Oblique rectangular sharp-crested weir

Abstract

Among discharge flowmeters, the rectangular sharp-crested weir is one of the most popular and accurate, as well as being easy to work with. Using the standard weir, with the help of the conventional weir equation and Rehbock's or a similar standard formula for the discharge coefficient (Cd), the flow can be easily calculated. In order to increase the efficiency of the weir, it can be placed obliquely to the flow, increasing the effective weir length (L) and, hence, increasing the discharge for the same water head with the same channel width (B). The results of a comprehensive set of tests for oblique weirs (1·14 ≤ L/B ≤ 2·26 or oblique angles from 29° to 64°) show that for L/B less than 1·41 (oblique angles < 45°), Cd increases with increasing head but the rate slows down as L/B increases. However, for L/B greater than 1·41 (oblique angles > 45°), Cd decreases with increasing head and is constant for all values of water head while L/B = 1·41 (oblique angle = 45°). On the whole, since with oblique weirs the effective length of the weir is extended, the efficiency of the weir is also increased. Additionally, this kind of weir is aerated automatically and therefore does not need aeration. Also similar tests have been carried out for submerged flow. Finally, a single formula for discharge coefficient relating all tested oblique angles in a free flow situation has been introduced and correction coefficients for submerged flow have been suggested.