 21 Sep 2022
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Sharp Crested Weir
 Updated on 21 Sep 2022
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The Sharp Crested Weir represents a full width sharp crested weir in free or drowned mode with forward or reverse flow. They are often used as measuring devices  particularly for measuring small flows.
Data
Field in Data Entry Field  Description  Name in Datafile 

Calibration Coefficient  Calibration coefficient  C_{c} 
Breadth of Crest  Breadth of weir at crest (m)  b 
Elevation of Crest  Elevation of weir crest (mAD)  z_{c} 
Downstream Height  Height of crest above bed of upstream channel (m)  p_{1} 
Upstream Height  Height of crest above bed at downstream channel (m)  p_{2} 
Upstream  Upstream node label  Label1 
Downstream  Downstream node label  Label2 
Theory and Guidance
The Sharp Crested Weir represents a full width sharp crested weir in free or drowned mode with forward or reverse flow.
Sharp crested weirs are often used as measuring devices  particularly for measuring small flows. The equations used for coefficient of discharge were derived by Kindsvater C.E. and Carter R.W.C. (1957) for weirs with crests of length less than 2mm in the direction of flow, under which condition the flow estimated is most accurate.
Drowned flow is modelled using the Villemont equations (Submerged Weir Discharge Studies, Engineering News Record, 1947).
The Sharp Crested Weir applies only to full width weirs and the discharge coefficient assume smooth vertical, plane, parallel channel sides. Deviation from the standard specification, which is described in greater detail in BS3680 (1981  90) Part 4A, will introduce an inaccuracy which should be accounted for through the coefficient C_{c}.
Reverse flow is permitted but assumes exactly the same equations, conditions and parameters as for forward flow.
Equations
Mode 0  Dry Crest
Condition  y_{1}_{ }£ Z_{c} y_{2}_{ }£ Z_{c} 
Equation  Q = 0 
Mode 3  Free Flow
Condition  y_{1}_{ }> Z_{c }and h_{2} £ 0 (for forward flow)  
Equation 
where: C_{e} = 0.602 + 0.075 h_{1} / p_{1} g = gravitational acceleration (m/s^{2}) 
Mode 4  Drowned Flow
Condition  y_{1} > Z_{c }and h_{2} > 0 (forward flow)  
Equation 
where: C_{e} = 0.602 + 0.075 h_{1} / p_{1} f_{r} = [1  (h_{2}/h_{1})^{1.5} ]^{0.385} g = gravitational acceleration (m/s^{2}) 
General
The equations are written in terms of water level and not total head. The coefficient of velocity (C_{c}) is specified by the user and no account is taken for variation of C_{c }with head.
For a well maintained weir constructed to the standard specification the calibration coefficient should be set to unity.
Datafile Format
Line 1  keyword `SCWEIR' [comment]
Line 2  Label1, Label2
Line 3  C_{c}, b, z_{c}
Line 4  p_{1}, p_{2}
Example
SCWEIR
UNIT023 UNIT024
1.100 10.000 1.000
1.000 2.000