WEIRS

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

WEIR

• its a barrier across the flowing fluid

designed to alter the flow characteristics.

• it is used to prevent flooding, measure

discharge, and render the river navigable.

• it take forms of obstructions smaller than

the conventional dams.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

• Since the geometry of top of the weir is

known and all water flows over the weir,

depth of water behind the weir can be

converted to a rate of flow.

• discharge can be summarized as;

Q=CLHn

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

• When used for flow measurement it is

important that the weir crest is free of rust

or impurities.

• Roughness of any form will cause the weir

to discharge more water than indicated by

the standard discharge equations or

tables.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

TYPES

1. LABYRINTHWEIR

It uses a trapezoidal shaped weir geometry

to increase the weir length. They are versatile

structures and can be modified to many

applications.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

2. BROAD CRESTED WEIR

It is a flat crested structure, with long

crest compared to flow thicknes. When the

crest is broad, the streamlines become

parallel to the crest invert and the

pressure distribution above the crest is

hydrostatic.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

3. SHARP CRESTED WEIR

It allows the water to flow cleanly away

from weir. Also known as FAYOUM weir.

These weirs are typically .25 inch (6.4

mm) or thinner metal plates. They are in

many shapes and styles like rectangular, v

notch and cipoletti weirs.... It can exhibit

accuracies to about +/-2%.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

4. COMPOUND WEIR

For accurate flow measurement

over a wider range of flow rates, a

compound weir combines two or more

types -typically a v-notch weir with a

rectangular weir. These weirs are intented

to measure no more than 35% of the

pipe's open channel flow capacity.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

5. V-NOTCH WEIR

Is a triangular channel section, used to

measure small discharge values. The upper

edge of the section is always above the water

level, so the channel is always triangular

simplifying calculation. V-notch weirs are

preferred for low discharges as head above the

weir crest is more sensitive to changes in flow

compared to rectangular weirs.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

V-notch weirs typically achieve

accuracies of 2-5% . V-notch weirs are

sized between 22-1/20 and 1200, with 22-

1/20,300,450,600,900 and 1200 the common

increments- although the free flow

discharge equations can be developed

from one universal equation for v-notch

weirs from 250 to 1200 in size.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

DRAW BACKS

• Typically increases the oxygen content of the water which brings a detrimental effect on the ecology of the local water bodies.

• Even though the water around weirs can often appear relatively calm, they can be extremely dangerous places to boat, swim, etc..as the circulation patterns on the downstream side typically known as hydraulic jump, can submerge a person indefinitely.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL

• It can become a point where the garbage

and other wastes accumulates.

CE8603 – IRRIGATION ENGINEERING/V.PRIYA/AP/CIVIL