weirs - sns courseware
TRANSCRIPT
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