Crump Weir
    • 21 Sep 2022
    • 3 Minutes to read
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    Crump Weir

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    Article summary

    The Crump Weir models a triangular profile weir with a 1:2 sloping front face and a 1:5 sloping back face.

    Data

    Field in Data Entry Form

    Description

    Name in Datafile

    Calibration Coefficient

    Calibration coefficient (should be set to unity for most cases)

    Cc

    Breadth of Weir

    Breadth of weir at crest (m)

    b

    Elevation of Weir

    Elevation of weir crest (m above datum)

    zc

    Modular Limit

    If Calculation Method set to FIXED, then a fixed modular limit value (eg 0.8) specified will be used; if set to VARIABLE (m=0 or blank in the dat file) then an internally calculated value will be used

    m

    Upstream Crest Height

    Height of crest above bed of upstream channel (m)

    p1

    Downstream Crest Height

    Height of crest above bed of downstream channel (m)

    p2

    Upstream Node

    Upstream node label

    Label1

    Downstream Node

    Downstream node label

    Label2

    Upstream Remote Node

    Upstream remote node label (must be a river or conduit section) - use if Label1 is not a river or conduit section

    Label3

    Downstream Remote Node

    Downstream remote node label (must be a river or conduit section) - use if Label2 is not a river or conduit section

    Label4

    Theory and Guidance

    The Crump Weir models a triangular profile weir with a 1:2 sloping front face and a 1:5 sloping back face.

    Crump weirs are used as measuring structures in open channels and have the advantage that the coefficient of discharge is predictable and that the downstream bed elevations have little effect on modular limits and modular coefficient, for one in two upstream and one in five downstream sloping faces.

    The design was originally prepared by Crump in 1952 and further investigated by W.R. White. The equations applied here are taken from White W.R. (1971); coefficient of discharge is taken from Fig.5 and the drowned flow reduction factor from Fig.11 (based on the curve for the ratio of upstream and downstream total head with no truncation of the weir).

    It must be noted that the Crump Weir operates in terms of total head and requires that the upstream and downstream nodes are conduit or river sections, from whence the velocities are determined to calculate total head; if not, for instance if either node is attached to a junction., then remote upstream and/or downstream nodes may be specified from which to obtain a representative velocity.

    Equations

    y³ y(forward flow) h1 = hu , etc

    y< y(reverse flow) h1 = hd , etc

    h1 = y- zc

    h2 = y- zc

    h= upstream head

    h= downstream head

    Mode 0 - Dry Crest

    Condition

    y< zc

    y< zc

    Equation

    Q = 0

    (1)


    RiverNodesimagesCrumpWeirMeasurements.gifFigure 1: Crump Weir parameters (slopes not to scale)

    Mode 3 - Free Flow

    Condition

    y1 < zc or y2 < zc

    H2/H1 ≤ m

    where:

    m is the modular limit

    Equation


    (2)

    where:

    Cd = discharge coefficient

    g = gravitational acceleration (m/s2)

    H1 = h1 + v12/2g

    H2 = h2 + v22/2g

    with:

    v1 = upstream flow velocity

    v2 = downstream flow velocity


    RiverNodesimagesCrumpWeirMode3.gifFigure 2: Crump Weir (free flow)

     Mode 4 - Drowned Flow


    Condition

    y1 > zc

    H2/H1 > m

    where:

    m is the modular limit

    Equation


    (3)

    where:

    Cd = modular discharge coefficient

    fr = drowned flow reduction factor

    g = gravitational acceleration (m/s2)

    H1 = h1 + v12/2g

    H2 = h2 + v22/2g

    with:

    v1 = upstream flow velocity

    v2 = downstream flow velocity


    RiverNodesimagesCrumpWeirMode4.gifFigure 3: Crump Weir (drowned flow)

    General

    A warning is generated if the Crump Weir is not attached (either directly or remotely) to nodes which have cross sections from which velocities can be established.

    The routine allows reverse flow but applies the same equation (it assumes that the upstream face has a slope of one in five and the downstream sloping face is one in two). In practice it is unlikely that reverse flow will occur over a Crump Weir unless it has been sited badly.

    If the weir complies with the British Standard and is free from blockages the calibration coefficient should be set to unity.

    Datafile Format

    Line 1 - Keyword 'CRUMP' [comment]

    Line 2 - Label1, Label2, Label3, Label4

    Line 3 - , , ,  

    Line 4 - p1, p2

    Example

    CRUMP
    UNIT029     UNIT030
         0.900    10.000     1.000     0.900
         1.000     2.000

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