orifice flow turns into weir flow, is found by equating the result of the orifice equation to that of the weir equation

    Cd*Area*sqrt(2g)*sqrt(Hcrit) = Cw*Length*sqrt(Hcrit)*Hcrit or

    Hcrit = (Cd * Area) / (Cw/sqrt(2g) * Length) The value of Cw/sqrt(2g) for a sharp crested weir is 0.414.

    c. Compute the flow coefficients (where A is the area of the opening):

    Corif = A*sqrt(2g)*Cd

    Cweir = A*sqrt(2g)*Cd*sqrt(Hcrit)

    2. During flow routing, compute the degree of inlet submergence (f) and head (H) at the current time step:

    a. Define:

    H1 = upstream head (from node with higher head),

    H2 = downstream head (from node with lower head) ,

    Hcrest = elevation of bottom of opening,

    Hcrown = elevation of top of opening,

    Hmidpt = elevation of midpoint of opening

    b. For side orifices:

    f = min{1.0, (H1 - Hcrest) / (Hcrown - Hcrest)}

    if f < 1.0 then H = H1 - Hcrest,

    else if H2 < Hmidpt then H = H1 - Hmidpt

    else H = H1 - H2

    c. For bottom orifices:

    if H2 > Hcrest then H = H1 - H2

    else H = H1 - Hcrest

    f = min{1.0, H/Hcrit}

    3. Compute the flow through the orifice (Q):

    if f < 1.0 then Q = Cweir*f^1.5

    else Q = Corif*sqrt(H)

    4: Villemonte correction

    If f < 1.0 and H2 > Hcrest then:

    r = (H2 - Hcrest) / (H1 - Hcrest)

    Q = Q * (1 - r^1.5)^0.385

    Weir Flow Computations

    1. Weir head calculations

    h1 = Upstream Node Depth + Upstream Invert Elevation

    h2 = Downstream Node Depth + Downstream Invert Elevation

    If h2 is greater than h1 then the flow is reversed and h2 = h1 and h1 = h2

    Weir Crest = Upstream Node Invert Elevation + Weir Offset Depth

    Head = h1 – Weir Crest

    2. Center Weir flow for Transverse Weirs

    Q = Cw * Weir Length * Head^3/2

    3. Center Weir flow for Side Flow Weirs

    Weir behaves as a transverse weir under reverse flow

    Q = Cw * Weir Length * Head^3/2

    And under normal flow

    Q = Cw * Weir Length * Head^5/3

    4. Center Weir flow for V Notch Weirs

    Q = Cw * Weir Slope * Head^5/2

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