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.

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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