The results of ε for the sidewall angles range from 6 to 35° and the quarter-round crest are shown in Figure 7. In general, ε is larger when α is small. According to total capacity, labyrinth weirs with larger sidewall angles are less efficient. For each α, the labyrinth weir efficiency (ε) increases with increasing H_t′/P′. This is because nappe interference and aeration behind the nappe flow decrease when H_t′/P′ increase. For comparison between the efficiency of the compound and conventional labyrinth weirs, the percentage of the improvement in efficiency was 10, 8.9, 7.8, 7.2, 6.6, 5, and 4% for α = 6, 8, 10, 12, 15, 20, and 35°, respectively. The best curve-fit coefficients of Equation (13) and the coefficient of determination (R²) are presented in Table 4. These equations are valid for 0.07 ≤ H_t′/P′ < ∼ 0.85. Equation (13) assists in determining the efficiency of the compound and conventional labyrinth weirs: