ABSTRACT
The overflow characteristics of semi-circular crested weir models were investigated. Twelve models were fabricated and tested. The models were categorized into two groups; normal weirs and oblique weir models. Both group of model had constant weir height and had the crest radius varied three times; 5cm, 7.5cm and 10cm. However, for the oblique weir, the oblique angle was varied three times; 60o , 30o and 15o . From experimental results, it was observed that for normal weirs, the coefficient of discharge (Cd) increases with corresponding increase in head to crest height ratio (h/P). There were increases in Cd by 3.75%, 3.11% and 3.12% for 5cm, 7.5cm and 10cm normal weirs respectively. However, the oblique weirs showed corresponding increases of (Cd) with the increase of (h/P) values; while the highest Cd values were obtained with weirs of small oblique angle (α =15o ) for all crest radius tested. There were increases in Cd values by 0.4%, 4.8% and 4.86% as the oblique angle was varied from 900 to 600 , 600 to 300 and 300 to 150 respectively. For normal weirs, the performance in terms of the discharge magnification factor (Qac/QNS) increases as (h/P) values increases. Hence, for all values of crest radius tested, normal weirs of semicircular crests had performance better than those of sharp crested weirs. While, for oblique weirs the performance in terms of the discharge magnification factor (Qac/QNS) increases with the increases in (h/P) values for all oblique weirs tested, hence, Weirs of small oblique angles give high discharge magnification factor and performance. Therefore, the model with radius 5cm and oblique angle 15o was selected as the most efficient with a discharge coefficient and magnification factor of 0.885 and 1.326 respectively. Expression for determining the flow rate over the semi-circular weir was developed through dimensional and regression analysis respectively. Similarly, linear and non-linear equations for estimating the discharge coefficients of the weir models were developed. It was found that the flow rate equation developed had correlation coefficient of 0.986 with maximum error of 1.4%. Also, the linear and non-linear equations developed had correlation coefficients of 0.664 and 0.574 with maximum errors of 33.5% and 42.6% respectively. Finally, the mathematical models developed for computing the discharge coefficient can be used for design of normal and oblique semi-circular crested weirs.