ABSTRACT
Linear proportional weirs refer to as sharp crested
weir in which the discharge is linearly proportional to the head over the weir
crest and tend to have high accuracy when compared with the non linear ones.
The main objective of the study was to investigate the flow characteristics of
a Sutro weir in an open channel.In this research work the flow characteristics
of a Sutro Weir were studied. The assumption of the linearity of the flow
through the weir was investigated. In addition, the effects of the rectangular
base height (s) and the radius (R) of the curved section on the head measured
above the weir sill were monitored. The experiments were conducted in a flume
having a working length of 6m with a cross section 0.3m wide and 0.3m deep. A
total of 45 laboratory experiments were carried out on nine different models
for five different discharges. The models made of wood were grouped in to two
sets of which set one(1) consists of five models of constant radius of 9cm and
varying base heights. The set 2 consists of four models of constant weir base
height of 4cm and varying curve radius.The results showed that the radius (R)
of the curved section has more effect on the head measured above the weir sill
compared with the rectangular base height(s). The set 2 models showed a better
linear relationship between the actual discharge and the head measured above
the weir sill compared to the set 1 model.
CHAPTER ONE
INTRODUCTION
1.1 Preamble
The ever growing demand for water
makes the understanding of water measuring techniques important and necessary.
Accurate flow measurement is very important for proper and equitable
distribution of water among water users. Gertrudys (2006) wrote that the
information concerning the volume of available water is very helpful in
planning for its future use and distribution.
The effective use of water for
irrigation requires that flow rates and volumes be measured and expressed
quantitatively. Measurement of flow rates in open channels is difficult because
of non-uniform channel dimensions and variations in velocities across the
channel. Weirs allow water to be routed through a structure of known
dimensions, permitting flow rates to be measured as a function of depth of flow
through the structure. Allens and Dalton (2002), mentioned that one of the
simplest and most accurate methods of measuring water flow in open channels is
by the use of weirs.
Open channel flow is flow in any
channel in which the liquid flows with a free surface, such as tunnels, partially
filled pipes, canals, streams, and rivers. Flow measurement is the
quantification of fluid movement parameters.Boiten (1993) stated that since the
early days of hydraulics, hydraulic structures have been installed in open
channels with a free water level to estimate discharge based on the measured
upstream water level.
A weir is basically an obstruction
in an open channel flow path. A weir functions by causing water to rise above
the obstruction in order to flow over it. The height of water above the obstruction
correlates with the flow rate, so that measurement of the height of the flowing
water above the top of the weir can be used to determine the flow rate by the
use of an equation, graph or table. The top of the weir, which is used as the
reference
level for the height of water flowing over it, is called the crestof the weir.
Weirs are typically classified as being either sharp-crested or broad-crested.
Sharp crested weirs are widely used for the purpose of flow measurement, flow
diversion and water level control in hydraulics, irrigation, and environmental
projects. Rectangular, triangular, cipolletti, circular and sutro are some of
the important shapes of sharp crested weirs (Novac, 2000).
Weirs are well suited for measuring
low flows, particularly where there is little head available. In addition to
being used to measure flows, weirs are commonly used in wastewater treatment
systems in secondary clarifiers to ensure uniform flow distribution along the
effluent channel. Weirs are not generally considered suitable for raw
wastewater (influent) flow measurement as solid materials can accumulate on the
upstream side of the weir that can disturb the conditions for accurate
discharge measurement or even block the weir (Novac, 2000).
Regardless of their performance,
properties, ages, or conditions, it should be noted that weirs are engineering
structures that have to function in difficult conditions. As one of the main
components of dam construction and water projects, weirs are important
structures built for various purposes. Two of the most important functions of
weirs are measurement of water discharge and adjustment of the water level in
primary and secondary channels. Considering the complex work they do, weirs
should be strong, reliable, and highly efficient so that they can readily be
put to use (Rasool and Ensiyeh, 2012).
Proportional weirs and their
discharge characteristics have been under investigation ever since the concept
of such weirs was first proposed by Stout (1897). These weirs have a profile
which ensures a certain relationship between the head on the weir and the
discharge. Linear proportional weirs are used as flow measuring devices, and as
outlets
for
settling basins, grit chambers and dosing siphons. There are various types of
linear proportional weirs depending on the shape of the base profile, which may
be parabolic, triangular, trapezoidal or rectangular.
The “sutro weir” is the most common
of the linear proportional weirs with rectangular bottom sections. The sutro
profile is asymptotic at the bottom (Fig.2.1) leading to an infinitely wide
base (Pratt, 1914).Sutro, 1914 tried to overcome this by assuming a known base
in the form of a rectangular weir of depths, above which the weir profile is
fitted. Recently, Keshava and Sashagiri (1968) presented a generalized
mathematical theory of proportional weirs, and supported their theory with
experimental verification. From the point of view of constructing the linear
weir profile, the profile computations suggested by earlier investigators
involve complex mathematical expressions. In engineering field applications, it
is necessary to seek a solution which ensures ease of construction of the weir
and provides the required accuracy in the linear head-discharge relationship.
This was the motivation for Sutroin 1914 to develop a practical linear
proportional weir and is known as the Sutro Weir. A designed shape is fitted
for the Sutro weir which has a rectangular base.
The proportional weir is defined as
a weir in which the discharge is linearly proportional to the head over an
arbitrary reference level which, for the Sutro Weir, has been selected at a
distance of one-third of the height of the rectangular section above the weir
crest.
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