Blockage Unit
- 15 Aug 2022
- 5 Minutes to read
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Blockage Unit
- Updated on 15 Aug 2022
- 5 Minutes to read
- Print
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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 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 ti | 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
and the Bernoulli equation representing both a contraction and an expansion,
or in terms of the Flood Modeller solution variables, q and h,
where
| = | flow at label 1 (m3/s) |
| = | flow at label 2 (m3/s) |
| = | stage at label 1 (mAD) |
| = | stage at label 2 (mAD) |
| = | inlet loss coefficient |
| = | outlet loss coefficient |
| = | blockage proportion |
| = | velocity at label 3, the upstream section, at a water level h1 and discharge q1 (m/s) |
| = | velocity at label 4, the downstream section, at a water level h2 and discharge q1 (m/s) |
| = | velocity at label 5, the section to be obstructed, at a water level h1 and discharge q1 (m/s) |
| = | flow area at label 3, at a water level h1 and discharge q |
| = | flow area at label 4, at a water level h2 and discharge q |
| = | flow area at label 5, at a water level h1 and discharge q. |
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For reverse flow, the upstream and downstream nodes will be reversed in the loss equation, giving.
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.
Culvert entrance without obstruction, or sudden narrowing of channel
With p= 0, the user can either
- set the constriction section to be the downstream section, giving,
or - set the constriction section to be the upstream section, giving.
or
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.
- 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.
- 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
- set the constriction section to be the downstream section, giving,
or
or- set the constriction section to be the upstream section, giving.
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.
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.
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