Blockage Unit
    • 15 Aug 2022
    • 5 Minutes to read

    Blockage Unit


    Article summary

    The blockage unit is intended to be simple to implement and widely applicable. It is based around a single time-varying parameter p, which represents the proportion of flow area obstructed.

    Data

    The DAT file format (with the reference to the fields in Data Entry Form) is as follows:

    Field in Data Entry Form

    Description

    Name in Datafile

    Upstream Label

    upstream node label

    label 1

    Downstream Label

    downstream node label 

    label 2

    Upstream Ref.

    upstream reference section, which will be used to provide velocities upstream of the obstruction (must be a river, conduit or bridge, defaults to label 1) 

    label 3

    Downstream Ref.

    downstream reference section, which will be used to provide velocities downstream of the obstruction (must be a river, conduit or bridge, defaults to label 2) 

    label 4

    Constriction

    constriction section, which will be used as the basis for the obstruction, i.e. should be the unobstructed section (must be a river, conduit or bridge, defaults to label 4) 

    label 5

    Inlet Loss Coefficient

    inlet loss coefficient (contraction), defaults to 1.5, must have 0.0 ≤ K

    K1

    Outlet Loss Coefficient

    outlet loss coefficient (expansion), defaults to 1.0, must have 0.0 ≤ K 

    K2

    Number of Lines in Data Table

    number of lines in following data table 

    n1

    Time Datum Adjustment

    optional time datum adjustment - tlag will be subtracted from each ti in the table below (adding rather than subtracting seems more intuitive, especially given the name "lag", but subtracting is consistent with the behaviour of the QTBDY) 

    tlag

    Units of Time

    optional keyword or value for units of time in the following data set.  Can be any numerical multiplier or one of the following: seconds (the default), minutes, hours, days, weeks, fortnight, lunar (month), months (of 30 days), quarter, years or decades. 

    tm

    Data Extending Method

    policy for extending data if the run finishes after the end of the time series data.

    Options are:     REPEAT - if the data are to be repeated from the beginning
    EXTEND - if the blockage proportion is to be fixed at the last given value
    NOEXTEND - no extension.

    If NOEXTEND is used or the field is left blank, then the program will stop with an error message if there are insufficient time series data. 

    repeat

    Time

    time (in units of tm

    ti

    Blockage

    blockage proportion at time t

    pi

    Line 1

     

    -

    keyword "BLOCKAGE #REVISION#1" [comment]

    Line 2

     

    -

    label 1, label 2, [label 3], [label 4], [label 5]

    Line 3

     

    -

    inlet loss coefficient K1, outlet loss coefficient K2

    Line 4

     

    -

    n1 , [tlag], [tm], [repeat]

    Line 5 to

    line 4+n1

    -

    ti,  pi

    where

    label1

    =

    upstream node label

    label2

    =

    downstream node label

    label3

    =

    upstream reference section, which will be used to provide velocities upstream of the obstruction (must be a river, conduit or bridge, defaults to label 1)

    label4

    =

    downstream reference section, which will be used to provide velocities downstream of the obstruction (must be a river, conduit or bridge, defaults to label 2)

    label5

    =

    constriction section, which will be used as the basis for the obstruction, i.e. should be the unobstructed section (must be a river, conduit or bridge, defaults to label 4)

    K1

    =

    inlet loss coefficient (contraction), defaults to 1.5, must have 0.0 ≤ K

    K2

    =

    outlet loss coefficient (expansion), defaults to 1.0, must have 0.0 ≤ K

    n1

    =

    number of lines in following data table

    tlag

    =

    optional time datum adjustment - tlag will be subtracted from each ti in the table below (adding rather than subtracting seems more intuitive, especially given the name "lag", but subtracting is consistent with the behaviour of the QTBDY)

    tm

    =

    optional keyword or value for units of time in the following data set.  Can be any numerical multiplier or one of the following: seconds (the default), minutes, hours, days, weeks, fortnight, lunar (month), months (of 30 days), quarter, years or decades.

    repeat

    =

    policy for extending data if the run finishes after the end of the time series data.

    Options are:

    REPEAT - if the data are to be repeated from the beginning

    EXTEND - if the flow is to be fixed at the last given value

    NOEXTEND - no extension

    If NOEXTEND is used or the field is left blank, then the program will stop with an error message if there are insufficient time series data.

    ti

    =

    time (in units of tm)

    pi

    =

    blockage proportion at time ti

    Theory and Guidance

    The blockage unit is intended to be simple to implement and use and to be widely applicable. It is based around a single time-varying parameter p , which will represent the proportion of the flow area obstructed. In effect, this will mean that the blockage is assumed to occupy the same proportion of the width of the section at all water levels, i.e. it is a vertical blockage. Users cannot specify an obstruction occupying only the lower part of the section, for example. The losses will be based on the Bernoulli equation, and will be similar to a combination of an inlet and an outlet loss.

    Methodology

    The blockage loss will be calculated using the continuity equation

    image002

    and the Bernoulli equation representing both a contraction and an expansion,

    image004

    or in terms of the Flood Modeller solution variables, q and h,

    image006

    where

    image008

    =

    flow at label 1 (m3/s)

    image010

    =

    flow at label 2 (m3/s)

    image012

    =

    stage at label 1 (mAD)

    image014

    =

    stage at label 2 (mAD)

    image016

    =

    inlet loss coefficient

    image018 (1)

    =

    outlet loss coefficient

    image020

    =

    blockage proportion

    image022

    =

    velocity at label 3, the upstream section, at a water level h1 and discharge q1 (m/s)

    image024

    =

    velocity at label 4, the downstream section, at a water level h2 and discharge q1 (m/s)

    image026

    =

    velocity at label 5, the section to be obstructed, at a water level h1 and discharge q1 (m/s)

    image028

    =

    flow area at label 3, at a water level h1 and discharge q

    image030

    =

    flow area at label 4, at a water level h2 and discharge q

    image032

    =

    flow area at label 5, at a water level h1 and discharge q.

    For reverse flow, the upstream and downstream nodes will be reversed in the loss equation, giving.

    image034

    Example uses

    Obstruction in a river section or culvert

    Here, the upstream and downstream sections are the same. The constriction section should be the downstream section, K1 should be used to specify the inlet loss and K2 the outlet loss. The equations become.

    image036

    image038

    Culvert entrance without obstruction, or sudden narrowing of channel

    With p= 0, the user can either

    1. set the constriction section to be the downstream section, giving,
      image040image042or
    2. set the constriction section to be the upstream section, giving.
      image044image046

    This example illustrates the importance of choosing the correct constriction section even when there is no obstruction. The choice of constriction section determines which loss coefficient is used.

    1. If the constriction section is the downstream section, then the entry loss coefficient will be used for forwards flow and the exit loss coefficient for reverse flow.
    2. If the constriction section is the upstream section, then the exit loss coefficient will be used for forwards flow and the entry loss coefficient for reverse flow.

    The intuitive and best choice in this case is (a).

    Culvert exit without obstruction, or sudden widening of channel

    With p= 0, the user can either

    1. set the constriction section to be the downstream section, giving,
      image040image048or
    1. set the constriction section to be the upstream section, giving.
      image044image050

    The intuitive and best choice in this case is (b), which will mean the exit loss coefficient is used. The default, however, is (a), so the user will need to set the constriction section explicitly to be the upstream section.

    Obstruction at the entrance to a culvert

    The upstream and downstream sections are different. The user should set the constriction section to be the downstream section, and we have.
    image040image052

    Obstruction at the exit from a culvert

    The upstream and downstream sections are different. The user should set the constriction section to be the upstream section, and we have.
    image044image055


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