 21 Sep 2022
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Notional Weir
 Updated on 21 Sep 2022
 3 Minutes to read

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The Notional Weir acts as a broad crested weir with a rectangular control section for free flow only. For drowned flow, the water levels are set as identical on each side of the weir
Data
Field in Data Entry Form  Description  Name in Datafile 

Discharge Coefficient  Coefficient of discharge  C_{d} 
Velocity Coefficient  Coefficient of approach velocity  C_{v} 
Exponent  Exponent of y (e = 1.5 for a rectangular control section) (e = 2 for a parabolic control section) (e = 2.5 for a triangular control section)  e 
Breadth of Crest  Breadth of weir at control section (normal to the flow direction) (m)  b 
Elevation of Crest  Elevation of weir crest (m Above Datum)  z_{c} 
Upstream  Upstream Label  Label 1 
Downstream  Downstream Label  Label 2 
Theory and Guidance
The Notional Weir acts as a broad crested weir with rectangular control section for free flow only. For drowned flow, the water levels are set as identical on each side of the weir.
The Notional Weir is extremely similar to the Weir in its simplified approach to modelling weirs, in that the effect of boundary layers and upstream dependence of the coefficients of approach velocity C_{v} and discharge C_{d} are neglected.
As with the Weir, triangular and parabolic control sections can be modelled by factoring the coefficient of discharge  see the General section below for details on how to do this.
Both forward and reverse flow can be modelled.
Equations
y_{1} ³ y_{2} (forward flow) h_{1} = h_{u}, etc
y_{1 }< y_{2} (reverse flow) h_{1} = h_{d}, etc
h_{1} = y_{1}  z_{c}
h_{2} = y_{2}  z_{c}
h_{u} = upstream head
h_{d} = downstream head
Mode 0  Dry Crest
Condition  y_{1} ³ z_{c} y_{2} ³ z_{c} 
Equation  Q = 0 
Mode 1  Free Flow (Positive Sense)
Condition  y_{1}_{ }> Z_{c} h_{2} £ 1.3 * hc_{rit} where:
 
Equation 

Mode 2  Free Flow (Negative Sense)
Condition  y_{2} > Z_{c} h_{1}_{ }£ 1.3 * h_{crit} where hcrit is defined as for Mode 1  
Equation 

Mode 3  Drowned Flow (Positive Sense)
Condition  y_{1} > Z_{c} h_{2} > 1.3 * h_{crit} where hcrit is defined as for Mode 1 
Equation  h_{1} = h2 
Mode 4  Drowned Flow (Negative Sense)
Condition  y_{2}_{ }> Z_{c} h_{1} > 1.3 * h_{crit} where hcrit is defined as for Mode 1 
Equation  h_{2} = h1 
General
Instead of an input modular limit, used in most of the Flood Modeller structures, a calculated value of


is used to differentiate between free and drowned flows. If the downstream water elevation above the weir crest exceeds 1.3 * y crit then the flow is assumed to be drowned. Otherwise it is free.
If a nonrectangular section is employed, the results must be examined carefully as the formula to calculate the critical height is approximate in this case.
The Notional Weir can be used to model approximately regions of supercritical flow at a location which acts as a channel control at low flows but is immaterial to water levels at higher flows.
Care must be taken when a Notional Weir is attached to other structures; the model results should be closely examined to ensure the correct behaviour of the model. Results could be compared by either using a round nosed weir instead, or inserting a reach of open channel or culvert between the structures.
Problems can arise if the weir is not acting as a control. For example, if the upstream cross section has supercritical flow due to the bed being higher than the weir or the weir being extremely wide compared to the width of the upstream cross section. Another problem may arise when the downstream water level oscillates around the critical depth; a warning message is given in this case.
Inaccuracies may be introduced if the weir crest is too short for critical flow to develop. Short crested weirs may be modelled approximately using a Notional Weir . Very short crested weirs may be better modelled with a sharpcrested weir.
Datafile Format
Line 1  Keyword `NOTWEIR'
Line 2  Label 1, Label 2
Line 3  e
Line 4  C_{d}, C_{v}, b, z_{c}
Example
NOTWEIR  west drayton mill 2.022a
UNIT072 UNIT073
2.500
0.900 0.800 10.000 1.000