Water Discharge - Essay Example

The Weirs Introduction Weirs are the controls or barriers placed in an open channel to permit measurement of water discharge (Encyclopedia Britannica2009). They are the type of dams that are used to raise the level of river of stream. Traditionally weirs were used to create millponds in some places, water flows over the top of the weirs although a few release water at the level below the top of the weir with the help of sluice gates present. The crest of the overflow spillway on a large dam is often called a weir. Weirs can be classified into various types; Wubbo Boiten in his book "Hydrometry", classifies weirs into the following categorize, 1. Broad crested weirs In this type the length of the crest is just sufficient to allow straight and parallel streamlines at least along a short distance along a crest. The crest height complies with the certain minimum value. The weirs under this category are; round nose horizontal broad crested weir, rectangular broad crested weir, Romign measuring and regulating weir, trapezoidal profile weir, Fayoum standard weir and V-shaped broad crested weir. 2. Sharp crested weir The length of the crest is 1 to 2 mm, for this reason they are also called thin plate weirs. The nape is free from weir body after passing the weir, and the streamline above the crest are strongly curved. The atmospheric pressure should prevail in the air filled area below the underside of the out flowing jet. Amongst these weirs are; horizontal sharp crested weir, rectangular sharp crested weir, V-shaped sharp crested weir, trapezoidal sharp crested weir, circular sharp crested weir and proportional weir (Sutro weir). 3. Short crested weirs They look like broad crested and sharp crested weirs. The streamline along the crest is curved. These include; weir still with rectangular control section, V-notch weir still, triangular profile weir, flat V-weir butchers movable standing wave weir, Wes- standard spill ways, cylindrical crested weir, streamlined triangular profile weir, flap gates and Rossum weir. Application and Importance of weirs Weirs are of great importance; broad crested weirs are used in measuring flow in open channels whenever the water surface can remain free. The throat perpendicular to the direction of flow can be shaped in such a way that the complete range of discharges can be measured accurately and without creating an excessive backwater effect. Weirs are used along with locks, to render a river navigable and to provide even flow for navigation. They allow hydrologist and engineers to measure the rate of flow of fluid in small to medium-sized streams, or in industrial discharge locations. Since the geometry of the top of the weir is known, and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. Because a weir increases the oxygen content of the water as it passes over the crest, a weir can have a detrimental effect on the local ecology of a river system(M.G.Bos, 1984). A weir will artificially reduce the upstream water velocity, which can lead to an increase in siltation. The weir may pose a barrier to migrating fish. Fish ladders provide a way for fish to get between the water levels. Mill ponds provide a water mill with the power it requires, using the difference in water level above and below the weir to provide the necessary energy. Because of their gradually converging transition these structures have few problems with floating debris. "Field and laboratory observations have shown that structures can be designed to pass sediments transported by channels with sub critical flow" (M.G.Bos, 1984). Among similar hydraulic and similar boundary conditions weirs are the economic of all structures for the accurate measurement of flow. As these advantages are being recognized the use of broad rest weirs is propagating, they are serving a variety of purposes; in hydrology they measure the discharge from catchments, in irrigation they measure they measure and control the distribution of water at canal bifurcation and at off take structure, in sanitary engineering they measure the flow from urban areas and industries into drainage systems and in both irrigation and drainage they can control the upstream water at a desired level( Sturm, T.W, 2000). Other applications are drop structures and weirs are small dams placed across a waterway to provide changes in gradient, slow water velocities and reduce erosion by water flow is directed through the weir into a stilling basin where the energy of the flow is dissipated. Critical factors in the success of such structures are the proper engineering of the drop structure itself to withstand hydraulic pressure and to prevent outflanking. Many weir structures will require a stilling basin. The downstream face of a vertical weir is flush. These structures are often used on small streams, usually in a system of weirs. "High vertical weirs require a still basin which may be created by constructing a scour apron and counter weir from gabion mattresses" (M.G.Bos, 1984). Further Bos states that stepped weirs differ slightly from vertical weirs. The addition of stepped downstream faces provides for some energy to dissipation at each level. Stepped weirs are appropriate for small structures in waters without heavy sediment loads. Stepped structures are often constructed with some degree of batter. Where larger structures are required or bearing capacity of soils is limited, sloped weirs are most appropriate. Sloped weirs are ramped on both the upstream and downstream faces. As with vertical structures, sloped weirs may require a stilling basin. (M.G.Boss, 1984). There are a number of weirs, a few are discussed below. Rectangular weirs There are various categorize of weirs, one such is the rectangular weir. "A rectangular weir has a rectangular opening and it can be either suppressed or unsuppressed" (Arcadio Pacquiao Sincero, 2002). A suppressed (subcontracted) weir has a rectangular opening channel width; a vent is often needed to maintain atmospheric pressure. An unsuppressed (contracted) weir has a rectangular opening which only spans part of the channel. It is an obstruction and backs up the flowing stream of liquid. The rectangular opening is composed of two vertical sides; one bottom called crest and no top side. To ensure the accurate measurement of flow the crest and vertical sides should be beveled into a sharp edge. "The flow rate measurement in the rectangular weir is based on the Bernoulli equation principles and can be expressed by; q = 2/3 cd b (2 g)1/2 h3/2 (1) Where; q = flow rate (m3/s), h = head on the weir (m), b = width of the weir (m), g = 9.81 (m/s2) - gravity, cd= discharge constant for the weir - must be determined, cd must be determined by analysis and calibration tests. For standard weirs - cd - is well defined or constant for measuring within specified head ranges" (Arcadio Pacquiao Sincero, 2002). Triangular weirs or V-Notch weir According to Arcadio Pacquiao Sincero, in triangular weirs the flow passes through in the form of triangle. They measure the rates of flow proportional to the cross sectional area of flow, thus they are also area meters. In addition they obstruct flows so triangular weirs are also intrusive flow meters. The hydraulic profile measured by these weirs is exactly similar to that measured by rectangular weirs. "Their flow rate is measured as q = 8/15 cd (2 g)1/2 tan(/2) h5/2 (2) Where = v-notch angle" (Arcadio Pacquiao Sincero, 2002). Sutro or Proportional weirs In these weirs the discharge is linearly proportional to the head over an arbitrary level which for the Sutro weir has been selected at the distance of one-third of the height of rectangular section above the weir crest. The rectangular portion joint to the curved portion provides the proportionality for all heads above the rectangular section. They have a varying cross sectional shape with depth. It provides high flow measurement accuracy for both small and large flow rates. The weir may be symmetrical or non symmetrical. They are designed to measure flow under free flow conditions. This type is not common in practice. (Arcadio Pacquiao Sincero, 2002). Polynomial weirs This is another uncommon weir, Polynomial weirs can be applied in agricultural and municipal engineering to produce a wide range of head-discharge behavior, including the proportional (linear) characteristics of the Sutro weir. In these weirs Variation of the ratios of longitudinal components of velocity vector of spill flow to the mean velocity of the main channel flow at upstream end of the side weir, exhibit a non-linear relationship. Then the variation of the discharge ratios of spill flow and main channel discharges is studied to find the final flow measurement. It usually uses a momentum principle. (Wubbo Boiten, 2000). Amongst many; these are the few weirs which exist in today's world, and with the increase in technological innovation weir are improving and their importance and application is increasing so that man race can take full benefits from it. Work cited Arcadio Pacquiao Sincero, Physical-chemical treatment of water an wastewater, Published by CRC Press, 2002 Marinus G. Bos, M. G. Bos, Long-throated Flumes And Broad-crested Weirs, Springer, Nov 1984 Sturm, T.W, Open Channel Hydraulics, Water Resources and Environmental Engineering Series McGraw Hill, Boston, USA,2000 Wubbo Boiten, Hydrometry, Edition: 2, Published by Taylor & Francis, 2000 "Encyclopedia Britannica. 2009, Encyclopedia Britannica Online. 24 May. 2009 http://www.britannica.com/EBchecked/topic/639087/weir Read More