Sharp Crested Weir
    • 21 Sep 2022
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    Sharp Crested Weir

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    Article summary

    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

    Cc

    Breadth of Crest

    Breadth of weir at crest (m)

    b

    Elevation of Crest

    Elevation of weir crest (mAD)

    zc

    Downstream Height

    Height of crest above bed of upstream channel (m)

    p1

    Upstream Height

    Height of crest above bed at downstream channel (m)

    p2

    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 Cc.

    Reverse flow is permitted but assumes exactly the same equations, conditions and parameters as for forward flow.

    Equations

    Mode 0 - Dry Crest

    Condition

    y1 £ Zc

    y2 £ Zc

    Equation

    Q = 0

    RiverNodesimagesSCWeirMeasurements.gifFigure 1: Sharp Crested Weir parameters

    Mode 3 - Free Flow

    Condition

    y1 > Zand h2 £ 0 (for forward flow)

    Equation

    Q = Ce Cc 2/3 Ö(2g) b h11.5

    (1)

    where:

    Ce = 0.602 + 0.075 h1 / p1

    g = gravitational acceleration (m/s2)

    RiverNodesimagesSCWeirMode3.gifFigure 2: Sharp Crested Weir (free flow)

    Mode 4 - Drowned Flow

    Condition

    y1 > Zand h2 > 0 (forward flow)

    Equation

    Q = fr Ce Cc 2/3 Ö (2g) b h11.5

    (2)

    where:

    Ce = 0.602 + 0.075 h1 / p1

    fr = [1 - (h2/h1)1.5 ]0.385

    g = gravitational acceleration (m/s2)

    RiverNodesimagesSCWeirMode4.gifFigure 3: Sharp Crested Weir (drowned flow)

    General

    The equations are written in terms of water level and not total head. The coefficient of velocity (Cc) is specified by the user and no account is taken for variation of Cwith 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 - Cc, b, zc

    Line 4 - p1, p2

    Example

    SCWEIR
    UNIT023     UNIT024
         1.100    10.000     1.000
         1.000     2.000

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