 21 Sep 2022
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FlowHead Control
 Updated on 21 Sep 2022
 2 Minutes to read

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The FlowHead Control imposes a specified flow discharge relationship at a channel control point such as a flume or weir.
Data
Field in Data Entry Form  Description  Name in Datafile 

Upstream Node  Upstream node label  Label1 
Downstream Node  Downstream node label  Label2 
Elevation of Weir  Elevation of weir crest (m above datum)  Z_{c} 
Modular Limit  modular limit (eg 0.9, range 0.999 to 0)  m 
Interpolation Method  SPLINE if a cubic spline is to be fitted to the data, or LINEAR to use linear interpolation. (If the field is blank then linear interpolation is used.)  smooth 
n/a  Number of ensuing flow and water level data pairs  n_{1} 
Discharge  Flow corresponding to water level y_{i} (in m^{3}/s)  Q_{i} 
Stage  Water level (in metres above datum)  y_{i} 
Theory and Guidance
The FlowHead Control imposes a specified flow discharge relationship at a channel control point such as a flume or weir.
It is not always possible to describe all channel controls such as weirs, sluices, flumes or even bridges in terms of simple equations for free and drowned flows. The FlowHead Control lets you define points on a rating curve corresponding to estimated or measured flow and head values at a channel control.
Equations
Mode 1  Free Flow (positive or negative)
Condition  h_{2}/ h_{1} < m  
Equation 
where f is the specified function (values are interpolated from the user entered table of Q:y). For negative flows y_{2} is substituted for y_{1}. 
Mode 2  Drowned Flow (positive or negative)
Condition  h_{2} / h_{1} ≥ m  
Equation 
where or drownf = (1  (y_{2}  z_{c}) / (y_{1}  z_{c})) / (0.3 * (1  m)) if the first formula gives drownf < 0.3 h_{1} = upstream depth of water above crest = y_{1}  z_{c} h_{2} = downstream depth of water above crest = y_{2}  z_{c} z_{c} = crest elevation m = modular limit For negative flows switch h_{2} and h_{1} 
General
If a SPLINE is specified, then a cubic spline is fitted to the data. This will help smooth data where the curvature is high. However, if the data are 'noisy' or if there are sudden changes in gradient, splines may lead to oscillations and may not be suitable.
In free flow (mode 1) the flows and levels at the FlowHead Control will comply with the user input table. However in drowned flow (mode 2) the table of values is altered by the drowned flow reduction factor (drownf). To encourage free flow you can set the modular limit to a high value (eg 0.999).
The second form of the drownf equation is an approximation used to avoid an infinite derivative as the downstream and upstream levels equalise. It is a linearisation of the drowning function between drownf = 0.3 and drownf = 0.0.
The maximum number of qh data pairs allowed in the unit is 500.
Datafile Format
Line 1  Keyword 'QH CONTROL' [comment]
Line 2  Label1, Label2
Line 3  zc , m, [smooth]
Line 4  n1
Line 5 to Line 4+n 1  Q_{i }, y_{i}
Example
QH CONTROL
UNIT027 UNIT028
1.000 0.900SPLINE
3
1.000 0.000
2.000 1.000
3.000 2.000