INTRODUCTIONThe word "weir" will be used to describe any structure used to determine the volume of flow of water from measurements of its depth on a crest or sill of known length and form. In this general sense timber and masonry dams having various shapes of section, reservoir overflows, and the like may be weirs. Terms, more or less synonymous, used to describe such weirs are "comb," "wasteway," "spillway," "overwash," "rollway," and "overfall" (Horton, 1907)Weirs have been widely used for the flow measurement, flow diversion and its control in the open channels (Kumar et al., 2011). Weirs are categorized in two main types: sharp-crested and broad-crested. Sharpcrested weirs normally consist of a vertical plate mounted at right angles to the flow and having a sharpedged crest (Henderson, 1966).In open channel hydraulics, weirs are commonly used to either regulate or to measure the volumetric flow rate. They are of particular use in large scale situations such as irrigation schemes, canals and rivers. For small scale applications, weirs are often referred to as notches and invariably are sharp edged and manufactured from thin plate material.

OBJECTIVES1. To observe the flow characteristic of open-channel flow over, firstly, a rectangular notch and then a triangular (vee) notch.2. To determine values of the discharge coefficient for both notches.

THEORYThe simplest form of these weirs consists of a plate set perpendicular to the flow in a rectangular channel, its horizontal upper edge running the full width of the channel. This last feature means that the flow is essentially two-dimensional, without lateral contraction effects. Also, so called contracted weirs exists which are contracted from the sides as well as in the vertical plane. This last type will involve three-dimensional flow (Henderson, 1966). Weirs have various shapes such as rectangular, triangular, trapezoidal, circular etc. and especial applications and governing equations. The Typical form of the governing equation of these weirs is as following (Bos, 1989): Q = kHn where Q = flow rate, k = coefficient depending on the size and shape of the weir and n = dimensionless number depending on the shape of the weir. For rectangular and triangular weirs n equals to 1.5 and 2.5, respectively.With different type of notch, the method of calculating the discharge coefficients of the fluid flow is also different. These are some theories that discuss about the reasoning of two different notches, which are:

1. Rectengular NotchA rectangular notch is a thin square edged weir plate installed in a weir channel as shown in figure below (Source: http://www.cussons.myzen.co.uk/SOFTWARE/Part5/PART5.HTM#3).Based on the figure above, consider that the flow in an element of height H at a depth, h below the surface. Then, assume that the fluid flow everywhere normal to the plane of the weir and that the free surface remains horizontal up to the plane of the weir.Discharge equation for a rectangular sharpcrested weir of the same width of the channel can be simplified as (Henderson, 1966): Qt= Cd Where;Qt= volume flow rate (m/s)H= height above notch base (m)B= width of rectangular notch (0.03m)Cd= the discharge coefficient which has to be determined by the experimentThe Cd depends on flow characteristics and geometry of the channel and weir (Kumar et al., 2011). For simplicity we can consider that Cd is just dependents on the ration H/W, for example we can use so called Rehbock equation for H/W