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 weir at crest (m)

b

Elevation of Crest

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 £ Zcy2 £ Zc Equation Q = 0
Figure 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)

Figure 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)

Figure 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``````