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.
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).
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 ...
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