Experimental Study of Drop Length for Vertical and Inclined Narrow Crested Weirs.

This study concerns with the determination of average drop length and some characteristics of nappe and brink section for stilling basin of narrow crested weirs. For a horizontal smooth rectangular channel, the experimental investigations were carried out under specific conditions such as a free nappe, which occurred when the atmospheric pressure exists beneath the drop; different discharges; different model dimensions and shapes; subcritical approaching flow; and no tail-water effect. This research work was conducted in a flume with dimensions of 12 m long, 0.4 m deep and 0.4 m wide. Two groups of models were made; the first group for narrow crested weir having height of 15, 19.5 and 25 cm; the second one of narrow crested weir with an inclined downstream face with slope of 1:2, having heights of 15, 19.5, and 25 cm and slopes of 1:2, 1:3, and 1:5 having height of 25 cm each. The momentum equation and Newton?s laws of motion were used to derive two theoretical equations to determine the average drop length. From this research, the experimental drop length in all cases of this study was less than that given by the theoretical equations by an average difference ranged from 5% to 7%. This difference is mainly due to the assumptions used for the derivation of the theoretical equation. A coefficient, CL (drop length correction coefficient) was determined from the comparison between the theoretical and experimental results and its value was found equal to 0.97 for narrow crested weirs. For the two groups of narrow crested weirs with vertical and inclined downstream faces, respectively the experimental drop length was found in a good agreement with the design equation. Comparison between the experimental and designed drop lengths indicated that 70.31% of the designed values were within an error of 5% of the experimental values, and 85.94% were within an error of 7%.