Crump Weir
• 21 Sep 2022
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Crump Weir

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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 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

Mode 0 - Dry Crest

Condition

y< zc

y< zc

Equation

 Q = 0 (1)

Figure 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

Figure 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

Figure 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