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FLOW CHARACTERISTICS OF SEMI-CIRCULAR BOTTOM CONTRACTION WEIRS FOR ACCURATE WATER MEASUREMENT IN OPEN CHANNELS

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Title FLOW CHARACTERISTICS OF SEMI-CIRCULAR BOTTOM CONTRACTION WEIRS FOR ACCURATE WATER MEASUREMENT IN OPEN CHANNELS
 
Creator SAI GANGADHARA RAO, D
 
Contributor SATYANARAYANA, T.V
 
Subject FLOW, CHARACTERISTICS, BOTTOM, CONTRACTION, WEIRS FOR ACCURATE WATER MEASUREMENT IN OPEN CHANNELS,
 
Description Water is life. Ever increasing population places new demands on our
water resources. More water is needed for all processes of life, food production,
municipal supply, industrial water use, navigation, recreation etc. Conservation
and associated water measurement are being recognized as important tools for
making best use of available water. Accurate water measurements systems
enable accurate accounting of water use, and permit the available water to be
supplied at optimum rates to the areas where it is intended to be used.
A perfect understanding of some of the primary principles relating to the
subject of water measurement is therefore necessary for establishing any water
measurement system in the canal commands of irrigated agriculture.
Measurement of inflow and outflow is the only way to determine how much
water is being lost from that portion of the irrigation system (Ahmad et al.,
1991). Due to failure in proper water measuring structures, in India, compared to
other countries, the farmers lag behind in receiving equitable distribution of
irrigation water in the canal commands and in other irrigation projects.
The most common structures used to measure flow in open channels
operate by creating critical flow or flow at critical depth through a control
section of known dimensions. The presence of critical flow in control sections
prevents the downstream water level and flow conditions from affecting the flow
through the critical sections, and the discharge can be computed as a function of
the measured upstream head. Measurements of irrigation water flows in field
channels have usually been expensive, too often of questionable accuracy and
otherwise difficult to apply to all field situations. The techniques available in
open channel hygrometry are the use of hydraulic structures (devices), velocityarea
methods, dilution techniques and slope-hydraulic radius and area methods.
Considering the constraints in the measurements of low discharges in open
channels, hydraulic structure technique is best suited. The difficulty in the
analysis of curvilinear flow, the complication in fabrication, the errors in
installation, the economy and the sensitivity towards submergence have limited
the use of these weirs.
Cylindrical weirs were common in late 19th century and early 20th century
prior to the introduction of Ogee shape. During 19th century, developments in
improving weir discharge capacity lead to the design of semi-circular crested
weirs. The characteristics of the flow over semi-circular crested weirs with
different discharges have been of interest to many investigators. A large numbers
of studies have been carried out on the flow characteristics and curvilinear flow
over the weir under critical conditions for rectangular, triangular and trapezoidal
channels. However, very few studies are available on the flow characteristics
and curvilinear flow in the circular channels. In most of the cases, it is not
possible to incorporate suitable parameters fully representing all the involving
variables such that strict mathematical equations could be formulated and
evolved.
The design and development of models is based on the design of simple
flume for flow measurement in open channel proposed by Castro-Orgaz and
others (2008).
Keeping in view the economy, required accuracy and simplicity in
operation, in the present experiment, four weirs with 50 cm, 40 cm, 30 cm, and
20 cm diameter and base width of 30 cm for each weir were prepared with babul
wood with fine finish in the department of Farm Machinery and Power of the
College of Agricultural Engineering, Bapatla. The weirs were painted to have
uniform roughness over the entire section and also to prevent from shrinkage and
any damage due to submergence in water. Before the starting of experiment, the
hydraulic flume was kept perfectly horizontal with motorized bed slope
alteration facility. A semi-circular bottom contraction Weir-I was fixed to the
side walls of hydraulic flume. A point gauge was used to take the water surface
profile before starting the experiment. The depth of water level was maintained
at 20.12, 16.62, 12.68, and 8.0 cm by operating valves for different discharges.
Flow stabilization was carried out in the hydraulic flume as explained above.
A constant head was maintained throughout the experimental run.
Initially the hydraulic flume was run to attain a constant discharge, after a
constant discharge was attained the water surface level drops were recorded at
each 2 cm intervals along the centre line of hydraulic flume by moving the point
gauge on the rails. The starting of the water surface profile measurement is from
55 cm away on upstream side of the weir to 33 cm on downstream side of the
weir. The readings were taken in three trials at each experimental setup. Starting
with the free flow condition, the submergence condition has been increased
gradually to 60% submergence, 75% submergence and 90% submergence with
the help of tail gate provided at the end of the flume.
The process of recording the water surface profiles has been repeated with
remaining three discharges of 18 Ls-1, 12 Ls-1, and 6 Ls-1 and depth of 16.62 cm,
12.68 cm, and 8.0 cm. The semi-circular crested Weir-I is replaced with Weir-II
by taking all precautions as in the installation of Weir-I. Again water surface
profile for discharges of 24 Ls-1, 18 Ls-1, 12 Ls-1, 6 Ls-1 and for depths of
20.12 cm, 16.62 cm, 12.68 cm and 8 cm are noted as in the previous run. The
semi-circular bottom contracted Weir-II has been replaced by Weir-III and
Weir-IV subsequently by taking all precautions as in the installation of Weir-I.
Again water surface profiles for discharges of 18 Ls-1, 12 Ls-1 and 6 Ls-1 and for
submergence conditions 60%, 75% and 90% have been noted as in the previous
run.
From the studies, the following conclusions could be drawn
Under Objective – 1: To design a semi-circular crested weir with critical
flow conditions
1. All the four weir types have enabled creation of critical flow conditions
within the throat section, which indicate their suitability for measurement
of water in open channels in general.
2. All the four design crest heights (25 cm (Weir-I), 20 cm (Weir-II), 15 cm
(Weir -III) and 10 cm (Weir-IV)) are found to be acceptable excepting for
90% submergence level condition. However, the 10 cm crest height of the
weir (Weir-IV) is found to be more advantageous in view of the higher
co-efficient of discharge accomplished.
Under Objective – 2: To determine the flow characteristics, critical depth,
location of critical depth and water surface profile on
the crest for different discharges and submergence
conditions
3. Critical depth (section) has occurred at only one location in the throat
section for all the weirs under all possible conditions.
4. The location of critical depth (section) moves downstream from the center
of the crest with decrease in discharge under free flow conditions and
submerged conditions.
5. The location of critical depth (section) moves towards upstream locations
from the center with increase in crest height.
Under Objective – 3: To study the curvilinear flow over the weir under the
critical conditions
6. The curvi-linear water surface profiles developed for all the weirs under
all conditions and these profiles are useful in understanding the flow
patterns.
7. Since the curvilinear flows exist on the crest, the classical hydrostatic
pressure distribution concept cannot be applied.
8. The coefficients of discharge have been computed for each type of weir.
They varied between 0.42 and 0.82 for different conditions of discharge,
crest height and submergence.
Under Objective – 4: To develop an easy method of computation of
discharges using the head-discharge relationships
by making simple measurements on depth of flow
and submergence
9. The head-discharge relationships developed for the weirs can be used for
determination of discharges with a single and simple measurement of
upstream head. These relationships will be unique since the critical
conditions exist within the throat section except at for 90% submergence.
This provides an easy method of computation of discharges.
10. The coefficient of discharge is found to increase with increase in
discharges. It can be concluded that these weirs will be best suited to
higher discharges (10 to 25 Ls-1) in the range tested.
11. The coefficient of discharge is found to increase with decrease in crest
height and submergence levels. However, this is to be confirmed by
further experimentation as the present study involves only four different
diameters of semi-circular weir.
Key words: Hydraulic flume, semi- circular crested weirs, characteristics of
Semi-circular weirs, open channels, point gauge, critical depth, crest height and
discharges.
1
 
Date 2016-08-17T14:30:03Z
2016-08-17T14:30:03Z
2010
 
Type Thesis
 
Identifier http://krishikosh.egranth.ac.in/handle/1/72789
 
Language en
 
Relation D8628;
 
Format application/pdf
 
Publisher ACHARYA N.G. RANGA AGRICULTURAL UNIVERSITY RAJENDRANAGAR, HYDERABAD